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Sample records for active shear zone

  1. Zipper and freeway shear zone junctions

    NASA Astrophysics Data System (ADS)

    Passchier, Cees; Platt, John

    2016-04-01

    Ductile shear zones are usually presented as isolated planar high-strain domains in a less deformed wall rock, characterised by shear sense indicators such as characteristic deflected foliation traces. Many shear zones, however, form branched systems and if movement on such branches is contemporaneous, the resulting geometry can be complicated and lead to unusual fabric geometries in the wall rock. For Y-shaped shear zone junctions with three simultaneously operating branches, and with slip directions at a high angle to the branch line, eight basic types of shear zone triple junctions are possible, divided into three groups. The simplest type, called freeway junctions, have similar shear sense on all three branches. If shear sense is different on the three branches, this can lead to space problems. Some of these junctions have shear zone branches that join to form a single branch, named zipper junctions, or a single shear zone which splits to form two, known as wedge junctions. Closing zipper junctions are most unusual, since they form a non-active high-strain zone with opposite deflection of foliations. Shear zipper and shear wedge junctions have two shear zones with similar shear sense, and one with the opposite sense. All categories of shear zone junctions show characteristic flow patterns in the shear zone and its wall rock. Shear zone junctions with slip directions normal to the branch line can easily be studied, since ideal sections of shear sense indicators lie in the plane normal to the shear zone branches and the branch line. Expanding the model to allow slip oblique and parallel to the branch line in a full 3D setting gives rise to a large number of geometries in three main groups. Slip directions can be parallel on all branches but oblique to the branch line: two slip directions can be parallel and a third oblique, or all three branches can have slip in different directions. Such more complex shear zone junctions cannot be studied to advantage in a

  2. Distributed Anelastic Strain and its Relationship to Compliant Zones Surrounding Active Faults of the Eastern California Shear Zone

    NASA Astrophysics Data System (ADS)

    Shelef, E.; Oskin, M.; Fialko, Y.

    2006-12-01

    Geologic measurements of distributed anelastic strain (DAS) adjacent to active strike slip faults of the Mojave Desert portion of the Eastern California shear zone quantify the magnitude, mechanism, temporal evolution, and relationship of DAS to fault compliant zones imaged via InSAR. Prefaulting markers (mylonitic lineation, dikes, and faults assumed linear prior to dextral faulting) in crystalline rocks next to the Harper Lake fault and Calico fault indicate that DAS accounts for 6 to 23 percent of total displacement and that this displacement scales with fault slip. We conclude that DAS is a significant, active process that is not restricted to the initial fault propagation stage. We find that the width of the zone of DAS is 400-700 m on each side of the faults studied, irrespective of total fault slip. 60 percent of the displacement due to DAS occurs within 100 m of the Calico fault. A similar zone of more intense deformation occurs adjacent to the Harper Lake fault. These 100m- wide-zones are of the same extent but much less intensely deformed compared to the damage zones surrounding the San Andreas fault. Based on these relationships, we hypothesize that damage feedback progressively focuses DAS into a stable, approximately 100-m-wide-zone where its intensity can increase proportionally to fault slip. Disruption of linear markers supports that DAS in crystalline rocks occurs via slip along secondary faults and small-scale block rotation with block sizes decreasing with proximity to faults. The widths of the geologically documented zones of DAS in the Eastern California shear zone are similar to the approximately 1 km width of compliant zones modeled from InSAR observations of surface deformation due to stress changes caused by nearby earthquakes. This correlation suggests a relationship between damage- reduction of shear modulus and displacement via DAS. Paleomagnetic measurements of prefaulting and syntectonically emplaced volcanic rocks in sedimentary

  3. Growth and interaction of active faults within a nascent shear zone, central Mojave Desert, California

    NASA Astrophysics Data System (ADS)

    Oskin, M.; Strane, M.

    2006-12-01

    Compilation of new slip-distribution and slip-rate data from the Mojave Desert portion of the Eastern California shear zone (ECSZ) lends insight into the role of fault growth and interaction of conjugate fault systems in accommodating shear. Dextral faults of the Mojave Desert ECSZ approach but do not appear to cut the bounding ENE-striking sinistral Pinto Mountain and Garlock faults. Differing styles of accommodation of these bounding faults occur at opposite ends of the 140 km-long NW-striking Hidalgo-Calico-Blackwater dextral fault system. Total slip and slip rate of the Blackwater fault gradually diminish northward. The fault terminates as a single strand with a zero-slip fault tip before intersecting the Garlock fault. In contrast, the Calico and Hidalgo faults spread displacement southward onto multiple fault strands spaced several kilometers apart. Active folding further distributes displacement onto the adjacent Bullion and Mesquite Lake faults. These mechanisms appear to maintain a uniform gradient of displacement approaching the Pinto Mountain fault. The highest displacement (9.8 ± 0.2 km) and slip rate (1.8 ± 0.3 mm/yr) occur in the central part of the Hidalgo-Calico-Blackwater fault system where strain is concentrated onto a single fault strand. A significant drop in total displacement and slip rate occurs along the northern Calico fault. Strain appears to be transferred here onto ENE-striking sinistral faults that separate domains of clockwise rotation in the central Mojave Desert. The kinematically incompatible intersection of sinistral and dextral faults is accommodated, at least in part, by active folding and uplift of the Calico Mountains and Mud Hills. Total slip and slip rate are not correlative for dextral faults of the Mojave ECSZ, indicating ongoing evolution of the fault network. For example, the Lenwood fault is a highly segmented, immature dextral fault with only 1.0 ± 0.1 km of total displacement yet its slip rate (1.5 ± 0.4 mm/yr) is

  4. Low resistivity and permeability in actively deforming shear zones on the San Andreas Fault at SAFOD

    NASA Astrophysics Data System (ADS)

    Morrow, C.; Lockner, D. A.; Hickman, S.

    2015-12-01

    The San Andreas Fault Observatory at Depth (SAFOD) scientific drill hole near Parkfield, California, crosses the San Andreas Fault at a depth of 2.7 km. Downhole measurements and analysis of core retrieved from Phase 3 drilling reveal two narrow, actively deforming zones of smectite-clay gouge within a roughly 200 m wide fault damage zone of sandstones, siltstones, and mudstones. Here we report electrical resistivity and permeability measurements on core samples from all of these structural units at effective confining pressures up to 120 MPa. Electrical resistivity (~10 Ω-m) and permeability (10-21 to 10-22 m2) in the actively deforming zones were 1 to 2 orders of magnitude lower than the surrounding damage zone material, consistent with broader-scale observations from the downhole resistivity and seismic velocity logs. The higher porosity of the clay gouge, 2 to 8 times greater than that in the damage zone rocks, along with surface conduction were the principal factors contributing to the observed low resistivities. The high percentage of fine-grained clay in the deforming zones also greatly reduced permeability to values low enough to create a barrier to fluid flow across the fault. Together, resistivity and permeability data can be used to assess the hydrogeologic characteristics of the fault, key to understanding fault structure and strength. The low resistivities and strength measurements of the SAFOD core are consistent with observations of low resistivity clays that are often found in the principal slip zones of other active faults making resistivity logs a valuable tool for identifying these zones.

  5. Alpine re-activation of pre-Alpine structures: details from a large-scale shear zone in the Aar massif (Central Alps)

    NASA Astrophysics Data System (ADS)

    Wehrens, Philip; Baumberger, Roland; Herwegh, Marco

    2013-04-01

    movement towards strike-slip shearing during a late stage of the shear zone activity. This highest strain event clearly shows a 160/80° orientation of mylonitic and ultramylonitic foliations. In more competent boudins-shaped rocks open fissures developed within the high strain domain. Shear sense indicators related to the subhorizontal lineations, i.e. C' structures, sheared boudins and asymmetric folds, indicate a dextral shear sense. This dextral shearing is coeval with dextral faulting along the Simplon line and represents the third reactivation of the pre-Alpine anisotropies. In sum, this shear zone illustrates the importance of mechanical anisotropies and pre-existing structures for strain distribution, localization and shear zone kinematics in case of the basement rocks.

  6. Age, tectonic evolution and origin of the Aswa Shear Zone in Uganda: Activation of an oblique ramp during convergence in the East African Orogen

    NASA Astrophysics Data System (ADS)

    Saalmann, K.; Mänttäri, I.; Nyakecho, C.; Isabirye, E.

    2016-05-01

    The Aswa Shear Zone (ASZ) is a major NW-SE trending structure of over 1000 km length in East Africa. In Uganda, the ASZ is a steeply NE-dipping, up to 11 km wide mylonitic shear zone that shows multiple stage brittle reactivation. On outcrop-scale, the fabric in the ASZ is characterized by a well-developed NW-SE striking and subvertical or steeply NE or SW dipping mylonitic foliation and a subhorizontal to moderately NW- or SE-plunging stretching lineation. Sinistral kinematics and fabric are very consistent along strike. The strain is heterogeneously distributed and partitioned into lens-shaped lower strain zones dominated by folding and characterized by pure shear, which are surrounded by high strain zones, some of them thick ultramylonites, with intense simple shear combined with flattening and strong transposition of pre-existing fabrics. Ductile shearing occurred during bulk E-W shortening, commenced at amphibolite facies conditions and continued with similar kinematics at greenschist and even lower grade conditions. A number of (sub-)parallel shear zones occur to the NE and SW of the main zone at a distance of up to 20-45 km. They show similar fabrics and kinematics and are thus related to activity along ASZ reflecting strain partitioning into simple shear and pure shear domains on a regional scale. Samples of mylonitic gneisses from the shear zone have been analyzed with U-Pb LA-MC-ICPMS and show Neoarchaean crystallisation ages between 2.66 and 2.61 Ga. Timing of ductile sinistral shearing is poorly constrained by lower intercept ages of 686 ± 62 and 640 ± 44 Ma. The fabric and structural relationship of the ca. 660 Ma Adjumani Granite exposed in the northern segment of ASZ suggest that the age of shear activity can be further limited to ca. 685 and 655 Ma. The Aswa Shear Zone is interpreted as an intra-cratonic, crustal-scale structure close to the northeastern margin of the Congo Craton, possibly inherited from previous continental extension. Early Aswa

  7. Age, tectonic evolution and origin of the Aswa Shear Zone in Uganda: Activation of an oblique ramp during convergence in the East African Orogen

    NASA Astrophysics Data System (ADS)

    Saalmann, K.; Mänttäri, I.; Nyakecho, C.; Isabirye, E.

    2016-05-01

    The Aswa Shear Zone (ASZ) is a major NW-SE trending structure of over 1000 km length in East Africa. In Uganda, the ASZ is a steeply NE-dipping, up to 11 km wide mylonitic shear zone that shows multiple stage brittle reactivation. On outcrop-scale, the fabric in the ASZ is characterized by a well-developed NW-SE striking and subvertical or steeply NE or SW dipping mylonitic foliation and a subhorizontal to moderately NW- or SE-plunging stretching lineation. Sinistral kinematics and fabric are very consistent along strike. The strain is heterogeneously distributed and partitioned into lens-shaped lower strain zones dominated by folding and characterized by pure shear, which are surrounded by high strain zones, some of them thick ultramylonites, with intense simple shear combined with flattening and strong transposition of pre-existing fabrics. Ductile shearing occurred during bulk E-W shortening, commenced at amphibolite facies conditions and continued with similar kinematics at greenschist and even lower grade conditions. A number of (sub-)parallel shear zones occur to the NE and SW of the main zone at a distance of up to 20-45 km. They show similar fabrics and kinematics and are thus related to activity along ASZ reflecting strain partitioning into simple shear and pure shear domains on a regional scale. Samples of mylonitic gneisses from the shear zone have been analyzed with U-Pb LA-MC-ICPMS and show Neoarchaean crystallisation ages between 2.66 and 2.61 Ga. Timing of ductile sinistral shearing is poorly constrained by lower intercept ages of 686 ± 62 and 640 ± 44 Ma. The fabric and structural relationship of the ca. 660 Ma Adjumani Granite exposed in the northern segment of ASZ suggest that the age of shear activity can be further limited to ca. 685 and 655 Ma. The Aswa Shear Zone is interpreted as an intra-cratonic, crustal-scale structure close to the northeastern margin of the Congo Craton, possibly inherited from previous continental extension

  8. Fluid-Assisted Shear Failure Within a Ductile Shear Zone

    NASA Astrophysics Data System (ADS)

    Kirkpatrick, J. D.; Compton, K.; Holk, G. J.

    2015-12-01

    Exhumed shear zones often contain folded and/or dynamically recrystallized structures such as veins and pseudotachylytes that record contemporaneous brittle and ductile deformation representing mixed bulk rheology. Here, we constrain the conditions that promote the transitions between ductile and brittle deformation by investigating quartz veins with shear offsets in the Saddlebag Lake shear zone in the central Sierra Nevada, California. Mesozoic metasedimentary rocks within the shear zone contain transposed bedding, strong cleavage, dextrally rotated porphyroclasts, and a steep mineral lineation, which together suggest an overall transpressive kinematic regime for the ductile deformation. Foliation sub-parallel veins are one subset of the veins in the shear zone. They have observed horizontal trace lengths of up to around 5 meters, though most are obscured by limited exposure, and displacements range from ~3-30 mm, with 1-5 mm of opening. Foliation sub-parallel veins are folded with the foliation and quartz microstructures and fluid inclusion thermobarometry measurements from vein samples indicate temperatures during vein formation by fracture were between 300-680°C. Quartz δ18O values (+5.9 to +16.5) suggest extended fluid-rock interaction that involved magmatic (δ18O ~ +8 to +10) and meteoric (δ18O down to -1) fluids. Foliation sub-parallel veins are most abundant in relatively massive, quartz-rich rocks where they are boudinaged, indicating they were rigid inclusions after formation. Based on the orientation and spatial distribution of the veins, we infer that they formed under high differential stress with pore pressures sufficiently high for the rocks to be critically stressed for shear failure along mechanically weak foliation planes. These observations suggest high pore pressures and mechanical heterogeneity at a variety of scales are necessary conditions for nucleation of shear fractures within ductile shear zones.

  9. New insights on the seismogenic potential of the Eastern Betic Shear Zone (SE Iberia): Quaternary activity and paleoseismicity of the SW segment of the Carrascoy Fault Zone

    NASA Astrophysics Data System (ADS)

    Martín-Banda, Raquel; García-Mayordomo, Julián.; Insua-Arévalo, Juan M.; Salazar, Ángel E.; Rodríguez-Escudero, Emilio; Álvarez-Gómez, Jose A.; Medialdea, Alicia; Herrero, María. J.

    2016-01-01

    The Carrascoy Fault (CAF) is one of the main active faults that form part of the Eastern Betic Shear Zone, a 450 km fault system that accommodates most of the convergence between the Eurasian (Iberia) and Nubian plates in the Betic Cordillera, south Spain. Although the CAF represents a major earthquake threat to the nearby City of Murcia, studies on its Quaternary tectonics and seismogenic potential are scarce to date. We present evidence that supports the division of the CAF into two overlapping segments with contrasting tectonic structure, Quaternary activity, and landform control: a SW segment, characterized by a broad fold-and-thrust zone similar to the forebergs defined in the Gobi-Altai region, and a NE segment, characterized by a sharp mountain front controlled by strike-slip tectonics. We attribute the differentiation into these two segments to the stresses associated with topography, which in turn is a consequence of the shortening component, at the middle Pleistocene, after circa 217.4 ka. For the SW segment we infer the occurrence of 9 to 11, Mw 6.7 paleoearthquakes in the last 30.2 kyr, and a slip rate of 0.37 ± 0.08 m/kyr. We date the occurrence of the last surface rupture event after 2750 B.P., and we estimate an average recurrence period of major events of 3.3 ± 0.7 kyr.

  10. Fault Population Analyses in the Eastern California Shear Zone: Insights into the Development of Young, Actively Evolving Plate Boundary Structures

    NASA Astrophysics Data System (ADS)

    Zhou, X.; Dawers, N. H.; Amer, R. M.

    2014-12-01

    Relationships between cumulative fault displacement, slip rate and length, along with fault population statistics are analyzed for faults located within the Eastern California Shear Zone (ECSZ), focusing on areas north of the Garlock fault. Here many faults are geologically young and in an early stage of evolution, while many older and larger faults are also still active. We analyze scaling relationships for both strike-slip and normal faults in order to determine whether the two fault populations share the same properties or not. Cumulative displacement, slip rate and length data are collected from published maps and literature sources. The dataset spans fault lengths from tens of meters to hundreds of kilometers. Results of fault scaling analyses indicate that displacement has a linear relationship with fault length for normal faults in this area over the entire length span, whereas strike-slip faults do not have a clear displacement-length scaling relation. For a given length, the subset of strike-slip faults typically exhibits a much larger displacement than that for the normal faults. The slip rate versus length trends are similar but are considerably more scattered. In addition, we define a subpopulation of normal faults that are kinematically related to the right-lateral strike-slip faults; these have a maximum length set by the spacing between the right-lateral faults. Fault size-frequency distributions also indicate differences between the normal and strike-slip fault populations. Overall, the normal faults have higher ratios of cumulative number to fault length than the strike-slip population does, which we relate to different patterns of localization of faulting. We interpret these trends as reflecting different tectonic histories, with the majority of normal faults being intraplate faults associated with Basin and Range extension and the strike-slip faults being kinematically connected with plate boundary.

  11. Vertical shear in the Jovian equatorial zone.

    NASA Technical Reports Server (NTRS)

    Layton, R. G.

    1971-01-01

    Jupiter photographs taken in two different wavelength regions (blue and red) are studied for clues to differing Jovian atmosphere motions. The relative motions of features visible on these photographs may be interpreted as a vertical shear at visible cloud level. The value obtained implies that the north equatorial zone must be about 0.35 deg K warmer than the adjacent equatorial zone. Deeper in the atmosphere the reverse must hold.

  12. Predicting km-scale shear zone formation

    NASA Astrophysics Data System (ADS)

    Gerbi, Christopher; Culshaw, Nicholas; Shulman, Deborah; Foley, Maura; Marsh, Jeffrey

    2015-04-01

    Because km-scale shear zones play a first-order role in lithospheric kinematics, accurate conceptual and numerical models of orogenic development require predicting when and where they form. Although a strain-based algorithm in the upper crust for weakening due to faulting appears to succeed (e.g., Koons et al., 2010, doi:10.1029/2009TC002463), a comparable general rule for the viscous crust remains unestablished. Here we consider two aspects of the geological argument for a similar algorithm in the viscous regime, namely (1) whether predicting km-scale shear zone development based on a single parameter (such as strain or shear heating) is reasonable; and (2) whether lithologic variability inherent in most orogenic systems precludes a simple predictive rule. A review of tectonically significant shear zones worldwide and more detailed investigations in the Central Gneiss belt of the Ontario segment of the Grenville Province reveals that most km-scale shear zones occur at lithological boundaries and involve mass transfer, but have fairly little else in common. As examples, the relatively flat-lying Twelve Mile Bay shear zone in the western Central Gneiss belt bounds the Parry Sound domain and is likely the product of both localized anatexis and later retrograde hydration with attendant metamorphism. Moderately dipping shear zones in granitoids of the Grenville Front Tectonic Zone apparently resulted from cooperation among several complementary microstructural processes, such as grain size reduction, enhanced diffusion, and a small degree of metamorphic reaction. Localization into shear zones requires the operation of some spatially restricted processes such as stress concentration, metamorphism/fluid access, textural evolution, and thermal perturbation. All of these could be due in part to strain, but not necessarily linearly related to strain. Stress concentrations, such as those that form at rheological boundaries, may be sufficient to nucleate high strain

  13. Kinematics and shear heat pattern of ductile simple shear zones with `slip boundary condition'

    NASA Astrophysics Data System (ADS)

    Mulchrone, Kieran F.; Mukherjee, Soumyajit

    2016-04-01

    Extrusion by Poiseuille flow and simple shear of hot lower crust has been deciphered from large hot orogens, and partial-slip boundary condition has been encountered in analogue models. Shear heat and velocity profiles are deduced from a simplified form of Navier-Stokes equation for simple shear together with extrusive Poiseuille flow and slip boundary condition for Newtonian viscous rheology. A higher velocity at the upper boundary of the shear zone promotes higher slip velocity at the lower boundary. The other parameters that affect the slip are viscosity and thickness of the shear zone and the resultant pressure gradient that drives extrusion. In the partial-slip case, depending on flow parameters (resultant pressure gradient, density and viscosity) and thickness of the shear zone, the velocity profiles can curve and indicate opposite shear senses. The corresponding shear heat profiles can indicate temperature maximum inside shear zones near either boundaries of the shear zone, or equidistant from them.

  14. Geochronological and Petrological Constraints on the Evolution of the Pan African Ajjaj Shear Zone, Saudi Arabia

    NASA Astrophysics Data System (ADS)

    Hassan, M.; Stuewe, K.; Abu-Alam, T. S.; Kloetzli, U. S.; Tiepolo, M.

    2014-12-01

    In the active tectonic regions, shear zones play an important role to re-configure the structure of the lithosphere. One of the largest shear zones on the Earth is the Najd Fault System of the Arabian-Nubian Shield. Literature data record the main active phase of this shear zone during the last stages of the Pan-African Orogeny (ca. 630 - 540 Ma). The Najd Fault System is composed of several shear zone segments, one of them is the Ajjaj shear zone. Determination of the age of variably deformed intrusions is expected to give approximated age of deformation in Ajjaj shear zone. Six samples of intrusive rocks showing variable composition were used to illustrate the time progress and evolution of the Ajjaj shear zone. One sample is from a very coarse grained diorite lying within the Ajjaj shear zone. It has very weak deformation and produces an intercept U-Pb zircon age of 696 ± 6 Ma. Two samples are from granodiorite-tonalite intrusions to the tenant of the Ajjaj shear zones. They show conspicuous degree of deformation and define two U-Pb clusters of concordia ages at 747 ± 12 Ma - 668 ± 8 Ma and 742 ± 5 Ma - 702 ± 12 Ma. Three samples are granites from variable plutons along the Ajjaj shear zone. Two of them show mylonitic foliation of flattened quartz and platy minerals such as biotite parallel to the main deformation trend of the shear zone. They yield U-Pb ages of 601 ± 6 Ma - 584 ± 3 Ma. The third sample is undeformed and has a cross-cut contact relationship with the foliation of the Ajjaj shear zone. It yield concordia ages of 581 ± 4 Ma. These data confine the activity of the Ajjaj shear zone to a limited period between 605 Ma and 577 Ma. As the activity of the Ajjaj shear zone was responsible for the exhumation of the Hamadat metamorphic complex, we also constrained the vertical motions that occurred during the shear zone activity using mmetamorphic rocks. It is shown that peak metamorphism occurred around 505 - 700 ºC at two ranges of pressure 8 - 11

  15. Recrystallization fabrics of sheared quartz veins with a strong pre-existing crystallographic preferred orientation from a seismogenic shear zone

    NASA Astrophysics Data System (ADS)

    Price, Nancy A.; Song, Won Joon; Johnson, Scott E.; Gerbi, Christopher C.; Beane, Rachel J.; West, David P.

    2016-07-01

    Microstructural investigations were carried out on quartz veins in schist, protomylonite, and mylonite samples from an ancient seismogenic strike-slip shear zone (Sandhill Corner shear zone, Norumbega fault system, Maine, USA). We interpret complexities in the microstructural record to show that: (1) pre-existing crystallographic preferred orientations (CPO) in the host rock may persist in the new CPO patterns of the shear zone and (2) the inner and outer parts of the shear zone followed diverging paths of fabric development. The host rocks bounding the shear zone contain asymmetrically-folded quartz veins with a strong CPO. These veins are increasingly deformed and recrystallized with proximity to the shear zone core. Matrix-accommodated rotation and recrystallization may position an inherited c-axis maximum in an orientation coincident with rhomb < a > or basal < a > slip. This inherited CPO likely persists in the shear zone fabric as a higher concentration of poles in one hemisphere of the c-axis pole figure, leading to asymmetric crossed girdle or paired maxima c-axis patterns about the foliation plane. Three observed quartz grain types indicate a general trend of localization with decreasing temperature: (1) large (> 100 μm), low aspect ratio (<~5) and (2) high aspect ratio (~ 5-20) grains overprinted by (3) smaller (<~80 μm), low aspect ratio (<~4) grains through subgrain rotation-dominated recrystallization. In the outer shear zone, subgrain rotation recrystallization led to a well-developed c-axis crossed girdle pattern. In the inner shear zone, the larger grains are completely overprinted by smaller grains, but the CPO patterns are relatively poorly developed and are associated with distinctively different misorientation angle histogram profiles ("flat" neighbor-pair profile with similar number fraction for angles from 10 to 90°). This may reflect the preferential activation of grain size sensitive deformation processes in the inner-most part of the

  16. Evolution of microstructures in Precambrian shear zones: An example from eastern India

    NASA Astrophysics Data System (ADS)

    Banerjee, Sayandeep; Matin, Abdul

    2013-05-01

    Shear zones are areas of intense deformation in localized zones which can be used as natural laboratories for studying deformation characteristics. Metre to-micro scale structures that develop in response to a progressive simple shear in a shear zone are characterized by a protracted history of deformation and are immensely useful in delineating the history of progressive deformation. To decipher these localized zones of deformation and to establish the continuous non-coaxial character of deformation, detail microstructural studies are very useful. Singhbhum shear zone (SSZ), a regional Precambrian tectonic dislocation zone in eastern India, depicting a top-to-south thrust movement of the hanging wall provides a scope for studying microstructural characteristics developed in response to a progressive shear at mid-crustal level. SSZ is characterized by intense stretching lineation, isoclinal folds, shear planes, superposed schistosity and deformed quartz veins. Quasi-plastic (QP) deformation mechanisms were predominantly active in the SSZ. The overprinting relationship between the earlier and later schistosity with a consistent sense of shear indicates that earlier schistosity is transposed to later schistosity through the intermediate stages of crenulation cleavage during a progressive non-coaxial deformation. The recrystallization of quartz in mylonitic quartzite suggests protracted history of deformation. The analysis of the character of quartz grains of both the porphyroclasts and recrystallized grains suggests that strain was partitioned between the most intensely deformed central part of the shear zone and the shear-related deformation zone outside the central part of the shear zone.

  17. Deformation of footwall rock of Phulad Shear Zone, Rajasthan: Evidence of transpressional shear zone

    NASA Astrophysics Data System (ADS)

    Choudhury, Manideepa Roy; Das, Subhrajyoti; Chatterjee, Sadhana M.; Sengupta, Sudipta

    2016-07-01

    Phulad Shear Zone (PSZ) of Delhi Fold Belt in Rajasthan is a northeasterly striking ductile shear zone with a well developed mylonitic foliation (035/70E) and a downdip stretching lineation. The deformation in the PSZ has developed in a transpressional regime with thrusting sense of movement. The northeastern unit, i.e., the hanging wall contains a variety of rocks namely calc-silicates, pelites and amphibolites and the southwestern unit, i.e., the footwall unit contains only granitic rocks. Systematic investigation of the granites of the southwestern unit indicate a gradual change in the intensity of deformation from a distance of about 1 km west of the shear zone to the shear zone proper. The granite changes from weakly deformed granite to a mylonite/ultramylonite as we proceed towards the PSZ. The weakly deformed granite shows a crude foliation with the same attitude of mylonitic foliation of the PSZ. Microscopic study reveals the incipient development of C and S fabric with angle between C and S varying from 15 ∘ to 24 ∘. The small angle between the C and S fabric in the least deformed granite variety indicates that the deformation has strong pure shear component. At a distance of about 1 m away from the PSZ, there is abrupt change in the intensity of deformation. The granite becomes intensely foliated with a strong downdip lineation and the rock becomes a true mylonite. In mesoscopic scale, the granite shows stretched porphyroclasts in both XZ and YZ sections indicating a flattening type of deformation. The angle between the C and S fabric is further reduced and finally becomes nearly parallel. In most places, S fabric is gradually replaced by C fabric. Calculation of sectional kinematic vorticity number ( W n) from the protomylonitic and mylonite/ultramylonite granites varies from 0.3 ± 0.03 to 0.55 ± 0.04 indicating a strong component of pure shear. The similarity of the geometry of structures in the PSZ and the granites demonstrates that the

  18. Periodic Viscous Shear Heating Instability in Fine-Grained Shear Zones: Mechanism for Intermediate Depth Earthquakes

    NASA Astrophysics Data System (ADS)

    Coon, E.; Kelemen, P.; Hirth, G.; Spiegelman, M.

    2005-12-01

    Kelemen and Hirth (Fall 2004 AGU) presented a model for periodic, viscous shear heating instabilities along pre-existing, fine grained shear zones. This provides an attractive alternative to dehydration embrittlement for explaining intermediate-depth earthquakes, especially those in a narrow thermal window within the mantle section of subducting oceanic plates (Hacker et al JGR03). Ductile shear zones with widths of cm to m are common in shallow mantle massifs and peridotite along oceanic fracture zones. Pseudotachylites in a mantle shear zone show that shear heating temperatures exceeded the mantle solidus (Obata & Karato Tectonophys95). Olivine grain growth in shear zones is pinned by closely spaced pyroxenes; thus, once formed, these features do not `heal' on geological time scales in the absence of melt or fluid (Warren & Hirth EPSL05). Grain-size sensitive creep will be localized within these shear zones, in preference to host rocks with olivine grain size from 1 to 10 mm. Inspired by the work of Whitehead & Gans (GJRAS74), we proposed that such pre-existing shear zones might undergo repeated shear heating instabilities. This is not a new concept; what is new is that viscous deformation is limited to a narrow shear zone, because grain boundary sliding, sensitive to both stress and grain size, may accommodate creep even at high stress and high temperature. These new ideas yield a new result: simple models for a periodic shear heating instability. Last year, we presented a 1D numerical model using olivine flow laws, assuming that viscous deformation remains localized in shear zones, surrounded by host rocks undergoing elastic deformation. Stress evolves due to elastic strain and drives viscous deformation in a shear zone of specified width. Shear heating and thermal diffusion control T. A maximum of 1400 C (substantial melting of peridotite ) was imposed. Grain size evolves due to recrystallization and diffusion. For strain rates of E-13 to E-14 per sec and

  19. Imaging Faults and Shear Zones Using Receiver Functions

    NASA Astrophysics Data System (ADS)

    Schulte-Pelkum, Vera; Mahan, Kevin H.

    2014-11-01

    The geometry of faults at seismogenic depths and their continuation into the ductile zone is of interest for a number of applications ranging from earthquake hazard to modes of lithospheric deformation. Teleseismic passive source imaging of faults and shear zones can be useful particularly where faults are not outlined by local seismicity. Passive seismic signatures of faults may arise from abrupt changes in lithology or foliation orientation in the upper crust, and from mylonitic shear zones at greater depths. Faults and shear zones with less than near-vertical dip lend themselves to detection with teleseismic mode-converted waves (receiver functions) provided that they have either a contrast in isotropic shear velocity ( V s), or a contrast in orientation or strength of anisotropic compressional velocity ( V p). We introduce a detection method for faults and shear zones based on receiver functions. We use synthetic seismograms to demonstrate common features of dipping isotropic interfaces and contrasts in dipping foliation that allows determination of their strike and depth without making further assumptions about the model. We proceed with two applications. We first image a Laramide thrust fault in the western U.S. (the Wind River thrust fault) as a steeply dipping isotropic velocity contrast in the middle crust near the surface trace of the fault; further downdip and across the range, where basin geometry suggests the fault may sole into a subhorizontal shear zone, we identify a candidate shear zone signal from midcrustal depths. The second application is the use of microstructural data from exhumed ductile shear zones in Scotland and in the western Canadian Shield to predict the character of seismic signatures of present-day deep crustal shear zones. Realistic anisotropy in observed shear fabrics generates a signal in receiver functions that is comparable in amplitude to first-order features like the Moho. Observables that can be robustly constrained without

  20. Brittle reactivation of ductile shear zones in NW Namibia

    NASA Astrophysics Data System (ADS)

    Salomon, Eric; Koehn, Daniel; Passchier, Cees

    2014-05-01

    Existing structural elements, such as shear zones, are regarded to have a significant influence on the orientation and extent of younger structures. Thus, shear zones are seen as a primary controlling factor on the development of rift zones (e.g. Piqué & Laville, 1996). Indeed, reactivation of such structures is observed in numerous areas. However, information on the amount of offset associated with a reactivation is often lacking. Reactivation of shear zones is also reported of in NW Namibia in relation to the South Atlantic Rifting (Marsh et al., 2001; Stanistreet & Charlesworth, 2001). Here, we present the results of a quantitative study on this reactivation. NW Namibia is characterized by the ~N-S trending Kaoko Belt which developed during the assemblage of Gondwana in the Neo-Proterozoic and incorporates a number of shear zones, e.g. the Purros Shear Zone and Three Palms Shear Zone, which run sub-parallel to the present-day continental margin. The shear zones are partly covered by the extrusives of the Paraná-Etendeka Large Igneous Province (~133 Ma) which extruded shortly before or during the onset of the Atlantic rifting (e.g. Blaich et al., 2011). Combining the analysis of satellite imagery and digital elevation models with extensive field work, we identified numerous faults tracing the old shear zones along which the Etendeka basalts were down-faulted. Listric faults developed along the shear zones and accumulated vertical offsets up to 900 m. The faults developed in areas where the foliation along the shear zones is steepest, indicating that a dip of >65° is necessary for normal reactivation. A basin developed along the section where the shear zone is supposedly widest. Our results contribute to the view that the basement inheritance plays a significant role on rifting processes and that the reactivation of shear zones can accumulate significant amounts of displacement. References: Blaich, O.A. et al. (2011). Journal of Geophysical Research - Solid

  1. Review on symmetric structures in ductile shear zones

    NASA Astrophysics Data System (ADS)

    Mukherjee, Soumyajit

    2016-07-01

    Symmetric structures in ductile shear zones range widely in shapes and geneses. Matrix rheology, its flow pattern, its competency contrast with the clast, degree of slip of the clast, shear intensity and its variation across shear zone and deformation temperature, and degree of confinement of clast in shear zones affects (independently) the degree of symmetry of objects. Kinematic vorticity number is one of the parameters that govern tail geometry across clasts. For example, symmetric and nearly straight tails develop if the clast-matrix system underwent dominantly a pure shear/compression. Prolonged deformation and concomitant recrystallization can significantly change the degree of symmetry of clasts. Angular relation between two shear zones or between a shear zone and anisotropy determines fundamentally the degree of symmetry of lozenges. Symmetry of boudinaged clasts too depends on competency contrast between the matrix and clast in some cases, and on the degrees of slip of inter-boudin surfaces and pure shear. Parasitic folds and post-tectonic veins are usually symmetric.

  2. How to create mylonitic shear zones in the presence of shear heating

    NASA Astrophysics Data System (ADS)

    Thielmann, Marcel; Rozel, Antoine; Kaus, Boris; Ricard, Yanick

    2013-04-01

    Lithospheric-scale shear zones are commonly defined as regions inhomogeneous and localized deformation. Strain softening has been demonstrated to be necessary for localization in those shear zones, but there is still debate about the physical cause of this softening. Here, we investigate the interplay between two mechanisms that have been suggested to have a significant impact on lithospheric localization: shear heating and grain size reduction. Shear heating has been suggested to play an important role in i) creating deep focus as well as intermediate-depth earthquakes (Ogawa (1987), Kelemen and Hirth (2007)) and ii) creating lithospheric-scale shear zones, thus creating a weak decoupling interface that enables subsequent subduction initiation (Kaus and Podlatchikov (2006), Crameri and Kaus (2010)). As natural shear zones typically have a significantly reduced grain size, it has been put forward that grain size reduction provides the necessary strain softening to localize deformation. As grain size reduces, the dominant deformation mechanism switches from dislocation to diffusion creep, thus requiring less stress to deform the rock. Usually, the equilibrium grain size is thought to follow a piezometric relationship, thus indicating the stress under which a shear zone deformed. Recent work (Austin and Evans (2007), Rozel et al. (2011)) suggests that the equilibrium grain size is not dependent on stress, but rather on the deformational work. In our study, we employ the grain size evolution law of Rozel et al. and use 1D viscoelastic numerical models of simple shear deformation to investigate the influence of both weakening mechanisms and their interaction for a variety of boundary conditions. We find that grain size reduction in pure olivine does not localize very efficiently, as grain size very rapidly reaches a steady state. Even when a fraction of the deformational work is used by grain size reduction processes, shear heating is found to localize very efficiently

  3. Coexistence and transition between shear zones in slow granular flows.

    PubMed

    Moosavi, Robabeh; Shaebani, M Reza; Maleki, Maniya; Török, János; Wolf, Dietrich E; Losert, Wolfgang

    2013-10-01

    We report experiments on slow granular flows in a split-bottom Couette cell that show novel strain localization features. Nontrivial flow profiles have been observed which are shown to be the consequence of simultaneous formation of shear zones in the bulk and at the boundaries. The fluctuating band model based on a minimization principle can be fitted to the experiments over a large variation of morphology and filling height with one single fit parameter, the relative friction coefficient μ(rel) between wall and bulk. The possibility of multiple shear zone formation is controlled by μ(rel). Moreover, we observe that the symmetry of an initial state, with coexisting shear zones at both side walls, breaks spontaneously below a threshold value of the shear velocity. A dynamical transition between two asymmetric flow states happens over a characteristic time scale which depends on the shear strength. PMID:24138274

  4. Shear Zone Development and Rheology in the Deep Orogenic Crust

    NASA Astrophysics Data System (ADS)

    Marsh, J. H.; Johnson, S. E.; Gerbi, C. C.; Culshaw, N. G.

    2008-12-01

    Within the Central Gneiss Belt (CGB) of the southwestern Grenville Province, Ontario, Canada, a number of allocthonous lithotectonic domains are juxtaposed along crustal-scale shear zones. Extensive exposure of variably reworked granulites of the interior Parry Sound domain (iPSD) has enabled investigation of the structural and petrologic character of domain-bounding shear zones within the deep orogenic crust. Recent detailed mapping and structural data collected along the southwestern margin of the iPSD is consistent with the suggestion of Culshaw et al. (in prep) that spaced outcrop-scale shear zones have coalesced and progressively reworked layered granulites into a transposed amphibolite-facies tectonite. The tectonites comprise the Twelve Mile Bay Shear Zone (TMBSZ), which separates the iPSD from para-autocthonous rocks to the south. This study investigates the grain- and outcrop-scale mechanisms involved in shear zone development and attempts to quantify the associated changes in rock rheology. Northwest of TMBSZ, samples collected across individual outcrop-scale shear zones (i.e., across large strain gradients) have distinct differences in mineralogy and microstructure. In mafic layers the original granulite texture and cpx + opx + pl + hbl +/- grt assemblage is commonly retained away from the shear zones within unsheared "panels". With proximity to the shear zones pyroxenes and garnet are progressively consumed in hydration reactions producing hornblende and biotite, which define a new planar foliation within the highly attenuated and deflected layering. Felsic layers generally have only minor mineralogical changes across the zones, but develop an increasingly intense and recrystallized structural fabric into the sheared margin. The shear zones are commonly cored by variably deformed pegmatite dikes that were emplaced prior to, or during the early stages of shearing. Evidence for incipient shear zone formation along mineralized fracture sets that cut

  5. Thermo-mechanical coupling of faults and mantle shear zones

    NASA Astrophysics Data System (ADS)

    Lambert, Valere; Barbot, Sylvain

    2016-04-01

    Paleo-seismological records suggest non-steady and potentially periodic trends in slip rates over time scales of the order of millennia. It is unclear whether the variability of recurrence times is due to fault processes alone or if they are modulated by off-fault deformation. Theoretical and numerical modeling of fault kinematics from geodetic data have enabled an explosion of new findings about the mechanics of the earthquake cycle. However, these models have been mostly confined to processes along the interface of a fault. Therefore many sources of off-fault deformation, such as thermoelasticity and viscoelasticity, cannot yet be accounted for in the earthquake cycle. Here, we couple fault kinematics and viscoelastic deformation within shear zones using the integral method to simulate unified earthquake cycles that combine fault and off-fault processes. We consider the modulation of slip rates along a fault within the brittle layer due to strain in a viscoelastic substrate beneath the brittle-ductile transition. By implementing a thermally-activated rheology accounting for thermal diffusion, we investigate the thermo-mechanical coupling of faults and mantle shear zones and its implications for earthquake recurrence.

  6. A new perspective on the significance of the Ranotsara shear zone in Madagascar

    NASA Astrophysics Data System (ADS)

    Schreurs, Guido; Giese, Jörg; Berger, Alfons; Gnos, Edwin

    2010-12-01

    The Ranotsara shear zone in Madagascar has been considered in previous studies to be a >350-km-long, intracrustal strike-slip shear zone of Precambrian/Cambrian age. Because of its oblique strike to the east and west coast of Madagascar, the Ranotsara shear zone has been correlated with shear zones in southern India and eastern Africa in Gondwana reconstructions. Our assessment using remote sensing data and field-based investigations, however, reveals that what previously has been interpreted as the Ranotsara shear zone is in fact a composite structure with a ductile deflection zone confined to its central segment and prominent NW-SE trending brittle faulting along most of its length. We therefore prefer the more neutral term “Ranotsara Zone”. Lithologies, tectonic foliations, and axial trace trajectories of major folds can be followed from south to north across most of the Ranotsara Zone and show only a marked deflection along its central segment. The ductile deflection zone is interpreted as a result of E-W indentation of the Antananarivo Block into the less rigid, predominantly metasedimentary rocks of the Southwestern Madagascar Block during a late phase of the Neoproterozoic/Cambrian East African Orogeny (c. 550-520 Ma). The Ranotsara Zone shows significant NW-SE striking brittle faulting that reactivates part of the NW-SE striking ductile structures in the flexure zone, but also extends along strike toward the NW and toward the SE. Brittle reactivation of ductile structures along the central segment of the Ranotsara Zone, confirmed by apatite-fission track results, may have led to the formation of a shallow Neogene basin underlying the Ranotsara plain. The present-day drainage pattern suggests on-going normal fault activity along the central segment. The Ranotsara Zone is not a megascale intracrustal strike-slip shear zone that crosscuts the entire basement of southern Madagascar. It can therefore not be used as a piercing point in Gondwana

  7. Structural Evidence for Fluid-Assisted Shear Failure within a Ductile Shear Zone

    NASA Astrophysics Data System (ADS)

    Compton, K.; Kirkpatrick, J. D.

    2014-12-01

    Recent observations of seismic slip occurring below the seismogenic zone of large fault zones have emphasized the significance of coeval ductile and brittle processes at high temperatures. We present observations of a shear zone contained within the Saddlebag Lake pendant of the eastern Sierra Nevada, CA, where Triassic and Jurassic metavolcanics and metasediments are highly strained in a high-temperature shear zone. Transposed bedding and cleavage that define a flattening fabric, dextrally rotated porphyroclasts, and a steep, pervasive lineation together suggest an overall transpressive kinematic regime for the ductile deformation. The high-strain rocks exhibit multiple episodes of vein formation, indicating a prolonged migration of hydrothermal fluids throughout the system. Crosscutting relationships and mineral assemblages define discrete sets of differently oriented veins. The veins form by fracture, but many veins are folded and boudinaged, showing synkinematic brittle and ductile deformation. We document foliation-parallel quartz veins that show shear displacement from the geometry of pull-apart structures and offsets of earlier veins. Synkinematic equilibrium mineral assemblages within the host rock and dynamic recrystallization of the quartz veins show they formed at temperatures around 400 to 500°C. The shear fractures have horizontal trace lengths of up to a few meters and displacements range from 2-3 mm to ~3 cm, with 1-5 mm of opening. Assuming the observed offset in the fractures occurred in a single event, these measurements are consistent with stress drops of 1 to 10 MPa. We interpret these observations to show that the veins formed as a result of high pore fluid pressure that caused shear failure at low effective stresses. Because foliated rocks are mechanically anisotropic, the foliation provided planes of weakness for failure with a preferred orientation. Evidence for shear failure occurring within crystal-plastic shear zones at high temperatures

  8. The brittle-viscous-plastic evolution of shear bands in the South Armorican Shear Zone

    NASA Astrophysics Data System (ADS)

    Bukovská, Zita; Jeřábek, Petr; Morales, Luiz F. G.; Lexa, Ondrej; Milke, Ralf

    2014-05-01

    Shear bands are microscale shear zones that obliquely crosscut an existing anisotropy such as a foliation. The resulting S-C fabrics are characterized by angles lower than 45° and the C plane parallel to shear zone boundaries. The S-C fabrics typically occur in granitoids deformed at greenschist facies conditions in the vicinity of major shear zones. Despite their long recognition, mechanical reasons for localization of deformation into shear bands and their evolution is still poorly understood. In this work we focus on microscale characterization of the shear bands in the South Armorican Shear Zone, where the S-C fabrics were first recognized by Berthé et al. (1979). The initiation of shear bands in the right-lateral South Armorican Shear Zone is associated with the occurrence of microcracks crosscutting the recrystallized quartz aggregates that define the S fabric. In more advanced stages of shear band evolution, newly formed dominant K-feldspar, together with plagioclase, muscovite and chlorite occur in the microcracks, and the shear bands start to widen. K-feldspar replaces quartz by progressively bulging into the grain boundaries of recrystallized quartz grains, leading to disintegration of quartz aggregates and formation of fine-grained multiphase matrix mixture. The late stages of shear band development are marked by interconnection of fine-grained white mica into a band that crosscuts the original shear band matrix. In its extremity, the shear band widening may lead to the formation of ultramylonites. With the increasing proportion of shear band matrix from ~1% to ~12%, the angular relationship between S and C fabrics increases from ~30° to ~40°. The matrix phases within shear bands show differences in chemical composition related to distinct evolutionary stages of shear band formation. The chemical evolution is well documented in K-feldspar, where the albite component is highest in porphyroclasts within S fabric, lower in the newly formed grains within

  9. Shear zones developed between extensional and compressional tectonic regimes: recent deformation of the Burdur Fethiye Shear Zone as a case study

    NASA Astrophysics Data System (ADS)

    Elitez, İrem; Yaltırak, Cenk; Aktuǧ, Bahadır

    2016-04-01

    The southwestern Turkey is one of the most tectonically active areas of the eastern Mediterranean and therefore is a controversial region from the geodynamic point of view. This complex tectonic regime is dominated by the westward escape of Anatolia related to North Anatolian Fault, Aegean back-arc extension regime due to roll-back of Hellenic Arc, the subduction transform edge propagator (STEP) fault zone related to the motion of Hellenic and Cyprus arcs and compressional regime of Tauride Mountains. In addition to that, an active subduction and seamounts moving towards the north determine the tectonic frame of the Eastern Mediterranean. Many researchers suggest either the existence of a single left lateral fault or the nonexistence of a fault zone between Western Anatolia and Western Taurides. According to the integration of digital elevation data, non-commercial GoogleEarth satellite images and field studies, a 300 km-long 75-90 km-wide NE-SW-trending left lateral shear zone, the Burdur-Fethiye Shear Zone, is located among these tectonic structures. By using GPS velocities and focal mechanism solutions of earthquakes, it is understood that most of the previous studies turn a blind eye to the hundreds of faults related to a left-lateral shear zone which will have an important role in the Mediterrenean tectonics. The Burdur-Fethiye Shear Zone is like a zipper driven by the relative velocity differences due to the Aegean back-arc extensional system and Western Taurides compressional region and presents a high seismic activity. The GPS vectors reflect remarkable velocity differences on land and relatedly the significant topographic differences can be clearly observed. According to the GPS vectors, the Aegean region moves 4-12 mm/yr faster than the wesward escape of the Anatolia towards southwest and the velocities are low in the Western Taurides. The left-lateral differential motion across the Burdur-Fethiye Shear Zone varies from 3-4 mm/yr in the north side to 8

  10. Orientations of faults determined by premonitory shear zones

    NASA Astrophysics Data System (ADS)

    Johnson, Arvid M.

    1995-07-01

    The postulate of premonitory shear zones that the orientations of many faults are controlled by previously formed shear zones is a combination of theoretical analysis and the concept that faulting is the result of a group of hereditary processes. The hereditary nature of faulting processes is evinced by detailed observations of faulting in several, quite different materials in which the faults are end products of irreversible, localized deformation sequences such as pressure solution, particle rearrangement, layer reorientation, plastic flow or grain fracturing. The localized deformation is concentrated within shear zones that premonish the formation of faults. Thus, the problem of determining the orientations of faults becomes one of determining the preferred orientations of shear zones, which is the focus of the postulate of premonitory shear zones. The postulate is based on one definition and two assumptions: by definition, deformation becomes localized within a shear zone (the reason need not be specified); by assumption, the virtual shearing and dilation within the shear zone are coupled and the orientation that develops corresponds to the preferred orientation. The preferred orientation of the shear zone is defined as that which satisfies the mechanical and kinematical boundary conditions and maximizes, in some sense, the virtual work accomplished by the shearing and dilation. With the exception of coupling of the shearing and dilation through a coefficient of dilation, the analysis is only implicitly dependent on rheology; the only rheological requirement is that the properties allow localization. Although the postulate is based on very simple assumptions, and so its predictions are necessarily simple, it has a power that belies its simplicity. The preferred orientations of premonitory shear zones are consistent with orientations of shear zones and faults in laboratory specimens of Chelmsford granite. The coefficient of dilatancy in granite specimens about

  11. Strain accumulation and rotation in the Eastern California Shear Zone

    NASA Astrophysics Data System (ADS)

    Savage, J. C.; Gan, Weijun; Svarc, J. L.

    2001-10-01

    Although the Eastern California Shear Zone (ECSZ) (strike ˜N25°W) does not quite coincide with a small circle drawn about the Pacific-North America pole of rotation, trilateration and GPS measurements demonstrate that the motion within the zone corresponds to right-lateral simple shear across a vertical plane (strike N33°W±5°) roughly parallel to the tangent to that local small circle (strike ˜N40°W). If the simple shear is released by slip on faults subparallel to the shear zone, the accumulated rotation is also released, leaving no secular rotation. South of the Garlock fault the principal faults (e.g., Calico-Blackwater fault) strike ˜N40°W, close enough to the strike of the vertical plane across which maximum right-lateral shear accumulates to almost wholly accommodate that accumulation of both strain and rotation by right-lateral slip. North of the Garlock fault dip slip as well as strike slip on the principal faults (strike ˜N20°W) is required to accommodate the simple shear accumulation. In both cases the accumulated rotation is released with the shear strain. The Garlock fault, which transects the ECSZ, is not offset by north-northwest striking faults nor, despite geological evidence for long-term left-lateral slip, does it appear at the present time to be accumulating left-lateral simple shear strain across the fault due to slip at depth. Rather the motion is explained by right-lateral simple shear across the orthogonal ECSZ. Left-lateral slip on the Garlock fault will release the shear strain accumulating there but would augment the accumulating rotation, resulting in a secular clockwise rotation rate ˜80 nrad yr-1 (4.6° Myr-1).

  12. Strain accumulation and rotation in the Eastern California Shear Zone

    USGS Publications Warehouse

    Savage, J.C.; Gan, Weijun; Svarc, J.L.

    2001-01-01

    Although the Eastern California Shear Zone (ECSZ) (strike ???N25??W) does not quite coincide with a small circle drawn about the Pacific-North America pole of rotation, trilateration and GPS measurements demonstrate that the motion within the zone corresponds to right-lateral simple shear across a vertical plane (strike N33??W??5??) roughly parallel to the tangent to that local small circle (strike ???N40??W). If the simple shear is released by slip on faults subparallel to the shear zone, the accumulated rotation is also released, leaving no secular rotation. South of the Garlock fault the principal faults (e.g., Calico-Blackwater fault) strike ???N40??W, close enough to the strike of the vertical plane across which maximum right-lateral shear accumulates to almost wholly accommodate that accumulation of both strain and rotation by right-lateral slip. North of the Garlock fault dip slip as well as strike slip on the principal faults (strike ???N20??W) is required to accommodate the simple shear accumulation. In both cases the accumulated rotation is released with the shear strain. The Garlock fault, which transects the ECSZ, is not offset by north-northwest striking faults nor, despite geological evidence for long-term left-lateral slip, does it appear at the present time to be accumulating left-lateral simple shear strain across the fault due to slip at depth. Rather the motion is explained by right-lateral simple shear across the orthogonal ECSZ. Left-lateral slip on the Garlock fault will release the shear strain accumulating there but would augment the accumulating rotation, resulting in a secular clockwise rotation rate ???80 nrad yr-1 (4.6?? Myr-1).

  13. Time-evolution and strain distribution of a major shear zone (SASZ, Brittany, France)

    NASA Astrophysics Data System (ADS)

    Augier, Romain; Raimbourg, Hugues; Bellanger, Mathieu; Turrillot, Paul; Monié, Patrick

    2014-05-01

    Mylonite-bearing crustal shear zones are exhumed extensions at depth of (potentially seismogenic) brittle faults at shallow structural level. By concentrating deformation, shear zones play a major role in the rheology and mechanical behavior of the continental crust. Analysis of shear zone geometry and microstructures is a useful tool to decipher deformation kinematics, though finite strain estimation is still relatively qualitative. Timing of the deformation has also been the focus of numerous studies using the complete spectrum of geochronometers. However, only rare examples provide geochronological data that can be unambiguously linked with deformation. Thus, time-scales over which major mylonite zones develop and remain active under ductile conditions as well as their strain rates often remain poorly documented. Several sections across the South Armorican Shear Zone (SASZ), a crustal-scale, several km-thick dextral shear zone were investigated by structural, petrological and Ar/Ar radiochronological methods. Finite strain profiles on these sections showed a strong deformation partitioning towards the ultramylonite core of the SASZ, with a decrease, of both the C'/S mean angle (from ca. 45° to less than 5°) and the Quartz grain-size (from ca. 150 to 5 µm). In parallel, conventional and in-situ Ar/Ar datings on the fabric-forming white micas were performed on compositionally complex white-micas. Inherited magmatic muscovites carried by the foliation and newly-formed, syn-tectonic substituted phengites located along the shear bands yielded 2 age groups for the less deformed samples. Along the shear zone cross-section, the difference between these two ages evolves from 10-15 Ma in weakly deformed domains to zero toward the SASZ core. In addition, the very fine-grained syn-tectonic phengites yielded the same ca. 300-298 Ma age irrespectively of their distance to the core of the shear zone, even in slightly deformed rocks distant by several km. These results

  14. Characteristics of faults and shear zones in deep mines

    USGS Publications Warehouse

    Wallace, R.E.; Morris, H.T.

    1986-01-01

    The characteristics of fault and shear zones to depths of 2.5 km are well documented in deep mines in North America. The characteristics may be summarized as follows. (a) Fault zones usually are irregular, branched, anastomosed, and curved rather than simple and planar. (b) Faults are generally composed of one or more clay or clay-like gouge zones in a matrix of sheared and foliated rock bordered by highly fractured rock. (c) The widths of fault zones appear to be greater when faults have greater displacement, probably as a result of a long history of repeated minor movements. Fault zones with kilometers of displacement tend to be 100 m or more wide, whereas those with only a few hundred meters of displacement commonly are only 1 m or less wide. (d) Some zones represent shear distributed across hundreds of meters without local concentration in a narrow gouge zone. (e) Many fault zones are wet even above the water table, and water moves along them at various rates, but some also serve as subsurface dams, ponding ground water as much as several hundred meters higher on one side than on the other. No striking differences in the characteristics of faults over the vertical range of 2.5 km are documented. ?? 1986 Birkha??user Verlag.

  15. Time constraints on faults activity in the Eastern California Shear Zone from U-Pb (SHRIMP-RG) dating of syntectonic opal

    NASA Astrophysics Data System (ADS)

    Nuriel, P.; Maher, K.; Miller, D. M.

    2013-12-01

    Absolute time constraints for fault activity are of fundamental importance in active fault systems. Such constraints are necessary for estimation of long-term slip-rates and earthquake recurrence intervals required for seismic-hazard assessments. Notwithstanding, paleoseismological records are often limited to the past 1 Ma, and important information such as fault initiation and early stage displacement are seldom determined. Here we present a novel methodological approach for direct dating of brittle deformation events over a geological time scale. We use in situ U-Pb SHRIMP-RG (Sensitive High Resolution Ion Microprobe - Reverse Geometry) analyses of opal precipitates in order to constrain the relative and absolute timing of brittle deformation events. The Mojave Desert fault segments within the Eastern California Shear Zone (ECSZ) are ideal faults to investigate the long-term history because of the need for improved constraints on the timing of fault initiation and the observed discrepancy between long-term and short-term estimates for strain accumulation rates in this area. We analyzed fault-related opal samples from ten different fault exposures within the Camp Rock, Cave Mountain, and the Cady fault systems. Millimeter size fragments of fault-related opal, occurring as fault coating, filling or fault-breccia cement, were imaged using cathodoluminescence and backscattering electron microscopy in order to identify distinct phases of opal associated with specific syntectonic microstructures. Sub-samples within each phase are then targeted with multiple SHRIMP-RG analyses (<50 μm in diameter) to allow the construction of 238U/208Pb-206Pb/208Pb and/or Tera-Wasserburg U-Pb isochrons. Of the 50 distinct phases that were identified, 20 were successfully dated and U-Pb age results range from 8.4 to 0.58 Ma. The timing of fault initiation along the Cave Mountain Fault system was previously estimated to be between 15 Ma and 5 Ma. Our results suggest that initial

  16. Complex strain patterns developed at the frontal and lateral tips to shear zones and thrust zones

    NASA Astrophysics Data System (ADS)

    Coward, M. P.; Potts, G. J.

    Many of the complex strain patterns seen in shear zones and thrust zones, such as variable fabric orientations, refolded folds and fabrics, together with folds with hinges almost parallel to the main transport direction, can be explained in terms of differential movement within the shear zones. These strains are developed at the frontal and lateral tips of the zones as they propagate. Examples are taken from the Moine thrust zone of Scotland which show variations in strains particularly at the lateral tips. The form of differential movement described here may lead to complex strain paths and non-plane strain ellipsoids and the spatial variations in finite strain may be used to delineate zones of extensional and compressional flow and differential movement in the shear zones or thrusts.

  17. Deformation and metamorphism of Hamadat complex: Shear zones reshaping the lithosphere

    NASA Astrophysics Data System (ADS)

    Hassan, Mahmoud; Abu-Alam, Tamer; Stüwe, Kurt

    2015-04-01

    In the active tectonic regions, shear zones play an important role to re-configure the structure of the lithosphere. One of the largest pre-Mesozoic shear zones on the Earth is the Najd Fault System of the Arabian-Nubian Shield. Literature data record exhumation of medium-pressure metamorphic complexes due to the activity of the Najd Fault System during the Pan-African Orogeny where the shear zone surrounds the boundaries of metamorphic complexes. The Hamadat complex of Northwestern part of Saudi Arabia shows an exception where the metamorphic rocks appear to occur within the shear zone itself instead of being surrounded by branches of it. The Hamadat complex records peak metamorphic conditions of 505 - 700 °C at two ranges of pressure 8 - 11 and 12.5 - 16.5 kbar. These pressures are higher than those of most other metamorphic complexes associated and exhumed by the Najd Fault system. The higher pressure conditions of the Hamadat complex relatively to other complexes in the shield can be interpreted in two different ways. The documented pressure can be due to lithostatic pressure or shear stress (i.e. in part non-lithostatic). Assuming the lithostatic condition, these pressure ranges indicate that the Hamadat complex consists rocks from two different crustal level (i.e. 28 - 38 km and 43 - 58 km) which shows the importance of the shear zone to reshape the lithosphere by bring rocks from different crustal level into one metamorphic complex.

  18. Rheology linked with phase changes as recorded by development of shear bands in the South Armorican Shear Zone

    NASA Astrophysics Data System (ADS)

    Jeřábek, Petr; Bukovská, Zita

    2015-04-01

    The South Armorican Shear Zone in France represents a major right-lateral strike slip shear zone formed in the late stages of Variscan orogeny. The active deformation in this shear zone is associated with the development of S-C fabrics in granitoids where thin shear bands (C) overprint an earlier higher grade metamorphic foliation (S). In the studied samples covering low to high intensity of shear band overprint, we identified three stages of shear band evolution associated with distinct microstructures and deformation mechanisms. The initiation of shear bands stage I is associated with the formation of microcracks crosscutting the S fabric and detected namely in the recrystallized quartz aggregates. The microcracks of suitable orientation are filled by microcline, albite, muscovite and chlorite which is a typical assemblage also for the well developed shear bands. Phase equilibrium modeling in PERPLEX indicates that this assemblage formed at pressure-temperature range of 0.1-0.4 GPa and 300-340 °C. Stage II of shear band evolution is characterized by dynamic recrystallization and grain size reduction of quartz aggregates along the microcracks and replacement of quartz by microcline along grain boundaries. This process leads to disintegration of quartz aggregate fabric and phase mixing in the shear bands. The inferred deformation mechanism for this stage is solution-precipitation creep although recrystallization of quartz is still active at the contact between quartz aggregates and shear bands. The coarse grained microstructure of quartz aggregates with ca ~250 microns average grain size reduces to ~10 microns grain size when recrystallized along extremely thin shear bands/microcracks and to ~20 microns grain size when recrystallized along the thicker shear bands. By using the flow law of Patterson and Luan (1990) for dislocation creep in quartz and the quartz piezometer of Stipp and Tullis (2003) corrected after Holyoke and Kronenberg (2010), the quartz

  19. Shear zone reactivation during South Atlantic rifting in NW Namibia

    NASA Astrophysics Data System (ADS)

    Koehn, D.; Passchier, C. W.; Salomon, E.

    2013-12-01

    Reactivation of inherited structures during rifting as well as an influence of inherited structures on the orientation of a developing rift has long been discussed (e.g. Piqué & Laville, 1996; Younes & McClay, 2002). Here, we present a qualitative and quantitative study of shear zone reactivation during the South Atlantic opening in NW Namibia. The study area comprises the Neo-Proterozoic rocks of the Kaoko Belt which was formed during the amalgamation of Gondwana. The Kaoko Belt encompasses the prominent ~500 km long ductile Purros shear zone and the Three Palms shear zone, both running sub-parallel to the present continental margin. The Kaoko Belt is partly overlain by the basalts of the Paraná-Etendeka Large Igneous Province, which with an age of ~133 Ma were emplaced just before or during the onset of the Atlantic rifting at this latitude. Combining the analysis of satellite imagery and digital elevation models with extensive field work, we identified numerous faults tracing the old shear zones along which the Etendeka basalts were down-faulted. The faults are often listric, yet we also found evidence for a regional scale basin formation. Our analysis allowed for constructing the geometry of three of these faults and we could thus estimate the vertical offsets to ~150 m, ~500 m, and ~1100 m, respectively. Our results contribute to the view that the basement inheritance plays a significant role on rifting processes and that the reactivation of shear zones can accumulate significant amounts of displacement. References: Pique, A. and E. Laville (1996). The Central Atlantic rifting: Reactivation of Paleozoic structures?. J. Geodynamics, 21, 235-255. Younes, I.A. and K. McClay (2002). Development of accommodation zones in the Gulf of Suez-Red Sea rift, Egypt. AAPG Bulletin, 86, 1003-1026.

  20. Small scale shear zone in calcite: AMS and microstructure

    NASA Astrophysics Data System (ADS)

    Roxerová, Zuzana; Machek, Matěj; Kusbach, Vladimír; Racek, Martin; Silva, Pedro F.

    2016-04-01

    Two structural profiles across thin shear zone in calcite from quarry in Estremoz (Portugal) were studied to find a relationship between AMS and strain in natural rocks. The mesoscopic fabric is characterized by the change from the subhorizontal coarse-grained foliation towards the ~2cm-wide shear zone center with subvertical fine-grained foliation. In microstructure, the shear zone records dynamic recrystallization of calcite aggregate which resulted in development of porphyroclastic microstructure with increasing proportion of fine-grained recrystallized matrix towards the shear zone center. Two distinct crystallographic preferred orientations of calcite were recorded. One related with porphyroclasts, characterized by subvertical orientation of calcite axes and another associated with recrystallized matrix showing subhorizontal calcite axes orientation. The magnetic susceptibility ranges from -8e-6SI to 9e-6SI, with the average -4e-6SI. The majority of the rock mass is diamagnetic, corresponding well with the thermomagnetic curves, with local paramagnetic accumulations in form of thin bands. The AMS of the both profiles exhibits stable subvertical foliation bearing vertical lineation which is locally alternated by the medium-angle foliation. We interpret the AMS fabric pattern which is perpendicular to the mineral one as a type of inverse AMS fabric, due to high iron content in major part of calcite grains The magnetic and microstructural description of the shear zone is accompanied by numerical modeling of AMS based on CPO and different proportion of porphyroclasts, matrix and mica for purposes of deciphering the influence of present microstructural features on AMS.

  1. Midnight velocity shear zone and the concept of Harang discontinuity

    SciTech Connect

    Koskinen, H.E.J.; Pulkkinen, T.I.

    1995-06-01

    The authors address the question of the origin and meaning of the Harang discontinuity. They consider mappings of the ionosphere, auroral zones, and magnetosphere. They argue for a relation between the velocity shear zone in the auroral ionosphere and the Harang discontinuity. However because of the upward directed field-aligned currents it is very hard to trace field lines well enough to show mappings of these zones. They argue however for the behavior of auroral arcs in relation to the appearance of substorm events to provide strong support for this relationship.

  2. Thermochronology of the South Cyclades Shear Zone, Ios, Greece: Effects of ductile shear in the argon partial retention zone

    NASA Astrophysics Data System (ADS)

    Baldwin, Suzanne L.; Lister, Gordon S.

    1998-04-01

    Micas and potassium feldspars from the South Cyclades Shear Zone, Ios, Cyclades, Greece, yield varied and complex 40Ar/39Ar apparent age spectra. A correlation exists between 40Ar/39Ar apparent ages and the relative timing of different episodes of recrystallization and grain growth, as indicated by fabric and microstructural analysis. The 40Ar/39Ar apparent age spectra record the effects of variation in the degree of recrystallization and grain growth, and partial to complete resetting of argon systematics in potassium-bearing minerals during Hercynian (M0), Alpine (M1) and late Oligocene - early Miocene (M2) metamorphism. Deformation was strongly partitioned within the shear zone and this led to localized recrystallization and heterogeneous resetting of argon systematics within preexisting minerals. Modeling suggests the Oligo-Miocène thermal events were of insufficient magnitude and/or duration to completely reset the isotopic systematics in these samples. Our data lead to the concept of the argon partial retention and resetting zone (PRZ), defined as that portion of the crust where temperatures are insufficient to completely reset argon systematics within preexisting potassium-bearing minerals. Within the PRZ, some radiogenic argon in preexisting potassium bearing minerals will be outgassed and only partially retained. Tectonic exhumation of the PRZ involves movement on crustal-scale ductile shear zones, accompanied by strongly partitioned deformation and localized recrystallization. Recrystallization leads to resetting of argon systematics, and thus will result in heterogeneous 40Ar/39Ar age distributions within these ductile shear zones (e.g., in the South Cyclades Shear Zone).

  3. The Pinto shear zone; a Laramide synconvergent extensional shear zone in the Mojave Desert region of the southwestern United States

    USGS Publications Warehouse

    Wells, M.L.; Beyene, M.A.; Spell, T.L.; Kula, J.L.; Miller, D.M.; Zanetti, K.A.

    2005-01-01

    The Pinto shear zone is one of several Late Cretaceous shear zones within the eastern fringe of the Mesozoic magmatic arc of the southwest Cordilleran orogen that developed synchronous with continued plate convergence and backarc shortening. We demonstrate an extensional origin for the shear zone by describing the shear-zone geometry and kinematics, hanging wall deformation style, progressive changes in deformation temperature, and differences in hanging wall and footwall thermal histories. Deformation is constrained between ???74 and 68 Ma by 40Ar/39Ar thermochronology of the exhumed footwall, including multi-diffusion domain modeling of K-feldspar. We discount the interpretations, applied in other areas of the Mojave Desert region, that widespread Late Cretaceous cooling results from refrigeration due to subduction of a shallowly dipping Laramide slab or to erosional denudation, and suggest alternatively that post-intrusion cooling and exhumation by extensional structures are recorded. Widespread crustal melting and magmatism followed by extension and cooling in the Late Cretaceous are most consistent with production of a low-viscosity lower crust during anatexis and/or delamination of mantle lithosphere at the onset of Laramide shallow subduction. ?? 2005 Elsevier Ltd. All rights reserved.

  4. The Pinto shear zone; a Laramide synconvergent extensional shear zone in the Mojave Desert region of the southwestern United States

    NASA Astrophysics Data System (ADS)

    Wells, Michael L.; Beyene, Mengesha A.; Spell, Terry L.; Kula, Joseph L.; Miller, David M.; Zanetti, Kathleen A.

    2005-09-01

    The Pinto shear zone is one of several Late Cretaceous shear zones within the eastern fringe of the Mesozoic magmatic arc of the southwest Cordilleran orogen that developed synchronous with continued plate convergence and backarc shortening. We demonstrate an extensional origin for the shear zone by describing the shear-zone geometry and kinematics, hanging wall deformation style, progressive changes in deformation temperature, and differences in hanging wall and footwall thermal histories. Deformation is constrained between ˜74 and 68 Ma by 40Ar/ 39Ar thermochronology of the exhumed footwall, including multi-diffusion domain modeling of K-feldspar. We discount the interpretations, applied in other areas of the Mojave Desert region, that widespread Late Cretaceous cooling results from refrigeration due to subduction of a shallowly dipping Laramide slab or to erosional denudation, and suggest alternatively that post-intrusion cooling and exhumation by extensional structures are recorded. Widespread crustal melting and magmatism followed by extension and cooling in the Late Cretaceous are most consistent with production of a low-viscosity lower crust during anatexis and/or delamination of mantle lithosphere at the onset of Laramide shallow subduction.

  5. Anisotropy across the Sorgenfrei Tornquist Zone from shear wave splitting

    NASA Astrophysics Data System (ADS)

    Wylegalla, K.; Bock, G.; Gossler, J.; Hanka, W.; TOR Working Group

    1999-12-01

    During the TOR-1 passive seismic experiment in 1996/97, a maximum of 139 temporary seismograph stations were operating over the Sorgenfrei-Tornquist Zone (STZ) in an area extending from northern Germany through Denmark to central Sweden. One of the objectives was to study horizontal anisotropy directions in the subcrustal lithosphere and asthenosphere across the Trans-European Suture Zone. To achieve this goal, broad-band and intermediate-period (5 s) data of the TOR-1 stations and additional stations of permanent networks (GRSN, GEOFON) were analysed for splitting of SKS and SKKS phases. As a result of the relatively dense station spacing, the method offers good lateral resolution of anisotropy. Preliminary results suggest that the directions of the fast horizontal S wave velocity are affected by the STZ. In central Europe and southern Sweden, far away from the STZ, fast S wave directions are approximately E-W while they turn more northerly closer to the STZ where they are approximately parallel to the trend of the STZ. No significant shear wave splitting was observed north of 57°N and east of 14°E. Small delay times between 0.2 and 0.5 s observed at the northernmost TOR-1 station T40S and T60S may be controlled by anisotropy in a thickened crust. The mantle contribution of horizontal anisotropy within the STZ is probably constrained to an approximately 60-km-thick zone in the depth range between 70 and 300 km. The observations are consistent with a model where azimuthal anisotropy is not governed by present-day mantle flow in the asthenosphere, but rather is frozen into the subcrustal lithosphere during the last episode of tectonic activity.

  6. Conjugate-shear folding: A model for the relationships between foliations, folds and shear zones

    NASA Astrophysics Data System (ADS)

    Aerden, Domingo G. A. M.; Sayab, Mohammad; Bouybaouene, Mohamed L.

    2010-08-01

    Microstructural mapping of whole thin sections cut from two samples of micaschist containing cm-scale folds plus garnet porphyroblasts has provided new insight in the relationships between folding, shearing and foliation development. The garnets exhibit coherent inclusion-trail patterns that place important constraints on the kinematic development of both samples, which are shown to be representative of coaxial versus non-coaxial deformation in rocks containing a pre-existing schistosity. A comparison of crenulations-cleavages geometries in both samples and a review of the geometry of natural and experimental multilayer folds leads to the conclusion that folding involves conjugate shearing at different scales. At microscopic scales, crenulation cleavages nucleate as conjugate-kink or shear instabilities and develop further as a function of the macroscopic partitioning of deformation. In fold-hinge domains, bulk-coaxial deformation results in equal development of conjugate crenulations that progressively coalescence into symmetrical crenulation patterns so that, macroscopically, parallelism is achieved between foliation, fold-axial planes and long axes of strain ellipses. Fold-limb domains represent a system of conjugate-shear zones where single sets of crenulation instabilities with synthetic shearing component preferentially develop producing oblique relationships between the aforementioned elements. Cleavage fanning is inferred as a direct consequence of this conjugate-shear origin of folds. The model implies that crenulation cleavages and S-C fabrics in shear zones form by analogous processes, in both cases involving a component of shearing along foliation planes. The development of conjugate sets of foliation planes surrounding porphyroblasts during early, relatively coaxial stages of deformation explains continued "gyrostatic" behaviour during more advanced non-coaxial stages, as indicated by consistently oriented inclusion trails in the studied samples.

  7. The Amount and Preferred Orientation of Simple-shear in a Deformation Tensor: Implications for Detecting Shear Zones and Faults with GPS

    NASA Astrophysics Data System (ADS)

    Johnson, A. M.; Griffiths, J. H.

    2007-05-01

    At the 2005 Fall Meeting of the American Geophysical Union, Griffiths and Johnson [2005] introduced a method of extracting from the deformation-gradient (and velocity-gradient) tensor the amount and preferred orientation of simple-shear associated with 2-D shear zones and faults. Noting the 2-D is important because the shear zones and faults in Griffiths and Johnson [2005] were assumed non-dilatant and infinitely long, ignoring the scissors- like action along strike associated with shear zones and faults of finite length. Because shear zones and faults can dilate (and contract) normal to their walls and can have a scissors-like action associated with twisting about an axis normal to their walls, the more general method of detecting simple-shear is introduced and called MODES "method of detecting simple-shear." MODES can thus extract from the deformation-gradient (and velocity- gradient) tensor the amount and preferred orientation of simple-shear associated with 3-D shear zones and faults near or far from the Earth's surface, providing improvements and extensions to existing analytical methods used in active tectonics studies, especially strain analysis and dislocation theory. The derivation of MODES is based on one definition and two assumptions: by definition, simple-shear deformation becomes localized in some way; by assumption, the twirl within the deformation-gradient (or the spin within the velocity-gradient) is due to a combination of simple-shear and twist, and coupled with the simple- shear and twist is a dilatation of the walls of shear zones and faults. The preferred orientation is thus the orientation of the plane containing the simple-shear and satisfying the mechanical and kinematical boundary conditions. Results from a MODES analysis are illustrated by means of a three-dimensional diagram, the cricket- ball, which is reminiscent of the seismologist's "beach ball." In this poster, we present the underlying theory of MODES and illustrate how it works by

  8. Horizontal shear zones: physical modeling of formation and structure

    NASA Astrophysics Data System (ADS)

    Bokun, A. N.

    2009-11-01

    On examples of ductile viscous materials (pastes), which demonstrated the deformational type of coagulation behavior and the reproduced zones of the horizontal shear of a brittle fracture, ductile flow, and intermediate types. The formation of coagulation agglomerates appeared well organized, both in terms of time and structurally. The found systems of fractures revealed a sequential course of the deformation process and the contribution of each system in the total structural transformation was established. By virtue of rheological analysis of coagulation structures, the basic parameters (yield strength, viscosity), and their input into the model of the deformational response (brittle, ductile), were determined. The substantial composition and its deformational properties of the material under question appeared to dictate the structure of shear zones and their general mutual organization. The rheological analysis of coagulation clusters of model materials allowed for the justified interpretation of experimental data to regulate deformation processes effectively and predict their results.

  9. Viscous shear heating instabilities in a 1-D viscoelastic shear zone

    NASA Astrophysics Data System (ADS)

    Homburg, J. M.; Coon, E. T.; Spiegelman, M.; Kelemen, P. B.; Hirth, G.

    2010-12-01

    Viscous shear instabilities may provide a possible mechanism for some intermediate depth earthquakes where high confining pressure makes it difficult to achieve frictional failure. While many studies have explored the feedback between temperature-dependent strain rate and strain-rate dependent shear heating (e.g. Braeck and Podladchikov, 2007), most have used thermal anomalies to initiate a shear instability or have imposed a low viscosity region in their model domain (John et al., 2009). By contrast, Kelemen and Hirth (2007) relied on an initial grain size contrast between a predetermined fine-grained shear zone and coarse grained host rock to initiate an instability. This choice is supported by observations of numerous fine grained ductile shear zones in shallow mantle massifs as well as the possibility that annealed fine grained fault gouge, formed at oceanic transforms, subduction related thrusts and ‘outer rise’ faults, could be carried below the brittle/ductile transition by subduction. Improving upon the work of Kelemen and Hirth (2007), we have developed a 1-D numerical model that describes the behavior of a Maxwell viscoelastic body with the rheology of dry olivine being driven at a constant velocity at its boundary. We include diffusion and dislocation creep, dislocation accommodated grain boundary sliding, and low-temperature plasticity (Peierls mechanism). Initial results suggest that including low-temperature plasticity inhibits the ability of the system to undergo an instability, similar to the results of Kameyama et al. (1999). This is due to increased deformation in the background allowing more shear heating to take place, and thus softening the system prior to reaching the peak stress. However if the applied strain rate is high enough (e.g. greater than 0.5 x 10-11 s-1 for a domain size of 2 km, an 8 m wide shear zone, a background grain size of 1 mm, a shear zone grain size of 150 μm, and an initial temperature of 650°C) dramatic

  10. Anatomy of an extensional shear zone in the mantle, Lanzo massif, Italy

    NASA Astrophysics Data System (ADS)

    Kaczmarek, Mary-Alix; Tommasi, AndréA.

    2011-08-01

    Analysis of the microstructures in the km-scale mantle shear zone that separates the northern and the central parts of the Lanzo peridotite massif provides evidence of an evolution in time and space of deformation processes accommodating shearing in the shallow mantle within an extensional setting. This shear zone displays an asymmetric distribution of deformation facies. From south to north, gradual reorientation of the foliation of coarse porphyroclastic plagioclase-bearing peridotites is followed by development of protomylonites, mylonites, and mm-scale ultramylonite bands. A sharp grain size gradient marks the northern boundary. Early deformation under near-solidus conditions in the south is recorded by preservation of weakly deformed interstitial plagioclase and almost random clinopyroxene and plagioclase crystal orientations. Feedback between deformation and melt transport probably led to melt focusing and strain weakening in the shear zone. Overprint of melt-rock reaction microstructures by solid-state deformation and decrease in recrystallized grain size in the protomylonites and mylonites indicate continued deformation under decreasing temperature. Less enriched peridotite compositions and absence of ultramafic dykes or widespread melt-impregnation microstructures north of the shear zone and clinopyroxene and amphibole enrichment in the mylonites and ultramylonites suggest that the shear zone acted as both a thermal barrier and a high-permeability channel for late crystallizing fluids. These observations, together with chemical data indicating faster cooling of central Lanzo relative to the northern body, corroborate that this shear zone is a mantle detachment fault. All deformation facies have crystal preferred orientations consistent with deformation by dislocation creep with dominant activation of the (010)[100] and (100)[001] systems in olivine and orthopyroxene, respectively. Dynamic recrystallization produces dispersion of olivine CPO but not a

  11. Kinematic, finite strain and vorticity analysis of the Sisters Shear Zone, Stewart Island, New Zealand

    NASA Astrophysics Data System (ADS)

    Ring, Uwe; Bernet, Matthias; Tulloch, Andy

    2015-04-01

    The Sisters Shear Zone (SSZ) on Stewart Island, New Zealand, is a greenschist-facies extensional shear zone active prior to and possibly during the development of the Pacific-Antarctica spreading ridge at ∼76 Ma. We report quantitative kinematic and rotation data as well as apatite fission-track (AFT) ages from the SSZ. Early kinematic indicators associated with the NNE-trending stretching lineation formed under upper greenschist-facies metamorphism and show alternating top-to-the-NNW and top-to-the-SSE senses of shear. During progressive exhumation lowermost greenschist-facies and brittle-ductile kinematic indicators depict a more uniform top-to-the-SSE sense of shear in the topmost SSZ just below the detachment plane. Deformed metagranites in the SSZ allow the reconstruction of deformation and flow parameters. The mean kinematic vorticity number (Wm) ranges from 0.10 to 0.89; smaller numbers prevail in the deeper parts of the shear zone with a higher degree of simple shear deformation in the upper parts of the shear zone (deeper and upper parts relate to present geometry). High finite strain intensity correlates with low Wm and high Wm numbers near the detachment correlate with relatively weak strain intensity. Finite strain shows oblate geometries. Overall, our data indicate vertical and possibly temporal variations in deformation of the SSZ. Most AFT ages cluster around 85-75 Ma. We interpret the AFT ages to reflect the final stages of continental break-up just before and possibly during the initiation of sea-floor spreading between New Zealand and Antarctica.

  12. Late Neogene kinematics of intra-arc oblique shear zones: The Petilia-Rizzuto Fault Zone (Calabrian Arc, Central Mediterranean)

    NASA Astrophysics Data System (ADS)

    van Dijk, J. P.

    1994-10-01

    The kinematics of intra-arc shear zones play a key role in the secondary shaping of orogenic arcs such as the Calabrian Arc (central Mediterranean). Comparison of the Neogene structural development of the Petilia-Rizzuto Fault Zone and the basement structure of the bordering Sila massif reveals that the fault zone is the surface expression of a deep NW-SE trending sinistral crustal oblique shear zone. This shear zone continues over a length of more than 130 km across the northern segment of the Calabrian Arc and shows a post-Eocene sinistral displacement of about 50 km. The late Neogene forearc basin development and syndepositional tectonics along the fault zone are reconstructed in great detail by analyzing the middle Miocene-Recent tectonic sequence stratigraphy. A strike-slip cycle can be recognized whereby the subsequent activity of Riedel shears, tensional faults, and P shears, positive flower structures and principle displacement wrench faults, can accurately be traced in time. Observed phenomena are discussed in terms of the activity of a conjugate system of oblique thrust zones within the growing accretionary complex. The evolution of special types of thrust belt basins is illustrated. These include oblique thin-skinned pull-apart basins, oblique rhomboidal "harmonica" basins, and "detached slab" basins (new terms introduced here), evolving one into the other. A new feature illustrated is the recurrent basin inversion which generated passive roof duplexes through back-shear motion and out-of-sequence thrusting along the wedge. The fault patterns and the style of inversion tectonics imply an E-W directed axis of effective compressive stress in this part of the arc. This resulted from an interaction of (1) local E-W directed compression related to a differential displacement of two parallel segments of the arc (generated by the migration to the southeast of the Calabrian Arc and opening of the Tyrrhenian backarc basin); (2) alternating NW-SE directed

  13. Spatial heterogeneities in tectonic stress in Kyushu, Japan and their relation to a major shear zone

    NASA Astrophysics Data System (ADS)

    Matsumoto, Satoshi; Nakao, Shigeru; Ohkura, Takahiro; Miyazaki, Masahiro; Shimizu, Hiroshi; Abe, Yuki; Inoue, Hiroyuki; Nakamoto, Manami; Yoshikawa, Shin; Yamashita, Yusuke

    2015-10-01

    We investigated the spatial variation in the stress fields of Kyushu Island, southwestern Japan. Kyushu Island is characterized by active volcanoes (Aso, Unzen, Kirishima, and Sakurajima) and a shear zone (western extension of the median tectonic line). Shallow earthquakes frequently occur not only along active faults but also in the central region of the island, which is characterized by active volcanoes. We evaluated the focal mechanisms of the shallow earthquakes on Kyushu Island to determine the relative deviatoric stress field. Generally, the stress field was estimated by using the method proposed by Hardebeck and Michael (2006) for the strike-slip regime in this area. The minimum principal compression stress ( σ3), with its near north-south trend, is dominant throughout the entire region. However, the σ 3 axes around the shear zone are rotated normal to the zone. This result is indicative of shear stress reduction at the zone and is consistent with the right-lateral fault behavior along the zone detected by a strain-rate field analysis with global positioning system data. Conversely, the stress field of the normal fault is dominant in the Beppu-Shimabara area, which is located in the central part of the island. This result and the direction of σ3 are consistent with the formation of a graben structure in the area.

  14. How does shear zone nucleate? An example from the Suretta nappe (Swiss Eastern Alps)

    NASA Astrophysics Data System (ADS)

    Goncalves, Philippe; Poilvet, Jean-Charles; Oliot, Emilien; Trap, Pierre; Marquer, Didier

    2016-05-01

    In order to address the question of the processes involved during shear zone nucleation, we present a petro-structural analysis of millimetre-scale shear zones within the Roffna rhyolite (Suretta nappe, Eastern central Alps). Field and microscopic evidences show that ductile deformation is localized along discrete fractures that represent the initial stage of shear zone nucleation. During incipient brittle deformation, a syn-kinematic metamorphic assemblage of white mica + biotite + epidote + quartz precipitated at ca. 8.5 ± 1 kbar and 480 ± 50 °C that represent the metamorphic peak conditions of the nappe stacking in the continental accretionary wedge during Tertiary Alpine subduction. The brittle to ductile transition is characterized by the formation of two types of small quartz grains. The Qtz-IIa type is produced by sub-grain rotation. The Qtz-IIb type has a distinct CPO such that the orientation of c-axis is perpendicular to the shear fracture and basal and rhombhoedric slip systems are activated. These Qtz-IIb grains can either be formed by recrystallization of Qtz-IIa or by precipitation from a fluid phase. The shear zone widening stage is characterized by a switch to diffusion creep and grain boundary sliding deformation mechanisms. During the progressive evolution from brittle nucleation to ductile widening of the shear zone, fluid-rock interactions play a critical role, through chemical mass-transfer, metasomatic reactions and switch in deformation mechanisms.

  15. A continuous transect through the lower seismogenic zone of a transform boundary: The Pofadder Shear Zone, Namibia and South Africa

    NASA Astrophysics Data System (ADS)

    Melosh, B. L.; Rowe, C. D.; Smit, L.

    2012-12-01

    The Pofadder Shear Zone (PSZ) is a ~400 km long, northwest to southeast oriented, dextral transform shear zone in southern Namibia and northwest South Africa. We present new geologic and structural data from mapped transects across the PSZ. These data demonstrate that, from the northwest toward the southeast, the PSZ narrows by 90%, retains a near vertical dip but changes in strike by 30 degrees, splays into a series of smaller shear zones, and displays a greater abundance of syn-kinematic greenschist facies minerals. Mineral stretching and aggregate lineations, measured on C surfaces, are defined by plastically deformed quartz and aligned hornblende porphyroblasts; these lineations display a polarity reversal in the northwest, no reversal in the southeast, and are folded by isoclinal folds. Within the mylonitic fault rock quartz, alkali feldspar and plagioclase feldspar record plastic, semi-plastic, and brittle deformation textures, respectively. Lastly, we observe monolithologic mylonite clast breccia and calc-schist cataclasites in foliation-parallel, tabular zones 1 - 50 cm thick. We interpret these data to suggest that: 1) the PSZ is tilted toward the southeast about a horizontal axis and this rotation caused differential exhumation from upper greenschist facies conditions, in the southeast, to amphibolite facies conditions, in the northwest, exposing the quartz brittle-plastic transition and the base of the seismogenic zone. Using approximate temperature estimates of these metamorphic conditions and assuming a geothermal gradient ranging between 25 - 30 °C/km we estimate the PSZ has been tilted by 17 - 11 degrees, respectively; 2) a series of discrete splay faults (possible horsetail faults) occurs on the southern side of the PSZ toward the southeast terminus where the fault approaches its tip; 3) horizontal mineral stretching lineations dominate the narrower shear zones in the southeast; and 4) fault breccias and cataclasites record either high strain rate

  16. Broad belts of shear zones: The common form of surface rupture produced by the 28 June 1992 Landers, California, earthquake

    SciTech Connect

    Johnson, A.M.; Cruikshank, K.M. |; Fleming, R.W.

    1993-12-31

    Surface rupturing during the 28 June 1992, Landers, California earthquake, east of Los Angeles, accommodated right-lateral offsets up to about 6 m along segments of distinct, en echelon fault zones with a total length of about 80 km. The offsets were accommodated generally not by faults -- distinct slip surfaces -- but rather by shear zones, tabular bands of localized shearing. In long, straight stretches of fault zones at Landers the rupture is characterized by telescoping of shear zones and intensification of shearing: broad shear zones of mild shearing, containing narrow shear zones of more intense shearing, containing even-narrower shear zones of very intense shearing, which may contain a fault. Thus the ground ruptured across broad belts of shearing with subparallel walls, oriented NW. Each broad belt consists of a broad zone of mild shearing, extending across its entire width (50 to 200 m), and much narrower (a few m wide) shear zones that accommodate most of the offset of the belt and are portrayed by en echelon tension cracks. In response to right-lateral shearing, the slices of ground bounded by the tension cracks rotated in a clockwise sense, producing left lateral shearing, and the slices were forced against the walls of the shear zone, producing thrusting. Even narrower shear zones formed within the narrow shear zones, and some of these were faults. Although the narrower shear zones probably are indicators to right-lateral fault segments at depth, the surface rupturing during the earthquake is characterized not by faulting, but by zones of shearing at various scales. Furthermore, understanding of the formation of the shear zones may be critical to understanding of earthquake faulting because, where faulting is associated with the formation of a shear zone, the faulting occurs late in the development of the shear zone. The faulting occurs after a shear zone or a belt of shear zones forms.

  17. The importance of strain localisation in shear zones

    NASA Astrophysics Data System (ADS)

    Bons, Paul D.; Finch, Melanie; Gomez-Rivas, Enrique; Griera, Albert; Llorens, Maria-Gema; Steinbach, Florian; Weikusat, Ilka

    2016-04-01

    The occurrence of various types of shear bands (C, C', C'') in shear zones indicate that heterogeneity of strain is common in strongly deformed rocks. However, the importance of strain localisation is difficult to ascertain if suitable strain markers are lacking, which is usually the case. Numerical modelling with the finite-element method has so far not given much insight in the development of shear bands. We suggest that this is not only because the modelled strains are often not high enough, but also because this technique (that usually assumes isotropic material properties within elements) does not properly incorporate mineral deformation behaviour. We simulated high-strain, simple-shear deformation in single- and polyphase materials with a full-field theory (FFT) model coupled to the Elle modelling platform (www.elle.ws; Lebensohn 2001; Bons et al. 2008). The FFT-approach simulates visco-plastic deformation by dislocation glide, taking into account the different available slip systems and their critical resolved shear stresses in relations to the applied stresses. Griera et al. (2011; 2013) have shown that this approach is particularly well suited for strongly anisotropic minerals, such as mica and ice Ih (Llorens 2015). We modelled single- and polyphase composites of minerals with different anisotropies and strengths, roughly equivalent to minerals such as ice Ih, mica, quartz and feldspar. Single-phase polycrystalline aggregates show distinct heterogeneity of strain rate, especially in case of ice Ih, which is mechanically close to mica (see also Griera et al. 2015). Finite strain distributions are heterogeneous as well, but the patterns may differ from that of the strain rate distribution. Dynamic recrystallisation, however, usually masks any strain and strain rate localisation (Llorens 2015). In case of polyphase aggregates, equivalent to e.g. a granite, we observe extensive localisation in both syn- and antithetic shear bands. The antithetic shear bands

  18. Evidence for fluid-assisted shear failure in a ductile shear zone: Tectonic tremor in the geologic record?

    NASA Astrophysics Data System (ADS)

    Compton, Katharine

    Recent direct observations of tectonic tremor below the seismogenic zone of large fault zones have emphasized the significance of coeval ductile and brittle processes at high temperatures. Tectonic tremor is defined as long-duration, low-amplitude, and low-frequency seismic signals produced at depths of 18-40 km. Because the source of tremor is currently unknown, the physical conditions and processes that cause tremor are unknown. This study presents observations of an exhumed shear zone system contained within the Saddlebag Lake pendant of the eastern Sierra Nevada, California. The high-strain rocks in this shear zone exhibit multiple episodes of vein formation, indicating a prolonged migration of hydrothermal fluids through the system. Crosscutting relationships and mineral assemblages define discrete sets of variously oriented veins that are folded and boudinaged. I document foliation-parallel quartz veins that show shear displacement parallel to the foliation. Textural evidence for dynamic recrystallization mechanisms, stable isotope data, and fluid inclusion thermobarometry measurements indicate temperatures between 300-680°C and relatively high fluid pressure conditions, greater than ?3, during fracture. Conditions of nucleation of shear fractures within this ductile shear zone suggest these structures may record similar processes to those under which tectonic tremor is observed in other continental transform fault zones. I interpret that these veins formed as shear fractures under increasing differential stress and fluctuations in pore pressure, with failure driven by heterogeneous materials within the shear zone.

  19. Microstructural and Rheological Constraints on the Mantle Strength of Strike-Slip Fault Systems: Evidence from the Bogota Peninsula Shear Zone, New Caledonia

    NASA Astrophysics Data System (ADS)

    Chatzaras, V.; Titus, S.; Tikoff, B.; Drury, M. R.

    2014-12-01

    Crust-mantle coupling along major strike-slip fault zones suggests that these two lithospheric layers act as an integrated system. In such a system, the spatial and temporal evolution of mantle strength across strike-slip shear zones has proven a key component in understanding lithospheric deformation and rheology. The Bogota Peninsula shear zone is exposed in the mantle section of the New Caledonia ophiolite. It contains a unique microstructural and textural record across a 4-km wide mylonitic zone bordered by a wider zone of weaker deformation. The shear zone is interpreted as a paleotransform fault, based on the orientations of fabrics and dikes inside and outside the zone. No ultramylonites or pseudotachylites were observed within the shear zone. Olivine grain size paleopiezometers suggest variation of the shear zone stresses, with the highest values recorded in the center of the shear zone, coincident with increasing olivine CPO strength toward the shear zone center. By estimating the finite strain in the zone, and assuming that all portions of the shear zone were active synchronously, we can correlate the increased stresses to increased strain rates. We compare the mantle strength in the Bogota Peninsula shear zone to other transform faults, such as the San Andreas fault (SAF) system. The differential stresses in the upper mantle of the SAF system, determined from xenoliths, is similar to those observed in the New Caledonia. Further, the width of shearing deformation in Bogota Peninsula shear zone is similar to that inferred for other transform zones, in both the upper crust and lithospheric mantle. These similarities suggest that viscous flow in the lithospheric mantle is in mechanical communication to brittle deformation in the upper crust. We propose a "Lithospheric Feedback" model, in which displacement due to mantle flow loads the crust during interseismic cycles, while the upper crust effectively limits the strength of the lithosphere.

  20. Periodic Viscous Shear Heating Instability in Fine-Grained Shear Zones: Possible Mechanism for Intermediate Depth Earthquakes and Slow Earthquakes?

    NASA Astrophysics Data System (ADS)

    Kelemen, P. B.; Hirth, G.

    2004-12-01

    Localized ductile shear zones with widths of cm to m are observed in exposures of Earth's shallow mantle (e.g., Kelemen & Dick JGR 95; Vissers et al. Tectonophys 95) and dredged from oceanic fracture zones (e.g., Jaroslow et al. Tectonophys 96). These are mylonitic (grain size 10 to 100 microns) and record mineral cooling temperatures from 1100 to 600 C. Pseudotachylites in a mantle shear zone show that shear heating temperatures can exceed the mantle solidus (e.g., Obata & Karato Tectonophys 95). Simple shear, recrystallization, and grain boundary sliding all decrease the spacing between pyroxenes, so olivine grain growth at lower stress is inhibited; thus, once formed, these shear zones do not "heal" on geological time scales. Reasoning that grain-size sensitive creep will be localized within these shear zones, rather than host rocks (grain size 1 to 10 mm), and inspired by the work of Whitehead & Gans (GJRAS 74), we thought these might undergo repeated shear heating instabilities. In this view, as elastic stress increases, the shear zone weakens via shear heating; rapid deformation of the weak shear zone releases most stored elastic stress; lower stress and strain rate coupled with diffusion of heat into host rocks leads to cooling and strengthening, after which the cycle repeats. We constructed a simple numerical model incorporating olivine flow laws for dislocation creep, diffusion creep, grain boundary sliding, and low T plasticity. We assumed that viscous deformation remains localized in shear zones, surrounded by host rocks undergoing elastic deformation. We fixed the velocity along one side of an elastic half space, and calculated stress due to elastic strain. This stress drives viscous deformation in a shear zone of specified width. Shear heating and thermal diffusion control temperature evolution in the shear zone and host rocks. A maximum of 1400 C (where substantial melting of peridotite would occur) is imposed. Grain size evolves during dislocation

  1. Strain localisation and thermal evolution of a thick ultramylonitic shear zone

    NASA Astrophysics Data System (ADS)

    Finch, Melanie; Hasalova, Pavlina; Weinberg, Roberto

    2013-04-01

    Ultramylonites are the ultimate product of mylonitisation in ductile shear zones where strain causes almost complete recrystallisation of the rock. They are the weakest rock in a shear zone and accommodate high amounts of strain, usually in a relatively narrow band. However, ultramylonitic shear zones hundreds of metres thick are occasionally reported in the literature, indicating inefficient localisation processes. This work describes the >3.5 km thick El Pichao shear zone of the Sierra de Quilmes, a metamorphic complex of the 470 Ma Famatinian orogeny in the Sierras Pampeanas of Argentina. The core to the El Pichao shear zone is a one kilometre thick band of continuous ultramylonite. Although rare, other shear zones of comparable thickness are reported in the literature and are often related to major orogenic fronts. The width of shear zones is determined by plate velocity and rock strength, with greater widths at high velocities and low rock strength. Shear zones widen when the degree of strain localisation decreases. This can be caused by hardening of the shear zone, weakening of the host rock, or an increase in temperature which decreases the yield stress of the rock. There are several mechanisms that can lead to each outcome but these are difficult to determine in studies of shear zones. El Pichao shear zone overprints granulite facies migmatites in the hanging wall, granites in the ultramylonitic shear zone core, and amphibolite facies Grt-schists in the footwall. Field relationships and mineralogy suggest that the migmatitic hanging wall was at higher temperatures during shearing than the ultramylonitic granitic core. Theoretically this makes the migmatites easier to deform than the granite. Additionally, the migmatite is very heterogenous with a mica-rich mesosome and syn-kinematic Qtz-Kfs rich leucosomes making it the ideal site for strain localisation and partitioning between strong and weak phases. Despite this, ultramylonite is localised to the

  2. Multi-scale analysis of Proterozoic shear zones: An integrated structural and geophysical study

    NASA Astrophysics Data System (ADS)

    Stewart, John R.; Betts, Peter G.; Collins, Alan S.; Schaefer, Bruce F.

    2009-11-01

    Structural mapping of poorly exposed shear zone outcrops is integrated with the analysis of aeromagnetic and Bouguer gravity data to develop a multi-scale kinematic and relative overprinting chronology for the Palaeoproterozoic Tallacootra Shear Zone, Australia. D 2 mylonitic fabrics at outcrop record Kimban-aged (ca. 1730-1690 Ma) N-S shortening and correlate with SZ 1 movements. Overprinting D 3 sinistral shear zones record the partitioning of near-ideal simple shear and initiated Riedel to regional-scale SZ 2 strike-slip on the Tallacootra Shear Zone (SZ 2). Previously undocumented NE-SW extension led to the emplacement of aplite dykes into the shear zone and can be correlated to the (ca. 1595-1575 Ma) Hiltaba magmatic event. D 4 dextral transpression during the (ca. 1470-1450 Ma) Coorabie Orogeny reactivated the Tallacootra Shear Zone (SZ 2-R4) exhuming lower crust of the northwestern Fowler Domain within a positive flower structure. This latest shear zone movement is related to a system of west-dipping shear zones that penetrate the crust and sole into a lithospheric detachment indicating wholesale crustal shortening. These methods demonstrate the value of integrating multi-scale structural analyses for the study of shear zones with limited exposure.

  3. Deriving strain from crystallographic preferred orientation for a ductile shear zone in north western Turkey.

    NASA Astrophysics Data System (ADS)

    Farrell, K.; Lloyd, G. E. E.; Wallis, D.; Phillips, R. J.

    2015-12-01

    Understanding the behaviour of active continental-scale fault zones at depth, and in particular how displacements observed at the Earth's surface are accommodated through the crust, is crucial to improving understanding of the earthquake cycle. This behaviour can be inferred by study of exhumed portions of ductile shear zones using methods such as recording strain profile(s) across the fault zone. However, due to the nature of mid-crustal rocks, strain markers tend to be rare and/or discontinuously distributed. The intensity (I) of crystallographic preferred orientation (CPO) of deformed minerals provides a proxy for strain that is continuous across fault zones. CPO are collected via electron back scattered diffraction in the scanning electron microscope. The strength of the CPO can be quantified using eigenvalue-based intensity parameters. Calibration of intensity with strain is achieved via comparison with visco-plastic self-consistency models of CPO evolution, although the temperature-dependent critical resolved shear stresses of potential crystal slip systems must be known. As an example, we consider the dextral strike-slip Eskişehir shear zone, NW Turkey, which was active during the Oligocene and accommodated ~100km of displacement, including a component of late oblique-normal slip. An exhumed mid-crustal section of this fault zone is exposed in the Uludağ Massif, comprising of high-grade metamorphic rocks of the Uludağ Group, intruded by the Central and South Uludağ granites. Sample transects focussed on the pure calcic marbles that dominate the stratigraphy. Fortunately, the availability of experimental data for calcite crystal slip behaviour at different temperatures makes the application of the CPO intensity strain proxy method relatively straightforward. The Uludağ Massif and Eskişehir shear zone provide a field based analogue for the ductile shear zone beneath the currently active North Anatolian Fault. The results of our CPO intensity-based strain

  4. Is the Chrystalls Beach Accretionary Melange a Fossil Subduction Channel Shear Zone?

    NASA Astrophysics Data System (ADS)

    Fagereng, A.; Sibson, R. H.

    2007-12-01

    In the northeast of the Hikurangi subduction margin, a 1-2 km thick layer of high Vp/Vs, low Qp and distributed microseismicity is present along the subduction megathrust interface (Eberhart-Phillips & Chadwick, 2002). This zone is interpreted as a 'subduction channel' consisting of a fluid-saturated, highly sheared mixture of trench-fill sediments, which have been subducted below (or eroded from) the accretionary prism (von Huene & Scholl, 1991). Seismic style within subduction channels may vary from large megathrust ruptures to aseismic slip associated with microseismic activity. The factors controlling these variations in style are not well understood due to the inaccessible nature of active subduction thrust interfaces. The Chrystalls Beach Complex, SE Otago, New Zealand, is a possible analogue for the seismogenic zone of an active subduction channel. This complex comprises an intensely sheared mixture of chert, terrigenous mud and sand, minor volcanogenic sediments and pillow lavas. It has a 'block-in-matrix' mélange structure, where asymmetric, dismembered beds of sand-rich competent material are enclosed within a relatively incompetent, cleaved pelitic matrix. The rock assemblage has been progressively deformed in a top-to-the-north shear zone, and is pervaded by an anastomosing network of quartz/calcite shear- and extension veins, where individual veins can be traced for tens of metres. The presence of extension veins indicates episodes where the tensile overpressure condition (Pf > σ3) was locally attained. Initially the sediments experienced compaction, volume loss and layer-parallel soft sediment shearing, developing a slaty cleavage and viscous S/C shear structures. The dense vein network developed during subsequent brittle deformation. The mineral assemblage (pumpellyite-chlorite to pumpellyite-actinolite), mica b0 spacing and illite crystallinity indicate deformation in a high pressure - low temperature environment ( ~ 3-6 kbar, ~ 200-300°C). This

  5. The Markoye Shear Zone in NE Burkina Faso

    NASA Astrophysics Data System (ADS)

    Tshibubudze, Asinne; Hein, Kim A. A.; Marquis, Pascal

    2009-12-01

    Birimian supracrustal sequences in NE Burkina Faso are dominated by meta-volcaniclastic greywacke, intercalated meta-conglomerate, siltstone and shale. The sequences where subjected to two phases of deformation and contact metamorphosed to hornblende-hornfels facies during emplacement of pyroxenite-gabbro-norite (Yacouba Mafic complex), granodiorite-tonalite (Tin Taradat granodiorite-tonalite) and dolerite dykes. Structural studies indicated that the NE-trending, first-order crustal-scale Markoye Shear Zone (MSZ; Markoye Fault of [Jeambrun, M., Delfour, J., Gravost, M., 1970. Carte géologique de L'Oudalan. Bureau De Recherches Geologiques et Miniéres, Burkina Faso.]) has undergone at least two phases of reactivation concomitant to two phases of regional deformation. The first phase of deformation, D1, resulted in the formation of NNW-NW trending folds and thrusts during dextral-reverse displacement on the MSZ. The deformation is termed the Tangaean Event and predates the Eburnean Orogeny. D2 phase involved a period of SE-NW crustal shortening and sinistral-reverse displacement on the MSZ, and is correlated to the Eburnean Orogeny ˜2.1 Ga. Deformation in D2 is characterised by NE-trending regional folds (F2) and a pervasive NE-trending foliation (S2-C to S2). Within the MSZ, deformation is characterised by NNE-trending zones of mylonite that are bordered in the hangingwall and footwall by pseudotachylite veins. Buck quartz-carbonate veins and quartz cataclasite veins crosscut the mylonite zones and are, in turn, crosscut by quartz-chlorite-(muscovite) shears that formed during reactivation of the MSZ late in D2. Several generations of veins are recognised at the Essakane main deposit (EMZ): Arsenopyrite-pyrite-gold mineralization in quartz veins formed in D1 during metasomatic alteration of the host rocks; Vein-stockwork gold mineralization is interpreted to have formed late in D2.

  6. Teleseismic shear wave tomography of the Japan subduction zone

    NASA Astrophysics Data System (ADS)

    Asamori, Koichi; Zhao, Dapeng

    2015-12-01

    We present a high-resolution shear wave tomography of the Japan subduction zone down to a depth of 700 km, which is determined by inverting a large number of high-quality S-wave arrival-time data from local earthquakes and teleseismic events. The subducting Pacific and Philippine Sea (PHS) slabs are revealed clearly as high-velocity (high-V) zones, whereas low-velocity (low-V) anomalies are revealed in the mantle wedge above the two slabs. The PHS slab has subducted aseismically down to a depth of 480 km under the Japan Sea and to a depth of 540 km under the Tsushima Strait. A window is revealed within the aseismic PHS slab, being consistent with P-wave tomography. Prominent low-V and high-Poisson's ratio (σ) anomalies exist below the PHS slab and above the Pacific slab, which reflect hot and wet mantle upwelling caused by the joint effect of deep dehydration of the Pacific slab and convective circulation process in the mantle wedge above the Pacific slab. The hot and wet mantle upwelling has caused the complex geometry and structure of the PHS slab in SW Japan, and contributed to the Quaternary volcanism along the Japan Sea coast. In eastern Japan, low-V zones are revealed at depths of 200-700 km below the Pacific slab, which may reflect hot upwelling from the lower mantle or even the core-mantle boundary.

  7. Scale invariant sheath folds in salt, sediments and shear zones

    NASA Astrophysics Data System (ADS)

    Alsop, G. I.; Holdsworth, R. E.; McCaffrey, K. J. W.

    2007-10-01

    Sheath folds are developed in a broad spectrum of geological environments in which material flow occurs, including gravity-driven surficial deformation in ignimbrites, unconsolidated sediments and salt, together with deeper level ductile shear zones in metamorphic rocks. This study represents the first geometric comparison of sheath folds in these different settings across a wide range of scales. Elliptical closures defining eye-folds represent ( y- z) cross sections through highly-curvilinear sheath folds. Our analysis of the published literature, coupled with field observations, reveals remarkably similar ellipticities ( R yz) for sheath folds in metamorphic shear zones ( R yz 4.23), salt ( R yz 4.29), sediment slumps ( R yz 4.34), glaciotectonites ( R yz 4.48), and ignimbrites ( R yz 4.34). Nested eye-folds across this range of materials ( N = 1800) reveal distinct and consistent differences in ellipticity from the outer- ( R yz) to the inner-most ( R y' z' ) elliptical "rings" of individual sheath folds. The variation in ratios from outer to inner rings ( R' = R yz/ R y' z' ) in gravity-driven surficial flows typically displays a relative increase in ellipticity to define cats-eye-folds ( R' < 1) similar to those observed during simple and general shear in metamorphic rocks. We show that sheath folds develop across a range of scales within these different environments, and display elliptical ratios ( R yz) that are remarkably constant ( R2 > 0.99) across 9 orders of magnitude (sheath y axes range from ˜0.1 mm to >75 km). Our findings lead us to conclude that the geometric properties of sheath folds are scale invariant and primarily controlled by the type and amount of strain, with R' also reflecting the rheological significance of layering associated with original buckle fold mechanisms. The scaling pattern of sheath folds reflects the length scales of the precursor buckle folds (and width of deformation zones) across a broad range of materials and

  8. Fluid-Controlled Metamorphism of Eclogitic Pseudotachylite-Bearing Shear Zones, FLAKSTADØY, Northern Norway

    NASA Astrophysics Data System (ADS)

    Leech, M.; Shulman, D. J.

    2012-12-01

    Eclogite-facies shear zones occur surrounding pseudotachylite veins and fractures in granulite-facies basement on Flakstadøy (Lofoten Islands, Norway). Omphacite and albite-diopside symplectite identified in the shear zones demonstrates shear zones formed at eclogite-facies conditions, estimated at 690-750°C and 1.3-1.5 GPa from garnet-omphacite pairs and the jadeite component of clinopyroxene. The brittle deformation accompanying pseudotachylite formation opened fluid pathways that overcame kinetic barriers for metamorphism in metastable lower crustal rocks. The advection of fluids likely facilitated deformation and localized strain within the shear zones: A strong alignment of amphibole <001> parallel to the shear zone and pseudotachylite fabric demonstrates fluid availability during shearing, and that hydrous phases grew during fluid-driven eclogitization. Hydration reactions in the shear zones consumed available fluid leaving the lower crust dry and preventing retrogression. The preservation of pseudotachylite textures and omphacite in shear zones show that shearing was not reactivated during exhumation and that retrogression was minimal. The eclogitized shear zones on Flakstadøy demonstrate that lower crustal rocks can remain metastable if they lack an influx of fluids to drive metamorphism and deformation, potentially controlling large-scale orogenic processes.

  9. Pseudotachylytes of the Deep Crust: Examples from a Granulite-Facies Shear Zone

    NASA Astrophysics Data System (ADS)

    Orlandini, O.; Mahan, K. H.; Regan, S.; Williams, M. L.; Leite, A.

    2013-12-01

    The Athabasca Granulite Terrane is an exhumed section of deep continental crust exposed in the western Canadian shield. The terrane hosts the 1.88 Ga Cora Lake shear zone, a 3-5 km wide sinistral and extensional oblique-slip system that was active at high-pressure granulite-grade conditions ( ~1.0 GPa, >800°C to ~0.8 GPa and 700 °C). Pseudotachylyte, a glassy vein-filling substance that results from frictional melting during seismic slip, is common in ultramylonitic strands of the shear zone, where veins run for tens of meters subparallel to foliation. Some but not all PST veins have been overprinted with the Cora Lake shear zone foliation, and undeformed PST locally bears microlitic garnet. The frictional melts that quench into PST may reach >1400 °C, but are extremely localized and cool to country rock temperatures within minutes, resulting in glass and/or microlitic mineral growths. The melt itself is thought by many to be in disequilibrium with the host rock due to its rapid nature, but during cooling equilibrium is probably reached at small scales. This allows for microprobe analysis of adjacent microlites for thermobarometric calculations. Preliminary results from undeformed (e.g., youngest of multiple generations) PST suggest that quenching occurred in upper amphibolite facies ambient conditions and is compatible with later stages of Cora Lake shear zone activity. Host-rock mylonites contain abundant garnet and pyroxene sigma clasts indicating sinistral shear, and where PST-bearing slip surfaces are found at low angles to the foliation, they display sinistral offset. The host rock contains abundant macroscopic and microscopic sinistral shear fracture systems (e.g., Riedel [R], Y, and P displacement surfaces) within the immediate proximity of PST veins, indicating a complex interplay of brittle and ductile behavior that is interpreted to be genetically related to the formation of the PST. The shear fracture systems are characterized by sharply bounded

  10. Controls of shear zone rheology and tectonic loading on postseismic creep

    NASA Astrophysics Data System (ADS)

    MontéSi, Laurent G. J.

    2004-10-01

    Postseismic deformation is well documented in geodetic data collected in the aftermath of large earthquakes. In the postseismic time interval, GPS is most sensitive to creep in the lower crust or upper mantle activated by earthquake-generated stress perturbations. In these regions, deformation may be localized on an aseismic frictional surface or on a ductile shear zone. These two hypotheses imply specific rheologies and therefore time dependence of postseismic creep. Hence postseismic creep constitutes a potential probe into the rheology of aseismic regions of the lithosphere. I present a simple shear zone model of postseismic creep in which the rheology of the creeping element can be varied. In the absence of tectonic loading during the postseismic time interval, the displacement history of the shear zone obeying a power law rheology with stress exponent n follows an analytical relaxation curve parameterized by 1/n. For a frictional surface, postseismic creep follows the same relaxation law in the limit 1/n → 0. A rough estimate of the apparent stress exponent can be obtained from continuous GPS records. Application to data collected after the 1994 Sanriku earthquake yields 1/n ˜ 0.1, which is consistent with dislocation creep mechanisms. However, the records of two other subduction zone events, the 2001 Peru event and the 1997 Kronotski earthquake, and a continental strike-slip earthquake, the 1999 İzmit earthquake, require negative 1/n. Rather than characterizing the shear zone rheology, these negative exponents indicate that reloading of the shear zone by tectonic forces is important. Numerical simulations of postseismic deformation with nonnegligible reloading produce curves that are well fit by the generalized relaxation law with 1/n < 0, although the actual stress exponent of the rheology is positive. While this prevents rheology from being tightly constrained by the studied GPS records, it indicates that reloading is important in the postseismic time

  11. Variscan granitoids related to shear zones and faults: examples from the Central Sudetes (Bohemian Massif) and the Middle Odra Fault Zone

    NASA Astrophysics Data System (ADS)

    Oberc-Dziedzic, T.; Kryza, R.; Pin, C.

    2015-07-01

    The granitoid intrusions of the Central Sudetes (CS) and of the Middle Odra Fault Zone (MOFZ), NE part of the Bohemian Massif, are both spatially and temporally related to large-scale shear zones and faults (including possible terrane boundaries) that provided effective channels for melt migration. Summarizing common features of the CS and MOFZ granitoids, we have delineated a set of characteristics of the fault-related and shear zone-related granitoids: (1) they are mainly generated by partial melting of crustal sources, with variable contribution (or no contribution) of mantle materials; (2) the sheet-like, steeply inclined, narrow and rather small granitoid intrusions are emplaced within shear zones at mid-crustal level (c. 20 km depth), whereas the larger, flat-lying plutons intrude into the upper crust, outside or above these shear zones; (3) the magmatic foliation and lineation in granitoids of the deeper, sheet-like intrusions are concordant with those in the surrounding metamorphic rocks, suggesting that the solidification of granitoids was coeval with the deformation in the shear zones; instead, the magmatic foliation in the shallower and larger dome-like plutons reflects magma flow; (4) ductile, transcurrent movements along the shear zones postdate medium-pressure regional metamorphism and are accompanied by an increase in the local thermal gradient, as documented by the crystallization of cordierite, andalusite and sillimanite; (5) the increase in the thermal gradient precedes the emplacement of granitoids and their concomitant thermal influence on the country rocks. The granitoids related to the final stages of tectonothermal activity of the shear zones are good-time markers of their evolutionary path.

  12. Modelling the role of graphite in development of a mineralised mid-crustal shear zone, Macraes mine, New Zealand

    NASA Astrophysics Data System (ADS)

    Upton, Phaedra; Craw, Dave

    2008-02-01

    The Hyde-Macraes Shear Zone (HMSZ) is a mineralised low-angle shear within the Otago Schist traceable for ˜ 30 km with a NW strike. It is being actively mined along 12 km of this strike, producing > 6 Moz of gold. The shear zone formed under lower greenschist facies conditions as the host schist was uplifted through the brittle-ductile transition. Sheared rocks consist of greenschist facies schists with mineralogy only subtly different from that of the host schist. Pervasive fluid flow has occurred along grain boundaries and within microshears whose development has been enhanced by the deposition of hydrothermal graphite. We model the development of these microshears using a three-dimensional numerical code. For an inhomogeneous initial rock with lithostatic fluid pressures and a dynamic permeability deformed under dextral non-coaxial flow, deformation is distributed into weak shear bands. Graphite deposition and associated reaction-softening have a dramatic effect on the development of model shear zones. Graphite is deposited into a shear band, weakening it and further localising the deformation into that shear band, producing an anastomosing and interconnected shear zone structure such as observed in the HMSZ. The degrees of graphite precipitation and graphite-induced weakening are varied in the models. Graphite precipitation with a low degree of reaction-softening results in more diffuse shear bands. In contrast less graphite deposition but with a high degree of reaction-softening leads to the development of more focused shear bands. Graphite deposition has clearly controlled the formation of the HMSZ structure, and is an important indicator of potentially mineralised mid-crustal structures elsewhere.

  13. Areas of Active Tectonic Uplift Are Sensitive to Small Changes in Fold Orientations within a Broad Zone of Left-lateral Transpression and Shearing, Dominican Republic and Haiti (Hispaniola)

    NASA Astrophysics Data System (ADS)

    Ambrosius, I.; Mann, P.

    2014-12-01

    Previous GPS studies have shown that the island of Hispaniola is a 250 km-wide zone of active, east-west, left-lateral shearing along two major strike-slip zones: the Septentrional-Oriente fault zone through the northern part of the island and the Enriquillo-Plantain Garden fault zone (EPGFZ) through the southern part of the island. The total interplate rate distributed on both faults is 21 mm/yr. Using a high-resolution DEM, we constructed fluvial channel profiles across transpression-related folds of late Miocene to recent age in the area of central and southern Dominican Republic and Haiti to determine controls of areas of relatively high, moderate, and slow uplift inferred from fluvial channel profiles. Fold axes in this area extend for 50-150 km and exhibit two different trends: 1) folds that occupy the area of the Sierra de Neiba-Chaine des Matheux north of the Enriquillo-Cul-de-Sac Valley and EPGFZ and folds that occupy the area of the Sierra de Bahoruco-Massif de la Selle all exhibit more east-west fold axes trending 110; 2) folds that occupy the area northwest of the EPGFZ in the western Chaine des Matheux and Sierra de Neiba all exhibit fold axes with more northwest trends of 125. River channel profiles show that the second group of more northwesterly-trending fold axes show relatively higher rates of tectonic uplift based on their convex-upward river profiles. Our interpretation for regional variations in river profiles and inferred uplift is that uplift is more pronounced on fold axes trending 15 degrees more to the northwest because their axes are more oblique to the interplate direction of east-west shearing. Longterm uplift rates previously measured from a stairstep of late Quaternary coral terraces at the plunging nose of the westernmost Chaine des Matheux have been previously shown to be occurring at a rate of 0.19 mm/yr. Onland exposures of Holocene corals are found only on one locality within the southern area of folds 30 km west of the epicenter

  14. Granular Shear Zone Formation: Acoustic Emission Measurements and Fiber-bundle Models

    NASA Astrophysics Data System (ADS)

    Michlmayr, Gernot; Or, Dani

    2013-04-01

    We couple the acoustic emissions method with conceptual models of granular material behavior for investigation of granular shear zone formation and to assess eminence of landslide hazard. When granular materials are mechanically loaded or sheared, they tend to produce discrete events of force network restructuring, and frictional interaction at grain contacts. Such abrupt perturbations within the granular lattice release part of the elastic energy stored in the strained material. Elastic waves generated by such events can be measured as acoustic emissions (AE) and may be used as surrogates for intermittent structural transitions associated with shear zone formation. To experimentally investigate the connection between granular shearing and acoustic signals we performed an array of strain-controlled shear-frame tests using glass beads. AE were measured with two different systems operating at two frequency ranges. High temporal resolution measurements of the shear stresses revealed the presence of small fluctuations typically associated with low-frequency (< 20 kHz) acoustic bursts. Shear stress jumps and linked acoustic signals give account of discrete events of grain network rearrangements and obey characteristic exponential frequency-size distributions. We found that statistical features of force jumps and AE events depend on mechanical boundary conditions and evolve during the straining process. Activity characteristics of high-frequency (> 30 kHz) AE events is linked to friction between grains. To interpret failure associated AE signals, we adapted a conceptual fiber-bundle model (FBM) that describes some of the salient statistical features of failure and associated energy production. Using FBMs for the abrupt mechanical response of the granular medium and an associated grain and force chain AE generation model provides us with a full description of the mechanical-acoustical granular shearing process. Highly resolved AE may serve as a diagnostic tool not only

  15. Ductile shear zones beneath strike-slip faults: Implications for the thermomechanics of the San Andreas Fault Zone

    NASA Astrophysics Data System (ADS)

    Thatcher, Wayne; England, Philip C.

    1998-01-01

    We have carried out two-dimensional (2-D) numerical experiments on the bulk flow of a layer of fluid that is driven in a strike-slip sense by constant velocities applied at its boundaries. The fluid has the (linearized) conventional rheology assumed to apply to lower crust/upper mantle rocks. The temperature dependence of the effective viscosity of the fluid and the shear heating that accompanies deformation have been incorporated into the calculations, as has thermal conduction in an overlying crustal layer. Two end-member boundary conditions have been considered, corresponding to a strong upper crust driving a weaker ductile substrate and a strong ductile layer driving a passive, weak crust. In many cases of practical interest, shear heating is concentrated close to the axial plane of the shear zone for either boundary condition. For these cases, the resulting steady state temperature field is well approximated by a cylindrical heat source embedded in a conductive half-space at a depth corresponding to the top of the fluid layer. This approximation, along with the application of a theoretical result for one-dimensional shear zones, permits us to obtain simple analytical approximations to the thermal effects of 2-D ductile shear zones for a range of assumed rheologies and crustal geotherms, making complex numerical calculations unnecessary. Results are compared with observable effects on heat flux near the San Andreas fault using constraints on the slip distribution across the entire fault system. Ductile shearing in the lower crust or upper mantle can explain the observed increase in surface heat flux southeast of the Mendocino triple junction and match the amplitude of the regional heat flux anomaly in the California Coast Ranges. Because ductile dissipation depends only weakly on slip rate, faults moving only a few millimeters per year can be important heat sources, and the superposition of effects of localized ductile shearing on both currently active and now

  16. Ductile shear zones beneath strike-slip faults: Implications for the thermomechanics of the San Andreas fault zone

    USGS Publications Warehouse

    Thatcher, W.; England, P.C.

    1998-01-01

    We have carried out two-dimensional (2-D) numerical experiments on the bulk flow of a layer of fluid that is driven in a strike-slip sense by constant velocities applied at its boundaries. The fluid has the (linearized) conventional rheology assumed to apply to lower crust/upper mantle rocks. The temperature dependence of the effective viscosity of the fluid and the shear heating that accompanies deformation have been incorporated into the calculations, as has thermal conduction in an overlying crustal layer. Two end-member boundary conditions have been considered, corresponding to a strong upper crust driving a weaker ductile substrate and a strong ductile layer driving a passive, weak crust. In many cases of practical interest, shear heating is concentrated close to the axial plane of the shear zone for either boundary condition. For these cases, the resulting steady state temperature field is well approximated by a cylindrical heat source embedded in a conductive half-space at a depth corresponding to the top of the fluid layer. This approximation, along with the application of a theoretical result for one-dimensional shear zones, permits us to obtain simple analytical approximations to the thermal effects of 2-D ductile shear zones for a range of assumed rheologies and crustal geotherms, making complex numerical calculations unnecessary. Results are compared with observable effects on heat flux near the San Andreas fault using constraints on the slip distribution across the entire fault system. Ductile shearing in the lower crust or upper mantle can explain the observed increase in surface heat flux southeast of the Mendocino triple junction and match the amplitude of the regional heat flux anomaly in the California Coast Ranges. Because ductile dissipation depends only weakly on slip rate, faults moving only a few millimeters per year can be important heat sources, and the superposition of effects of localized ductile shearing on both currently active and now

  17. Seismic reflection imaging of two megathrust shear zones in the northern Cascadia subduction zone.

    PubMed

    Calvert, Andrew J

    2004-03-11

    At convergent continental margins, the relative motion between the subducting oceanic plate and the overriding continent is usually accommodated by movement along a single, thin interface known as a megathrust. Great thrust earthquakes occur on the shallow part of this interface where the two plates are locked together. Earthquakes of lower magnitude occur within the underlying oceanic plate, and have been linked to geochemical dehydration reactions caused by the plate's descent. Here I present deep seismic reflection data from the northern Cascadia subduction zone that show that the inter-plate boundary is up to 16 km thick and comprises two megathrust shear zones that bound a >5-km-thick, approximately 110-km-wide region of imbricated crustal rocks. Earthquakes within the subducting plate occur predominantly in two geographic bands where the dip of the plate is inferred to increase as it is forced around the edges of the imbricated inter-plate boundary zone. This implies that seismicity in the subducting slab is controlled primarily by deformation in the upper part of the plate. Slip on the shallower megathrust shear zone, which may occur by aseismic slow slip, will transport crustal rocks into the upper mantle above the subducting oceanic plate and may, in part, provide an explanation for the unusually low seismic wave speeds that are observed there. PMID:15014496

  18. Microstructures, deformation mechanisms and seismic properties of a Palaeoproterozoic shear zone: The Mertz shear zone, East-Antarctica

    NASA Astrophysics Data System (ADS)

    Lamarque, Gaëlle; Bascou, Jérôme; Maurice, Claire; Cottin, Jean-Yves; Riel, Nicolas; Ménot, René-Pierre

    2016-06-01

    The Mertz shear zone (MSZ) is a lithospheric scale structure that recorded mid-crustal deformation during the 1.7 Ga orogeny. We performed a microstructural and crystallographic preferred orientation (CPO) study of samples from both mylonites and tectonic boudins that constitute relics of the Terre Adélie Craton (TAC). The deformation is highly accommodated in the MSZ by anastomosed shear bands, which become more scattered elsewhere in the TAC. Most of the MSZ amphibolite-facies mylonites display similar CPO, thermal conditions, intensity of deformation and dominant shear strain. Preserved granulite-facies boudins show both coaxial and non-coaxial strains related to the previous 2.45 Ga event. This former deformation is more penetrative and less localized and shows a deformation gradient, later affected by a major phase of recrystallization during retrogression at 2.42 Ga. Both MSZ samples and granulite-facies tectonic boudins present microstructures that reflect a variety of deformation mechanisms associated with the rock creep that induce contrasted CPO of minerals (quartz, feldspar, biotite, amphibole and orthopyroxene). In particular, we highlight the development of an "uncommon" CPO in orthopyroxene from weakly deformed samples characterized by (010)-planes oriented parallel to the foliation plane, [001]-axes parallel to the stretching lineation and clustering of [100]-axes near the Y structural direction. Lastly, we computed the seismic properties of the amphibolite and granulite facies rocks in the MSZ area in order to evaluate the contribution of the deformed intermediate and lower continental crust to the seismic anisotropy recorded above the MSZ. Our results reveal that (i) the low content of amphibole and biotite in the rock formations of the TAC, and (ii) the interactions between the CPO of the different mineralogical phases, generate a seismically isotropic crust. Thus, the seismic anisotropy recorded by the seismic stations of the TAC, including the

  19. The Sefwi-Comoé belt Ghana/Ivory Coast : a major crustal shear zone ?

    NASA Astrophysics Data System (ADS)

    Jessell, Mark

    2010-05-01

    The Palaeoproterozoic Sefwi-Comoé region that straddles Ghana and the Ivory Coast in West Africa has been characterised as resulting from a combination of compression and simple shear during late synkinematic leucogranite intrusion. The analysis of regional geophysical datasets allows us to better define the geometry of the major lithostratigraphic packages and their structural contacts in this region. This analysis reveals a series of well defined leucogranites intrusions enveloped by high strain zones. Recent finite element modelling of two-phase aggregates has shown that we can analyse the geometry of these systems both in terms of their finite defomation and their mechanical contrast. We interpret the geometries we see in the Sefwi-Comoé region as reflecting the activity of a major crustal deformation zone which was dominated by simple shear. The comparison with the modelling suggests a finite shear strain of approximately 5 gamma, which in turn implies a lateral displacement of 400 km across the belt. Our analysis suggests that the leucogranites were already acting as more rigid bodies during the (dextral?) shearing, suggesting that their emplacement was predominantly pre-kinematic, and which has implications for their potential subsequent remobilization by gravitational forces.

  20. On the effects of thermally weakened ductile shear zones on postseismic deformation

    NASA Astrophysics Data System (ADS)

    Takeuchi, Christopher S.; Fialko, Yuri

    2013-12-01

    We present three-dimensional (3-D) numerical models of postseismic deformation following repeated earthquakes on a vertical strike-slip fault. Our models use linear Maxwell, Burgers, and temperature-dependent power law rheology for the lower crust and upper mantle. We consider effects of viscous shear zones that result from thermomechanical coupling and investigate potential kinematic similarities between viscoelastic models incorporating shear zones and elastic models incorporating rate-strengthening friction on a deep aseismic fault root. We find that the thermally activated shear zones have little effect on postseismic relaxation. In particular, the presence of shear zones does not change the polarity of vertical displacements in cases of rheologies that are able to generate robust postseismic transients. Stronger rheologies can give rise to an opposite polarity of vertical displacements, but the amplitude of the predicted transient deformation is generally negligible. We conclude that additional (to thermomechanical coupling) mechanisms of strain localization are required for a viscoelastic model to produce a vertical deformation pattern similar to that due to afterslip on a deep extension of a fault. We also investigate the discriminating power of models incorporating Burgers and power law rheology. These rheologies were proposed to explain postseismic transients following large (M7) earthquakes in the Mojave desert, Eastern California. Numerical simulations indicate that it may be difficult to distinguish between these rheologies even with high-quality geodetic observations for observation periods less than a decade. Longer observations, however, may potentially allow discrimination between the competing models, as illustrated by the model comparisons with available GPS and interferometric synthetic aperture radar data.

  1. Structural analysis of high-pressure shear zones (Bacariza Formation, Cabo Ortegal, NW Spain)

    NASA Astrophysics Data System (ADS)

    Puelles, P.; Mulchrone, K. F.; Ábalos, B.; Ibarguchi, J. I. Gil

    2005-06-01

    High-pressure granulites of the Bacariza Formation (Cabo Ortegal Complex, NW Spain) exhibit spectacular examples of ductile shear zones developed at different scales in rocks containing pre-existing foliations. A detailed structural analysis was carried out on these shear zones in order to unravel and compare the role of various parameters controlling the deformation process (i.e. heterogeneous simple shear, components of homogeneous deformation, heterogeneous volume change and degree of non-coaxiality). Although heterogeneous simple shear largely dominated, negligible deviations from the ideal simple shear model were detected involving shortening along the structural directions perpendicular to the stretching axis (within the foliation plane) of the finite strain ellipsoid. The relationship between displacement parallel to a half-shear zone and the normal distance from its boundary provided the basis for the estimation of the stress exponent in the power-law constitutive flow equation associated with each shear zone, which is interpreted as a rheological indicator. These geometric and rheological results, and the thermobaric conditions of high-pressure shear zone deformation, indicate that these shear zones accommodated dominant plastic rock flow coeval with high-pressure and high-temperature deformations under moderate stress levels concomitant with elevated strain rates.

  2. Multiscale model for predicting shear zone structure and permeability in deforming rock

    NASA Astrophysics Data System (ADS)

    Cleary, Paul W.; Pereira, Gerald G.; Lemiale, Vincent; Piane, Claudio Delle; Clennell, M. Ben

    2016-04-01

    A novel multiscale model is proposed for the evolution of faults in rocks, which predicts their internal properties and permeability as strain increases. The macroscale model, based on smoothed particle hydrodynamics (SPH), predicts system scale deformation by a pressure-dependent elastoplastic representation of the rock and shear zone. Being a continuum method, SPH contains no intrinsic information on the grain scale structure or behaviour of the shear zone, so a series of discrete element method microscale shear cell models are embedded into the macroscale model at specific locations. In the example used here, the overall geometry and kinematics of a direct shear test on a block of intact rock is simulated. Deformation is imposed by a macroscale model where stresses and displacement rates are applied at the shear cell walls in contact with the rock. Since the microscale models within the macroscale block of deforming rock now include representations of the grains, the structure of the shear zone, the evolution of the size and shape distribution of these grains, and the dilatancy of the shear zone can all be predicted. The microscale dilatancy can be used to vary the macroscale model dilatancy both spatially and temporally to give a full two-way coupling between the spatial scales. The ability of this model to predict shear zone structure then allows the prediction of the shear zone permeability using the Lattice-Boltzmann method.

  3. Strain localization in shear zones during exhumation: a graphical approach to facies interpretation

    NASA Astrophysics Data System (ADS)

    Cardello, Giovanni Luca; Augier, Romain; Laurent, Valentin; Roche, Vincent; Jolivet, Laurent

    2015-04-01

    formation of smaller asymmetric syn-blueschist boudins and is associated with epidote grain-size reduction, synonym of more intense strain localization. In the latest stage (3), newly formed chlorite-rich shear bands bound new boudins of the previously produced terms having blueschist fine-grained heritage. Sense of shear is consistently top-to-the-ENE through the entire localization process. Greenschist-related boudinage is associated with quartz precipitation along the main shear bands and at the necks of boudins, places of strain localization. Metamorphic reactions and transpositions gradually obliterate the former texture, being progressively more active towards the top, where minor shear zones have been deforming at the expenses of similar folds and epidote-rich levels. The resulting qualitative analysis means to be a proxy for further numerical models and a comparison to similar field-study areas.

  4. Scaling of viscous shear zones with depth-dependent viscosity and power-law stress-strain-rate dependence

    NASA Astrophysics Data System (ADS)

    Moore, James D. P.; Parsons, Barry

    2015-07-01

    One of the unresolved questions concerning fault deformation is the degree and cause of localization of shear at depth beneath a fault. Geologic observations of exhumed shear zones indicate that while the motion is no longer planar, it can still be localized near the down-dip extension of the fault; however, the degree of localization is uncertain. We employ simple analytic and numerical models to investigate the structural form of distributed shear beneath a strike-slip fault, and the relative importance of the physical mechanisms that have the potential to localize a shear zone. For a purely depth dependent viscosity, η = η0 exp (-z/z0), we find that a shear zone develops with a half-width δ _w˜ √{z_0} for small z0 at the base of the layer, where lengths are non-dimensionalized by the layer thickness (d km). Including a non-linear stress-strain-rate relation (dot{ɛ }∝ σ ^n) scales δw by 1/√{n}, comparable to deformation length scales in thin viscous sheet calculations. We find that the primary control on the shear-zone width is the depth dependence of viscosity that arises from the temperature dependence of viscosity and the increase in temperature with depth. As this relationship is exponential, scaling relations give a dimensional half-width that scales approximately as tilde{δ}_w≈ T_{1/2}√{Rd/nQβ } km, where T_{1/2} (K) is the temperature at the midpoint of the layer, R (J mol-1 K-1) the gas constant, Q (J mol-1) the activation energy and β (K km-1) the geothermal gradient. This relation predicts the numerical results for the parameter range consistent with continental rheologies to within 2-5 per cent and shear-zone half-widths from 2 to 6 km. The inclusion of shear-stress heating reduces δw by only an additional 5-25 per cent, depending on the initial width of the shear zone. While the width of the shear zone may not decrease significantly, local temperature increases from shear-stress heating range from 50 to 300 °C resulting in a

  5. Strain localisation and thermal evolution of a thick ultramylonitic shear zone

    NASA Astrophysics Data System (ADS)

    Finch, M.; Hasalova, P.; Weinberg, R. F.

    2013-12-01

    Ultramylonites are the ultimate result of mylonitisation in a ductile shear zone. They accommodate high amounts of strain because they are the weakest rock in the shear zone and usually form thin bands within rocks of lower strain (mylonites and protomylonites). Thick ultramylonites are rare; where they are reported in the literature they are up to a few hundred metres thick and indicate inefficient localisation processes. This work describes an ultramylonitic shear zone within the Sierras Pampeanas of Argentina: the El Pichao shear zone. The shear zone is part of the 470 Ma Sierra de Quilmes, a metamorphic complex of the Famatinian orogeny. The El Pichao shear zone is >3.5-km thick and contains a 1-km core of continuous ultramylonite. Globally, shear zones of similar width are rare and where reported are related to major orogenic fronts. Shear zone width is determined by plate velocity and rock strength, such that greater widths occur at high velocities and lower rock strengths. Shear zones widen when the degree of strain localisation decreases which can be caused by weakening of the host rock, hardening of the shear zone, or a decrease in the yield stress of the rock due to an increase in temperature. Different mechanisms lead to each of these processes and their determination is difficult in studies of shear zones. El Pichao shear zone overprints amphibolite faces Grt-schists in the footwall, granites in the ultramylonitic shear zone core, and migmatites in the hanging wall. Syn-kinematic anatexis in the migmatitic hanging wall indicates that the migmatites should have been weaker during shearing than the crystallised granite which formed the protolith to the ultramylonitic core. Additionally, the migmatites are very heterogenous with mica-rich melanosomes and syn-kinematic Qtz-Fsp leucosomes, making them the ideal site for strain localisation and strain partitioning between weaker and stronger phases. However, strain localised to the granite of the

  6. Shear-wave velocity structure of the crust and uppermost mantle in the Shanxi rift zone

    NASA Astrophysics Data System (ADS)

    Song, Meiqing; Zheng, Yong; Liu, Chun; Li, Li; Wang, Xia

    2015-04-01

    The Shanxi rift zone is one of the largest and active Cenozoic grabens in the world, studying the velocity structure of the crust and upper mantle in this region may help us to understand the mechanisms of rift processes and the seismogenic environment of active seismicity in continental rifts. In this work, using the broadband seismic data of Shanxi, Hebei, Henan, Shaanxi provinces, and the Inner Mongolia Autonomous Region from February 2009 to November 2011, we have picked out 350 high-quality phase velocity dispersion curves of fundamental mode Rayleigh waves at periods from 8 to 75 s, and Rayleigh wave phase velocity maps have been constructed from 8 to 75 s period with horizontal resolution ranging from 40 to 50 km by two-station surface-wave tomography. Then, using a genetic algorithm, a 3D shear-wave speed model of the crust and uppermost mantle have been derived from these maps with a spatial resolution of 0.4° × 0.4°. Four characteristics can be outlined from the results: (1) Except in the Datong volcanic zone, in the depth range of 11-30 km, the location of a transition zone between the high- and low-velocity regions is in agreement with the seismicity pattern in the study region, and the earthquakes are mostly concentrated near this transition zone; (2) In the depth range of 31-40 km, shear-wave velocities are higher to the south of the Taiyuan Basin and lower to the north, which is similar to the distribution pattern of Moho depth variations in the Shanxi region; (3) The shear-wave velocity pattern of higher velocities to the south of 38°N and lower velocities to the north is found to be consistent with that from the upper crustal levels to depth of 70 km. At the deeper depths, the spatial scale of the low-velocity anomalies zone in the north is gradually shrinking with depth increasing, the low-velocity anomalies are gradually disappearing beneath the Datong volcanic zone at the depth of 151-200 km. We proposed that the root of the Datong volcano

  7. Structural setting and magnetic properties of pseudotachylyte in a deep crustal shear zone, western Canadian shield

    NASA Astrophysics Data System (ADS)

    Orlandini, O. F.; Mahan, K. H.; Brown, L. L.; Regan, S.; Williams, M. L.

    2012-12-01

    Seismic slip commonly produces pseudotachylytes, a glassy vein-filling substance that is typically interpreted as either a frictional melt or an ultra-triturated cataclasite. In either form, pseudotachylytes are commonly magnetite enriched, even in magnetite-free host rocks, and therefore are potentially useful as high fidelity recorders of natural magnetic fields at the time of slip in a wide array of lithologies. Pseudotachylytes generally have high magnetic susceptibility and thus should preserve the dominant field present as the material passes the Curie temperatures of magnetic minerals, primarily magnetite. Two potential sources have been proposed for the dominant magnetic field recorded: the earth's magnetic field at the time of slip or the temporary and orders of magnitude more intense field created by the presence of coseismic currents along the failure plane. Pseudotachylytes of the Cora Lake shear zone (CLsz) in the Athabasca Granulite Terrain, western Canadian shield, are consistently hosted in high strain ultramylonitic orthogneiss. Sinistral and extensional oblique-slip in the CLsz occurred at high-pressure granulite-grade conditions of ~1.0 GPa and >800°C and may have persisted to somewhat lower P-T conditions (~0.8 GPa, 700 °C) during ductile deformation. Pseudotachylyte-bearing slip surfaces have sinistral offset, matching the larger shear zone, and clasts of wall rock in the more brecciated veins display field evidence for ductile shear along the same plane prior to brittle failure. The presence of undeformed pseudotachylyte in kinematically compatible fracture arrays localized in ultramylonite indicates that brittle failure may have occurred in the waning stages of shear zone activity and at similar deep crustal conditions. Field-documented occurrences of pseudotachylyte include 2 cm-thick veins that run subparallel to mylonitic foliation and contain small flow-aligned clasts and large, heavily brecciated foliation-crosscutting zones up to

  8. High-T Detachment Shear Zone in Mirdita Ophiolite (albania)

    NASA Astrophysics Data System (ADS)

    Jousselin, D.; Nicolas, A. A.; Boudier, F. I.; Meshi, A.

    2013-12-01

    Two oceanic core complex (OCC) extending over 50km have been mapped in the northern part of the Mirdita ophiolite. Despite the fact that the ophiolite is encased between major Dinaric thrusts, a late Jurassic marine topography is still preserved, as demonstrated by the nearly horizontal sedimentary cover. The study area exposes two peridotite domes, aligned on a N-S trend, separated by a talweg 1km wide, occupied by gabbros. This alignment is parallel to the paleoridge indicated by the sheeted dike complex, exposed at the eastern margin, and trending N-S. The two mantle domes composed of clinopyroxene bearing harzburgite with high-T porphyroclastic textures are roofed by a ~1km thick mylonitic shell, particularly well exposed at the limit with the gabbros, and interpreted as an oceanic high-T detachment shear zone. Six mylonite samples are studied for textures and crystal preferred orientation (CPO) aiming to improve the kinematics of the oceanic detachment. The mylonitic peridotite are exceptionally fresh, serpentine minerals being restricted to the bordering porphyroclastic harzburgites. They exhibit a tight millimetric layering formed by olivine / olivine+orthopyroxene / olivine+clinopyroxene or pargasitic amphibole, with grain-size 100-200μm in olivine bands vs 20-50μm in polyphase bands; plagioclase is ubiquitous. Orthopyroxene porphyroclasts show both body rotation and slip with boudinage in the flowing matrix. Electron back scattering diffraction (EBSD) maps provide precise modal composition and phase distribution. Although olivine CPO is not strong, it consistently records solid state flow on the [100](0kl)(010) slip system; the slight obliquity of [100] slip line on the mineral lineation marks the sense of shear. Pargasitic amphibole having grown in the mylonitic development has a strong CPO with [001]pg parallel to [100]ol. The most surprising result is a weak but constant orientation of [001]opx, known as the unique slip direction in orthopyroxene, at

  9. The nucleation and evolution of Riedel shear zones as deformation bands in porous sandstone

    NASA Astrophysics Data System (ADS)

    Ahlgren, S. G.

    2001-08-01

    The term Riedel shear zone refers to a geometric fracture pattern commonly associated with strike-slip fault systems. Using field mapping and three-dimensional digital modeling, the progressive temporal evolution of natural Riedel shear zones within exposures of the Jurassic Navajo Sandstone is interpreted from the spatial evolution of small-scale, incipient Proto-Riedel Zones (PRZs) to more completely developed systems. PRZs nucleate as a tabular zone of localized shearing marked by en échelon deformation bands, each of which is no more than a few mm wide and tens of cm long. These initial deformation bands have an opposite (antithetic) sense of slip with respect to the zone, and are oriented at an acute angle of 55-85° to the trend of the zone. With increasing strain, deformation bands and sedimentary markers become sheared through granular flow across the zone and assume a sigmoidal form. Subsequent bands develop as conjugate shear fractures within the strain-hardened axis of the PRZ. These antithetic driven systems are not directly linked to pre-existing or external structural elements and are not compatible with traditional synthetic driven models of Riedel shear zones.

  10. The evolution of faults formed by shearing across joint zones in sandstone

    NASA Astrophysics Data System (ADS)

    Myers, Rodrick; Aydin, Atilla

    2004-05-01

    The evolution of strike-slip and normal faults formed by slip along joint zones is documented by detailed field studies in the Jurassic Aztec Sandstone in the Valley of Fire State Park, Nevada, USA. Zones of closely spaced planar sub-parallel joints arranged en échelon are sheared, forming faults. Fracturing occurs as a result of shearing, forming new joints. Later shearing along these joints leads to successively formed small faults and newer joints. This process is repeated through many generations of fracturing with increasing fault slip producing a hierarchical array of structures. Strain localization produced by shearing of joint zones at irregularities in joint traces, fracture intersections, and in the span between adjacent sheared joints results in progressive fragmentation of the weakened sandstone, which leads to the formation of gouge along the fault zone. The length and continuity of the gouge and associated slip surfaces is related to the slip magnitude and fault geometry with slip ranging from several millimeters to about 150 m. Distributed damage in a zone surrounding the gouge core is related to the original joint zone configuration (step sense, individual sheared joint overlaps and separation), shear sense, and slip magnitude. Our evolutionary model of fault development helps to explain some outstanding issues concerning complexities in faulting such as, the variability in development of fault rock and fault related fractures, and the failure processes in faults.

  11. Mechanics of shear rupture applied to earthquake zones

    NASA Technical Reports Server (NTRS)

    Li, Victor C.

    1986-01-01

    The mechanics of shear slippage and rupture in rock masses are reviewed. The essential ideas in fracture mechanics are summarized emphasizing the interpretation and relation among the fracture parameters in shear cracks. The slip-weakening model is described. The general formulation of the problem of nonuniform slip distribution in a continuum is covered.

  12. A low-temperature ductile shear zone: The gypsum-dominated western extension of the brittle Fella-Sava Fault, Southern Alps

    PubMed Central

    Bartel, Esther Maria; Neubauer, Franz; Heberer, Bianca; Genser, Johann

    2014-01-01

    Based on structural and fabric analyses at variable scales we investigate the evaporitic gypsum-dominated Comeglians-Paularo shear zone in the Southern Alps (Friuli). It represents the lateral western termination of the brittle Fella-Sava Fault. Missing dehydration products of gypsum and the lack of annealing indicate temperatures below 100 °C during development of the shear zone. Despite of such low temperatures the shear zone clearly exhibits mylonitic flow, thus evidencing laterally coeval activity of brittle and viscous deformation. The dominant structures within the gypsum rocks of the Lower Bellerophon Formation are a steeply to gently S-dipping foliation, a subhorizontal stretching lineation and pure shear-dominated porphyroclast systems. A subordinate simple shear component with dextral displacement is indicated by scattered σ-clasts. Both meso- and microscale structures are characteristic of a subsimple shear type of deformation with components of both coaxial and non-coaxial strain. Shortening in a transpressive regime was accommodated by right-lateral displacement and internal pure shear deformation within the Comeglians-Paularo shear zone. The shear zone shows evidence for a combination of two stretching faults, where stretching occurred in the rheologically weaker gypsum member and brittle behavior in enveloping lithologies. PMID:26523080

  13. Effects of Shear Zone Development on Seismic Anisotropy in the Lower Grenvillian Crust

    NASA Astrophysics Data System (ADS)

    Song, W. J.; Gerbi, C. C.; Johnson, S. E.; Vel, S. S.

    2014-12-01

    Deep crustal structure, particularly the geometry of shear zones, affects the degree of crust-mantle coupling and the kinematics of crustal deformation. In principle, shear zones in the deep crust can be visible using seismic imaging due to the change in the orientations and modes of anisotropic minerals. However, matching the seismic signals to structures present remains a challenge. This work seeks to bridge some of that gap. We utilize the Parry Sound domain in the western Central Gneiss Belt of the Grenville orogen, Ontario, Canada, to develop quantitative relationships between geologic structures and seismic anisotropy. This region provides excellent examples of granulite and amphibolite facies shear zones up to several km wide. We investigated three rock types: (1) regionally deformed mafic and felsic granulite facies orthogneiss, (2) granulite facies shear zones, and (3) amphibolite facies shear zones. Both of the latter two derived from (1). Using the numerical architecture of asymptotic expansion homogenization (which considers grain-scale elastic interactions), we computed much higher precision seismic velocities than is possible with conventional Voigt-Reuss-Hill algorithms. In all sheared felsic rocks, the dominant quartz slip system was prism + rhomb indicating slowest Vp direction paralleled lineation because in quartz a-axis is near the slowest direction. In contrast, in all sheared mafic rocks, the fastest amphibole direction is strongly parallel to the lineation. As a consequence of combining the quartz and amphibole deformation, rocks comprising felsic and mafic layers have a weak seismic anisotropy. In monolithological shear zones, anisotropy can exceed 10%. Despite the promise this work illustrates, we must continue to consider the influence of inherited fabrics in the host rock. In a second line of investigation, we explored how shear zone volume fraction affects seismic anisotropy.

  14. Mechanical and Microstructural Evolution of Ductile Shear Zones: Implications for the Deep Structure of Lithospheric Faults

    NASA Astrophysics Data System (ADS)

    Platt, J. P.; Behr, W. M.

    2010-12-01

    We offer three new concepts that help place constraints on the mechanics and width of plate-boundary shear zones below the brittle-ductile transition. 1. Lithospheric shear zones operate at approximately constant stress at any given depth (temperature). This is because shear zones form by microstructural changes that cause weakening and hence strain localization. These changes occur when the ambient stress reaches the yield strength σy of intact rock. As a result, the cumulative width w of shear zones reaches a value such that they can accommodate plate motion at a flow stress equal to σy. 2. Exhuming shear zones preserve a record of the stress-temperature profile through the deforming crust. Increasing strain localization as the rocks cool, and quenching of the microstructure outside the narrowing shear zone, allow preservation of the microstructure and mineral chemistry at various stages in their evolution. If the flow stress in the shear zone at any depth is a measure of the yield strength of the surrounding rock, we can use this information to construct strength-depth profiles through the lithosphere. 3. Dislocation creep causes dynamic recrystallization and grainsize reduction. This may result in a switch to grainsize-sensitive creep, which is the main cause of weakening and strain localization in shear zones. At constant strain-rate, this results in a stress drop, which may be followed by grain growth, preventing a permanent switch in mechanism. If shear zones operate at constant stress, however, dislocation density in the deforming grains remains the same after the switch, so that dynamic recrystallization and grain-boundary migration driven by dislocation strain energy continue at the same rate as before. This inhibits grain growth driven by surface energy, so that the deformation mechanism switch is permanent. We calculate shear zone widths at depth in the lithosphere based on these concepts. We use a stress-temperature profile obtained from the Whipple

  15. Kinematics, fabrics and geochronology analysis in the Médog shear zone, Eastern Himalayan Syntaxis

    NASA Astrophysics Data System (ADS)

    Dong, Hanwen; Xu, Zhiqin

    2016-01-01

    The uplift history and tectonic evolution of the Eastern Himalayan Syntaxis (EHS) have been extensively studied in the last several decades. The Médog shear zone, the eastern boundary of the EHS, has preserved considerable significant information on the structural and tectonic evolution of the EHS. In this study, we report kinematics, fabrics and geochronology data of the Médog shear zone in the EHS. Analyses of the crystallographic preferred orientation (CPO) of quartz (EBSD analysis) demonstrated that there are three major slip systems: (1) basal < a > slip, (2) prism [c] slip, and (3) prism < a > slip. These slip systems are consistent with microstructures of low-temperature shearing, medium-temperature shearing and high-temperature shearing, respectively. Zircons from the two gneiss samples possess inherited magmatic cores and metamorphic overgrown rims, yielding a metamorphic age of 29.4-28.6 Ma. It is suggested that the dextral shearing along the Médog shear zone was not earlier than the Early Oligocene. The 40Ar/39Ar analysis indicates that the Médog shear zone experienced three thermo-tectonic events from the Late Oligocene to the Pliocene, e.g., ~ 23.4 Ma, 16.9-12.6 Ma and ~ 5.3 Ma. We correlate the Oligocene metamorphic event and the Late Oligocene to the Pliocene multi-stage thermo-tectonic events that resulted from subduction of the northeast corner of the India plate.

  16. Ductile shear zones related to crustal shortening and domain boundary evolution in the central Fennoscandian Shield

    NASA Astrophysics Data System (ADS)

    HöGdahl, Karin; SjöStröM, HâKan; Bergman, Stefan

    2009-02-01

    The Paleoproterozoic part of the Fennoscandian Shield is composed of crustal components formed in different tectonic settings and generally separated by well-defined shear zone systems. An anomalous transitional boundary has been investigated by integrating structural analysis and geochronology with published geophysical data. The nature of this boundary is interpreted to be a consequence of an apparent stacking in the lower and middle crust initiating 1.87-1.86 Ga dextral shear along the Gävle-Rättvik Zone (GRZ) and adjacent shear zones, resulting in an arcuate northern boundary of the Bergslagen province. This boundary coincides with geophysical anomalies and temporal and metamorphic breaks. Owing to continuous convergence the pure-shear overprint component increased on the GRZ and caused a shift of dextral shear to the Hagsta Gneiss Zone with recorded shear at 1809 ± 2 Ma. Most likely, both these structures are related to coeval shear zones farther to the east as a part of an ˜1500 km long crustal, or possibly terrane, boundary.

  17. Shear zone nucleation and deformation transient: effect of heterogeneities and loading conditions in experimentally deformed calcite

    NASA Astrophysics Data System (ADS)

    Morales, L. F. G.; Rybacki, E.; Dresen, G. H.; Kilian, R.

    2015-12-01

    In the Earth's middle to lower crust, strain is frequently localized along ductile shear zones, which commonly nucleate at structural and material heterogeneities. To investigate shear zone nucleation and development due to heterogeneities, we performed constant strain-rate (CSR) and constant stress (CS) simple shear (torsion) deformation experiments on Carrara marble samples containing weak (limestone) inclusions. The experiments were conducted in a Paterson-type gas deformation apparatus at 900 °C temperature and 400 MPa confining pressure and maximum bulk shear strains of 3. Peak shear stress was about 20 MPa for all the samples, followed by smooth weakening and steady state behavior. The strain is predominantly localized in the host marble within the process zone in front of the inclusion, defined by a zone of intense grain size reduction due to dynamic recrystallization. In CS tests a narrow shear zone developed in front of the inclusion, whereas in CSR experiments the deformation is more heterogeneously distributed, up to g=3.. In the later, secondary foliations oblique to the process zone and alternating thin, high-strain layers are common. In samples deformed at the same shear strain (g=1), the average recrystallized grain size in the process zone is similar for CS and CSR conditions. Crystallographic preferred orientation (CPO) measurements shows that different grain sizes have slightly different CPO patterns. CPO strength varies for different grain sizes, with a CPO strength peak between 40-50 μm, decreasing progressively within smaller grain size, but with secondary peaks for different coarse-grained sizes. Our observations suggest that the initial formation and transient deformation of shear zones is strongly affected by loading conditions.

  18. Microstructural study of the Mertz shear zone, East Antarctica. Implications for deformation processes and seismic anisotropy.

    NASA Astrophysics Data System (ADS)

    Lamarque, Gaëlle; Bascou, Jérôme; Maurice, Claire; Cottin, Jean-Yves; Ménot, René-Pierre

    2015-04-01

    The Mertz Shear Zone (MSZ; 146°E 67°S; East Antarctica) is one major lithospheric-scale structure which outcrops on the eastern edge of the Terre Adélie Craton (Ménot et al., 2007) and that could connected with shear zones of South Australia (e.g., Kalinjala or Coorong shear zone (Kleinschmidt and Talarico, 2000; Gibson et al., 2013)) before the Cretaceous opening of the Southern Ocean. Geochronological and metamorphic studies indicated an MSZ activity at 1.7 and 1.5 Ga respectively in amphibolite and greenschists facies conditions. The deformation affects both the intermediate and lower crust levels, without associated voluminous magma injection. Granulite crop out in the area of the MSZ. They were dated at 2.4 Ga (Ménot et al., 2005) and could represent some preserved Neoarchean tectonites. These rocks show various degrees of deformation including penetrative structures that may display comparable features with that observed in amphibolite and greenschists facies rocks, i.e. NS-striking and steeply dipping foliation with weekly plunging lineation. In the field, cinematic indicators for the MSZ argue for a dominant dextral shear sense. We proceed to optical analysis and crystallographic preferred orientation (CPO) measurements using EBSD technique in order to better constrain the deformation processes. Our results highlight (1) a microstructural gradient from highly deformed rocks (mylonites), forming plurimetric large shear bands and showing evidences of plastic deformation, to slightly deformed rocks in preserved cores with no evidences of plastic deformation or with a clear strong static recrystallization; (2) CPO of minerals related with variations on deformation conditions. Feldspar and quartz CPO argue for plastic deformation at high temperature in the most deformed domains and for the absence of deformation or an important stage of static recrystallization in preserved cores; (3) uncommon CPO in orthopyroxene which are characterized by [010]-axes

  19. Using drill cutting separates to estimate the strength of narrow shear zones at SAFOD

    USGS Publications Warehouse

    Morrow, C.; Solum, J.; Tembe, S.; Lockner, D.; Wong, T.-F.

    2007-01-01

    A technique is presented for estimating frictional strength of narrow shear zones based on hand selection of drillhole cuttings separates. Tests were conducted on cuttings from the SAFOD scientific drillhole near Parkfield, California. Since cuttings are mixed with adjacent material as they travel up the drillhole, these fault-derived separates give a better representation of the frictional properties of narrow features than measurements from the bulk material alone. Cuttings from two shear zones (one an active trace of the San Andreas fault) contain a significant weight percent of clay-rich grains that exhibit deformation-induced slickensides. In addition, cuttings from the active SAF trace contain around 1% serpentine. Coefficients of friction for clay-rich and serpentine grains were 0.3-0.5 and 0.4-0.45, respectively. These values are around 0.12 lower than the friction coefficient of the corresponding bulk cuttings, providing an improved estimate of the frictional strength of the San Andreas fault. Copyright 2007 by the American Geophysical Union.

  20. Auriferous mineralization in some shear-zones: A three-stage model of metallogenesis

    NASA Astrophysics Data System (ADS)

    Bonnemaison, M.; Marcoux, E.

    1990-05-01

    Studies of numerous shear zones hosting auriferous quartz veins suggest a three-stage model of formation for such veins. These stages are indicative of the progressive increase of the gold concentration with the evolution of the shear zone and comprise an early stage with “invisible” gold, an intermediate stage with finegrained gold, and a late stage with gold nuggets. The early stage, consisting of two consecutive episodes, comprises the development of the shear zone sensu stricto which results in the formation of a structure characterized by mylonite. This structure subsequently acts as a drain for hydrothermal fluids. Under the combined action of deformation and hydrothermal circulation, the rocks in this structure undergo mineralogical transformations that depend on the initial composition of the rock. These phenomena occur with increasing intensity towards the cores of the structures, the sites of substantial silicification and sulfidation. Gold is first fixed in the crystal lattice of pyrrhotite (up to 30 ppm of gold), disseminated throughout the structure as a whole. In the core, it is destabilized to pyrite-marcasite during the second episode of this early stage and the gold thus freed is trapped in the lattice of locally abundant ferriferous sulfides: pyrite, arsenopyrite (up to 1.6 wt% gold), and berthierite. At the intermediate stage, again consisting of two consecutive episodes, the shear zone can generate openings, allowing the emplacement of lenses and veins of milky white quartz. When these various veins are in turn crushed by continuing tectonic activity, microsaccharoidal quartz forms by cataclasis, and acts as a “receptacle” for the gold mineralization. Therefore textbook “auriferous quartz veins” are only one particular aspect of this stage, characterized by the appearance of visible native gold caused by destabilization of the auriferous sulfides of the early stage. This gold is generally very fine-grained (several micrometres) and

  1. The effects of heterogeneities and loading conditions in the development of shear zones

    NASA Astrophysics Data System (ADS)

    Morales, Luiz F. G.; Rybacki, Erik; Dresen, Georg

    2016-04-01

    Shear zones are regions of localized deformation and are frequently nucleated by material and/or structural heterogeneities and may develop under transient boundary conditions of strain rate and stress. Here we investigate shear zone nucleation and development due to mechanical heterogeneities. Experiments were performed in constant twist rate (CTR) and constant torque (CT) torsion tests to simulate the end member conditions of constant strain rate and constant stress. We have used hollow cylinders of Carrara marble samples containing weak inclusions of Solnhofen limestone. The experiments were conducted in a Paterson-type gas deformation apparatus at 900 °C temperature and 400 MPa confining pressure to maximum bulk shear strains of 3. Peak shear stress was about 20 MPa for all the samples, followed by smooth weakening and steady state behavior. The strain is predominantly localized in the host marble within the process zone in front of the inclusion, defined by a zone of intense grain size reduction due to dynamic recrystallization. Local shear strain values in the process zone are between 5 to 10 times higher than the bulk applied strain. In CT experiments, a narrow shear zone marked by intense grain size reduction is developed in front of the inclusion and the surrounding material remains relatively intact, whereas in CTR experiments the deformation is more widely distributed. The volume of recrystallized grains is nevertheless similar in the samples deformed at same bulk strains in both CT and CTR. At similar bulk strain, the crystallographic preferred orientation (CPO) in the process zone of CT experiments is stronger than in CTR experiments and CPO strength varies with grain size. Our observations suggest that the initial formation and transient deformation of shear zones is strongly affected by loading conditions.

  2. Evolution of crustal stress, pressure and temperature around shear zones during orogenic wedge formation: a 2D thermo-mechanical numerical study

    NASA Astrophysics Data System (ADS)

    Markus Schmalholz, Stefan; Jaquet, Yoann

    2016-04-01

    maximal shear heating in shear zones is approximately 200 °C. Marker points can migrate through the main shear zone in the lower crust which remains active throughout lithospheric shortening. Some pressure-temperature paths show an anti-clockwise evolution. The impact of various model parameters on the results is discussed as well as applications of the results to geological data.

  3. The Cora Lake Shear Zone: Strain Localization in an Ultramylonitic, Deep Crustal Shear Zone, Athabasca Granulite Terrain, Western Churchill Province, Canada

    NASA Astrophysics Data System (ADS)

    Regan, S.; Williams, M. L.; Mahan, K. H.; Orlandini, O. F.; Jercinovic, M. J.; Leslie, S. R.; Holland, M.

    2012-12-01

    Ultramylonitic shear zones typically involve intense strain localization, and when developed over large regions can introduce considerable heterogeneity into the crust. The Cora Lake shear zone (CLsz) displays several 10's to 100's of meters-wide zones of ultramylonite distributed throughout its full 3-5 km mylonitized width. Detailed mapping, petrography, thermobarometry, and in-situ monazite geochronology suggest that it formed during the waning phases of granulite grade metamorphism and deformation, within one of North America's largest exposures of polydeformed lower continental crust. Anastomosing zones of ultramylonite contain recrystallized grain-sizes approaching the micron scale and might appear to suggest lower temperature mylonitization. However, feldspar and even clinopyroxene are dynamically recrystallized, and quantitative thermobarometry of syn-deformational assemblages indicate high P and T conditions ranging from 0.9 -10.6 GPa and 775-850 °C. Even at these high T's, dynamic recovery and recrystallization were extremely limited. Rocks with low modal quartz have extremely small equilibrium volumes. This is likely the result of inefficient diffusion, which is further supported by the unannealed nature of the crystals. Local carbonate veins suggests that H2O poor, CO2 rich conditions may have aided in the preservation of fine grain sizes, and may have inhibited dynamic recovery and recrystallization. The Cora Lake shear zone is interpreted to have been relatively strong and to have hardened during progressive deformation. Garnet is commonly fractured perpendicular to host rock fabric, and statically replaced by both biotite and muscovite. Pseudotachylite, with the same sense of shear, occurs in several ultramylonitized mafic granulites. Thus, cataclasis and frictional melt are interpreted to have been produced in the lower continental crust, not during later reactivation. We suggest that strengthening of rheologically stiffer lithologies led to

  4. Electrical resistivity structure of the Great Slave Lake shear zone, northwest Canada: implications for tectonic history

    NASA Astrophysics Data System (ADS)

    Yin, Yaotian; Unsworth, Martyn; Liddell, Mitch; Pana, Dinu; Craven, James A.

    2014-10-01

    Three magnetotelluric (MT) profiles in northwestern Canada cross the central and western segments of Great Slave Lake shear zone (GSLsz), a continental scale strike-slip structure active during the Slave-Rae collision in the Proterozoic. Dimensionality analysis indicates that (i) the resistivity structure is approximately 2-D with a geoelectric strike direction close to the dominant geological strike of N45°E and that (ii) electrical anisotropy may be present in the crust beneath the two southernmost profiles. Isotropic and anisotropic 2-D inversion and isotropic 3-D inversions show different resistivity structures on different segments of the shear zone. The GSLsz is imaged as a high resistivity zone (>5000 Ω m) that is at least 20 km wide and extends to a depth of at least 50 km on the northern profile. On the southern two profiles, the resistive zone is confined to the upper crust and pierces an east-dipping crustal conductor. Inversions show that this dipping conductor may be anisotropic, likely caused by conductive materials filling a network of fractures with a preferred spatial orientation. These conductive regions would have been disrupted by strike-slip, ductile deformation on the GSLsz that formed granulite to greenschist facies mylonite belts. The pre-dominantly granulite facies mylonites are resistive and explain why the GSLsz appears as a resistive structure piercing the east-dipping anisotropic layer. The absence of a dipping anisotropic/conductive layer on the northern MT profile, located on the central segment of the GSLsz, is consistent with the lack of subduction at this location as predicted by geological and tectonic models.

  5. Seismic Refraction Profiles Across Shear Zones: What Can We Expect To See ?

    NASA Astrophysics Data System (ADS)

    Rumpel, H.-M.; Levander, A. R.

    In recent years more high-resolution refraction / wide-angle reflection data sets have been recorded to elucidate our picture of the Earth. For the 1999 seismic refraction/ wide-angle reflection CDROM profile more than 1200 stations were deployed be- tween New Mexico and Wyoming (Southern Rocky Mountain Region) at an average receiver spacing of 800m. A ray tracing code was used to analyse the data set. Though we have a good velocity model for the crust with a vertical resolution of less than 1km for the sediments and circa 2km at Moho depths, explaing more than 90% of the travel time data, we still find features we cannot explain by ray tracing only. One of those is the influence of shear zones on seismic sections. It is well known that the CDROM profile crosses many shear zones which formed in Phanerozoic time or even earlier. Therefore we test this idea in the following study. A two-dimensional visco-elastic finite-difference code was used to calculate synthetic seismogramms. Several models were created. The shear zone was realized as an elongate tabular body with velocities reduced at a constant percentage relative to the positive linear velocity gradient back- ground model. Variable parameters were the thickness of the shear zone, the relative location to the source, the dip angle and the velocity reduction within the shear zone relative to the background model. Recorder spacing was chosen to be comparable to the CDROM wide-angle data set and the source was located along the profile. Beside the normal first arrivals the synthetic seismogramms show diffuse arrivals between the front and back shear zone reflections. The time difference between front and back reflections depends on the shear zone thickness and the amplitudes on the velocity contrast. This band of arrivals comes in steeply crossing other reflections, thus creat- ing ambiguties in the data. A small influence on the first arrivals is seen as well, but might not be resolvable in field data. Varying

  6. Grain size in lithospheric-scale shear zones: Chicken or Egg?

    NASA Astrophysics Data System (ADS)

    Thielmann, M.; Rozel, A.; Kaus, B. J. P.; Ricard, Y.

    2012-04-01

    Lithospheric-scale shear zones are commonly defined as regions inhomogeneous and localized deformation. Strain softening has been demonstrated to be necessary for localization in those shear zones, but there is still debate about the physical cause of this softening. As natural shear zones typically have a significantly reduced grain size, it has been proposed that grain size reduction provides the necessary strain softening to localize deformation. As grain size reduces, the dominant deformation mechanism switches from dislocation to diffusion creep, thus requiring less stress to deform the rock. Until recently, the equilibrium grain size has been thought to follow a piezometric relationship, thus indicating the stress under which a shear zone deformed. More recent work (Austin and Evans (2007), Rozel et. al. (2011)) suggests that the equilibrium grain size is not dependent on stress, but rather on the deformational work. Using this relationship, we use numerical models to investigate the effect of grain size evolution on lithospheric deformation. We focus on the question if grain size provides sufficient weakening to effectively localize deformation under lithospheric conditions or if it's effect is rather passive and as such a marker for the deformational work done in a shear zone. We then compare the localization potential of grain size reduction to shear heating and investigate the interplay between the two weakening mechanisms.

  7. Unusually large shear wave anisotropy for chlorite in subduction zone settings

    NASA Astrophysics Data System (ADS)

    Mookherjee, Mainak; Mainprice, David

    2014-03-01

    Using first principle simulations we calculated the elasticity of chlorite. At a density ρ~ 2.60 g cm-3, the elastic constant tensor reveals significant elastic anisotropy: VP ~27%, VS1 ~56%, and VS2 ~43%. The shear anisotropy is exceptionally large for chlorite and enhances upon compression. Upon compression, the shear elastic constant component C44 and C55 decreases, whereas C66 shear component stiffens. The softening in C44 and C55 is reflected in shear modulus, G, and the shear wave velocity, VS. Our results on elastic anisotropy at conditions relevant to the mantle wedge indicates that a 10-20 km layer of hydrated peridotite with serpentine and chlorite could account for the observed shear polarization anisotropy and associated large delay times of 1-2 s observed in some subduction zone settings. In addition, chlorite could also explain the low VP/VS ratios that have been observed in recent high-resolution seismological studies.

  8. Dating the longevity of ductile shear zones: Insight from 40Ar/39Ar in situ analyses

    NASA Astrophysics Data System (ADS)

    Schneider, Susanne; Hammerschmidt, Konrad; Rosenberg, Claudio L.

    2013-05-01

    We attempt to improve temporal constraints on the longevity and the termination of ductile shear zones by performing texturally-controlled in situ 40Ar/39Ar analyses of pre-kinematic muscovite, biotite and K-feldspars, of syn-kinematic phengite and K-feldspar, and of post-kinematic phengite within the same samples of sinistral shear zones from the western Tauern Window (Eastern Alps). Additionally two samples were dated by the Rb/Sr method (microsampling). Relative sequences of mineral formation based on microstructural, cross-cutting relationships were confirmed by in situ 40Ar/39Ar analyses, showing that syn-kinematic minerals are, in general, younger than pre-kinematic minerals and older or of equal age than the post-kinematic minerals of the same sample. From the rim to the core of the western Tauern Window syn-kinematic phengite and K-feldspar reveal a set of formation ages varying between 33 and 15 Ma for the northernmost and peripheral shear zone (Ahorn Shear Zone), between 24 and 12 Ma for the intermediate shear zone network (Tuxer Shear Zones), and between 20 and 7 Ma for the southernmost and central shear zone (Greiner Shear Zone). The age variation of syn-kinematic phengite and K-feldspar analyses is larger than the analytical error of each age obtained. In addition, isochron calculations of the syn-kinematic minerals reveal atmospheric-like 40Ar/36Ar intercepts. Therefore, the obtained age values of the syn-kinematic minerals are interpreted as formation ages which date increments of a long lasting deformation period. The time range of deformation of each shear zone system is bracketed by the oldest and youngest formation ages of syn-kinematic phengite and K-feldspar. Post-kinematic phengite laths show the youngest formation ages and overlap with the youngest syn-kinematic formation ages. This relationship indicates that post-kinematic growth occurred immediately after syn-kinematic mineral formation at the end of ductile sinistral shear. Hence, the

  9. New tectono-geochronological constraints on timing of shearing along the Ailao Shan-Red River shear zone: Implications for genesis of Ailao Shan gold mineralization

    NASA Astrophysics Data System (ADS)

    Liu, Junlai; Chen, Xiaoyu; Wu, Wenbin; Tang, Yuan; Tran, My-Dung; Nguyen, Quang-Luat; Zhang, Zhaochong; Zhao, Zhidan

    2015-05-01

    Several world class gold deposits are located along the Ailao Shan (ALS) belt in eastern Tibet, China. The genesis of gold mineralization along the belt, however, has been the subject of debates in the last decades, which highlights the importance of dating shearing, magmatism and mineralization along the Ailao Shan-Red River shear zone (ASRR). Through detailed field observations and microscopic analysis, a group of leucocratic intrusions from within and outside the shear zone along the ALS belt are investigated in the present paper. Pre-, syn- and post-shearing intrusions are grouped based on structural and microstructural analysis. LA-ICP-MS and SIMS dating of the intrusions revealed the existence of two age populations, a group of ages older than 30 Ma and the other younger than 28 Ma. The former are distributed both within and outside the shear zone, and the latter, in contrast, occur only within the shear zone. Our new results show that the ductile shearing along the ASRR shear zone initiated since ca. 30 Ma ago. The dating results place major constraints on timing of shearing along the ASRR shear zone and have profound implications on the genesis of gold mineralization along the ALS belt. The present study reveals that ductile shearing along the ASRR shear zone was resulted from extrusion of the Indochina block late during the Indian-Eurasian plate collision. Meanwhile, we conclude that the gold mineralization took place prior to the shearing, but in close relation to an early magmatism (>30 Ma) ascribed to post-collisional extension collapse involving mantle processes, early during the plate collision.

  10. Metasomatism-controlled nucleation and development of paired-shear zone: an example from the Neves area (Eastern Alps, Italy)

    NASA Astrophysics Data System (ADS)

    Link, Gaétan; Goncalves, Philippe; Lanari, Pierre; Oliot, Emilien

    2016-04-01

    garnet, only presents in the bleached zone, is made of a Mn-rich core and a Ca-rich rim. The core of atoll garnets were formed during the bleaching event, then they have been partially resorbed during the development of the paired-shear zone that is coeval with a new metasomatic event. This latter forms the rim of atoll garnets, which are similar in compositions with euhedral garnets. The center of the shear zone, located at the interface between the granodiorite and the bleached zone, shows evidences of late brittle reactivation, with breakdown of biotite into chlorite, the cataclasis of garnet with crystallization of K-feldspar (or adularia) and epidote in the cracks, and crystallization of a large amount of Fe and Ti-oxydes. The temperature conditions of this reactivation - maybe linked to a seismic event - are estimated at ~225°C. Our results demonstrate that early fluid-rock interaction is responsible for the formation of a bleached zone and the development of viscosity contrasts, within an initially isotropic rock, sufficient to localize deformation. Furthermore, the transient brittle deformation is also associated with fluid and chemical mass transfer. This contribution shows that both ductile and brittle deformations are associated with fluid-rock interactions. Thus, understanding strain localization processes requires to take into consideration not only mechanical processes, like active deformation mechanisms, but also fluid-assisted chemical processes.

  11. Paleozoic transcurrent conjugate shear zones in the central Appalachian piedmont of southeastern Pennsylvania

    NASA Astrophysics Data System (ADS)

    Valentino, D. W.; Valentino, R. W.; Hill, M. L.

    1995-07-01

    Well-documented tectonic events in the central Appalachians of Pennsylvania are: (1) the early Paleozoic Taconian orogeny that occurred during convergence of Laurentian and the Chopawamsic-Wilmington complex magmatic arc over an east-dipping subduction zone, and resulted in intense metamorphism and deformation in the Piedmont, and (2) the late Paleozoic Alleghanian orogeny that resulted in the thrust and fold belt in the Valley and Ridge province and dextral shear zones in the Piedmont. Unlike the Paleozoic tectonic history for the northern and southern Appalachians, the north-central part of the orogen in Pennsylvania lacks evidence for Acadian deformation and metamorphism. The relative chronological order of deformation and metamorphic events in the eastern Piedmont of Pennsylvania, combined with published geochronology suggests the previously undocumented Acadian deformation possibly exists as a transcurrent conjugate shear zone pair. The Rosemont shear zone is a dextral transcurrent shear zone that is the boundary between the type-section Wissahickon Formation of the Philadelphia structural block (to the southeast) and the West Chester and Avondale Grenvillian basement massifs (to the northwest). The Crum Creek shear zone is the sinistral antithetic structure to the Rosemont zone, and developed internal to the Philadelphia block. Geometric and metamorphic history similarities, opposing offsets, angular relationships, and relative timing of local deformation events supports a conjugate model for these shear zones. East-west oriented bulk shortening and north-south oriented bulk elongation directions are inferred from the conjugate geometry. The Rosemont-Crum Creek system crosscuts and deforms regional Taconian structures and metamorphic zones. In turn, the Rosemont zone is truncated by the Alleghanian Pleasant Grove-Huntingdon Valley shear zone. The available geochronology brackets the movement on the Rosemont-Crum Creek system from Devonian to early

  12. The Border Ranges shear zone, Glacier Bay National Park, Alaska: An example of an ancient brittle-ductile transition zone

    SciTech Connect

    Smart, K.J. . Dept. of Geological Sciences)

    1992-01-01

    The Border Ranges fault system in southern Alaska forms the tectonic boundary between the Peninsular-Alexander-Wrangellia (PAW) composite terrane and the Chugach terrane. In Glacier Bay National Park, the Border Ranges fault system is a north-northwest trending, 10 kilometer wide zone of ductile shear zones and brittle faults hereafter referred to as the Border Ranges shear zone. Three-dimensional strain analyses of plagioclase lathes in the foliated calc-alkaline plutons reveals a strong flattening fabric with the plane of maximum flattening (XY-plane) oriented northwest-southeast and dipping steeply to the southwest. The distribution and shapes of sub-elliptical mafic enclaves in the calc-alkaline plutons show a similarly oriented flattening fabric. Coeval brittle and ductile deformational processes are indicated by: (1) ductile shear zones narrowing to brittle faults at the outcrop scale; and (2) undulose quartz with subgrain development, kinked biotite, twinned and undulose feldspar, and fractured and twinned hornblende often within a single thin-section. Amphibole geobarometry indicates that two of the calc-alkaline plutons deformed by the shear zone crystallized at pressures of approximately 3 kilobars equivalent to 10 to 12 kilometers depth. Metamorphic mineral assemblages within the mylonites indicate deformation under lower greenschist facies conditions (300--400 C). The shear zone may represent a snapshot of the brittle-ductile transition of an ancient convergent-transform plate boundary. As such, this unique exposure may be an ancient analogue for the brittle-ductile transition of the present day San Andreas fault system.

  13. Pseudotachylyte-bearing faults and shear zones along the Norumbega fault system in Maine

    NASA Astrophysics Data System (ADS)

    West, D. P.; Price, N. A.; Swanson, M.; Pollock, S. G.

    2012-12-01

    The Norumbega fault system represents the eroded roots of a >400 km long fault system that records a Middle Paleozoic through Mesozoic history of superimposed deformational processes. The Paleozoic history is dominated by right lateral shearing that evolved from a wide zone of regional scale dextral transpression to more highly focused and in many cases seismogenic slip along relatively narrow faults and shear zones. The Mesozoic history, largely deduced through geochronological studies, likely involved localized reactivation in association with the transition between Late Paleozoic dextral transpression and Early Mesozoic extension in the northern Appalachians. Pseudotachylyte, found in a variety of structural settings, has been identified on numerous fault strands along the southern 250 km of the fault system in Maine and provides an excellent opportunity to evaluate evolving seismogenic processes, at a variety of depths, along a regional scale fault system. The oldest pseudotachylyte (Late Devonian-Early Carboniferous) formed at frictional-to-viscous transitional depths during dextral deformation and is most commonly characterized by multiple generations of foliation-parallel frictional melt veins that were subsequently deformed through viscous shearing and transformed into thin ultramylonite layers. While this variety of pseudotachylyte has only been positively identified along three ~25 km long fault segments within the central portion of the Norumbega (e.g., Sandhill Corner), we speculate it may be more widespread in the fault system owing to difficulties in recognition in the field and a lack of detailed imaging and laboratory studies of mylonite-hosted pseudotachylyte-bearing rocks in the fault system. Younger (Permian-Early Triassic) undeformed pseudotachylyte-bearing faults have been found discontinuously along much of the Norumbega and presumably these occurrences reflect countless high velocity coseismic slip events at shallower depths. Spectacularly

  14. Microstructures and kinematic vorticity analysis from the mylonites along the Karakoram Shear Zone, Pangong Mountains, Karakoram

    NASA Astrophysics Data System (ADS)

    Roy, P.

    2012-04-01

    The Karakoram Shear Zone is a northwest-southeast trending dextral ductile shear zone, which has affected the granitic and granodioritic bodies of the southern Asian Plate margin in three distinct episodes. The ductile shearing of the granitic bodies at Tangste and Darbuk has resulted in the development of mylonites with mylonitic foliation and stretching lineation. More intense deformation is noted in the Tangste granite grading upto orthomylonite, as compared to the Darbuk granite. Kinematic indicators include S-C foliation, synthetic C' and C" antithetic shear bands, Type A σ-mantled porphyroclasts, oblique quartz foliation, micro-shears with bookshelf gliding, mineral fishes including Group 2 mica fishes, and Type 1 and 2a pull-apart microstructures, and exhibit strong dextral sense of ductile shearing towards southeast. The textural features of the minerals especially that of quartz and feldspar, indicate temperature of mylonitisation ranging between 300° C and 500° C in the upper greenschist facies. The mylonitic rocks of the KSZ provide an opportunity for the possible utilization of the deformational structures namely that of quartz and feldspar porphyroclast as well as, well developed shear bands for kinematic vorticity studies. Well developed quartz and feldspar porphyroclasts and synthetic and antithetic shear bands from six different mylonitic samples of the mylonitic Tangste granite has been used to estimate the bulk kinematic vorticity (Wk) involved in the overall deformation of the KSZ using the Porphyroclast Hyperbolic Distribution (PHD) method and Shear band (SB) analysis. The PHD method yields Wk values that range from Wk = 0.29 to Wk =0.43, where as the Shear bands yields values ranging from Wk = 0.45 to Wk =0.93, thus indicating distinct pure and simple shear regimes at different stages of the evolution of the KSZ.

  15. Stretching lineations, shear zone kinematics and dextral transpression along the Flying Point/Norumbega fault zone, Casco Bay, Maine

    SciTech Connect

    Swanson, M.T. . Dept. of Geosciences)

    1993-03-01

    Stretching lineations (L2) throughout the high-grade metamorphic rocks of the Casco Bay area are defined by the alignment of grain aggregates and elongate minerals generally parallel to subhorizontal upright F2 fold hinges. L2 lineations were developed due to regional layer-parallel shear related to dextral transpression along the Flying Point segment of the Norumbega Fault Zone during the later Paleozoic. The reorientation of boudin partings, quartz veins and pegmatite intrusions, the asymmetry of boudin pods, late vein folds and crenulations as well as a range of microscopic kinematic indicators within these rocks clearly indicate an overall dextral shear sense and a variable dip-slip component with local transport directions parallel to L2 during deformation. The distribution of L2 lineations about the trace of the NE- trending Flying Point Fault Zone shows: (a) E-plunging L2 in a broad zone on the NW side within SE-dipping, locally, pegmatite-injected, porphyroclastic schists and gneisses and; (c) sub-horizontal L2 within subordinate fault slices of folded Casco Bay Group lithologies to the SE. The Flying Point Fault zone itself consists of the straight planar gneisses and related rocks as a 2 km wide corridor of high shear strain reflected in the development of quartz-vein sheath folds parallel to L2. Variably-deformed mafic and felsic intrusions preserved as asymmetric pods and lenses within the flanking lithologies have been obliterated within this zone of high shear strain. This kinematic pattern and distribution of lineations is interpreted as an asymmetric transpressional uplift dominated by a broad NW front suffering oblique escape toward the west under dextral reverse motions and a major near-vertical zone of decoupling that developed at a restraining bend at the southwest end of the Norumbega Fault Zone.

  16. Kinematic interpretation of shearband boudins: New parameters and ratios useful in HT simple shear zones

    NASA Astrophysics Data System (ADS)

    Pamplona, Jorge; Rodrigues, Benedito C.

    2011-01-01

    Shearband boudins (asymmetric boudins showing slip along the inter-boudin surface, which is synthetic with respect to the bulk shear sense) are ubiquitous and well exposed in HT simple shear zones. The present work aims to extend the methodology of analysis of shearband boudins developed by Goscombe and Passchier (2003). Such shearband boudins represent complex objects that require an adequate methodology for unambiguous kinematic interpretation. We propose new geometric parameters (Bbs, B-t, c', D', d, ψ') in order to describe and identify, with confidence, the kinematics of boudinage in this particular geological framework. The key-observation for kinematic interpretation in simple shear regimes is the boudin axis (Lb) that is commonly present, excluding the necessity to identify the regional stretching lineation (Lx) in the metasedimentary matrix. In monoclinic HT simple shear zones, this approach involves the use of a local displacement plane (Sx), which is always normal to the boudin axis (Lb).

  17. The Suruli shear zone and regional scale folding pattern in Madurai block of Southern Granulite Terrain, south India

    NASA Astrophysics Data System (ADS)

    Srinivasan, V.; Rajeshdurai, P.

    2010-04-01

    Through the application of remote sensing techniques followed by field checks, the exact extension and nature of Suruli shear zone in Madurai block of southern granulite terrain (SGT) in south India is brought out for the first time in this work. The dominant rock type exposed in this area is charnockite intruded by granites. The Suruli ductile shear zone extends from just west of Kadaiyanallur in the south to Ganguvarpatti in the north over a length of 150 km. Between Kadaiyanallur and Kambam, the shear zone extends roughly in N-S direction. From Kambam, it swerves towards NE and then towards ENE near Ganguvarpatti. The strongly developed transposed foliation and mylonite foliation within the shear zone dip towards east only and so the eastern block (Varushanad hills) is the hanging wall and the western block (Cardamom hills) is the footwall of the shear zone. In the eastern block, three distinct phases of regional scale folding (F1, F2 and F3) are recognized. In complete contrast, the western block recorded only the last phase (F3) regional scale folding. As the more deformed eastern block (older terrain) moved over the relatively less deformed western block (younger terrain) along the Suruli shear zone, it is proposed that this shear zone is a thrust or reverse fault, probably of Proterozoic age. As there are evidences for decreasing displacement from north to south (i.e., from Ganguvarpatti to Kadaiyanallur), the Suruli shear zone could be a rotational thrust or reverse fault with the pivot located close to Kadaiyanallur. As the pivot is located near Achankovil shear zone which trends WNW-ESE (dip towards SSW), the Suruli shear zone could be splaying (branching) out from Achankovil shear zone. In a nutshell, the Suruli shear zone could be a splay, rotational thrust or reverse fault.

  18. Pan-African shear zone-hosted gold mineralization in the Arabian-Nubian shield

    NASA Astrophysics Data System (ADS)

    Abu-Alam, Tamer; Grosch, Eugene; Abd El Monsef, Mohamed

    2013-04-01

    A new tectonic model of the exhumation mechanism of the Arabian-Nubian Shield will be presented at the EGU2013 by Abu-Alam and Stüwe (2013). According to this new tectonic model, the shear zones of the Arabian-Nubian Shield can be classified into two types; deep-seated and relatively shallow shear zones. The deep-seated shear zones are accompanied with deep sub-horizontal crustal channel flows which are response to the exhumation of the metamorphic complexes from the peak condition depth to a shallower crustal level (ductile-brittle transition). An example of these deep-seated shear zones is the Najd Fault System - the largest shear zone on the Earth. At the ductile-brittle transition crustal level, the deep-seated shear zones were overprinted by a greenschist facies condition or the ?2 and ?3 of the principle stresses may be flipped with each other. This flipping can produce other conjugate shallow shear zones in a greenschist facies conditions. The Egyptian gold deposits can be classified into three main types (Botros, 2004), These are stratabound deposits, non-stratabound deposits and placer gold deposits. The non-stratabound deposits are the most common (ex: Sukari, Wadi Allaqi, Abu Marawat, Atalla, El-Sid and Atud gold mines). They are found in form of vein type mineralization or as disseminated mineralization hosted in volcanics and volcaniclastic rocks (volcanogenic massive sulphides). Spatial and temporal relationships between gold veins and structures in the Arabian-Nubian Shield suggest a genetic relationship between mineralization and major tectonic events. At Sukari, Wadi Allaqi and Abu Marawat areas, the gold is hosted in quartz veins parallel to a deep-seated NW-SE to NNW-SSE shear zones. For Atud, El-Sid and Atalla area, the gold is hosted in NE-SW veins parallel to a shallow shear zone but at the conjugate point with a deep-seated NW-SE shear zone. According to the new tectonic model, we propose the following model for gold formation (non

  19. Fluid flow and polymetallic sulfide mineralization in the Kettara shear zone (Jebilet Massif, Variscan Belt, Morocco)

    NASA Astrophysics Data System (ADS)

    N'diaye, I.; Essaifi, A.; Dubois, M.; Lacroix, B.; Goodenough, K. M.; Maacha, L.

    2016-07-01

    The Kettara shear zone is a regional wrench shear zone within the Jebilet massif of Western Morocco, part of the Variscan orogenic belt. This massif is characterized by bimodal magmatism, largely intrusive, and by a number of polymetallic massive sulfide deposits. A syntectonic mafic-ultramafic intrusion and an adjacent, deformed pyrrhotite-rich massive sulfide deposit are located within a 'compressional jog' of the shear zone. Hydrothermal alteration in both the intrusion and the wall rocks adjacent to the deposit is characterized by syntectonic replacement processes leading to formation of chlorite-schists and quartz ± calcite veins. Fluid inclusions in mineralized (pyrrhotite-bearing) quartz veins from the wall rocks adjacent to the deposit and in veins associated with chlorite-schists within the intrusion indicate a prevalence of H2O-CO2-CH4-N2 and H2O-salt fluid systems. In the mineralized veins the fluid shows reducing conditions, with gas dominated by CH4 and N2 and salinities around 7.5 wt% NaCl, whereas in the chlorite shear zones fluid is CO2 dominated and salinities are higher than 23 wt% NaCl. Hydrogen and oxygen isotopic compositions of chlorite and quartz are similar and demonstrate involvement of metamorphic water in both the deposit and the intrusion. The data are consistent with a regional metamorphic fluid flow through the Kettara shear zone. The migrating metamorphic fluids were reduced in the organic matter-rich host rocks leading to deposition of sulfides in the mineralized veins. There are two possible hypotheses for the origin of these mineralized veins: either they were formed during deformation and remobilization of a syn-sedimentary massive sulfide deposit, or they were formed synchronously with the sulfide deposit during development of the Kettara shear zone.

  20. The Freyenstein Shear Zone - Implications for exhumation of the South Bohemian Batholith (Moldanubian Superunit, Strudengau, Austria)

    NASA Astrophysics Data System (ADS)

    Griesmeier, Gerit; Iglseder, Christoph; Konstantin, Petrakakis

    2016-04-01

    The Moldanubian superunit is part of the internal zone of the Variscan Orogen in Europe and borders on the Saxothuringian and Sudetes zones in the north. In the south, it is blanketed by the Alpine foreland molasse. Tectonically it is subdivided into the Moldanubian Nappes (MN), the South Bohemian Batholith (SBB) and the Bavarian Nappes. This work describes the ~ 500 m thick Freyenstein shear zone, which is located at the southern border of the Bohemian Massif north and south of the Danube near Freyenstein (Strudengau, Lower Austria). The area is built up by granites of Weinsberg-type, which are interlayered by numerous dikes and paragneisses of the Ostrong nappe system. These dikes include medium grained granites and finegrained granites (Mauthausen-type granites), which form huge intrusions. In addition, smaller intrusions of dark, finegrained diorites und aplitic dikes are observed. These rocks are affected by the Freyenstein shear zone und ductily deformed. Highly deformed pegmatoides containing white mica crystals up to one cm cut through the deformed rocks and form the last dike generation. The Freyenstein shear zone is a NE-SW striking shear zone at the eastern edge of the SBB. The mylonitic foliation is dipping to the SE with angles around 60°. Shear-sense criteria like clast geometries, SĆ structures as well as microstructures show normal faulting top to S/SW with steep (ca. 50°) angles. The Freyenstein shear zone records a polyphase history of deformation and crystallization: In a first phase, mylonitized mineral assemblages in deformed granitoides can be observed, which consist of pre- to syntectonic muscovite-porphyroclasts and biotite as well as dynamically recrystallized potassium feldspar, plagioclase and quartz. The muscovite porphyroclasts often form mica fishes and show top to S/SW directed shear-sense. The lack of syntectonic chlorite crystals points to metamorphic conditions of lower amphibolite-facies > than 450° C. In a later stage fluid

  1. Evidence for fluid flow in ductile shear zones, Granite Wash Mountains, Maria fold and thrust belt

    SciTech Connect

    Marin, B.A.; Mosher, S. . Dept. of Geological Sciences)

    1993-04-01

    Synkinematic fluids that accompanied Mesozoic (D2) deformation in the Granite Wash Mountains, eastern Maria fold and thrust belt, had profound effects on the metamorphic mineralogy and deformation mechanisms of Paleozoic metsedimentary rocks. The D2 thrusts are discrete brittle faults that commonly bound highly sheared, ductilely deformed rocks. These low-angle faults cross cut a vertical section of Paleozoic and Mesozoic supracrustal rocks that were multiply repeated and upturned by earlier Mesozoic deformation, D1. Kinematic analysis of a series of these ductile shear zones has confirmed that the dominant sense of motion during thrusting was to the SW, however, some isolated kinematic indicators and indicators along the lowest zone suggest NE-thrusting. Microstructures in samples collected both within D2 ductile shear zones and outside D2 zones show fluids played an important role in D2 deformation. The abundant evidence for fluids along D2 ductile shear zones includes: quartz veins in the Kaibab Fm (a dolomitic marble), zones of metasomatic alteration ( bleached'' zones) within D2 zones, an increase in the abundance of metamorphic minerals within D2 zones, and the presence of synkinematic wollastonite in silicones limestones of the Redwall Fm. The most dramatic and significant effect of synkinematic fluids in on the style of deformation mechanisms operating during thrusting. Evidence for intracrystalline plastic deformation and rotational recrystallization is not well-developed: instead, microstructures are dominated by migrational recrystallization fabrics. In some samples, quartz grain boundaries have migrated over a preexisting foliation defined by aligned micas, indicating that grain boundaries were highly mobile. Fluid-enhanced grain boundary mobility has allowed internally deformed grains or parts of grains to be consumed rapidly with no or little record of the deformation left behind.

  2. Modeling the influence of tectonic extrusion and volume loss on the geometry, displacement, vorticity, and strain compatibility of ductile shear zones

    NASA Astrophysics Data System (ADS)

    Baird, Graham B.; Hudleston, Peter J.

    2007-10-01

    Oblate strains are often observed in meso-scale ductile shear zones and this is generally taken to indicate narrowing across the shear zone during formation. Volume loss is one mechanism that could produce shear zone narrowing. However, not all shear zones display characteristics consistent with volume loss, and in such cases, the narrowing must be accomplished by the extrusion of material from within the shear zone. To explore the relationship between shear zone geometry, volume loss, and extrusion, shear zones were mathematically modeled. Results demonstrate the important influence of pure shear and volume loss on controlling the geometry, displacement, and vorticity of ductile shear zones. Further, volume loss does not preclude extrusion unless, for a given volume loss, the strain is of a specific geometry. Extrusion is a likely mechanism important in shear zone development, even if volume loss occurs. Extrusion presents strain compatibility problems because, unlike crustal-scale shear zones, meso-scale ductile shear zones do not possess a free surface. If extrusion occurs, bulk strain compatibility can be maintained if shear zones interlink in anastomosing arrays or change in thickness, though not all shear zone systems display such characteristics. Modeling results elucidate the deformation style of shear zone in the northwest Adirondacks in NY and in the Kebnekaise region in northern Sweden.

  3. Dextral strike-slip along the Kapıdağ shear zone (NW Turkey): evidence for Eocene westward translation of the Anatolian plate

    NASA Astrophysics Data System (ADS)

    Türkoğlu, Ercan; Zulauf, Gernold; Linckens, Jolien; Ustaömer, Timur

    2016-07-01

    The northern part of the Kapıdağ Peninsula (Marmara Sea, NW Turkey) is affected by the E-W trending Kapıdağ shear zone, which cuts through calc-alkaline granitoids of the Ocaklar pluton resulting in mylonitic orthogneiss. Macroscopic and microscopic shear-sense indicators, such as SC fabrics, shear bands, σ-clasts and mica fish, unequivocally suggest dextral strike-slip for the Kapıdağ shear zone. Based on petrographic data, deformation microfabrics of quartz and feldspar, and the slip systems in quartz, the dextral shearing should have been active at T = 500-300 °C and P < 5 kbar. Published K-Ar and 39Ar-40Ar cooling ages of hornblende and biotite suggest that cooling below 500-300 °C occurred during the Eocene (ca. 45-ca. 35 Ma), meaning that the Kapıdağ shear zone should have been active during Middle to Late Eocene times. The differential stress related to the shearing was <50 MPa as is indicated by the size of recrystallized quartz grains. Based on the new and published data, it is concluded that the westward movement of the Anatolian plate might have been active almost continuously from the Middle Eocene until recent times.

  4. Quaternary layer anomalies around the Carlsberg Fault zone mapped with high-resolution shear-wave seismics south of Copenhagen

    NASA Astrophysics Data System (ADS)

    Kammann, Janina; Hübscher, Christian; Nielsen, Lars; Boldreel, Lars Ole

    2015-04-01

    The Carlsberg Fault zone is located in the N-S striking Höllviken Graben and traverses the city of Copenhagen. The fault zone is a NNW-SSE striking structure in direct vicinity to the transition zone of the Danish Basin and the Baltic Shield. Recent small earthquakes indicate activity in the area, although none of the mapped earthquakes appear to have occurred on the Carlsberg Fault. We examined the fault evolution by a combination of very high resolution onshore shear-wave seismic data, one conventional onshore seismic profile and marine reflection seismic profiles. The chalk stratigraphy and the localization of the fault zone at depth was inferred from previous studies by other authors. We extrapolated the Jurassic and Triassic stratigraphy from the Pomeranian Bay to the area of investigation. The fault zone shows a flower structure in the Triassic as well as in Cretaceous sediments. The faulting geometry indicates strong influence of Triassic processes when subsidence and rifting prevailed in the Central European Basin System. Growth strata within the surrounding Höllviken Graben reveal syntectonic sedimentation in the lower Triassic, indicating the opening to be a result of Triassic rifting. In the Upper Cretaceous growth faulting documents continued rifting. This finding contrasts the Late Cretaceous to Paleogene inversion tectonics in neighbouring structures, as the Tornquist Zone. The high-resolution shear-wave seismic method was used to image structures in Quaternary layers in the Carlsberg Fault zone. The portable compact vibrator source ElViS III S8 was used to acquire a 1150 m long seismic section on the island Amager, south of Copenhagen. The shallow subsurface in the investigation area is dominated by Quaternary glacial till deposits in the upper 5-11 m and Danian limestone below. In the shear-wave profile, we imaged the 30 m of the upward continuation of the Carlsberg Fault zone. In our area of investigation, the fault zone appears to comprise

  5. Creep cavitation bands control porosity and fluid flow in lower crustal shear zones

    SciTech Connect

    Menegon, Luca; Fusseis, Florian; Stunitz, Holger; Xiao, Xianghui

    2015-03-01

    Shear zones channelize fluid flow in Earth’s crust. However, little is known about deep crustal fluid migration and how fluids are channelized and distributed in a deforming lower crustal shear zone. This study investigates the deformation mechanisms, fluid-rock interaction, and development of porosity in a monzonite ultramylonite from Lofoten, northern Norway. The rock was deformed and transformed into an ultramylonite under lower crustal conditions (temperature = 700–730 °C, pressure = 0.65–0.8 GPa). The ultramylonite consists of feldspathic layers and domains of amphibole + quartz + calcite, which result from hydration reactions of magmatic clinopyroxene. The average grain size in both domains is <25 mm. Microstructural observations and electron backscatter diffraction analysis are consistent with diffusion creep as the dominant deformation mechanism in both domains. Festoons of isolated quartz grains define C'-type bands in feldspathic layers. These quartz grains do not show a crystallographic preferred orientation. The alignment of quartz grains is parallel to the preferred elongation of pores in the ultramylonites, as evidenced from synchrotron X-ray microtomography. Such C'-type bands are interpreted as creep cavitation bands resulting from diffusion creep deformation associated with grain boundary sliding. Mass-balance calculation indicates a 2% volume increase during the protolith-ultramylonite transformation, which is consistent with synkinematic formation of creep cavities producing dilatancy. Thus, this study presents evidence that creep cavitation bands may control deep crustal porosity and fluid flow. Nucleation of new phases in creep cavitation bands inhibits grain growth and enhances the activity of grain size–sensitive creep, thereby stabilizing strain localization in the polymineralic ultramylonites.

  6. Seismicity, shear failure and modes of deformation in deep subduction zones

    NASA Technical Reports Server (NTRS)

    Lundgren, Paul R.; Giardini, Domenico

    1992-01-01

    The joint hypocentral determination method is used to relocate deep seismicity reported in the International Seismological Center catalog for earthquakes deeper than 400 km in the Honshu, Bonin, Mariannas, Java, Banda, and South America subduction zones. Each deep seismic zone is found to display planar features of seismicity parallel to the Harvard centroid-moment tensor nodal planes, which are identified as planes of shear failure. The sense of displacement on these planes is one of resistance to deeper penetration.

  7. Relationship of quartz LPO fabrics in mylonites near the Alpine Fault, New Zealand to the attitude of the shear zone boundary

    NASA Astrophysics Data System (ADS)

    Little, T. A.; Prior, D. J.; Toy, V.

    2015-12-01

    The active Alpine fault self-exhumes its own ductile shear zone roots and has known kinematics. Within ~1 km of the fault, the foliation is subparallel to the shear zone boundary in which it formed at amphibolite-facies conditions. Using EBSD, we analysed quartz Lattice Preferred Orientations (LPOs) of mylonites along a central part of the fault. The samples were mostly taken from naturally outcropping rocks, complemented by a few sections from core from the DFDP-2B hole—rocks that accommodated a range of finite strains and that have diverse quartz contents. All the LPOs feature a single (or strongest) girdle of c-axes hat is inclined ~28 ±4° away from the pole to the shear zone boundary (SZB) in a sense that is synthetic to the bulk shear. A point maximum of a-axes is inclined at the same angle relative to the shearing direction. This girdle is perpendicular to C' extensional shear bands in the rock, not to the bulk shear zone boundary, whereas the maximum is parallel to the slip direction of the shear bands. These relationships prevail across large variations in quartz content and finite shear strain magnitude. We infer that quartz LPOs are not always reliable indicators of SZB attitude, and they do not necessarily undergo an obvious rotation relative to the SZB as a function of increasing finite strain. Both the C' shears and the LPOs formed as late-incremental features at orientations controlled by the instantaneous geometry of a non-simple shear flow. We suggest that that the C' planes were aligned to planes of maximum shear-strain-rate. The data can be explained by flow in a thinning and stretching shear zone that deforms in plane strain at a Wk of ~0.7 to ~0.85. In support of this, inversions of seismic focal mechanism data yield an orientation of σ1 for the brittle crust of the central Southern Alps "natural laboratory" that approximately coincides with the predicted orientation of the contractional instantaneous stretching axis for the above

  8. A Middle Paleozoic shear zone in the Sierra de Valle Fértil, Argentina: Records of a continent-arc collision in the Famatinian margin of Gondwana

    NASA Astrophysics Data System (ADS)

    Cristofolini, E. A.; Otamendi, J. E.; Walker, B. A.; Tibaldi, A. M.; Armas, P.; Bergantz, G. W.; Martino, R. D.

    2014-12-01

    Geological, petrological and structural observations were obtained along a 30-km-long traverse across a segment of the Valle Fértil shear zone, central-western Argentina. On a regional scale, the shear zone appears as numerous discontinues belts over 25 km in width and is approximately 140 km in length, extended on the western section of the Sierras Valle Fértil - La Huerta mountain range. The steeply dipping shear zone with a vertical mylonitic lineation is composed of amphibolite facies ribbon mylonites and amphibolite to greenschist facies ultramylonites derived from Early Ordovician plutonic and metasedimentary parent rocks. Locally, syn-kinematic retrogression of mylonites formed greenschist facies phyllonites. During the later stages of deformation, unstrained parent rocks, mylonites, ultramylonites and phyllonites were affected by pervasive cataclasis under low greenschist facies conditions associated with localized faulting. One new 40Ar/39Ar age on biotite and published 40Ar/39Ar ages on amphibole in the shear zone yield an average cooling rate of 6.2 °C/Ma for a time period that crosses the Silurian-Devonian boundary. Since in metasedimentary rocks the youngest zircon's rims dated at 465 Ma marks the beginning of cooling, nearly continuous uplift of rocks within the shear zone occurred over a minimum time span of 55 Ma. During the period of active deformation, dip-slip movement can explain uplift of several kilometers of the Early Ordovician arc crust. The Valle Fértil shear zone, which was formed near above the inferred suture zone between the Famatinian arc and Cuyania microcontinent, is a major structural boundary nucleated within the Early Ordovician crust. The simplest geodynamic model to explain the evolution of the Valle Fértil shear zone involves the collision of the composite Cuyania/Precodillera microcontinent against the Famatinian arc.

  9. Scaling of Viscous Shear Zones with Depth Dependent Viscosity and Power Law Stress-strain Rate Dependence

    NASA Astrophysics Data System (ADS)

    Moore, J. D. P.; Parsons, B.

    2014-12-01

    One of the unresolved questions concerning fault deformation is the degree and cause of localisation of shear at depth beneath a fault. Geologic observations of exhumed shear zones indicate that whilst the motion is no longer planar, it can still be localised near the down-dip extension of the fault; however, the degree of localisation is uncertain. We employ simple analytic and numerical models to investigate the structural form of distributed shear beneath a strike-slip fault, and the relative importance of the physical mechanisms that have the potential to localise a shear zone. As we are concerned with long-term structure the model is time-averaged across the earthquake cycle, consisting of an idealised strike-slip fault within a rigid lid over a viscous layer. For a depth dependent viscosity, η = η0 exp (-z/z0), we find a shear zone develops with a half-width δw √z0 for small z0, where lengths are non-dimensionalised by the layer thickness (d km). Including a non-linear stress-strain rate relation (ɛ ˙ ∝ σn) scales δw by 1/√n, comparable to deformation length scales in thin viscous sheet calculations. We find that the primary control on δw is the depth dependence of viscosity arising from the increase in temperature with depth. As this relationship is exponential, scaling relations give a half-width that scales approximately as δw≈T(z=1/2)RdnQβ-----√km,delta_wapprox T(z=1/2){sqrtfrac{Rd}{nQbeta}} km, with T (K), gas constant R (J/mol K), activation energy Q (J/mol), and geotherm β (K/km). Figure illustrates shear zones for a dry olivine composition. For n = 1 the shear zone half-width is δw = 4 km, which reduces to δw = 2.3 km when n = 3; other parameter choices consistent with laboratory-derived rheological properties give δwfrom 2-6 km. The inclusion of shear-stress heating only reduces δw by an additional 5-25%, depending on the initial width of the shear zone; in the case of dry olivine with n = 3 we get δw = 1.8 km. This

  10. Effect of depth-dependent shear modulus on tsunami generation along subduction zones

    USGS Publications Warehouse

    Geist, E.L.; Bilek, S.L.

    2001-01-01

    Estimates of the initial size of tsunamis generated by subduction zone earthquakes are significantly affected by the choice of shear modulus at shallow depths. Analysis of over 360 circum-Pacific subduction zone earthquakes indicates that for a given seismic moment, source duration increases significantly with decreasing depth (Bilek and Lay, 1998; 1999). Under the assumption that stress drop is constant, the increase of source duration is explained by a 5-fold reduction of shear modulus from depths of 20 km to 5 km. This much lower value of shear modulus at shallow depths in comparison to standard earth models has the effect of increasing the amount of slip estimated from seismic moment determinations, thereby increasing tsunami amplitude. The effect of using depth dependent shear modulus values is tested by modeling the tsunami from the 1992 Nicaraguan tsunami earthquake using a previously determined moment distribution (lhmle??, 1996a). We find that the tide gauge record of this tsunami is well matched by synthetics created using the depth dependent shear modulus and moment distribution. Because excitation of seismic waves also depends on elastic heterogeneity, it is important, particularly for the inversion of short period waves, that a consistent seismic/tsunami shear modulus model be used for calculating slip distributions.

  11. Initiation and propagation of shear zones in a heterogeneous continental lithosphere

    SciTech Connect

    Tommasi, A.; Vauchez, A.

    1995-11-10

    Numerical methods were used to investigate the deformation of a continental plate in northeastern Brazil. Of particular interest are the perturbations induced by a stiff compressional deformation of a highly heterogeneous continental lithosphere on the development of a shear zone formed at the termination of a stiff block.

  12. Evolution of Continental Lower Crust Recorded By an Exhumed Deep Crustal Intracontinental Shear Zone

    NASA Astrophysics Data System (ADS)

    Dumond, G.; Mahan, K. H.; Regan, S. P.; Williams, M. L.; Goncalves, P.; Wood, V. R.

    2014-12-01

    Exposures of deep crustal shear zones are fundamental records of strain localization and the temporal evolution of ductile to brittle behavior as these tectonites were exhumed to the surface. We present results from a decade of field-based research on a deeply exhumed (~35 km-paleodepths) strike-slip shear zone in the western Churchill province of the Canadian Shield. The Grease River shear zone is a >400 km-long and 7 km-thick structure that cuts the Athabasca granulite terrane, North America's largest exposure of continental lower crust (>20,000 km2). The shear zone is dominated by granulite- to amphibolite-grade L-S and L>S tectonites characterized by penetrative NE-striking steeply-dipping foliations with gently-plunging to sub-horizontal stretching and intersection lineations. These fabrics are locally overprinted by pseudotachylyte and narrow (<500 m-thick) greenschist-grade zones of cataclasite. Dextral kinematics are defined by deflected foliation trajectories, C' shear bands, and well-developed σ- and δ-type porphyroclasts of Kfs + Pl + Opx + Grt + Hb in felsic to intermediate granulite paragneisses and orthogneisses. Data collected along a well-exposed, nearly 150 km-long segment of the shear zone documents a >100 m.y. episodic record of transpressive to strike-slip intracontinental strain accumulation that coincided with two oppositely convergent orogenies: the east-vergent arc-continent collision of the 1.94-1.90 Ga Taltson orogen and the west-vergent continent-continent collision of the 1.9-1.8 Ga Trans-Hudson orogen. Deformation mechanisms evolved from distributed ductile dynamic recrystallization and grain-size reduction to localized pseudotachylyte development, cataclastic flow, and brittle faulting. Lower crustal behavior during strain localization was dynamic. Melt-weakened mono-cyclic crust was juxtaposed against strong isobarically-cooled poly-cyclic crust along the shear zone at 1.92-1.90 Ga. Brittle-ductile reactivation of the structure

  13. The Pennsbury-Concord shear zone: A deformed ductile thrust, central Appalachian Piedmont

    SciTech Connect

    Solar, G.S.; Hill, M.L. . Dept. of Geology)

    1993-03-01

    The contact between the Baltimore Gneiss and the Wissahickon Group in southeastern Pennsylvania is a ductile shear zone. The contact extends from just west of Pennsbury to just east of Concord, Pennsylvania. The strike of the shear zone contact varies along its length from a general orientation of N60--65E in the west to N75--80E near Concord and then to N30E adjacent to the Rosemont Shear Zone. Microstructural analysis reveals deformation in units on both sides of the contact that is characterized by plastic deformation mechanisms associated with depths below the brittle-ductile transition. Retrograde metamorphism and grain-size reduction as well as continuity across the zone indicate that this deformation was later than the metamorphism of the individual units. The contact has experienced at least two phases of deformation. The early phase was ductile thrusting juxtaposing the Wissahickon Group gneiss on top of the Precambrian Baltimore Gneiss and producing a high temperature pervasive planar and linear fabric. This fabric is most preserved in the west where foliation is shallowly dipping with a lineation trending S54E and plunging 35[degree] to the SE. The later phase was characterized by transposition of the ductile thrust fabric to a more strike-parallel orientation. Along the contact zone to the east, strike of the foliation gradually changes 10--15[degree], dip becomes steeper and the orientation of lineation changes to S85E with plunge of 17[degree] to the east. The deformation of the contact therefore confirms that the ductile thrusting is earlier than the major transcurrent shearing of the Appalachian Piedmont. Named the Pennsbury-Concord Zone, this deformed ductile thrust could be a root zone of brittle thrust faulting to the west in the fold-and-thrust-belt of central PA.

  14. Midcrustal shear zones in postorogenic extension: Example from the northern Tyrrhenian Sea

    NASA Astrophysics Data System (ADS)

    Jolivet, Laurent; Faccenna, Claudio; Goffé, Bruno; Mattei, Massimo; Rossetti, Federico; Brunet, Christophe; Storti, Fabrizio; Funiciello, Renato; Cadet, Jean Paul; D'Agostino, Nicola; Parra, Teddy

    1998-06-01

    Metamorphic core complexes of the Aegean region have revealed midcrustal, shallow-dipping extensional shear zones. These shear zones display constant kinematic indicators over large regions (100-200 km). We analyze the example of the northern Tyrrhenian Sea and then compare it to the Aegean region. We first summarize our observations on ductile extension and metamorphic evolution in the northern Tyrrhenian Sea from Alpine Corsica to Tuscany. (1) Extension migrated from west to east from the early Miocene in Corsica to the Recent in the Apennines; (2) Extension is accommodated by shallow east dipping extensional shear zones at the depth of the brittle-ductile transition, from the early Miocene to the Pliocene. (3) West dipping normal faults accommodate extension on the eastern side of the volcanic arc. (4) Extension is preceded along the convergence front by the formation of a thrust wedge, where high-pressure and low-temperature conditions are recorded; maximum PT conditions decrease toward the east, and PT paths are systematically very cold, suggesting that a large part of the exhumation occurred during synorogenic extension. We discuss the possible mechanisms that account for constant shear sense over large domains. The model involves retreat of the slab and migration of the volcanic arc. Partially molten lower crust acts as a low strength zone where extensional strain is localized. Eastward motion of the upper mantle as a consequence of the migration of the slab induced a component of shear toward the volcanic arc at the base of the stronger upper crust. In the weak upper mantle and lower crust, to the west of the volcanic arc, extensional stresses are not transmitted; this produces a top-to-the-east sense of shear at the base of the upper crust that migrates eastward, following arc migration.

  15. Initiation of deformation of the Eastern California Shear Zone: Constraints from Garlock fault geometry and GPS observations

    USGS Publications Warehouse

    Gan, Weijun; Zhang, P.; Shen, Z.-K.; Prescott, W.H.; Svarc, J.L.

    2003-01-01

    We suggest a 2-stage deformation model for the Eastern California Shear Zone (ECSZ) to explain the geometry of the Garlock fault trace. We assume the Garlock fault was originally straight and then was gradually curved by right-lateral shear deformation across the ECSZ. In our 2-stage deformation model, the first stage involves uniform shear deformation across the eastern part of the shear zone, and the second stage involves uniform shear deformation across the entire shear zone. In addition to the current shape of the Garlock fault, our model incorporates constraints on contemporary deformation rates provided by GPS observations. We find that the best fitting age for initiation of shear in eastern part of the ECSZ is about 5.0 ?? 0.4 Ma, and that deformation of the western part started about 1.6 Myr later.

  16. Distributed extensional deformation in a zone of right-lateral shear: Implications for geodetic versus geologic rates of deformation in the eastern California shear zone-Walker Lane

    NASA Astrophysics Data System (ADS)

    Foy, T. Andrew; Frankel, Kurt L.; Lifton, Zachery M.; Johnson, Christopher W.; Caffee, Marc W.

    2012-08-01

    The eastern California shear zone (ECSZ)-Walker Lane belt represents an important, evolving component of the Pacific-North America plate boundary. Geodetic data suggest the northern ECSZ is accumulating dextral shear at a rate of ˜9.3 mm/a, more than double the total measured late Pleistocene rate at ˜37.5°N. At this latitude, the Silver Peak-Lone Mountain (SPLM) extensional complex plays an important role in accommodating and transferring slip among the strike-slip and normal faults of the ECSZ and Walker Lane. To better understand the recent geodynamic evolution of this region, we determined late Pleistocene extension rates for the Clayton Valley fault zone, one of a series of down-to-the-northwest normal faults comprising the SPLM, using geologic mapping, differential GPS fault scarp surveys, and cosmogenic nuclide geochronology. Extension rates along the Clayton Valley fault zone are time-invariant at 0.1 ± 0.1 to 0.3 ± 0.1 mm/a (depending on fault dip) since ˜137 ka. When combined with other published fault slip rates at this latitude, the cumulative late Pleistocene geologic slip rate is ˜3.3 to 5.2 mm/a. This rate is lower than both the geodetic rate of dextral shear and other long-term slip rate budgets in the northern ECSZ. Our results suggest that deformation in Clayton Valley is spread across a diffuse set of normal faults and that not all of the deformation is recorded in the surficial geology. We suggest that the low cumulative geologic slip rate in the northern ECSZ may be a result of this distributed extension, which can cause long-term rates of deformation to be significantly underestimated.

  17. Relating rheology to geometry in large-scale natural shear zones

    NASA Astrophysics Data System (ADS)

    Platt, John

    2016-04-01

    The geometry and width of the ductile roots of plate boundary scale faults are very poorly understood. Some field and geophysical data suggests widths of tens of km in the lower crust, possibly more in the upper mantle. Other observations suggest they are much narrower. Dip slip shear zones may flatten out and merge into zones of subhorizontal lower crustal or asthenospheric flow. The width of a ductile shear zone is simply related to relative velocity and strain rate. Strain rate is related to stress through the constitutive relationship. Can we constrain the stress, and do we understand the rheology of materials in ductile shear zones? A lot depends on how shear zones are initiated. If they are localized by pre-existing structures, width and/or rheology may be inherited, and we have too many variables. If shear zones are localized primarily by shear heating, initial shear stress has to be very high (> 1 GPa) to overcome conductive heat loss, and very large feedbacks (both positive and negative) make the system highly unstable. Microstructural weakening requires a minimum level of stress to cause deformation and damage in surrounding rock, thereby buffering the stress. Microstructural weakening leads to grain-size sensitive creep, for which we have constitutive laws, but these are complicated by phase mixing in polyphase materials, by viscous anisotropy, by hydration, and by changes in mineral assemblage. Here are some questions that need to be addressed. (1) If grain-size reduction by dynamic recrystallization results in a switch to grain-size sensitive creep (GSSC) in a stress-buffered shear zone, does dynamic recrystallization stop? Does grain growth set in? If grain-size is still controlled by dislocation processes, then the effective stress exponent for GSSC is 4-5, even though the dominant mechanism may be diffusion and/or grain-boundary sliding (GBS). (2) Is phase mixing in ultramylonites primarily a result of GBS + neighbour switching, creep cavitation and

  18. Conditions of kyanite formation from fluid in an alpine shear zone

    NASA Astrophysics Data System (ADS)

    Schultze, Dina; Loges, Anselm; Franz, Gerhard

    2014-05-01

    Hydrothermally formed quartz-kyanite rocks from the WSW-ENE-striking Greiner shear zone (Pfitscher Joch, Italy) on the south-western border of Tauern window were investigated in order to reconstruct conditions and processes of kyanite formation from metamorphic fluids. It is known from experiments that kyanite does not precipitate spontaneously from a qz-saturated, Al-rich fluid in spite of p-T conditions well inside the thermodynamic stability field of kyanite. Natural samples where kyanite nucleates spontaneously from fluid were investigated in order to understand what factors may control inhibition of nucleation in experiments. The Greiner shear zone cuts a variety of metasedimentary (metapelites, -psammites) units of the lower Schieferhülle, which lie between two distinct Zentralgneis units (Tuxer gneiss core in the north, Zillertaler gneiss core in the south). At the Pfitscher Joch near to the Rotbachlspitze (2897 m), the shear zone intersects the tectonic contact between the Schieferhülle metasediments and the Zillertaler gneiss in an acute angle. A number of segregations (up to 1m in diameter) composed mainly of quartz, feldspar and tourmaline are found along strike in the most silica-rich sheared stratigraphic layers. Besides kyanite and quartz the vein rocks show a range of other refractory mineral phases, especially pyrophyllite, rutile and zircon. Minor amounts of xenotime and monazite represent the REE carrier in these rocks. Growth textures indicate simultaneous crystallization of quartz, kyanite, rutile and zircon, whereas pyrophyllite may represent alteration processes in a later stage during obduction of the rocks. Additionally, in some samples muscovite and tourmaline were observed. The potassium and boron supply is likely provided by the metapelite or gneiss units adjacent to the shear zone. In the metapelites tourmaline is most abundant in the vicinity of the shear zone, suggesting migration of boron-rich fluids either to or from the shear zone

  19. The significance of geological and zircon age data derived from the wall rocks of the Ailao Shan-Red River Shear Zone, NW Vietnam

    NASA Astrophysics Data System (ADS)

    Żelaźniewicz, Andrzej; Hòa, Trần Trọng; Larionov, Alexander N.

    2013-09-01

    This paper offers new evidence on whether the Ailao Shan-Red River Shear Zone of NW Vietnam is part of a suture zone between two continental blocks (the IndoChina Block and the South China Block) or whether it is itself of intracontinental origin, developed within the South China margin. To help clarify the role that the Ailao Shan-Red River Shear Zone plays in South China tectonic reconstructions, we gathered new whole-rock geochemistry, structural field data, and zircon U-Pb (SHRIMP) ages from granites, rhyodacites, and migmatites that occur within geological units adjacent to both the SW and NE sides of the Red River Fault Zone, a segment of the larger shear zone. The new zircon ages show that both walls of the Red River Fault Zone contain metamorphic and intraplate A-type granitoid rocks of Late Permian-Early Triassic age (263-240 Ma) and are of Indosinian origin. In the SW wall, the Fan Si Pan complex is a Neoproterozoic basement of metagranites and metasediments that was intruded by Late Permian (˜260 Ma), peralkaline, A-type granites and by subalkaline, A-type, biotite granite of Eocene age (˜35 Ma), containing xenoliths of gneissified Permian granitoids. The two intrusive episodes were separated by regional tectonic deformations occurring within a transpressional regime of a NW/W-vergent thrusting with a left-lateral oblique component, that was associated with greenschist to amphibolite facies metamorphism, presumably also of Eocene age (˜50-35 Ma), and that may have been related to the left-lateral movement on the Ailao Shan-Red River Shear Zone. In the NE wall, the Lo Gam complex is a Neoproterozoic basement (˜767 Ma) that was repeatedly subjected to tectonothermal activity throughout the Palaeozoic (at ˜450-420 Ma, ˜350 Ma, ˜265 Ma), ending in the Early Triassic (˜248 Ma). There was no thermal overprint during the Cenozoic. In this wall, a significant part of the Permo-Triassic thermotectonism was ductile shearing that was concentrated along

  20. Role of the eastern California shear zone in accommodating Pacific-North American plate motion

    SciTech Connect

    Dokka, R.K.; Travis, C.J. )

    1990-08-01

    The newly recognized Eastern California shear zone (ECSZ) of the Mojave Desert-Death Valley region has played a major, but previously underappreciated role in accommodating the dextral shear between the Pacific and North American plates in late Cenozoic time. Comparison of integrated net slip along the shear zone with motion values across the entire transform boundary indicates that between 9% and 23% of the total relative plate motion has occurred along the ECSZ since its probably inception {approximately}10-6 Ma. Long-term integrated shear along the ECSZ (6-12 mm yr{sup {minus}1}) is similar to historic measurements (6.7 {plus minus} 1.3 mm yr{sup {minus}1}). Time-space patterns of faulting suggest that shear was concentrated in the eastern part of the Mojave Desert block and Death Valley during late Miocene and early Pleistocene time, but that the locus of faulting in the south-central Mojave jumped westward between 1.5 and 0.7 Ma.

  1. Ductile shear zone rheology: the viewpoint of experimentally crept lower crustal rocks and analogues

    NASA Astrophysics Data System (ADS)

    Dimanov, Alexandre; Raphanel, Jean; Bornert, Michel; Bourcier, Mathieu; Gaye, Ababacar; Ludwig, Wolfgang

    2015-04-01

    With respect to lithosphere rheology, we are especially interested in the mechanical behavior and evolution of ductile shear zones at depth, which present polyphase and heterogeneous character and multi-scale strain localization patterns. According to structural geology, most strain concentrates in ultramylonitic layers, which exhibit along with metamorphism overprinted or concomitant microstructural signatures from several deformation mechanisms. The latter are either active in volume (crystal slip plasticity and dislocation recovery processes), or in the vicinity and along interfaces (grain sliding, phase transformations and solution mass transfer). Because all of these contribute to the drastic evolution of microstructures with respect to the wall rock and the protomylonite, and because the chronology of their activation and their interactions are unclear, inference of the overall rheology from these microstructural records seems illusory. Therefore, since more than a decade we investigate experimentally and numerically the rheology of synthetic rocks representative of lower crustal mineralogy (namely plagioclases and clinopyroxenes). Samples are elaborated with different microstructures and with variable phases, fluid and melt contents for the purpose of being representative of diverse geodynamical contexts. Experiments were performed either at constant stress or strain rate, in co-axial compression or in torsion. For macroscopic non-Newtonian flow we clearly identified dislocation glide and creep mechanisms. That is to say that power law rheology relates to dominant crystal slip plasticity accommodated by recovery processes, including dislocation climb and pile-up, sub-grain rotation and marginal recrystallization. We further refer to this regime as RCSP (recovery crystal slip plasticity). Conversely, Newtonian (linear viscous) behavior mostly involves grain boundary sliding (GBS) accommodated by diffusional mass transfer and grain boundary dislocation

  2. Transient strain accumulation and fault interaction in the Eastern California shear zone

    NASA Astrophysics Data System (ADS)

    Peltzer, Gilles; Crampé, Fréderic; Hensley, Scott; Rosen, Paul

    2001-11-01

    Satellite synthetic aperture radar interferometry reveals transient strain accumulation along the Blackwater Little Lake fault system within the Eastern California shear zone. The surface strain map obtained by averaging eight years (1992 2000) of Earth Resource Satellite (ERS) radar data shows a 120-km-long, 20-km- wide zone of concentrated shear between the southern end of the 1872 Owens Valley earthquake surface break and the northern end of the 1992 Landers earthquake surface break. The observed shear zone is continuous through the Garlock fault, which does not show any evidence of left-lateral slip during the same time period. A dislocation model of the observed shear indicates right-lateral slip at 7 ± 3 mm/yr on a vertical fault below ˜5 km depth, a rate that is two to three times greater than the geologic rates estimated on northwest-trending faults in the eastern Mojave area. This transient slip rate and the absence of resolvable slip on the Garlock fault may be the manifestation of an oscillatory strain pattern between interacting, conjugate fault systems.

  3. Fluid assisted shearing at the depth of the Brittle-Ductile Transition: an integrated structural, petrological, fluid inclusions study of the Erbalunga shear zone, Schistes Lustrés Nappe, Alpine Corsica (France).

    NASA Astrophysics Data System (ADS)

    Maggi, M.; Rossetti, F.; Tecce, F.; Vignaroli, G.

    2009-04-01

    In this work we present structural, petrological and fluid inclusion studies performed in a major retrogressive shear zone (the Erbalunga shear zone), which occurs within the HP/LT domain of the Schistes Lustrés Nappe of eastern Alpine Corsica. This shear zone is part of the post-orogenic network of shear zones that favoured the exhumation of the HP core of Alpine Corsica (Daniel et al., 1996) during Late Oligocene/Early Miocene times (Brunet et al., 2000). The shear zone is characterised by a progressive ductile-to-brittle top-to-the-E shearing, starting at greenschist facies conditions (ca. 600 MPa, 400-450 °C). Evidence for vigorous fluid flow through the shear zone is documented by widespread quartz and quartz-calcite vein segregations, which accompanied the progressive evolution of shearing. Textural characteristics of three main generations of veins record the incremental evolution of the shear zone tracing the continuum transition from ductile- to brittle-dominated deformation environments. Regardless of the vein generation, fluid inclusions hosted in quartz grains hosted within the three different sets of veins document a low-salinity (<5% NaCl eq.) fluid circulation. Fluid trapping occurred under pore pressure conditions fluctuating from lithostatic to hydrostatic values, as also attested by the crack-sealing textures preserved in most of the veins. The findings of this study suggest that the main source of fluid was of meteoric origin and argue for fluid percolation and infiltration at the brittle-ductile depths. Such a fluid supply cause the availability of a higher amount of fluids in the deforming rock volume, working against ductile deformation and tendency to pore space reduction by recovery during progressive deformation. This impose definition of the (i) mechanism through which superficial fluids infiltrate the mid-lower crust; and (ii) the modes (fracturing vs. ductile creep) of creation and maintenance of the structural permeability moving from

  4. Brittle-ductile shear zone formation in the McKim Limestone: eastern Monument Upwarp, Utah

    NASA Astrophysics Data System (ADS)

    Seyum, S.; Pollard, D. D.

    2011-12-01

    The McKim Limestone is part of a regressive, marine sedimentary sequence of strata that was deposited in the Pennsylvanian to Permian periods. It is well-exposed across large portions of Raplee anticline and Comb monocline; a pair of kilometer-scale folds that mark the Monument Upwarp of the Colorado Plateau in southeastern Utah. Two conjugate sets of echelon vein arrays, with complementary echelon pressure solution seam arrays, occur as bed-perpendicular, systematic deformation features in the 1-3 m thick McKim Limestone unit. Based on large vein to vein array angles, large vein aperture to length ratios, and the presence of vein-perpendicular pressure solution seams, these structures are interpreted to have developed within localized, brittle-ductile shear zones. Topics of debate among structural geologists regarding the formation mechanism of echelon veins include the initiation mode of vein segments (tensile or shear), the relative age between shear zone initiation and vein formation, the interpretation of strain within a shear zone, and the development of sigmoidal veins as being indicative of rotation. These concepts often are founded on geometric observations and kinematic models of deformation (e.g. simple shear) that are independent of the constitutive properties of the rock, are not constrained by the equations of motion, and do not honor the boundary conditions on the vein surfaces. Here we show a more realistic representation of brittle-ductile shear zone formation by introducing numerical models that consider the mechanical properties of limestone, are constrained by the equations of motion, and explicitly define the vein surfaces and their boundary conditions. The commercial finite element software, Abaqus FEA, is used to investigate the deformed geometry of model echelon vein arrays as a function of the remotely applied stress, the initial geometry of the vein arrays, and the constitutive properties of the solid. These geometric patterns are compared

  5. Development of a deep-crustal shear zone in response to syntectonic intrusion of mafic magma into the lower crust, Ivrea-Verbano zone, Italy

    USGS Publications Warehouse

    Snoke, A.W.; Kalakay, T.J.; Quick, J.E.; Sinigoi, S.

    1999-01-01

    A 1 to 1.5 km-thick, high-temperature shear zone is localized in wall rocks subparallel to the eastern intrusive contact of the Permian Mafic Complex of the Ivrea-Verbano zone (IVZ), Italy. The shear zone is characterized by concentrated ductile deformation manifested by a penetrative foliation subparallel to the intrusive contact and a northeast-plunging sillimanite lineation. Evidence of noncoaxial strain and transposition is widespread in the shear zone including such features as rootless isoclinal folds, dismemberment of competent layers, and scattered kinematic indicators. The metasedimentary rocks in the shear zone are migmatitic, and the accumulation of leucosome is variable within the shear zone. Near the intrusive contact with the Mafic Complex leucosome forms ~20 vol% of the wall rock, whereas leucosome concentrations may locally reach ~60 vol% of the wall rock near the outer limits of the shear zone. This variation in vol% leucosome suggests melt/magma migration from the inferred site of anatexis along the intrusive contact to lower-strain regions within and near the margins of the shear zone. The leucosome accumulations chiefly occur as layer-parallel concentrations, but are also folded and boudined, and locally are associated with tension gashes and fracture arrays. Networks of granitic dikes and small plutons in the eastern IVZ suggest that some magmas migrated out of the high-temperature shear zone. Some magma apparently migrated laterally along the strike of the shear zone and concentrated in areas of lower strain where the intrusive contact takes a major westward bend. The high-temperature shear zone is interpreted as a 'stretching fault' (or stretching shear zone) after Means [W.D. Means, Stretching faults, Geology 17 (1989) 893-896], whereupon the metasedimentary wall rocks and associated leucosome deformed synchronously with the multistage emplacement and deformation flow of the Mafic Complex. The recognition of a high-temperature shear zone

  6. The Pliny-Strabo trench region: A large shear zone resulting from slab tearing

    NASA Astrophysics Data System (ADS)

    Özbakır, Ali D.; Şengör, A. M. C.; Wortel, M. J. R.; Govers, R.

    2013-08-01

    The eastern part of the Hellenic subduction zone is composed of the Pliny and Strabo "trenches" that have been regarded as a zone of convergence between the subducting African lithosphere and the overriding Anatolian-Aegean plate. In the Pliny and Strabo "trenches", the oblique relative plate motion is generally thought to be accommodated by a typical strain partitioning consisting of strike-slip and convergence components. Notwithstanding the occurrence of strike-slip motion parallel with the Pliny-Strabo "trenches", trench-normal thrusting is not observed so far. Therefore, we conducted a detailed analysis to investigate the deformation mechanisms of the eastern part of the Hellenic Trench system. Our analyses of offshore faulting and mechanisms of earthquakes in the overriding Aegean lithosphere show that the region of the Pliny and Strabo "trenches" obeys the mechanics of the sinistral shear zone model of Tchalenko (1970). We propose that the trench perpendicular convergence is taken up by the Rhodes fold and thrust belt, which has been postulated off the southeast coast of Rhodes. Several regional P-wave tomography results give indications of a slow seismic anomaly under this zone, which is interpreted as a tear between the Hellenic and Cyprus subduction zones. The primary reason for such tear and its propagation is the ongoing rollback of the subducted part of the African lithosphere, also referred to as "the Aegean slab". Our work elucidates the surface expression of this tearing process in the form of the development of a shear zone between the Aegean lithosphere in the NW and the African lithosphere in the SE, the Pliny-Strabo Shear Zone.

  7. Faults and Shear Zones: Constraints on the Extrapolation of Laboratory Tests

    NASA Astrophysics Data System (ADS)

    Dresen, Georg; Bohnhoff, Marco; Kwiatek, Grzegorz; Rybacki, Erik

    2016-04-01

    Geological structures and processes often show strong geometric and physical similarities if observed on different scales. Examples range from fractures, faults and shear zones to seismic characteristics such as b-value or seismic source properties. Laboratory tests on small-scale rock samples allow studying aspects of processes that govern earthquake nucleation and rupture propagation, strain localization in shear zones, and high-temperature rheology. However, upscaling of laboratory results to the field scale requires that dominant deformation processes remain the same on vastly different scales, and that potential effects of changing kinematic and thermodynamic boundary conditions may successfully be accounted for by appropriate constitutive equations. A key observational strategy relies on analysis of deformation processes on different scales. In this presentation we will illustrate the approach with two examples from very different geological environments: 1. Scaling of earthquake mechanisms observed in the laboratory, in mines and along major fault zones and 2. High-temperature creep processes governing the deformation in highly localized shear zones in the lower crust and upper mantle. Our results show that constitutive models capturing fundamental physical processes on the laboratory scale may be successfully applied to model deformation on the field scale.

  8. Pseudotachylite in eclogite-facies shear zones from the Lofoten Islands, northern Norway

    NASA Astrophysics Data System (ADS)

    Leech, M. L.; Shulman, D.; Dutra, S.

    2009-12-01

    The Lofoten Islands of northern Norway are one of two areas on Earth where eclogite-facies pseudotachylite has been found. Pseudotachylite-bearing eclogite shear zones indicate brittle faulting below the brittle/ductile transition existing at approximately 40 km depth in mafic lithosphere. Localized eclogite-facies shear zones, containing omphacite and garnet, have been found in various types of host rock on the islands of Flakstadøy, Vestvagøya, and Austvagøya. These eclogite shear zones are thought to be the result of deep crustal processes during Caledonian subduction and continent-continent collision. Pseudotachylite veins within these ESZ were observed in two locations on Flakstadøy near Flakstad and Nusfjord. At both locations, pseudotachylite- and non-pseudotachylite-bearing eclogite shear zones were sampled with a one-inch core drill. Using Fe-Mg exchange between garnet and omphacite, eclogitization took place at approximately 1.5 GPa and 680° C or approximately 45 km depth. Mineral chemistry and microstructural analyses of the host rock to shear zone transitions will provide information on the relationship between deformation and high-grade metamorphism. The microstructural relationship between the eclogite shear zones and pseudotachylite formation will be analyzed using electron backscatter diffraction (EBSD) at San Francisco State University. We expect mineral chemistry and EBSD analyses will distinguish between deformation-controlled or fluid-induced eclogite-facies metamorphism in otherwise metastable host rocks. Based on field observations, the ESZ and pseudotachylite formed coseismically, indicating brittle faulting occurred at eclogite-facies P-T conditions in the lower crust. Previous attempts to date the ESZ have been hampered by a retrograde amphibolite-facies overprint during post-Caledonian extension and exhumation. We will use 40Ar/39Ar to date crystal-free portions (i.e., unretrogressed) of pseudotachylite veins and Rb-Sr or Lu-Hf to

  9. The Neoproterozoic Trans-Saharan/Trans-Brasiliano shear zones: Suggested Tibetan Analogs

    NASA Astrophysics Data System (ADS)

    Attoh, K.; Brown, L. D.

    2008-05-01

    The Trans-Saharan Borborema (TSB) belt is a product of the assembly of Gondwana, a supercontinent that formed from cratonic fragments derived from Rodinia and other vagrant lithospheric blocks. Recent reconstructions show the West African craton (WAC) and Congo- San-Francisco craton juxtaposed by the closure of the Brasiliano (Pharuside, Adamastor) ocean during early stages of the Pan-African orogenic cycle in northwest Gondwana. The Dahomeyide and Pharuside segments of the resulting orogenic belt preserve well- organized lithotectonic units on the eastern margin of the WAC. The foreland units consist of craton-verging nappe stacks formed from the deformed margin of the WAC and its cover rocks. The near-hinter land is underlain by granitoid gneisses postulated to represent ca 600 Ma juvenile crust, exposed in the Accra-Benin plain. Further east from the suture zone is the Nigerian province, which includes rocks that were extensively reworked apparently during the Pan-African. The Borborema province of northeastern Brasil is the correlative/ extension of Nigerian. It is underlain by rocks intensively reworked during the Brasiliano (Pan-African) orogeny and juxtaposed along a series of shear zones. A distinct feature of the TBS are these extensive shear zones, many of which are typified by dextral wrench shear. In West Africa the prominent examples extend from the Sahara to the coastline and include the Hoggar, which splays into the Kandi fault, which itself has numerous splays in Benin, Togo and southeastern Ghana. In Brazil, nearly all reconstructions show that the continuation of the Kandi Fault is the Sobral fault which is inferred to be the northern segment of the Trans-Brasiliano lineament (TBL). If correct, the TBL and its TBS extensions constitute a 4000 km long dextral shear zone, perhaps the longest coherent shear zone on earth. We suggest that the geometry of these shear zones and associated Pan African sutures have instructive analogs in the Tibet

  10. Creep cavitation can establish a dynamic granular fluid pump in ductile shear zones.

    PubMed

    Fusseis, F; Regenauer-Lieb, K; Liu, J; Hough, R M; De Carlo, F

    2009-06-18

    The feedback between fluid migration and rock deformation in mid-crustal shear zones is acknowledged as being critical for earthquake nucleation, the initiation of subduction zones and the formation of mineral deposits. The importance of this poorly understood feedback is further highlighted by evidence for shear-zone-controlled advective flow of fluids in the ductile lower crust and the recognition that deformation-induced grain-scale porosity is a key to large-scale geodynamics. Fluid migration in the middle crust cannot be explained in terms of classical concepts. The environment is considered too hot for a dynamic fracture-sustained permeability as in the upper crust, and fluid pathways are generally too deformed to be controlled by equilibrium wetting angles that apply to hotter, deeper environments. Here we present evidence that mechanical and chemical potentials control a syndeformational porosity generation in mid-crustal shear zones. High-resolution synchrotron X-ray tomography and scanning electron microscopy observations allow us to formulate a model for fluid migration in shear zones where a permeable porosity is dynamically created by viscous grain-boundary sliding, creep cavitation, dissolution and precipitation. We propose that syndeformational fluid migration in our 'granular fluid pump' model is a self-sustained process controlled by the explicit role of the rate of entropy production of the underlying irreversible mechanical and chemical microprocesses. The model explains fluid transfer through the middle crust, where strain localization in the creep regime is required for plate tectonics, the formation of giant ore deposits, mantle degassing and earthquake nucleation. Our findings provide a key component for the understanding of creep instabilities in the middle crust. PMID:19536262

  11. Preservation of Blueschist Facies Minerals Along a Shear Zone By Fast Flowing CO2-Bearing Fluids - a Field Study from the Cycladic Blueschist Unit on Syros, Greece

    NASA Astrophysics Data System (ADS)

    Kleine, B. I.; Skelton, A.; Huet, B.; Pitcairn, I.

    2014-12-01

    Our study was undertaken at Fabrika Beach on the southeastern shore of Syros which belongs to the Greek Cycladic archipelago in the Aegean Sea. The island is situated within the Attic-Cycladic metamorphic core complex belt and is now located in the back-arc of the active Hellenic subduction zone. At Fabrika Beach, blueschist facies minerals are observed in haloes fringing a shear zone within greenschist facies rocks. The approximately vertical shear zone cuts a near horizontal layer of greenschist facies rocks. The blue haloes are ca. 1 m wide, and are seen on both sides of the shear zone. The haloes consist of a carbonated blueschist facies mineral assemblage. Based on petrological, geochemical and thermodynamic evidence we show that these haloes were preserved at greenschist facies conditions in response to fast flowing CO2-bearing fluid. Furthermore, we use a simple mass balance to calculate the fluid flux within the shear zone which would be required to cause the observed preservation of blueschist facies minerals. We constructed a simplified P-T vs. XCO2 pseudosection using PerPlex 6.6.6 to confirm that preservation of carbonated blueschist can occur at greenschist facies conditions in the presence of CO2-bearing fluid. The flux of CO2-bearing fluid along the shear zone was rapid with respect to the fluid flux in the surrounding rocks. Mass balance calculations reveal that the fluid flux within the shear zone was at least 100 - 2000 times larger than the fluid flux within the surrounding rocks. Mineral textures show greenschist facies minerals partially replacing blueschist minerals in the haloes supporting our interpretation that blueschist facies minerals were preserved during greenschist facies retrogression.

  12. Mineralogically triggered strain localization: inferences from ductile paired shear zones (Tauern Window, Eastern Alps)

    NASA Astrophysics Data System (ADS)

    Duprat-Oualid, Sylvia; Grasemann, Bernhard; Huet, Benjamin; Yamato, Philippe; Habler, Gerlinde

    2016-04-01

    Lithosphere is mainly constituted by polyphase rocks whose mineralogical, structural and textural characteristics control the spatial distribution of strain, and so, its effective mechanical strength. Variations in local mineralogical compositions may lead to drastic changes in the local microstructures and texture leading, by mechanical feedback processes, to strain hardening or weakening. Understanding these small-scale relations between the petrological characteristics, the rheological properties and the development of progressive deformation is thus of fundamental importance for understanding the strength of rocks at large-scale, especially the mechanical behavior of plates boundaries. We acknowledge the importance of brittle precursors for the localization of strain in the viscous deforming part of the crust. In this study, we focus in centimeter-wide paired ductile shear zones shaped nearby along on both sides of ep-grt-qtz veins within a late Variscan metagranodiorite of the "Zentralgneis" in the Tauern Window (Berlinerhütte, Zillertal, Austria). The paired shear zones, underlined by biotite bands, localized at some centimeters away from the veins associated with a metasomatic domain, within the relatively undeformed host rock. According to their spatial orientations, they exhibit different intensities of shearing (from incipient linking of biotites to anastomosing ultra-mylonitic bands) and thus can be explored as successive strain domains of a shear zone developing in space and time. Here, we present a combination of high-resolution petro-chemical section across the paired shear zones with microstructural and textural measurements in order to constrain mineral reactions and deformation processes associated with the development of localized shear zones. Whole rock chemical analyzes combined with continuous mineralogical mapping revealed small chemical variations induced by fluid-rock interactions in the vicinity of the veins. Although macroscopically

  13. Isotopic dating of strain fringe increments: duration and rates of deformation in shear zones

    PubMed

    Muller; Aerden; Halliday

    2000-06-23

    The time scales over which deformation in the Earth's crust remains localized in shear zones are poorly known, as are the associated strain rates. We have determined the longevity and rates of deformation using rubidium-strontium (Rb-Sr) microsampling dating of increments of fibrous strain fringes from a Pyrenean shear zone. The fibers grew quasi-continuously through a protracted deformation history between 87 and 50 million years ago, over a period comparable to that of an orogeny. During a short interval between 66 and 62 million years, a rise in strain rate from 1.1 x 10(-15) to 7. 7 x 10(-15) seconds(-1) occurred. This acceleration correlates with an abrupt change in fiber-growth direction and a stress-field inversion from gravitational collapse to renewed horizontal crustal shortening. PMID:10864865

  14. Properties of Ductile Shear Zones Below Strike-Slip Faults: Insights From Numerical Experiments Incorporating Laboratory-Derived Rheologies (Invited)

    NASA Astrophysics Data System (ADS)

    Fialko, Y. A.; Takeuchi, C. S.

    2013-12-01

    We investigate the long-term evolution of stress and strain in a ductile substrate driven by far-field plate motion and slip on a vertical transform fault cutting through the brittle crust. Numerical models that incorporate laboratory-derived power-law rheologies with Arrhenius temperature dependence, viscous dissipation, and conductive heat transfer give rise to long-lived fault "roots" that localize deformation below the brittle-ductile transition. Strain localization in the viscoelastic medium in this case results from thermomechanical coupling and power law dependence of strain rate on stress. For conditions corresponding to the San Jacinto and San Andreas Faults in Southern California, the predicted width of the shear zone in the lower crust is a few kilometers; this shear zone accommodates more than 50% of the far-field plate motion. Coupled thermomechanical models predict a single-layer lithosphere in case of "dry" composition of the lower crust and upper mantle, and a "jelly sandwich" lithosphere in case of "wet" composition. Deviatoric stress in the lithosphere in our models is relatively insensitive to the water content, the far-field loading rate, and the fault strength, and is of the order of 102 MPa. Furthermore, stress in the lithosphere is found to inversely correlate with the velocity of relative plate motion. Somewhat surprisingly, we find that the thermally-activated shear zones have little effect on postseismic relaxation. In particular, the presence of such zones does not change the polarity of vertical displacements in cases of rheologies that are able to generate robust postseismic transients. We conclude that additional (to thermomechanical coupling) mechanisms of strain localization are required for a viscoelastic model to produce a vertical deformation pattern similar to that due to afterslip on a deep extension of a fault. Possible candidates include dynamic grain re-crystallization, and fabric development (mylonitization).

  15. Mechanical anisotropy control on strain localization in upper mantle shear zones

    NASA Astrophysics Data System (ADS)

    Herwegh, Marco; Mercolli, Ivan; Linckens, Jolien; Müntener, Othmar

    2016-05-01

    Mantle rocks at oceanic spreading centers reveal dramatic rheological changes from partially molten to solid-state ductile to brittle deformation with progressive cooling. Using the crustal-scale Wadi al Wasit mantle shear zone (SZ, Semail ophiolite, Oman), we monitor such changes based on quantitative field and microstructural investigations combined with petrological and geochemical analyses. The spatial distribution of magmatic dikes and high strain zones gives important information on the location of magmatic and tectonic activity. In the SZ, dikes derived from primitive melts (websterites) are distributed over the entire SZ but are more abundant in the center; dikes from more evolved, plagioclase saturated melts (gabbronorites) are restricted to the SZ center. Accordingly, harzburgite deformation fabrics show a transition from protomylonite (1100°C), mylonite (900-800°C) to ultramylonite (<700°C) and a serpentine foliation (<500°C) from the SZ rim to the center. The spatial correlation between solid-state deformation fabrics and magmatic features indicates progressive strain localization in the SZ on the cooling path. Three stages can be discriminated: (i) Cycles of melt injection (dunite channels and websterite dikes) and solid-state deformation (protomylonites-mylonites; 1100-900°C), (ii) dominant solid-state deformation in harzburgite mylonites (900-800°C) with some last melt injections (gabbronorites) and ultramylonites (<700°C), and (iii) infiltration of seawater inducing a serpentine foliation (<500°C) followed by cataclasis during obduction. The change of these processes in space and time indicates that early dike-related ridge-parallel deformation controls the onset of the entire strain localization history promoting nucleation sites for different strain weakening processes as a consequence of changing physicochemical conditions.

  16. Metamorphic signature of the Gneiss Canyon Shear Zone, Lower Granite Gorge, Grand Canyon, Arizona

    SciTech Connect

    Robinson, K.; Williams, M.L. . Dept. of Geology and Geography)

    1992-01-01

    The Proterozoic orogen in Arizona consists of structural blocks separated by NE trending shear zones. The Gneiss Canyon Shear Zone (GCSZ) is important because it appears to define in part the boundary between the amphibolite facies Yavapai Province and the granulite facies Mojave Province. An early NW striking foliation is clearly visible in many samples from the Lower Granite Gorge (LGG). In Travertine Canyon, east of the GCSZ, pelitic schists contain And-Sil-Crd-Bi and Gar-Sil-Sta-Bi. Mafic rocks exhibit complex phase relations between cummingtonite, anthophyllite, gedrite, garnet, and cordierite. The coexistence of cordierite-cummingtonite is indicative of low pressure metamorphism. Microprobe analyses of garnets reveal prograde growth zoning profiles. Temperature and pressure estimates of peak metamorphism are 550--600 C and 3 kb. Just east of the GCSZ, pelitic assemblages contain Gar-Bi [+-] Sil [+-] Mus, and garnet zoning profiles are flat in the cores. In Spencer Canyon, west of the GCSZ, samples commonly contain Gar-Bi-Sil-Crd, and in many samples cordierite is being replaced by sillimanite. Thermobarometric calculations yield temperature and pressure estimates of 650 C and 3.5 kb. Mineral assemblages and quantitative thermobarometry suggest higher peak metamorphic temperature west of the GCSZ but relatively constant pressures across the LGG. On the east side of the GCSZ, temperatures increase toward the Shear Zone, probably due to the presence of extensive dikes, pods, and veins of variably deformed granite. Peak mineral assemblages are syntectonic with respect to the NE-striking GCSZ fabric. If a suture exists in the LGG, the GCSZ fabrics apparently reflect post-accretionary tectonism, with accretion occurring prior to the peak of metamorphism.

  17. Kinematics of the Eastern California shear zone: Evidence for slip transfer from Owens and Saline Valley fault zones to Fish Lake Valley fault zone

    USGS Publications Warehouse

    Reheis, M.C.; Dixon, T.H.

    1996-01-01

    Late Quaternary slip rates and satellite-based geodetic data for the western Great Basin constrain regional fault-slip distribution and evolution. The geologic slip rate on the Fish Lake Valley fault zone (the northwest extension of the Furnace Creek fault zone) increases northward from about 3 to 5 mm/yr, in agreement with modeled geodetic data. The increase coincides with the intersections of the Deep Springs fault, connected to the Owens Valley fault zone, and of other faults connected to the Saline Valley fault. The combined geologic and geodetic data suggest that (1) the northwest-striking faults of the Eastern California shear zone north of the Garlock fault are connected by north- to northeast-striking normal faults that transfer slip in a series of right steps, and (2) the amount and distribution of slip among the many faults of this broad, complex plate boundary have changed through time.

  18. Grain size reduction of feldspar and pyroxene, phase mixing, and strain localization in lower crustal shear zones (Lofoten, Norway)

    NASA Astrophysics Data System (ADS)

    Menegon, L.; Stunitz, H.; Nasipuri, P.; Svahnberg, H.; Heilbronner, R.

    2011-12-01

    compositional domains are indistinguishable and the degree of phase mixing is higher. Fractured fragments of mesoperthite are not preserved, the grain size of plagioclase, K-feldspar and quartz is further reduced compared to the mylonite (20 μm Vs. 25-30 μm), and the main constituent phases do not show a CPO. Diffusion creep is interpreted to be the dominant deformation mechanism. In summary, shear zone formation is invariably associated with a preliminary stage of cracking and fluid infiltration, which triggers syndeformational metamorphic reactions, strong grain size reduction, and activation of diffusion creep. Initial cracking at the estimated deformation conditions requires high differential stresses (in the absence of high pore pressures), and indicates a high strength of the lower continental crust at the onset of the deformation. A strain-dependent transition from dislocation creep to diffusion creep is not observed, and diffusion creep appears to be the dominant deformation mechanism in all compositional domains from the mylonite to ultramylonite stage.

  19. Les unités gneissiques et la zone de cisaillement crustal du Sud-Togo (Gneissic units and crustal shear zone of South Togo)

    NASA Astrophysics Data System (ADS)

    Castaing, C.; Aregba, A.; Assih-Edeou, P.; Chevremont, P.; Godonou, K. S.; Sylvain, J. P.

    This publication is the result of recent geological mapping in Togo. An important crustal shear-zone in the South Togo is reviewed and the overall structural evolution of the area is examined in the light of current ideas on the chain. The geological evolution of the area occurred in three tectono-metamorphic stages in which the foliation developed and was folded under upper amphibolite facies conditions, with anatexis and the emplacement of granites, followed by transcurrent shearing under retrograde metamorphic conditions. The shear-zones, recognised in southeastern Togo, are the southward continuation of the great submeridian shear zones of the Hoggar and the Adrar des Iforas. They take the form of kilometres wide, blasto to ultramylonitic zones in which a mylonitic foliation indicates dextral ductile deformation under greenschist facies conditions. The ductile deformation evolves towards late cataclasis in the center of the shear zones. The geological evolution of the South Togo accords with the geotectonic framework of the internal Pan-African zones of the central Hoggar and Nigeria, where a polycyclic tectono-metamorphic history, with important granitization and late shearing appears to be the general case.

  20. Structural mapping and analysis of a Madagascar Precambrian shear zone using enhanced Landsat Thematic Mapper Data

    SciTech Connect

    Kilmer, D.S.; Duncan, I.J. )

    1990-05-01

    Recently, the west coast of Madagascar has become a frontier region for petroleum exploration. Major structures in the Precambrian shield of Madagascar may have a strong control on the development of sedimentary basins, as has been documented in the Morondava basin. The 2.5-3.0+ Ga shield of Madagascar is an amphibolite- to granulite-grade metamorphic gneiss terrain, intruded by anorthosites and 550-Ma granites and pegmatites. Landsat Thematic Mapper data provides a cost-effective method for regional-scale structural mapping of this poorly known terrain. A five-component linear mixing model has been used to enhance the lithologic information in this six-band data. Lithologic component images thus produced utilize the full geologic spectral range of the data. A preliminary structural geologic map compiled from the component images has greater detail than existing maps at 1:100,000 scale, to which it has been compared. The Ankafotra-Saririaky shear zone has been identified as a north-northeast-trending, 15- to 20-km-wide region of appressed folds, attenuated layering, and subparallel faults on the western side of the shield. Two anorthosite massifs that occur within this shear zone have the structural characteristics of boudins in a ductile matrix. The shear deformed a preexisting terrain of poly-phase folding, characterized by tight folds and complex fold interference structures displayed by basins and domes on a scale of 10 km. Enhanced remote sensing data can be used to characterize the nature and mechanism of shear deformation in such zones.

  1. High strength semi-active energy absorbers using shear- and mixedmode operation at high shear rates

    NASA Astrophysics Data System (ADS)

    Becnel, Andrew C.

    This body of research expands the design space of semi-active energy absorbers for shock isolation and crash safety by investigating and characterizing magnetorheological fluids (MRFs) at high shear rates ( > 25,000 1/s) under shear and mixed-mode operation. Magnetorheological energy absorbers (MREAs) work well as adaptive isolators due to their ability to quickly and controllably adjust to changes in system mass or impact speed while providing fail-safe operation. However, typical linear stroking MREAs using pressure-driven flows have been shown to exhibit reduced controllability as impact speed (shear rate) increases. The objective of this work is to develop MREAs that improve controllability at high shear rates by using pure shear and mixed shear-squeeze modes of operation, and to present the fundamental theory and models of MR fluids under these conditions. A proof of concept instrument verified that the MR effect persists in shear mode devices at shear rates corresponding to low speed impacts. This instrument, a concentric cylinder Searle cell magnetorheometer, was then used to characterize three commercially available MRFs across a wide range of shear rates, applied magnetic fields, and temperatures. Characterization results are presented both as flow curves according to established practice, and as an alternate nondimensionalized analysis based on Mason number. The Mason number plots show that, with appropriate correction coefficients for operating temperature, the varied flow curve data can be collapsed to a single master curve. This work represents the first shear mode characterization of MRFs at shear rates over 10 times greater than available with commercial rheometers, as well as the first validation of Mason number analysis to high shear rate flows in MRFs. Using the results from the magnetorheometer, a full scale rotary vane MREA was developed as part of the Lightweight Magnetorheological Energy Absorber System (LMEAS) for an SH-60 Seahawk helicopter

  2. The Ailao Shan-Red River shear zone (Yunnan, China), Tertiary transform boundary of Indochina

    NASA Astrophysics Data System (ADS)

    Leloup, Philippe Hervé; Lacassin, Robin; Tapponnier, Paul; Schärer, Urs; Zhong, Dalai; Liu, Xiaohan; Zhang, Liangshang; Ji, Shaocheng; Trinh, Phan Trong

    1995-12-01

    The Red River Fault zone (RRF) is the major geological discontinuity that separates South China from Indochina. Today it corresponds to a great right-lateral fault, following for over 900 km the edges of four narrow (< 20 km wide) high-grade gneiss ranges that together form the Ailao Shan-Red River (ASRR) metamorphic belt: the Day Nui Con Voi in Vietnam, and the Ailao, Diancang and Xuelong Shan in Yunnan. The Ailao Shan, the longest of those ranges, is fringed to the south by a strip of low-grade schists that contain ultramafic bodies. The ASRR belt has thus commonly been viewed as a suture. A detailed study of the Ailao and Diancang Shan shows that the gneiss cores of the ranges are composed of strongly foliated and lineated mylonitic gneisses. The foliation is usually steep and the lineation nearly horizontal, both being almost parallel to the local trend of the gneissic cores. Numerous shear criteria, including asymmetric tails on porphyroclasts, C-S or C'-S structures, rolling structures, asymmetric foliation boudinage and asymmetric quartz axis fabrics, indicate that the gneisses have undergone intense, progressive left-lateral shear. P-T studies show that left-lateral strain occurred under amphibolite-facies conditions (3-7 kb and 550-780°C). In both ranges high-temperature shear was coeval with emplacement of leucocratic melts. Such deformed melts yield {U}/{Pb} ages between 22.4 and 26.3 Ma in the Ailao Shan and between 22.4 and 24.2 Ma in the Diancang Shan, implying shear in the Lower Miocene. The mylonites in either range rapidly cooled to ≈ 300°C between 22 and 17 Ma, before the end of left-lateral motion. The similarity of deformation kinematics, P-T conditions, and crystallization ages in the aligned Ailao and Diancang Shan metamorphic cores, indicate that they represent two segments of the same Tertiary shear zone, the Ailao Shan-Red River (ASRR) shear zone. Our results thus confirm the idea that the ASRR belt was the site of major left

  3. Serpentine in active subduction zones

    NASA Astrophysics Data System (ADS)

    Reynard, Bruno

    2013-09-01

    Serpentinization is a key phenomenon for understanding the geodynamics of subduction zones in the 10-200 km depth range. Serpentines are a major water carrier, and their rheological properties have a strong influence on deformation partitioning and seismicity at depths. I review experimental investigations that have been conducted on serpentines, with emphasis on the large body of data acquired over the past decade. Determinations of physical properties at the pressure and temperature conditions of subductions allow interpreting geophysical data in active subduction in terms of mineralogy and petrology, and to link the presence of serpentinites with deformation and fluid circulation. The fluid budget can be partially constrained from geophysical data. Elasticity data provide a quantitative basis for mapping serpentinization in the mantle wedge and slab from seismic tomography. Anisotropy suggests the existence of thin serpentinite channels above the plate interface, that account for mechanical decoupling inferred from down-dip limit of the seismogenic zone and heat flow. Strain-rate dependent rheology of antigorite serpentine is consistent with stable deformation of this thin layer or channel over timescales ranging from those of the seismic cycle to those of thermal equilibration and exhumation of high-pressure rocks, and with the geological record of subduction-related deformation. Circulation of serpentinizing fluids depends on the permeability structure, and is imaged by electrical conductivity tomography. It could be controlled by fracturing in the undeformed cold nose of the mantle wedge, and by plastic deformation along the plate interface. Fluid migration mechanisms are similar to those inferred from petrological and geochemical data on exhumed serpentinites. Estimation of the fluid budget associated with serpentine formation will rely on numerical simulations for which coupling of kinetics of hydration and dehydration at scales ranging from grain size up

  4. A new mechanism for fluid migration in midcrustal shear zones based on viscous grain boundary sliding and creep cavitation

    NASA Astrophysics Data System (ADS)

    Fusseis, F.; Regenauer-Lieb, K.; Liu, J.

    2009-04-01

    associated volume changes. Deformation, pore formation and growth dissipate a total power that is the time rate of the pressure work done by the granular fluid pump minus the time rate of the energy stored in the pore surface. If the rate of dissipation is maximized and constant, pore growth is stable. Note that this granular fluid pump relies on the opening and closing of pores and consequently describes a dynamic permeability. It should occur wherever viscous grain boundary sliding is active in shear zones with a free fluid phase. This is the case in many midcrustal shear zones that are either characterized by episodic cataclastic/mylonitic deformation (during the earthquake cycle) or by reaction softening associated with retrogression. The granular fluid pump provides comparatively steady, non-episodic fluid transfer and does not require pervasive fracturing. References: Fliervoet et al., 1997, JSG 19/12, 1495-1520, Fusseis et al., in review, Nature.

  5. Reconciling viscoelastic models of postseismic and interseismic deformation: Effects of viscous shear zones and finite length ruptures

    NASA Astrophysics Data System (ADS)

    Hearn, Elizabeth H.; Thatcher, Wayne R.

    2015-04-01

    We have developed a suite of earthquake cycle models for strike-slip faults to investigate how finite ruptures and lithosphere-scale viscous shear zones affect interseismic deformation. In particular, we assess whether localized and stationary interseismic deformation and large-scale, rapidly decaying postseismic transients may be explained with models incorporating either or both of these features. Models incorporating viscous shear zones give more stationary interseismic deformation than layered half-space, Maxwell viscoelastic models producing similar early postseismic deformation in the near field. This tendency is accentuated when the effective viscosity per unit width of the shear zone increases either with depth or with interseismic time. Models with finite (200 km long) ruptures produce time-dependent deformation similar to that from models with infinitely long ruptures, though with smaller-magnitude and somewhat more localized surface velocity fluctuations. Models incorporating a stiff lithosphere, a low-viscosity mantle asthenosphere, and a crust- to lithosphere-scale viscous shear zone can replicate postseismic and interseismic deformation typical of large, strike-slip earthquakes. Such models require depth-dependent, power law, or transient rheologies for the viscous shear zone material in the crust, as long as the effective viscosity per unit shear zone width is approximately 1015 Pa s/m at crustal depths during the postseismic interval. Below the Moho, the shear zone effective viscosity per unit width must be higher throughout the seismic cycle, at least over a short depth interval. For example, if shear zone viscosity per unit width is 5×1016 Pa s/m in the mantle lithosphere, a model with a 50 km thick lithosphere and asthenosphere effective viscosities of 1 to 5×1018 Pa s can reproduce reference velocity profiles for both postseismic and later interseismic deformation.

  6. Sub-layers inside the entrainment zone of a dry, shear-free convective boundary layer

    NASA Astrophysics Data System (ADS)

    Garcia, Jade Rachele; Mellado, Juan Pedro

    2013-11-01

    The entrainment zone of a dry, shear-free convective boundary layer growing into a homogeneously stably-stratified fluid is studied using direct numerical simulation. Based on the self-similar analysis of the mean and variance buoyancy profiles, we identify two sub-layers within the entrainment zone, defined as the region of negative buoyancy flux: i) an upper sub-layer with a thickness comparable to the penetrative length scale based on the convective velocity and the buoyancy frequency of the free troposphere and ii) a lower sub-layer acting as a transition towards the mixed layer, with a thickness equal to a constant fraction of the boundary layer height. The capping region of the penetrative thermals belongs to the upper sub-layer of the entrainment zone, and the troughs between the penetrating thermals belong to the lower sub-layer of the entrainment zone. Correspondingly, different buoyancy scales are identified in the different regions; parametrizations thereof are provided and explained. This multiplicity of characteristic scales inside the entrainment zone helps to explain the uncertainty associated with previous analysis of entrainment zone properties and the difficulty to parametrize them based on a single length scale and a single buoyancy scale. Juelich Research Centre for the computing time.

  7. Scaling of the critical slip distance for seismic faulting with shear strain in fault zones

    USGS Publications Warehouse

    Marone, C.; Kilgore, B.

    1993-01-01

    THEORETICAL and experimentally based laws for seismic faulting contain a critical slip distance1-5, Dc, which is the slip over which strength breaks down during earthquake nucleation. On an earthquake-generating fault, this distance plays a key role in determining the rupture nucleation dimension6, the amount of premonitory and post-seismic slip7-10, and the maximum seismic ground acceleration1,11. In laboratory friction experiments, Dc has been related to the size of surface contact junctions2,5,12; thus, the discrepancy between laboratory measurements of Dc (??? 10-5 m) and values obtained from modelling earthquakes (??? 10-2 m) has been attributed to differences in roughness between laboratory surfaces and natural faults5. This interpretation predicts a dependence of Dc on the particle size of fault gouge 2 (breccia and wear material) but not on shear strain. Here we present experimental results showing that Dc scales with shear strain in simulated fault gouge. Our data suggest a new physical interpretation for the critical slip distance, in which Dc is controlled by the thickness of the zone of localized shear strain. As gouge zones of mature faults are commonly 102-103 m thick13-17, whereas laboratory gouge layers are 1-10 mm thick, our data offer an alternative interpretation of the discrepancy between laboratory and field-based estimates of Dc.

  8. Texture and elastic anisotropy of a mylonitic anorthosite from the Morin Shear Zone (Quebec, Canada)

    NASA Astrophysics Data System (ADS)

    Gómez Barreiro, Juan; Wenk, Hans-Rudolf; Vogel, Sven

    2015-02-01

    A sample of anorthosite from the granulite facies Morin Shear Zone (Quebec, Canada) was investigated for crystal preferred orientation and elastic anisotropy. Time-of-flight neutron diffraction data obtained with the HIPPO diffractometer at LANSCE were analyzed with the Rietveld method to obtain orientation distribution functions of the principal phases (plagioclase, clinopyroxene and orthopyroxene). Texture and microstructures are compatible with the plastic deformation of the aggregate under high-T conditions. All mineral phases depict a significant preferred orientation that could be related to the general top-to-the north shearing history of the Morin Shear Zone. Texture patterns suggest that (010)[001] in plagioclase and (110)[001] in clinopyroxene are likely dominant slip systems. Using preferred orientation data P- and S-waves velocities and elastic anisotropy were calculated and compared with previous studies to explore elastic properties of rocks with different pyroxene-plagioclase mixtures. P-wave velocity, S-wave splitting and anisotropy increase with clinopyroxene content. Seismic anisotropy is linked to the texture symmetry which can lead to large deviations between actual anisotropy and that measured along Cartesian XYZ sample directions (lineation/foliation reference frame). This is significant for the prediction and interpretation of seismic data, particularly for monoclinic or triclinic texture symmetries.

  9. An empirical method to estimate shear wave velocity of soils in the New Madrid seismic zone

    USGS Publications Warehouse

    Wei, B.-Z.; Pezeshk, S.; Chang, T.-S.; Hall, K.H.; Liu, Huaibao P.

    1996-01-01

    In this study, a set of charts are developed to estimate shear wave velocity of soils in the New Madrid seismic zone (NMSZ), using the standard penetration test (SPT) N values and soil depths. Laboratory dynamic test results of soil samples collected from the NMSZ showed that the shear wave velocity of soils is related to the void ratio and the effective confining pressure applied to the soils. The void ratio of soils can be estimated from the SPT N values and the effective confining pressure depends on the depth of soils. Therefore, the shear wave velocity of soils can be estimated from the SPT N value and the soil depth. To make the methodology practical, two corrections should be made. One is that field SPT N values of soils must be adjusted to an unified SPT N??? value to account the effects of overburden pressure and equipment. The second is that the effect of water table to effective overburden pressure of soils must be considered. To verify the methodology, shear wave velocities of five sites in the NMSZ are estimated and compared with those obtained from field measurements. The comparison shows that our approach and the field tests are consistent with an error of less than of 15%. Thus, the method developed in this study is useful for dynamic study and practical designs in the NMSZ region. Copyright ?? 1996 Elsevier Science Limited.

  10. Kinematics of the Torcal Shear Zone: Transpressional tectonics in a salient-recess transition at the northern Gibraltar Arc

    NASA Astrophysics Data System (ADS)

    Barcos, L.; Balanyá, J. C.; Díaz-Azpiroz, M.; Expósito, I.; Jiménez-Bonilla, A.

    2015-11-01

    Complex strain patterns in the Gibraltar Arc derive from the interaction between the westward drift - and concomitant back-arc extension - of the arc hinterland (Alboran Domain) and the Europe-Africa convergence. In order to explore strain partitioning modes within the arc and the role played by large-scale oblique structures, we have studied the kinematics of the Torcal Shear Zone located at the northern branch of the Gibraltar Arc. The Torcal Shear Zone is a 70 km-long, E-W brittle-ductile shear zone that underwent overall dextral transpression during the Late Miocene to Quaternary time. Within the Torcal Shear Zone strain is highly partitioned at multiple scales into shortening, oblique, extensional and strike-slip structures. Moreover, strain partitioning is heterogeneous along-strike giving rise to four distinct structural domains. In the central sector the strain is pure-shear dominated, although narrow sectors parallel to the shear walls are simple-shear dominated. A single N99°E-N109°E trending horizontal velocity vector (V→) could explain the kinematics of the entire central sector of the Torcal Shear Zone. Lateral domains have different strain patterns and are comparable to splay-dominated and thrust-dominated strike-slip fault tips. The Torcal Shear Zone provokes the subvertical extrusion of the External Betics units against the Alboran Domain and a dextral deflection of the structural trend. Moreover, the estimated V→ points to the importance of the westward motion of the hinterland relative to the external wedge and fits well with the radial outward thrusting pattern identified in the arc.

  11. Shear-transformation-zone theory of yielding in athermal amorphous materials

    NASA Astrophysics Data System (ADS)

    Langer, J. S.

    2015-07-01

    Yielding transitions in athermal amorphous materials undergoing steady-state shear flow resemble critical phenomena. Historically, they have been described by the Herschel-Bulkley rheological formula, which implies singular behaviors at yield points. In this paper, I examine this class of phenomena using an elementary version of the thermodynamic shear-transformation-zone (STZ) theory, focusing on the role of the effective disorder temperature, and paying special attention to scaling and dimensional arguments. I find a wide variety of Herschel-Bulkley-like rheologies but, for fundamental reasons not specific to the STZ theory, conclude that the yielding transition is not truly critical. In particular, for realistic many-body models with short-range interactions, there is a correlation length that grows rapidly but ultimately saturates near the yield point.

  12. Shear-transformation-zone theory of yielding in athermal amorphous materials

    SciTech Connect

    Langer, J. S.

    2015-07-22

    Yielding transitions in athermal amorphous materials undergoing steady-state shear flow resemble critical phenomena. Historically, they have been described by the Herschel-Bulkley rheological formula, which implies singular behaviors at yield points. In this paper, I examine this class of phenomena using an elementary version of the thermodynamic shear-transformation-zone (STZ) theory, focusing on the role of the effective disorder temperature, and paying special attention to scaling and dimensional arguments. I find a wide variety of Herschel-Bulkley-like rheologies but, for fundamental reasons not specific to the STZ theory, conclude that the yielding transition is not truly critical. Specifically, for realistic many-body models with short-range interactions, there is a correlation length that grows rapidly but ultimately saturates near the yield point.

  13. Shear-transformation-zone theory of yielding in athermal amorphous materials

    DOE PAGESBeta

    Langer, J. S.

    2015-07-22

    Yielding transitions in athermal amorphous materials undergoing steady-state shear flow resemble critical phenomena. Historically, they have been described by the Herschel-Bulkley rheological formula, which implies singular behaviors at yield points. In this paper, I examine this class of phenomena using an elementary version of the thermodynamic shear-transformation-zone (STZ) theory, focusing on the role of the effective disorder temperature, and paying special attention to scaling and dimensional arguments. I find a wide variety of Herschel-Bulkley-like rheologies but, for fundamental reasons not specific to the STZ theory, conclude that the yielding transition is not truly critical. Specifically, for realistic many-body models withmore » short-range interactions, there is a correlation length that grows rapidly but ultimately saturates near the yield point.« less

  14. Nappe-Bounding Shear Zones Initiated On Syn-Tectonic, Pegmatite-Filled Extensional Shear Fractures During Deep-Crustal Nappe Flow In A Large Hot Orogen

    NASA Astrophysics Data System (ADS)

    Culshaw, Nicholas; Gerbi, Christopher; Marsh, Jeffrey; Regan, Peter

    2014-05-01

    The Central Gneiss Belt (CGB) of the Proterozoic western Grenville Province is an extensive exposure of the mid-crustal levels (upper amphibolite facies, lesser granulites) of a large hot orogen. Numerical models give a credible prediction of structure and metamorphism accompanying CGB deep-crustal nappe flow and define a temporal framework based on four developmental phases: thickening, heating, nappe-flow and post convergence extensional spreading. These phases are diachronous in direction of orogen propagation and imply a spatial framework: externides (close to orogen-craton boundary) containing moderately inclined thickening and/or extensional structures, and internides containing thickening structures overprinted by sub-horizontal nappe flow structures, which may be locally overprinted by those due to extensional spreading. Although on average of granitoid composition, CGB nappes differ in rheology, varying from fertile and weak (unmetamorphosed before Grenville, meltable) to infertile and strong (metamorphosed at high grade before Grenville, unmeltable) or mixed fertile-infertile protoliths. Deformation style varies from diffuse in fertile nappes, weakened by pervasive melting, to localised in shear zones on boundaries or interiors of infertile nappes. Specifically, in terms of deformation phase and location within the orogen, shear zones occur as: thickening structures of externides, early thickening- and later overprinting nappe-flow structures of infertile internide nappes, and extension-related shear zones in externides and internides. Many of the nappe-flow shear zones of the internides are associated with pegmatites. One example has been recognized of a preserved progression from small-scale fracture arrays to regional shear zone. The sequence is present on a km-scale and initiates in the interior of a nappe of layered granulite with arrays of pegmatite filled extensional-shear fractures (mm to cm width) displaying amphibolized margins. The fracture

  15. Strain accumulation across the Eastern California Shear Zone at latitude 36°30'N

    USGS Publications Warehouse

    Gan, Weijun; Svarc, Jerry L.; Savage, J.C.; Prescott, W.H.

    2000-01-01

    The motion of a linear array of monuments extending across the Eastern California Shear Zone (ECSZ) has been measured from 1994 to 1999 with the Global Positioning System. The linear array is oriented N54°E, perpendicular to the tangent to the local small circle drawn about the Pacific-North America pole of rotation, and the observed motion across the ECSZ is approximated by differential rotation about that pole. The observations suggest uniform deformation within the ECSZ (strike N23°W) (26 nstrain yr−1 extension normal to the zone and 39 nstrain yr−1 simple right-lateral shear across it) with no significant deformation in the two blocks (the Sierra Nevada mountains and southern Nevada) on either side. The deformation may be imposed by right-lateral slip at depth on the individual major fault systems within the zone if the slip rates are: Death Valley-Furnace Creek fault 3.2±0.9 mm yr−1, Hunter Mountain-Panamint Valley fault 3.3±1.6 mm yr−1, and Owens Valley fault 6.9±1.6 mm yr−1. However, this estimate of the slip rate on the Owens Valley fault is 3 times greater than the geologic estimate.

  16. Simultaneous inversion for mantle shear velocity and the topography of transition zone discontinuities

    NASA Astrophysics Data System (ADS)

    Gu, Y. J.; Dziewonski, A. M.

    2001-05-01

    A method is presented for the simultaneous inversions of shear velocity in the mantle and the topography of transition zone discontinuities. Each travel time residual, corrected for crust and free surface topography, is modeled as resulting from contributions from three-dimensional shear velocity perturbations to a spherical Earth model and boundary undulations to the 410 and 660 km discontinuities. This approach minimizes tradeoffs between velocity and topography. We expand the lateral variations in velocity and the topography of each discontinuity using 362 spherical B-splines; we expand the radial variations using 14 cubic B-splines. To increase the reliability of the measurements, particularly in the undersampled southern hemisphere, we re-examine the topography of the 410- and 660 km discontinuities from more than 21,000 SH-component records. This new data set is significantly larger than those used earlier studies of SS precursors. The long-wavelength features of our new topography maps of the 410- and 660-km discontinuities are compatible with results of earlier studies: the large-scale patterns are dominated by low degree spherical harmonics, particularly at degrees 1 and 2. We also include an independent measurement of the global transition zone thickness for additional constraints on the structure in the transition zone. The best-fit model from the joint inversion reduces the variance of the absolute and differential travel times of S, SS and ScS by 40 to 70 %, and the differential travel times of SS precursors by up to 90%.

  17. Spatial variations of shear wave anisotropy near the San Jacinto Fault Zone in Southern California

    NASA Astrophysics Data System (ADS)

    Li, Zefeng; Peng, Zhigang; Ben-Zion, Yehuda; Vernon, Frank L.

    2015-12-01

    We examine crustal anisotropy at several scales along and across the San Jacinto Fault Zone (SJFZ) by systematically measuring shear wave splitting (SWS) parameters. The analyzed data are recorded by 86 stations during 2012-2014, including five linear dense arrays crossing the SJFZ at different locations and other autonomous stations within 15 km from the main fault trace. Shear phase arrivals and SWS parameters (fast directions and delay times) are obtained with automated methods. The measurement quality is then assessed using multiple criteria, resulting in 23,000 high-quality measurements. We find clear contrast of fast directions between the SW and NE sides of the SJFZ. Stations on the SW side have fast directions consistent overall with the maximum horizontal compression direction (SHmax), while stations on the NE side show mixed patterns likely reflecting lithological/topographic variations combined with fault zone damage. The fast directions in the Anza gap section with relatively simple fault geometry agree with the inferred SHmax, and the delay times at an array within that section are smaller than those observed at other across-fault arrays. These indications of less pronounced damage zone in the Anza section compared to other segments of the SJFZ are correlated generally with geometrical properties of the surface traces. Significant variations of fast directions on several across-fault arrays, with station spacing on the orders of a few tens of meters, suggest that shallow fault structures and near-surface layers play an important role in controlling the SWS parameters.

  18. Constraints on strain rates during large-scale mid-crustal shearing: An example from the basal Vaddas shear zone, northern Caledonides

    NASA Astrophysics Data System (ADS)

    Gasser, Deta; Stünitz, Holger; Nasipuri, Pritam; Menegon, Luca

    2013-04-01

    The Caledonian orogen in Scandinavia is characterized by large-scale crustal nappe stacks which were emplaced east-/southeast-wards onto the Baltica shield. Whereas original thrust relationships are generally obscured by syn- to post-collisional extensional deformation in the southern and central Scandinavian Caledonides, several large-scale thrust systems are well-preserved in the northern Scandinavian Caledonides in Troms and Finnmark. One example is the mid-crustal Vaddas shear zone, which emplaced the Vaddas nappe on top of the Kalak nappe complex. In this contribution we present a structural, petrological and geochronological analysis of the rocks under- and overlying the Vaddas shear zone in northern Troms, in order to estimate the strain rate associated with thrusting along this major shear zone. The Vaddas nappe above the investigated shear zone consists mainly of Upper Ordovician to Silurian metasediments, which were deposited in a marine environment and which were intruded by voluminous gabbroic intrusions, before they were sheared off from their substratum and transported on top of the Kalak nappe complex during the Caledonian orogeny. PT conditions from one of these gabbroic bodies indicate that the body intruded the metasediments at ~9 kbar (Getsinger et al., subm to G3), which corresponds to a depth of ~34 km. U-Pb SIMS dating of zircons from this gabbro indicate that intrusion occurred at 439±2 Ma. The Vaddas nappe is separated from the Kalak nappe by an at least ~150 m thick, amphibolite-facies shear zone with a subhorizontal fabric and top-to-the-SE shear sense. It has developed within the lowest part of the Vaddas nappe as well as the upper part of the Kalak nappe complex and PT calculations indicate that final shearing occurred at ~450° C and ~6 kbar (depth of ~23 km). U-Pb TIMS dating of titanites, which grow parallel to the shear fabric in the Kalak nappe complex, gives 206Pb/238U ages ranging from 442±1 to 429±1 Ma, indicating that

  19. The influence of metasomatic reactions on distributed vs. localized slip in ultramafic shear zones

    NASA Astrophysics Data System (ADS)

    Tarling, Matthew S.; Tulley, Chris J.; Smith, Steven A. F.

    2016-04-01

    The Livingstone Fault is a >1000 km long terrane boundary in New Zealand that juxtaposes ultramafic rocks of the Dun Mountain Ophiolite Belt against quartzofeldspathic rocks of the continental Caples Terrane. The fault is characterized by a zone of sheared serpentinite mélange tens to several hundreds of meters wide with a generally well-defined scaly fabric, containing entrained pods of massive serpentinite, volcanic rocks and quartzofeldspathic rocks. Talc- and tremolite- forming metasomatic reactions occurred frequently within the mélange zone, along the margins of the mélange and at the edges of entrained pods. These reactions were the result of the interaction between the serpentine minerals and silica bearing fluids derived from the quartzofeldspathic Caples Terrane. In the bulk of the mélange, structures such as distributed scaly fabrics, S-C fabrics, and networks of fibrous serpentine veins suggest a broad delocalization of strain, likely accommodated by pressure-solution mechanisms along the serpentinite- and talc-bearing fabrics. However, at the margins of the mélange zone and the edges of pods, layers of tremolite tens of centimeters thick are characterized by a highly indurated microstructure consisting of networks of tightly interwoven, acicular tremolite crystals forming a semi-nephritic to nephritic texture. In these metasomatic regions, discrete cataclastic slip zones associated with well-polished slickenlined surfaces are observed at the interfaces of the serpentinite and Caples Terrane quartzofeldspathics. In the Livingstone Fault, this style of highly-localized slip is uniquely associated withthe development of the indurated nephritic textures. Because tremolite is a frictionally-strong and generally velocity-weakening calc-silicate, we speculate that the tremolite-forming metasomatic reactions may have promoted localized and unstable fault slip within a shear zone that was otherwise deforming by creep. Employing scanning and transmission

  20. Deep heterogeneity of the stress state in the horizontal shear zones

    NASA Astrophysics Data System (ADS)

    Rebetsky, Yu. L.; Mikhailova, A. V.

    2014-11-01

    The formation structures of brittle destruction in a rock layer above an active strike-slip fault in the crystalline basement is studied. The problem is analyzed from the standpoint of loading history, when after the stage of pure gravitational loading, an additional strain state of uniform horizontal shear of both the layer and underlying basement develops, which is further followed by a vertically nonuniform shear caused by the activation of the deep fault. For the studied object, irreversible fracture deformations on macro- and microlevels arise as early as the initial stage of loading under the action of gravitational stresses. These deformations continue evolving on the megascopic level in the course of horizontal shearing that is quasi-uniform both along the depth and laterally. The final formation of the structural ensemble occurs after a long stage of horizontal displacement of the blocks of the crystalline basement—the stage of localized shear. The theoretical analysis of the evolution of the stress state and morphology of the failure structures established the presence of numerous fractures with the normal dip-slip components in the intermediate-depth part of the rock mass, which are formed at the stages of uniform and localized horizontal shearing. The fractures with a strike-slip component mainly arise in the upper and near-axial deep parts of the section.

  1. Shear-wave splitting in Quaternary sediments: Neotectonic implications in the central New Madrid seismic zone

    USGS Publications Warehouse

    Harris, J.B.

    1996-01-01

    Determining the extent and location of surface/near-surface structural deformation in the New Madrid seismic zone (NMSZ) is very important for evaluating earthquake hazards. A shallow shear-wave splitting experiment, located near the crest of the Lake County uplift (LCU) in the central NMSZ, shows the presence of near-surface azimuthal anisotropy believed to be associated with neotectonic deformation. A shallow fourcomponent data set, recorded using a hammer and mass source, displayed abundant shallow reflection energy on records made with orthogonal source-receiver orientations, an indicator of shear-wave splitting. Following rotation of the data matrix by 40??, the S1 and S2 sections (principal components of the data matrix) were aligned with the natural coordinate system at orientations of N35??W and N55??E, respectively. A dynamic mis-tie of 8 ms at a two-way traveltime of 375 ms produced an average azimuthal anisotropy of ???2% between the target reflector (top of Quaternary gravel at a depth of 35 m) and the surface. Based on the shear-wave polarization data, two explanations for the azimuthal anisotropy in the study area are (1) fractures/cracks aligned in response to near-surface tensional stress produced by uplift of the LCU, and (2) faults/fractures oriented parallel to the Kentucky Bend scarp, a recently identified surface deformation feature believed to be associated with contemporary seismicity in the central NMSZ. In addition to increased seismic resolution by the use of shear-wave methods in unconsolidated, water-saturated sediments, measurement of near-surface directional polarizations, produced by shear-wave splitting, may provide valuable information for identifying neotectonic deformation and evaluating associated earthquake hazards.

  2. The role of discrete intrabasement shear zones during multiphase continental rifting

    NASA Astrophysics Data System (ADS)

    Phillips, Thomas B.; Jackson, Christopher A.-L.; Bell, Rebecca E.; Duffy, Oliver B.; Fossen, Haakon

    2016-04-01

    Rift systems form within areas of relatively weak, heterogeneous lithosphere, containing a range of pre-existing structures imparted from previous tectonic events. The extent to which these structures may reactivate during later rift phases, and therefore affect the geometry and evolution of superposed rift systems, is poorly understood. The greatest obstacle to understanding how intrabasement structures influence the overlying rift is obtaining detailed constraints on the origin and 3D geometry of structures within crystalline basement. Such structures are often deeply buried beneath rift systems and therefore rarely sampled directly. In addition, due to relatively low internal acoustic impedance contrasts and large burial depths, crystalline basement typically appears acoustically transparent on seismic reflection data showing no resolvable internal structure. However, offshore SW Norway, beneath the Egersund Basin, intrabasement structures are exceptionally well-imaged due to large impedance contrasts within a highly heterogeneous and shallow basement. We use borehole-constrained 2D and 3D seismic reflection data to constrain the 3D geometry of these intrabasement reflections, and examine their interactions with the overlying rift system. Two types of intrabasement structure are observed: (i) thin (c. 100 m) reflections displaying a characteristic trough-peak-trough wavetrain; and (ii) thick (c. 1 km), sub-parallel reflection packages dipping at c. 30°. Through 1D waveform modelling we show that these reflection patterns arise from a layered sequence as opposed to a single interface. Integrating this with our seismic mapping we correlate these structures to the established onshore geology; specifically layered mylonites associated with the Caledonian thrust belt and cross-cutting extensional Devonian shear zones. We observe multiple phases of reactivation along these structures throughout multiple rift events, in addition to a range of interactions with

  3. Investigation of the logarithmic model applied to bed shear stresses in the swash zone

    NASA Astrophysics Data System (ADS)

    Allis, M.; Blenkinsopp, C. E.; Turner, I. L.; Baldock, T. E.; Puleo, J. A.

    2014-12-01

    Accurate understanding of beach face sediment transport in the swash zone is essential to improve existing models for predicting beach morphological changes. In the swash zone, bed shear stresses are the dominant driving mechanism of both bed-load and suspended-load sediment transport. A detailed comparison is presented of swash zone bed shear stresses obtained from direct measurements and velocimetry derived estimates, as measured in the large-scale GWK wave flume facility in Hannover, Germany. Bed shear stresses were measured directly by flush mounted shear plates and estimated using the logarithmic model for velocity profiles obtained from Acoustic Doppler Velocity Profilers (ADVP). The swashes measured were generated by large-scale (H > 0.9m, T > 8s) monochromatic and solitary waves on a planar fixed-bed beach with a rough surface (d50 = 4.6mm). The logarithmic model and its application to swash flows are investigated in detail for the ensemble and individual swash events. The results confirm the concerns of others about log-law suitability in the swash zone and extend the prior works to fully prototype scale. The logarithmic model proves reasonably valid in uprush but increasing invalid through backwash where there is clear evidence of a systematic departure from log-law theory. The cause of the disparity is investigated and considered to be the result of unsteady hydrodynamics, free-surface pressure gradients and complex boundary layer evolution. In the latter stages of backwash the boundary layer becomes emergent further disrupting the flow, re-aerating and tending towards more complex turbulent sheet-flow behaviour. Adjustment to the depth-averaged void fraction cannot account for the magnitude of the discrepancy, indicating that the formulation of the logarithmic model itself is decreasingly valid as the flow thins and decelerates throughout backwash. Though it is conceptually appealing and relatively simple to apply, the results further confirm the

  4. The Main Shear Zone in Sør Rondane, East Antarctica: Implications for the late-Pan-African tectonic evolution of Dronning Maud Land

    NASA Astrophysics Data System (ADS)

    Ruppel, Antonia S.; Läufer, Andreas; Jacobs, Joachim; Elburg, Marlina; Krohne, Nicole; Damaske, Detlef; Lisker, Frank

    2015-06-01

    Structural investigations in western Sør Rondane, eastern Dronning Maud Land (DML), provide new insights into the tectonic evolution of East Antarctica. One of the main structural features is the approximately 120 km long and several hundred meters wide WSW-ENE trending Main Shear Zone (MSZ). It is characterized by dextral high-strain ductile deformation under peak amphibolite-facies conditions. Crosscutting relationships with dated magmatic rocks bracket the activity of the MSZ between late Ediacaran to Cambrian times (circa 560 to 530 Ma). The MSZ separates Pan-African greenschist- to granulite-facies metamorphic rocks with "East African" affinities in the north from a Rayner-age early Neoproterozoic gabbro-tonalite-trondhjemite-granodiorite complex with "Indo-Antarctic" affinities in the south. It is interpreted to represent an important lithotectonic strike-slip boundary at a position close to the eastern margin of the East African-Antarctic Orogen (EAAO), which is assumed to be located farther south in the ice-covered region. Together with the possibly coeval left-lateral South Orvin Shear Zone in central DML, the MSZ may be related to NE directed lateral escape of the EAAO, whereas the Heimefront Shear Zone and South Kirwanveggen Shear Zone of western DML are part of the south directed branch of this bilateral system.

  5. Is There any Relationship Between Active Tabriz Fault Zone and Bozkush Fault Zones, NW Iran?

    NASA Astrophysics Data System (ADS)

    ISIK, V.; Saber, R.; Caglayan, A.

    2012-12-01

    Tectonic plate motions and consequent earthquakes can be actively observed along the northwestern Iran. The Tabriz fault zone (TFZ), also called the North Tabriz fault, active right-lateral strike-slip fault zone with slip rates estimated as ~8 mm/yr, has been vigorously deforming much of northwestern Iran for over the past several million years. Historical earthquakes on the TFZ consist of large magnitude, complimentary rupture length and changed the landscape of regions surrounding the fault zone. The TFZ in the city of Bostanabad is more segmented with several strands and joined by a series of WNW-ESE trending faults, called the Bozkush fault zones. The Bozkush fault zones (BFZ's) (south and north), bounding arch-shaped Bozkush mountains, generates not only hundreds of small earthquakes each year but also has provided significant earthquakes that have been historically documented. The rock units deformed within the BFZ's include Eocene-Oligocene volcanic rocks with intercalation limestone, Oligo-Miocene clastic rocks with intercalation gypsiferous marl and Plio-Quaternary volcano-sedimentary rocks, travertine and alluvium. The North and South Bozkush fault zones are characterized by development of structures typically associated with transpression. These include right-lateral strike-slip faults, thrust faults and foldings. Our field studies indicate that these zones include step to sub-vertical fault surfaces trending NW and NE with slickenlines. Slickensides preserve brittle kinematic indicators (e.g., Riedel shear patterns, slickenside marks) suggesting both dextral displacements and top-to-the-NE/NW and-SE/SW sense of shearing. Besides, mesoscopic and microscopic ductile kinematic indicators (e.g., asymmetric porphyroclasts, C/S fabrics) within Miocene gypsum marl show dextral displacements. Fault rocks along most of these faults consist of incohesive fault breccia and gauge. Adjacent to the fault contact evidence of bedding in Oligo-Miocene and Plio

  6. Neoproterozoic structural evolution of the NE-trending Ad-Damm Shear Zone, Arabian Shield, Saudi Arabia

    NASA Astrophysics Data System (ADS)

    Hamimi, Zakaria; El-Sawy, El-Sawy K.; El-Fakharani, Abdelhamid; Matsah, Mohamed; Shujoon, Abdulrahman; El-Shafei, Mohamed K.

    2014-11-01

    The Ad-Damm Shear Zone (AdSZ) is a major NE- (to NNE-) trending fault zone separating Jiddah and Asir tectonic terranes in the Neoproterozoic Juvenile Arabian Shield (AS). AdSZ is characterized by the development of dextral transcurrent shear-sense indicators and moderately to steeply NW plunging stretching lineations. It is mainly developed under high amphibolite-to greenschist-facies conditions and extends ∼380 km, with an average width ∼2-4 km, from the conspicuous Ruwah Fault Zone in the eastern shield to the Red Sea Coastal plain. It was believed to be one of the conjugate shears of the NW- to NNW-trending sinistral Najd Shear System. This assumption is, based on the noteworthy dextral shear criteria recorded within the 620 Ma mylonitic granite of No'man Complex. A total shear-zone strike length exceeding 117 km is carefully investigated during this study to reconstruct its structural evolution. Shear-sense indicators and other field observations including overprinting relations clearly demonstrate a complicated Neoproterozoic history of AdSZ, involving at least three phases of deformations (D1-D3). Both D1 and D2 phases were of contractional regime. During D1 phase a NW-SE compression led to the formation of NE-oriented low-angle thrusts and tight-overturned folds. D2 is represented by a NE-SW stress oriented that led to the development of an open folding. D3 is expressed by the NE-SW intensive dextral transcurrent brittle-ductile shearing. It is overprinting the early formed fabrics and played a significant role in the creation of AdSZ and the mega-scale related folds. Such deformation history reflects the same Neoproterozoic deformation regime recognized in the NE-trending shear zones in the Arabian-Nubian Shield (ANS).

  7. High temperature pseudotachylytes and ductile shear zones in dry rocks from the continental lower crust (Lofoten, Norway)

    NASA Astrophysics Data System (ADS)

    Menegon, Luca; Pennacchioni, Giorgio; Harris, Katherine; Wood, Elliot

    2014-05-01

    Understanding the mechanisms of initiation and growth of shear zones under lower crustal conditions is of fundamental importance when assessing lithosphere rheology and strength. In this study we investigate brittle-ductile shear zones developed under lower crustal conditions in anorthosites from Nusfjord, Lofoten (northern Norway). Steep ductile shear zones trend E-W to ESE-WSW and have a stretching lineation plunging steeply to the SSW or SSE. The shear sense is normal (south block down to the south) as indicated by SC and SC' fabrics and sigmoidal foliations. The shear zone show a mylonitic to ultramylonitic fabric, sharp boundaries to the host anorthosites, and abundant anastomosing dark fine-grained layers along the main foliation. The fine-grained layers localized much of the strain. Relatively lower strain domains within or adjacent to shear zones indicate that the fine dark bands of mylonites represent transposed pseudotachylyte which still locally preserve the pristine structures such as chilled margins, breccia textures with angular clasts of the host rock and injection veins; intersecting veins of pseudotachylyte record multiple stages of seismic slip. The orientation of injection veins and marker offset along the most preserved pseudotachylyte fault veins indicate approximately a sinistral strike slip kinematic during faulting event responsible for the friction-induced melting. These observations indicate that ductile shear zones exploited pre-existing brittle fault zones including a network of pseudotachylytes, and that the fine-grained "ultramylonites" derive from former fine-grained pseudotachylytes. The pseudotachylyte microstructure is dominated by plagioclase microlites dispersed in a groundmass of fine-grained clinopyroxene. Clinopyroxene recrystallizes in the damage zone flanking the pseudotachylytes, indicating high metamorphic grade during pseudotachylyte formation. Small idioblastic or cauliflower garnet are scattered through the matrix and

  8. Shear Wave Splitting from Local Earthquakes in the New Madrid Seismic Zone

    NASA Astrophysics Data System (ADS)

    Martin, P.; Arroucau, P.; Vlahovic, G.

    2012-12-01

    In this study we investigate crustal anisotropy in the New Madrid seismic zone (NMSZ), by analyzing shear wave splitting from local earthquake data. The NMSZ is centrally located in the United States, spanning portions of western Tennessee, northeastern Arkansas, and southeastern Missouri. The NMSZ is also the location in which three of the largest known earthquakes took place in North America, occurring in 1811-1812. Although many seismic studies have been performed in this region, there is no consensus about which driving mechanism could satisfy both the current observations, as well as the historically observed seismicity. Therefore, it is important to continue investigating the NMSZ, to gain a better understanding of its seismicity, and the possible mechanisms that drive it. The automated technique developed by Savage et al. (2010) is used to perform the shear wave splitting measurements at 120 seismic stations within the NMSZ. The Center for Earthquake Research and Information (CERI) at the University of Memphis provided data for 1151 earthquakes spanning the years 2003-2011. The initial event selection was reduced to 245 earthquakes ranging in magnitude from 2.0 to 4.6, which fell within the shear wave window of one or more of the stations. The results of this study provide information about orientation of microcracks in the upper portion of the crust; future work will include analysis for temporal and spatial variations in order to assess the state of stress in the region.

  9. Ductile extensional shear zones in the lower crust of a passive margin

    NASA Astrophysics Data System (ADS)

    Clerc, Camille; Jolivet, Laurent; Ringenbach, Jean-Claude

    2015-12-01

    We describe and interpret an unpublished set of ION Geophysical seismic reflection profile showing strong organized seismic reflectors at the base of the continental crust of the Uruguayan volcanic rifted margin. We distinguish two main groups of reflectors in the lowermost continental crust. A first group, at depths ranging from 32 km below the continent to 16 km in the continent-ocean transition, comprises reflectors continuous over tens of kilometers, peculiarly visible near the mantle-crust boundary. A second group of reflectors dipping toward the ESE (oceanward) is widely distributed in the lower crust. These reflectors are slightly curved and tend to merge and become sub-parallel with the first group of reflectors. Together they draw the pattern of thick shallow-dipping top-to-the continent shear zones affecting the lower continental crust. Such sense of shear is also consistent with the continentward dip of the normal faults that control the deposition of the thick syn-tectonic volcanic formations (SDR). A major portion of the continental crust behaved in a ductile manner and recorded a component of top-to-the continent penetrative simple shear during rifting indicative of a lateral movement between the upper crust and the mantle.

  10. Shear mixing in stellar radiative zones. II. Robustness of numerical simulations

    NASA Astrophysics Data System (ADS)

    Prat, V.; Guilet, J.; Viallet, M.; Müller, E.

    2016-07-01

    Context. Recent numerical simulations suggest that the model by Zahn (1992, A&A, 265, 115) for the turbulent mixing of chemical elements due to differential rotation in stellar radiative zones is valid. Aims: We investigate the robustness of this result with respect to the numerical configuration and Reynolds number of the flow. Methods: We compare results from simulations performed with two different numerical codes, including one that uses the shearing-box formalism. We also extensively study the dependence of the turbulent diffusion coefficient on the turbulent Reynolds number. Results: The two numerical codes used in this study give consistent results. The turbulent diffusion coefficient is independent of the size of the numerical domain if at least three large turbulent structures fit in the box. Generally, the turbulent diffusion coefficient depends on the turbulent Reynolds number. However, our simulations suggest that an asymptotic regime is obtained when the turbulent Reynolds number is larger than 103. Conclusions: Shear mixing in the regime of small Péclet numbers can be investigated numerically both with shearing-box simulations and simulations using explicit forcing. Our results suggest that Zahn's model is valid at large turbulent Reynolds numbers.

  11. Helium distribution in a mantle shear zone from the Josephine Peridotite

    NASA Astrophysics Data System (ADS)

    Recanati, A.; Kurz, M. D.; Warren, J. M.; Curtice, J.

    2012-12-01

    A previous study of oceanic mylonites suggested that peridotite helium concentrations are correlated with the degree of high-temperature ductile deformation in the mantle. In order to test this result, this study combines helium measurements with characterization of the deformation state of harzburgite samples in a small (6 m width) ductile mantle shear zone from the Josephine Peridotite, Oregon, USA. All measurements were made by coupled in vacuo crushing and melting, demonstrating that most of the helium (>80%) resides within the solid phases rather than fluid or melt inclusions. The present study confirms the influence of deformation on helium contents, but only at the highest shear strain (γ>20) are helium contents significantly higher. The highest helium concentration, by roughly a factor of two, is found in the center-most sample, which also has grain size reduction by a factor of ∼4. Dislocations and sub-grain boundaries are present in all samples and do not correlate with helium concentrations. Mineralogy also appears to have a negligible influence in this shear zone, as modal mineralogy is relatively homogeneous, with all samples being harzburgites. These observations suggest that the increase in helium concentration is related to grain size reduction, with grain boundaries proposed as an additional storage site for helium in the mantle. The present data also characterize the isotopic composition of the Josephine Peridotite: 3He/4He=6.7±0.2 Ra (n=33, between 6.3 and 7.1 Ra). The presence of cosmogenic 3He in the matrix is indicated by the helium isotopic composition released by melting: 3He/4He=8.5±0.3 Ra (n=10; from 7.9 to 10.9). This corresponds to an exposure age of 10 Kyr, which is approximately concordant with the end of the last glacial maximum. Very little radiogenic helium is present in the samples, suggesting extremely low uranium and thorium contents ([U]<0.3 ppb). Helium isotope measurements in four samples outside the shear zone suggest

  12. Large-magnitude transient strain accumulation on the Blackwater fault, Eastern California shear zone

    NASA Astrophysics Data System (ADS)

    Oskin, Michael; Iriondo, Alex

    2004-04-01

    We investigate the Quaternary slip rate for the Blackwater fault, Eastern California shear zone, through mapping and geochronology of offset volcanic rocks. Basalt flows of the Black Mountains support the presence of faulting at 3.77 ± 0.11 Ma, 1.8 ± 0.1 km of subsequent slip, and a well-constrained long-term slip rate of 0.49 ± 0.04 mm/yr. Total slip diminishes northward, evidenced by a 0.3 1.8 km offset of a 7.23 ± 1.07 Ma dacite flow in the Black Hills and fault termination in the Lava Mountains, 5 km short of the Garlock fault. Slow long-term slip rate together with sparse evidence for Holocene rupture contradict predictions of rapid slip rate from tectonic geodesy. These results support the conclusion that as much as 95% of geodetic strain accumulation across the Blackwater fault, and thus from 1 to 6 mm/yr of geodetic strain measured across the Eastern California shear zone, is a transitory phenomenon. Discrepant geologic and geodetic results may indicate an increased near-term seismic hazard, but merit caution for interpretation of fault slip rates from geodesy alone.

  13. Preliminary Investigation of the Footwall Transition of the Borrego Spring Shear Zone

    NASA Astrophysics Data System (ADS)

    Vadman, M.

    2014-12-01

    The Borrego Springs Shear Zone in the Peninsular Range Batholith, California, is an eastover west thrusting regime that is part of the Eastern Peninsular Ranges Mylonite Zone. Tothe west, undeformed tonalite is dominant. Moving east, tonalite becomes foliated andmylonitized in places. Further east, the full transition to the footwall is unclear as it is coveredby Quaternary deposits. While roughly continuous on a regional level, there is no clear cutband of mylonite that can be followed locally for greater than several hundred meters. Themylonitized tonalite shows lower amphibolite to upper greenschist facies metamorphism,which may indicate why there is no local uninterrupted band of mylonitization, as itmetamorphosed at the brittle/ductile transition. Dated at ~92MA, it is surmised thatdeformation ended no later than 65-70MA due to 40Ar/40K dates from biotite in themylonitized tonalite. Felsic pegmatite dikes also cut through the area, showing evidence ofductile shear in some places, but again, not consistently throughout the region. Detailed mapping of the area shows a synformal structure of tonalitic mylonite betweenundeformed tonalite. Foliations are roughly perpendicular to the hinge line of the synform.This evidence, along with intermittent mylonitization, indicates non-uniform deformation. Noevidence of overprinting was seen in the synform, suggesting that the fold formed after themajority of ductile deformation.

  14. The Main Shear Zone in Sør Rondane: A key feature for reconstructing the geodynamic evolution of East Antarctica

    NASA Astrophysics Data System (ADS)

    Ruppel, Antonia; Läufer, Andreas; Lisker, Frank; Jacobs, Joachim; Elburg, Marlina; Damaske, Detlef; Lucka, Nicole

    2013-04-01

    Structural investigations were carried out along the Main Shear Zone (MSZ) of western Sør Rondane (22°-25°E, 71.5°-72.5°S) to gain new information about the position of the East-/West-Gondwana suture and the ancient plate tectonic configuration during Gondwana amalgamation. The WSW-ENE striking MSZ divides south-western Sør Rondane in a northern amphibolite-facies terrane and a southern tonalite-trondhjemite-granodiorite (TTG) terrane. The structure can be traced over a distance of ca. 100 km and reaches several hundred meters in width. It is characterized by a right-lateral sense of movement and marked by a transpressional and also transtensional regime. Ductilely deformed granitoids (ca. 560 Ma: SHRIMP U-Pb of zircon) and ductile - brittle structures, which evolved in a transitional ductile to brittle regime in an undeformed syenite (ca. 499-459 Ma, Ar-Ar mica), provide a late Proterozoic/ early Paleozoic time limit for the activity of the shear zone (Shiraishi et al., 2008; Shiraishi et al., 1997). Documentation of ductile and brittle deformation allows reconstructing up to eight deformation stages. Cross-cutting relationships of structural features mapped in the field complemented by published kinematic data reveal the following relative age succession: [i] Dn+1 - formation of the main foliation during peak metamorphism, [ii] Dn+2 - isoclinal, intrafolial folding of the main foliation, mostly foliation-parallel mylonitic shear zones (1-2 meter thick), [iii] Dn+3 - formation of tight to closed folds, [iv] Dn+4 - formation of relatively upright, large-scale open folds, [v] Dn+5 - granitoid intrusion (e.g. Vengen granite), [vi] Dn+6 - dextral shearing between amphibolite and TTG terranes, formation of the MSZ, [vii] Dn+7 - intrusion of late- to post-tectonic granitoids, first stage of brittle deformation (late shearing along MSZ), intrusion of post-kinematic mafic dykes, [viii] Dn+8 - second stage of brittle deformation including formation of conjugate fault

  15. An apparent shear zone trending north-northwest across the Mojave Desert into Owens Valley, eastern California

    SciTech Connect

    Savage, J.C.; Lisowski, M.; Prescott, W.H. )

    1990-11-01

    Strain rates measured at four geodetic networks in eastern California situated between northern Owens Valley and Transverse Ranges along a small circle drawn about the Pacific-North America pole of rotation are remarkably consistent. Each exhibits 0.14 {mu}rad/yr simple right-lateral engineering-shear-strain accumulation across the local vertical plane tangent to the small circle. Local faults (e.g., Owens Valley, Garlock, Helendale) traversing these networks are not as closely aligned with the vertical planes of maximum shear-strain accumulation as is the local tangent to the small circle. A fifth network slightly east of the small circle shows no significant strain accumulation. Thus, a shear zone trending N35{degree}W from near the eastern end of the big bend of the San Andreas fault to northern Owens Valley is indicated by these data. This corresponds to the Eastern California shear zone proposed on geological evidence by Dokka and Travis. The shear zone carries {approximately}8 mm/yr of the Pacific-North America relative plate motion from the San Andreas fault north-northwest across the Mojave Desert into Owens Valley and the northern Basin and Range province. The shear zone observed at the surface may be a manifestation of a through-going subcrustal fault.

  16. A brittle (normal?) shear zone cored in Site C0002 of Nankai Trough Seismogenic Zone Experiment (IODP Expedition 348)

    NASA Astrophysics Data System (ADS)

    Crespo-Blanc, Ana; Sample, James; Brown, Kevin; Otsubo, Makoto; Yamamoto, Yuzuru

    2016-04-01

    Integrated Ocean Discovery Program (IODP) Expedition 348, which belongs to the Nankai Trough Seismogenic Zone Experiment, conducted riser-drilling to make deeper an existing hole at Site C0002, up to 3058.5 meters below seafloor (mbsf). This site is located 80 km SE of the Kii Peninsula (Japan) in the Kumano forearc basin, in turn situated on top of the Nankai accretionary prism. Cuttings (875.5-3058.5 mbsf) and cores (2163.0-2217.5 mbsf) were collected in the upper Miocene to Pliocene turbiditic silty claystone with few intercalations of sandstone which characterize the accretionary prism lithological units. A remarkably preserved fault zone has been cored around 2205 mbsf (core section Hole C0002P-348-5R-4). It is characterized by 34 cm of fault breccia, in which an anastomosed cataclastic foliation is present. The rocks of the damaged zone are formed by silty claystone with an incipient scaly fabric and scarce levels of sandstones. Extra-large thin sections were made along the whole core section. In the brittle shear zone, they reveal a catalogue of deformation structures characteristic of a high structural level. In particular, almond-type structures and arrays of microfaults cutting the stratification are the most common structures and outline the cataclastic foliation. The occurrence of calcite veins in the recovered cores is limited to this fault zone, which is indicative of its role as fluid path, accompanied by carbonate cementation. Generally fault veins have lower δ18O values than carbonate cements in the sedimentary matrix, consistent with veins forming at higher temperatures and/or from a fluid more strongly depleted in 18O. A continuum of the relationships between calcite veins and cataclastic deformation is observed, from veins that precipitated early in the fault history, with calcite grains broken during subsequent deformation, to late veins which seal the almond-type structures within the claystones. The geometry of the calcite grains within the

  17. Shear zones in the upper mantle - relation between geochemical enrichment and deformation in mantle peridotites

    SciTech Connect

    Downes, H. )

    1990-04-01

    Textural variations in mantle-derived spinel peridotites have previously been interpreted as evidence of the existence of asthenospheric mantle diapirs, indicating deformational heterogeneity on a large lateral scale (kilometers to tens of kilometers). However, many volcanic vents entrain both deformed and undeformed xenoliths, and field relations in peridotite massifs show the scale of alternation between deformed and undeformed peridotite to be small (centimeters to meters) because of the presence of numerous lithospheric shear zones. Some rare xenoliths contain both deformed and undeformed peridotite. These is also an apparent relation between deformation and the growth of metasomatic minerals; amphibole is often concentrated in strongly deformed zones in peridotite massifs and deformed xenoliths, although it is also found in crosscutting veins and aureoles around such veins. A relation can also be seen between deformation and indicators of geochemical enrichment. Clinopyroxenes from many deformed spinel peridotites show light rate earth element (REE) enrichment, whereas clinopyroxenes from undeformed spinel peridotites commonly have mid-ocean ridge basalt (MORB)-type light REE-depleted patterns. Sr and Nd radiogenic isotopic compositions of undeformed peridotites are generally MORB-like, and have low {epsilon}Sr and high {epsilon}Nd. In contrast, deformed peridotites, with or without amphibole, often have higher {epsilon}Sr and low {epsilon}Nd values, indicating geochemical enrichment by large ion lithophile (LIL)- and light REE-enriched fluids or melts. These observations can be used to infer that the shallow mantle contains lithospheric ductile shear zones in which metasomatic fluids precipitated amphibole and clinopyroxene.

  18. Structural history of the crustal-scale Coast shear zone north of Portland Canal, southeast Alaska and British Columbia

    NASA Astrophysics Data System (ADS)

    Klepeis, Keith A.; Crawford, Maria Luisa; Gehrels, George

    1998-07-01

    Structural, metamorphic and U-Pb geochronologic data reveal how a steep, crustal-scale shear zone influenced the evolution of the Paleogene Coast Mountains batholith during and since its emplacement. We document two distinct stages of deformation ( DCSZ3 and DCSZ4) that produced the Coast shear zone north of Portland Inlet. Between 65 Ma and 57 Ma, deformation now preserved within the eastern side of the Coast shear zone ( DCSZ3) produced a moderately to gently, north-northeast-dipping foliation and north-east-plunging mineral lineations. DCSZ3 involved dominantly east-side-up, top-to-the-southwest displacements during and after the intrusion of tabular tonalite and granodiorite plutons. Widespread crustal thickening followed by rapid exhumation, east-side-up tilting of the batholith, and decompression of rocks equilibrating at 5.6±0.4 kbars, 710±30°C occurred at this time. Prior to DCSZ3, deformation ( DWTB1-2) now preserved west of the Coast shear zone resulted in tectonic imbrication of lithologically distinctive crustal fragments at 8-9 kbars, and west- to southwest-vergent ductile thrust faults before ˜92 Ma. From ˜57 Ma to 55 Ma, deformation in the western Coast shear zone ( DCSZ4) produced a narrow, 1-2 km wide, zone comprised of a steeply-dipping to subvertical foliation that overprints and transposes all DWTB1-2 and DCSZ3 structures. DCSZ4 involved bulk east-side-down displacements parallel to a steeply-plunging, down-dip sillimanite lineation and regional tilting of the batholith. This east-side-down displacement may reflect a final period of crustal readjustment and collapse following an earlier period of crustal thickening during batholith construction. The variable history of motion within the Coast shear zone appears to reflect a response to different periods of batholith development within a convergent to obliquely-convergent continental margin.

  19. Geophysical characterization of transtensional fault systems in the Eastern California Shear Zone-Walker Lane Belt

    NASA Astrophysics Data System (ADS)

    McGuire, M.; Keranen, K. M.; Stockli, D. F.; Feldman, J. D.; Keller, G. R.

    2011-12-01

    The Eastern California Shear Zone (ECSZ) and Walker Lane belt (WL) accommodate ~25% of plate motion between the North American and Pacific plates. Faults within the Mina deflection link the ECSZ and the WL, transferring strain from the Owens Valley and Death Valley-Fish Lake Valley fault systems to the transcurrent faults of the central Walker Lane. During the mid to late Miocene the majority of strain between these systems was transferred through the Silver Peak-Lone Mountain (SPLM) extensional complex via a shallowly dipping detachment. Strain transfer has since primarily migrated north to the Mina Deflection; however, high-angle faults bounding sedimentary basins and discrepancies between geodetic and geologic models indicate that the SPLM complex may still actively transfer a portion of the strain from the ECSZ to the WL on a younger set of faults. Establishing the pattern and amount of active strain transfer within the SPLM region is required for a full accounting of strain accommodation, and provides insight into strain partitioning at the basin scale within a broader transtensional zone. To map the active structures in and near Clayton Valley, within the SPLM region, we collected seismic reflection and refraction profiles and a dense grid of gravity readings that were merged with existing gravity data. The primary goals were to determine the geometry of the high-angle fault system, the amount and sense of offset along each fault set, connectivity of the faults, and the relationship of these faults to the Miocene detachment. Seismic reflection profiles imaged the high-angle basin-bounding normal faults and the detachment in both the footwall and hanging wall. The extensional basin is ~1 km deep, with a steep southeastern boundary, a gentle slope to the northwest, and a sharp boundary on the northwest side, suggestive of another fault system. Two subparallel dip-slip faults bound the southeast (deeper) basin margin with a large lateral velocity change (from ~2

  20. Imaging Water in Deformed Quartzites: Examples from Caledonian and Himalayan Shear Zones

    NASA Astrophysics Data System (ADS)

    Kronenberg, Andreas; Ashley, Kyle; Hasnan, Hasnor; Holyoke, Caleb; Jezek, Lynna; Law, Richard; Thomas, Jay

    2016-04-01

    Infrared IR measurements of OH absorption bands due to water in deformed quartz grains have been collected from major shear zones of the Caledonian and Himalayan orogens. Mean intragranular water contents were determined from the magnitude of the broad OH absorption at 3400 cm‑1 as a function of structural position, averaging over multiple grains, using an IR microscope coupled to a conventional FTIR spectrometer with apertures of 50-100 μm. Images of water content were generated by scanning areas of up to 4 mm2 of individual specimens with a 10 μm synchrotron-generated IR beam and contouring OH absorptions. Water contents vary with structural level relative to the central cores of shear zones and they vary at the grain scale corresponding to deformation and recrystallization microstructures. Gradients in quartz water content expressed over structural distances of 10 to 400 m from the centers of the Moine Thrust (Stack of Glencoul, NW Scotland), the Main Central Thrust (Sutlej valley of NW India), and the South Tibetan Detachment System (Rongbuk valley north of Mount Everest) indicate that these shear zones functioned as fluid conduits. However, the gradients differ substantially: in some cases, enhanced fluid fluxes appear to have increased quartz water contents, while in others, they served to decrease water contents. Water contents of Moine thrust quartzites appear to have been reduced during shear at greenschist facies by processes of regime II BLG/SGR dislocation creep. Intragranular water contents of the protolith 70 m below the central fault core are large (4078 ± 247 ppm, H/106 Si) while mylonites within 5 mm of the Moine hanging wall rocks have water contents of only 1570 (± 229) ppm. Water contents between these extremes vary systematically with structural level and correlate inversely with the extent of dynamic recrystallization (20 to 100%). Quartz intragranular water contents of Himalayan thrust and low-angle detachment zones sheared at upper

  1. Quantifying the effect of pyroxene on peridotite deformation in a natural shear zone

    NASA Astrophysics Data System (ADS)

    Hansen, L. N.; Warren, J. M.

    2013-12-01

    The rheological behavior of peridotites is key to understanding convective processes in the upper mantle, the long-term strength of the lithosphere, and the dynamics of tectonic plate boundaries. Much work has been dedicated to elucidating the rheological behavior of olivine aggregates, which are often assumed to behave the same as pyroxene-bearing peridotites. Some field studies, however, demonstrate that the addition of pyroxene can significantly affect deformation behavior. Unfortunately, laboratory studies disagree on the strength of pyroxene at lithospheric conditions by many orders of magnitude. Thus, it remains unclear whether pyroxene primarily affects peridotite deformation through its increased (or decreased) strength relative to olivine or through its effect on olivine microstructure. We provide constraints on the viscosity of pyroxene relative to olivine through examination of paired harzburgites and dunites from a natural shear zone exposed in the Josephine Peridotite. Shear zone P deforms centimeter- to meter-scale alternating layers of harburgites and dunites. Microstructures in paired harzburgite and dunite samples from a transect across the shear zone were analyzed by electron-backscatter diffraction to obtain mean grain size, mean subgrain size, and crystallographic fabric for olivine. Olivine grain sizes are about a factor of 1.5 larger in dunites than in harzburgites, whereas olivine subgrain sizes are about a factor of 1.2 smaller in dunites than in harzburgites. We estimate the average stress in olivine grains for each sample using a laboratory-derived subgrain-size piezometer. Average stresses in olivine are inversely correlated with pyroxene volume fraction and directly correlated with mean olivine grain size. The evolution of crystallographic fabric shape and orientation with increasing shear strain is comparable in both harzburgites and dunites, indicating the operation of similar deformation mechanisms in olivine. The ratio of pyroxene

  2. Mineralogical constraint for metamorphic conditions in a shear zone affecting the Archean Ngoulemakong tonalite, Congo craton (Southern Cameroon) and retentivity of U-Pb SHRIMP zircon dates

    NASA Astrophysics Data System (ADS)

    Tchameni, R.; Lerouge, C.; Penaye, J.; Cocherie, A.; Milesi, J. P.; Toteu, S. F.; Nsifa, N. E.

    2010-08-01

    In the Ngoulemakong region of the Ntem unit (South Cameroon), tonalite crops out as intrusions of various sizes cross-cutting the charnockite suite. Both of these granitoids are affected by NE-SW and WNW-ESE sinistral and dextral shear zones. Tonalite in the WNW-ESE shear zone are deformed and shows metamorphic assemblages represented by quartz-microcline-biotite-garnet-plagioclase-scapolite-fluoro-apatite and chlorite-sulfides-epidote-muscovite-quartz-calcite not recorded by the undeformed rocks outside it. These mineralogical assemblages provide evidence of decreasing pressure-temperature conditions from granulite-amphibolite-facies in the moderate deformed part to greenschist-facies in the central part of the shear plane. The higher fluid (H 2O, CO 2, S, F, Cl, K, and Na) activities and high-grade recrystallizations recorded in the shear zone favour a Pb loss hypothesis in the zircons. U-Pb SHRIMP zircon dating yield an emplacement age of 2865 ± 4 Ma for the tonalite but does not permit the determination of the age of the high-grade event responsible for the Pb loss. These results provide evidence of the retentivity of U-Pb zircon dates under high grade conditions.

  3. Role of the Western Anatolia Shear Zone (WASZ) in Neotectonics Evolution of the Western Anatolia Extended Terrain, Turkey

    NASA Astrophysics Data System (ADS)

    Cemen, I.; Gogus, O. H.; Hancer, M.

    2013-12-01

    The Neotectonics period in western Anatolia Extended Terrain, Turkey (WAET) may have initiated in late Oligocene following the Eocene Alpine collision which produced the Izmir-Ankara suture zone. The Western Anatolia Shear Zone (WASZ) bounds the WAET to the east. The shear zone contains mostly normal faults in the vicinity of the Gulf of Gokova. However, its movement is mostly oblique slip from the vicinity of Tavas towards the Lake of Acigol where it makes a northward bend and possibly joins the Eskisehir fault zone to the north of the town of Afyon. The shear zone forms the southern and eastern margins of the Kale-Tavas, Denizli and Acigol basins. The shear zone is similar in its structural/tectonics setting to the Eastern California Shear zone (ECSZ) of the Basins and Ranges of North America Extended terrain which is also composed of many normal to oblique-slip faults and separates two extended terrains with different rates of extension. Western Anatolia experienced many devastating earthquakes within the last 2000 years. Many of the ancient Greek/Roman city states, including Ephesus, Troy, and Hierapolis were destroyed by large historical earthquakes. During the second half of the 20th century, the region experienced two major large earthquake giving normal fault focal mechanism solutions. They are the 1969, M=6.9 Alasehir and the 1970, M=7.1 Gediz earthquakes. These earthquakes had caused substantial damage and loss of life in the region. Therefore, a comprehensive understanding of the kinematics of the Cenozoic extensional tectonics and earthquake potential of the WASZ in the region, is very important, especially since the fault zone is very close to the major towns in eastern part of western Turkey, such as Mugla, Denizli, Sandikli, Dinar and Afyon.

  4. Characterizing fractures and shear zones in crystalline rock using seismic and GPR methods

    NASA Astrophysics Data System (ADS)

    Doetsch, Joseph; Jordi, Claudio; Laaksonlaita, Niko; Gischig, Valentin; Schmelzbach, Cedric; Maurer, Hansruedi

    2016-04-01

    Understanding the natural or artificially created hydraulic conductivity of a rock mass is critical for the successful exploitation of enhanced geothermal systems (EGS). The hydraulic response of fractured crystalline rock is largely governed by the spatial organization of permeable fractures. Defining the 3D geometry of these fractures and their connectivity is extremely challenging, because fractures can only be observed directly at their intersections with tunnels or boreholes. Borehole-based and tunnel-based ground-penetrating radar (GPR) and seismic measurements have the potential to image fractures and other heterogeneities between and around boreholes and tunnels, and to monitor subtle time-lapse changes in great detail. We present the analysis of data acquired in the Grimsel rock laboratory as part of the In-situ Stimulation and Circulation (ISC) experiment, in which a series of stimulation experiments have been and will be performed. The experiments in the granitic rock range from hydraulic fracturing to controlled fault-slip experiments. The aim is to obtain a better understanding of coupled seismo-hydro-mechanical processes associated with high-pressure fluid injections in crystalline rocks and their impact on permeability creation and enhancement. GPR and seismic data have been recorded to improve the geological model and characterize permeable fractures and shear zones. The acquired and processed data include reflection GPR profiles measured from tunnel walls, single-borehole GPR images, and borehole-to-borehole and tunnel-to-tunnel seismic and GPR tomograms. The reflection GPR data reveal the geometry of shear zones up to a distance of 30 m from the tunnels and boreholes, but the interpretation is complicated by the geometrical ambiguity around tunnels and boreholes and by spurious reflections from man-made structures such as boreholes. The GPR and seismic traveltime tomography results reveal brittle fractured rock between two ductile shear zones. The

  5. Grainsize evolution in ductile shear zones: Implications for strain localization and the strength of the lithosphere

    NASA Astrophysics Data System (ADS)

    Platt, J. P.; Behr, W. M.

    2011-04-01

    At high stresses and low temperatures, grainsize reduction by dynamic recrystallization profoundly modifies rock rheology. Strain energy driven grain-boundary migration (ρGBM) is involved both in the nucleation of new grains by the grain-boundary bulging mechanism (BLG), and in the subsequent evolution of the microstructure. Above the Dmin line, which is a line in stress/grainsize space that defines the minimum size of nucleus that can form by BLG, ρGBM dominates the microstructure, and grain growth by surface energy driven grain-boundary migration (γGBM) is inhibited. The recrystallized grainsize is therefore dominated by the nucleation process, possibly controlled by the size of subgrains or dislocation cells within the old grains. This provides a first-order explanation for the experimentally observed grainsize-stress relationship. ρGBM is an important agent of recovery in rocks deformed by dislocation creep, sweeping out dislocations and counteracting work-hardening. We have derived a new flow law (DRX-assisted dislocation creep) based on this process, which exhibits grainsize sensitivity as a result of the role of ρGBM. If grainsize obeys the empirically-determined grainsize-stress relationship, DRX creep has an effective stress exponent of a little over 4, consistent with experimental observations and inferences from naturally deformed rocks. DRX creep may be an important agent in weakening quartz at low temperatures, whereas current flow law data suggest it may not be important in olivine. Rocks deformed and dynamically recrystallized above the Dmin line may switch from climb-assisted dislocation creep to grainsize-sensitive creep (Coble creep, DRX creep, or creep dominated by grain-boundary sliding), resulting in weakening. Lithospheric-scale shear zones are likely to evolve at approximately constant stress; under these conditions weakening results in an increase in strain rate, not a stress drop. The rate of dislocation motion, the dislocation density

  6. Effect of Machining on Shear-Zone Microstructure in Ti-15V-3Cr-3Al-3Sn: Conventional and Ultrasonically Assisted Turning

    NASA Astrophysics Data System (ADS)

    Shi, Q.; Tse, Y. Y.; Muhammad, R.; Roy, A.; Silberschmidt, V. V.; Higginson, R. L.

    2016-07-01

    This work systematically studied morphology of nano- and microstructures in primary and secondary shear zones of machining chips produced with two different machining methods: conventional and ultrasonically assisted turning. Electron backscatter diffraction and transmission electron microscopy showed that chips had similar microstructures for both machining techniques. The nanostructure in secondary shear zones was less homogeneous than that in primary shear zones. In addition, a heavily deformed layer was formed in a subsurface of Ti-15V-3Cr-3Al-3Sn work-pieces, replicating the microstructure of secondary shear zones of the machining chips, and elongated nanocrystalline grains in this layer were aligned with a tangential direction of turning.

  7. Channelized Fluid Flow and Metasomatism in Subducted Oceanic Lithosphere recorded in an Eclogite-facies Shear Zone (Monviso Ophiolite, Italy)

    NASA Astrophysics Data System (ADS)

    Angiboust, S.; Agard, P.; Pettke, T.

    2012-04-01

    The Monviso ophiolite Lago Superiore Unit (LSU) constitutes a well-preserved, almost continuous fragment of upper oceanic lithosphere subducted down to ca. 80 km (between 50 and 40 Ma) and later exhumed along the subduction interface. The LSU is made of (i) a variably thick (50-500 m) section of eclogitized mafic crust (associated with minor calcschist lenses) overlying a 100-400 m thick metagabbroic body, and of (ii) a serpentinite sole (ca. 1000 m thick). This section is cut by two 10 to 100m thick eclogite-facies shear zones, found at the boundary between basalts and gabbros (Intermediate Shear Zone: ISZ), and between gabbros and serpentinites (Lower Shear Zone: LSZ). Fragments of mylonitic basaltic eclogites and marbles were dragged and dismembered within serpentinite schists along the LSZ during eclogite-facies deformation [Angiboust et al., Lithos, 2011]. Metasomatic rinds formed on these fragments at the contact with the surrounding antigorite schists during lawsonite-eclogite facies metamorphism, testifying to prominent fluid-rock interaction along with deformation. We present new petrological and geochemical data on four types of metasomatically altered eclogites (talc-, chlorite-, lawsonite- and phengite-bearing eclogites) and on a (serpentinite-derived) talc schist from the block rind. Bulk-rock compositions, in situ LA-ICP-MS analysis and X-ray Cr/Mg maps of garnet demonstrate that (i) these samples underwent significant B, Cr, Mg, Ni and Co enrichment and Fe, V and As depletion during eclogite-facies metamorphism (while Li and Pb behaved inconsistently) and (ii) garnet composition and chemistry of inclusions show extreme variation from core to rim. These compositional patterns point to a massive, pulse-like, fluid-mediated element transfer along with deformation, originating from the surrounding serpentinite (locally, with contributions from metasediments-equilibrated fluids). Antigorite breakdown, occurring ca. 10 km deeper than the maximum depth

  8. CLMSZ, Garnet Mountain area, southern California: A collisionally generated contractional shear zone

    SciTech Connect

    Bracchi, K.A.; Girty, G.H.; Girty, M.S. . Dept. of Geological Sciences)

    1993-04-01

    The Harper Creek gneiss (HCg) and Oriflamme Canyon unit (OCu) underlie the central portion of the Cuyamaca Laguna Mountains shear zone (CLMSZ) in and around Garnet Mountain, Peninsular Ranges, California, and may have been deformed during Cretaceous arc-continent collision. U-Pb zircon work and petrological and geochemical analyses suggest that in the Garnet Mountain area, the 140 Ma HCg is derived from granite and granodiorite, whereas the 122 [+-] 1 Ma OCu is a protomylonite derived from a granite. Both units appear to be per aluminous calc-alkaline magmatic arc granitoids. Mineral assemblages suggest uppermost greenschist to lower amphibolite grade conditions during deformation. In the HCg, S-1hc is a mylonitic gneissosity with a mean attitude of N11W, 60 NE. A mineral streaking lineation lies within the plane of S-1hc and has a mean attitude of 61[degree] N76E. In the OCu, S-1oc strikes about N13W and dips 52 NE and contains a mineral streaking lineation with an attitude of 49 N52E. Dextral and sinistral shear bands, S-2d and S-2s (looking NW), transect S-1hc and S-1oc. S-2d and S-2s strike subparallel to S-1. In the HCg S-2s is weakly developed and dips about 32 NE, whereas S-2d is more dominant and dips about 76 NE. On the OCu these relationships are reversed. S-2d does not cross cut S-2s: hence, the two sets of shear bands are interpreted to be conjugates reflecting NE-SW contraction and subvertical extension during collisional development of the CLMSZ.

  9. Athermal shear-transformation-zone theory of amorphous plastic deformation. II. Analysis of simulated amorphous silicon

    NASA Astrophysics Data System (ADS)

    Bouchbinder, Eran; Langer, J. S.; Procaccia, Itamar

    2007-03-01

    In the preceding paper, we developed an athermal shear-transformation-zone (STZ) theory of amorphous plasticity. Here we use this theory in an analysis of numerical simulations of plasticity in amorphous silicon by Demkowicz and Argon (DA). In addition to bulk mechanical properties, those authors observed internal features of their deforming system that challenge our theory in important ways. We propose a quasithermodynamic interpretation of their observations in which the effective disorder temperature, generated by mechanical deformation well below the glass temperature, governs the behavior of other state variables that fall in and out of equilibrium with it. Our analysis points to a limitation of either the step-strain procedure used by DA in their simulations, or the STZ theory in its ability to describe rapid transients in stress-strain curves, or perhaps to both. Once we allow for this limitation, we are able to bring our theoretical predictions into accurate agreement with the simulations.

  10. Shear wave velocity variation across the Taupo Volcanic Zone, New Zealand, from receiver function inversion

    USGS Publications Warehouse

    Bannister, S.; Bryan, C.J.; Bibby, H.M.

    2004-01-01

    The Taupo Volcanic Zone (TVZ), New Zealand is a region characterized by very high magma eruption rates and extremely high heat flow, which is manifest in high-temperature geothermal waters. The shear wave velocity structure across the region is inferred using non-linear inversion of receiver functions, which were derived from teleseismic earthquake data. Results from the non-linear inversion, and from forward synthetic modelling, indicate low S velocities at ???6- 16 km depth near the Rotorua and Reporoa calderas. We infer these low-velocity layers to represent the presence of high-level bodies of partial melt associated with the volcanism. Receiver functions at other stations are complicated by reverberations associated with near-surface sedimentary layers. The receiver function data also indicate that the Moho lies between 25 and 30 km, deeper than the 15 ?? 2 km depth previously inferred for the crust-mantle boundary beneath the TVZ. ?? 2004 RAS.

  11. Characterizing Seismic Anisotropy across the Peruvian Flat-Slab Subduction Zone: Shear Wave Splitting from PULSE

    NASA Astrophysics Data System (ADS)

    Eakin, C. M.; Long, M. D.; Beck, S. L.; Wagner, L. S.; Tavera, H.

    2013-12-01

    Although 10% of subduction zones worldwide today exhibit shallow or flat subduction, we are yet to fully understand how and why these slabs go flat. An excellent study location for such a problem is in Peru, where the largest region of flat-subduction currently exists, extending ~1500 km in length (from 3 °S to 15 °S) and ~300 km in width. Across this region we investigate the pattern of seismic anisotropy, an indicator for past and/or ongoing deformation in the upper mantle. To achieve this we conduct shear wave splitting analyzes at 40 broadband stations from the PULSE project (PerU Lithosphere and Slab Experiment). These stations were deployed for 2+ years across the southern half of the Peruvian flat-slab region. We present detailed shear wave splitting results for deep and teleseismic events, making use of a wide variety of available phases that sample the upper mantle directly beneath the stations (such as SKS, SKKS, PKS, sSKS, SKiKS, ScS and local/direct S). We analyze the variability of our results with respect to initial polarizations and ray paths, as well as spatial variability between stations as the underlying slab morphology changes. Preliminary results show predominately NW-SE fast polarizations (trench oblique to sub-parallel) over the flat-slab region east of Lima. These results are consistent with observations of more complex multi-layered anisotropy beneath a nearby permanent station (NNA). Further south, towards the transition to steeper subduction, the splitting pattern becomes increasingly dominated by null measurements. Over to the east however, beyond Cuzco, where the mantle wedge might begin to play a role, we record fast polarizations quasi-parallel to the local slab contours. We carefully evaluate the different possible source locations within the subduction zone for this seismic anisotropy and observe increasing evidence for distinct anisotropy within the slab as well as the sub-slab mantle.

  12. Sheared active fluids: thickening, thinning, and vanishing viscosity.

    PubMed

    Giomi, Luca; Liverpool, Tanniemola B; Marchetti, M Cristina

    2010-05-01

    We analyze the behavior of a suspension of active polar particles under shear. In the absence of external forces, orientationally ordered active particles are known to exhibit a transition to a state of nonuniform polarization and spontaneous flow. Such a transition results from the interplay between elastic stresses, due to the liquid crystallinity of the suspension, and internal active stresses. In the presence of an external shear, we find an extremely rich variety of phenomena, including an effective reduction (increase) in the apparent viscosity depending on the nature of the active stresses and the flow-alignment property of the particles, as well as more exotic behaviors such as a nonmonotonic stress-strain-rate relation and yield stress for large activities. PMID:20866262

  13. Strain localization in the middle- to upper continental crust: examples from the Patos and Pernambuco shear zones (Borborema Province, NE Brazil)

    NASA Astrophysics Data System (ADS)

    Viegas, G.; Archanjo, C. J.; Hollanda, M. H.; Vauchez, A. R.

    2014-12-01

    The accommodation of deformation in the Earth's lithosphere typically results in a heterogeneous distribution of strain in the continental crust, which is a function of effective pressure, temperature and strain rate at different structural levels. In Northeast Brazil, the Borborema Province is characterized by an interconnected, crustal-scale shear zone system associated with a widespread granitic plutonism. Two of the most prominent structures of this system, the Patos and Pernambuco shear zones, are characterized by ~ 600 km long E-W striking mylonite belts in which strain localization processes are observed either in association with partial melting in the Patos strike-slip fault, or as zones of overprinting brittle-ductile deformation in the Pernambuco shear zone. Deformation mechanisms are distinct across the Patos shear zone, mainly marked by crystalline plasticity and diffusion creep in the high-temperature northern border, magmatic flow in the central region and dislocation creep coupled with microfracturing in the southern sector. The Espinho Branco migmatite (~ 565 Ma) acts as a weak rheological layer that accumulates strain in the northern portion of the fault. Alternatively, the absence of partial melting and the dominant cataclastic/plastic flow regime lead to grain-size sensitive strain localization at the southern border. The Pernambuco shear zone was nucleated at the vicinities of two granitoid batholiths at c.a. 588 Ma. Low-temperature mylonites adjacent to the batholiths show several microstructures indicating coeval activity of brittle-ductile deformation. Recent zircon U-Pb (SHRIMP) data on these mylonites yielded mean ages of ~ 539 Ma, suggesting successive events of thermal input and shearing within the structure. These features suggest that strain localization processes exert an important control on the rheology of the continental lithosphere; the accommodation of deformation in the middle crust is mainly attained by the presence of weak

  14. Analogue modelling of inclined, brittle-ductile transpression: Testing analytical models through natural shear zones (external Betics)

    NASA Astrophysics Data System (ADS)

    Barcos, L.; Díaz-Azpiroz, M.; Balanyá, J. C.; Expósito, I.; Jiménez-Bonilla, A.; Faccenna, C.

    2016-07-01

    The combination of analytical and analogue models gives new opportunities to better understand the kinematic parameters controlling the evolution of transpression zones. In this work, we carried out a set of analogue models using the kinematic parameters of transpressional deformation obtained by applying a general triclinic transpression analytical model to a tabular-shaped shear zone in the external Betic Chain (Torcal de Antequera massif). According to the results of the analytical model, we used two oblique convergence angles to reproduce the main structural and kinematic features of structural domains observed within the Torcal de Antequera massif (α = 15° for the outer domains and α = 30° for the inner domain). Two parallel inclined backstops (one fixed and the other mobile) reproduce the geometry of the shear zone walls of the natural case. Additionally, we applied digital particle image velocimetry (PIV) method to calculate the velocity field of the incremental deformation. Our results suggest that the spatial distribution of the main structures observed in the Torcal de Antequera massif reflects different modes of strain partitioning and strain localization between two domain types, which are related to the variation in the oblique convergence angle and the presence of steep planar velocity - and rheological - discontinuities (the shear zone walls in the natural case). In the 15° model, strain partitioning is simple and strain localization is high: a single narrow shear zone is developed close and parallel to the fixed backstop, bounded by strike-slip faults and internally deformed by R and P shears. In the 30° model, strain partitioning is strong, generating regularly spaced oblique-to-the backstops thrusts and strike-slip faults. At final stages of the 30° experiment, deformation affects the entire model box. Our results show that the application of analytical modelling to natural transpressive zones related to upper crustal deformation

  15. Major softening at brittle-ductile transition due to interplay between chemical and deformation processes: An insight from evolution of shear bands in the South Armorican Shear Zone

    NASA Astrophysics Data System (ADS)

    Bukovská, Zita; Jeřábek, Petr; Morales, Luiz F. G.

    2016-02-01

    The formation of S-C/C' fabrics in the South Armorican Shear Zone has been evaluated by detailed microstructural study where the focus was given to initiation and early evolution of the C/C' fabric shear bands. Our observations suggest that the S-C/C' fabrics formed at distinct temperature conditions indicating >550°C for the S fabric and 300-350°C at 100-400 MPa for the C/C' fabric shear bands. The evolving microstructure within shear bands documents switches in deformation mechanisms related to positive feedbacks between deformation and chemical processes and imposes mechanical constraints on the evolution of the brittle-ductile transition in the continental transform fault domains. Three stages of shear band evolution have been identified. Stage I corresponds to initiation of shear bands via formation of microcracks with possible yielding differential stress of up to 250 MPa. Stage II is associated with subgrain rotation recrystallization and dislocation creep of quartz and coeval dissolution-precipitation creep of microcline. Recrystallized quartz grains show continual increase in size and decrease in stress and strain rates from 94 MPa to 17-26 MPa and 1.8 × 10-1 s-1-9 × 10-17 s-1 associated with deformation partitioning into weaker microcline layer and shear band widening. The quartz mechanical data allowed us to set some constrains for coeval dissolution-precipitation of microcline which at our estimated pressure-temperature conditions suggests creep at 17-26 MPa differential stress and 1.8 × 10-15 s-1 strain rate. Stage III is characterized by localized slip along white mica bands accommodated by dislocation creep at strain rate 1.8 × 10-14 s-1 and stress 5.75 MPa. Our mechanical data point to dynamic evolution of the studied brittle-ductile transition characterized by major weakening to strengths >10 MPa. Such nonsteady state evolution may be common in crustal shear zones especially when phase transformations are involved.

  16. Applicability of Channel flow as an extrusion mechanism of the Higher Himalayan Shear Zone from Sutlej, Zanskar, Dhauliganga and Goriganga Sections, Indian Himalaya

    NASA Astrophysics Data System (ADS)

    Mukherjee, Soumyajit

    2010-05-01

    Applicability of Channel flow as an extrusion mechanism of the Higher Himalayan Shear Zone from Sutlej, Zanskar, Dhauliganga and Goriganga Sections, Indian Himalaya Soumyajit Mukherjee Department of Earth Sciences, Indian Institute of Technology Bombay Powai, Mumbai- 400076, INDIA, e-mail: soumyajitm@gmail.com Mukherjee & Koyi (1,2) evaluated the applicability of channel flow extrusion of the Higher Himalayan Shear Zone (HHSZ) in the Zanskar and the Sutlej sections based on field- and micro-structural studies, analytical- and analog models. Further work on the Dhauliganga and the Goriganga sections of the HHSZ reveal complicated structural geology that is untenable to explain simply in terms of channel flow. For example, in the former section, flexure slip folds exist in a zone spatially separated from the upper strand of the South Tibetan Detachment System (STDSU). On the other hand, in the later section, an STDSU- in the sense of Mukherjee and Koyi (1)- is absent. Instead, a steep extensional shear zone with northeasterly dipping shear plane cuts the pre-existing shear fabrics throughout the HHSZ. However, the following common structural features in the HHSZ were observed in these sections. (1) S-C fabrics are the most ubiquitous ductile shear sense indicators in field. (2) Brittle shearing along the preexisting ductile primary shear planes in a top-to-SW sense. (3) Less ubiquitous ductile compressional shearing in the upper part of the shear zone including the STDSU. (4) A phase of local brittle-ductile extension throughout the shear zone as revealed by boudins of various morphologies. (5) The shear zone is divisible into a southern non-migmatitic and a northern migmatitic zone. No special structural dissimilarity is observed across this lithological boundary. Keywords: Channel flow, Extrusion, Higher Himalaya, Structural Geology, Shear zone, Deformation References 1. Mukherjee S, Koyi HA (in press) Higher Himalayan Shear Zone, Sutlej section: structural geology

  17. Ascent and emplacement of pegmatitic melts in a major reverse shear zone (Sierras de Córdoba, Argentina)

    NASA Astrophysics Data System (ADS)

    Demartis, Manuel; Pinotti, Lucio Pedro; Coniglio, Jorge Enrique; D'Eramo, Fernando Javier; Tubía, José María; Aragón, Eugenio; Agulleiro Insúa, Leonardo Alfredo

    2011-09-01

    Ordovician to Devonian aged crustal-scale Guacha Corral shear zone (GCSZ), central Argentina, hosts rare element pegmatites of the Comechingones pegmatitic field (CPF). In the CPF an eastwards decreasing strain gradient related to the GCSZ deformation is defined, with a high strain domain (HSD) to the west and a low strain domain (LSD) to the east. Pegmatites of the CPF were emplaced in both HSD and LSD synkinematically during ductile GCSZ deformation. Two main mechanisms for pegmatitic melt ascent and emplacement are recognized: "fracture-controlled" and "magma pumping" mechanisms. The former implies fracturing generated due to simple shear deformation not related to any previous heterogeneity. With further deformation, pegmatites were emplaced in low-dip surfaces of anisotropy (C'-planes or T-fractures), that might behave as releasing bends connecting adjacent high-dip conduits or shear zones. Displacements along staggered shear zones with releasing bends induce the local development of domains with negative pressure gradients, where open spaces could form transiently attracting for the collection of buoyant melts, a mechanism similar to "magma pumping". With ongoing deformation pegmatites were progressively rotated, sheared and transposed to the mylonitic foliation. Late pegmatites emplaced by either of the two mechanisms in the HSD and the TZ have retained their original orientations.

  18. The Hurd Peak gneiss of the Long Lake shear zone, eastern Sierra Nevada, California

    SciTech Connect

    Holland, K.S.; Reed, W.E. . Dept. of Earth and Space Sciences)

    1993-04-01

    The Hurd Peak gneiss is located within the Long Lake valley of the east-central Sierra Nevada, California. This unit is the principle orthogneiss in Hathaway's (1993) Long Lake shear zone. The rock shows porphyroclasts of plagioclase and quartz, abundant mafic enclaves, and cross-cutting field associations which suggest that the gneiss had a plutonic protolith. The gneiss varies from biotite-poor nearest the contact with the Lamarck to biotite-rich nearest Long Lake. The contact zone between the gneiss and the Lamarck pluton ranges from sharp to gradational and from migmatitic to mixed, i.e., the mixed zone being greater than 50% intermingled dikes of 10 cm or greater thickness. In places this contact is marked by a quartz-free biotite hornfels approximately 5 m thick. Based on their relative deformation, at least 3 suites of aplite dikes cross-cut the gneiss, and 5 other lithologies, including basaltic, mixed, composite, andesitic, and quartz dioritic compositions, also cross-cut the gneiss. The Rb-Sr whole rock isochron age of the Hurd Peak gneiss has been determined to be 90.2 Ma. The authors interpret this isochron to be the result of mobilization of the Rb-Sr isotopic system during intrusion of the Lamarck Granodiorite (90 Ma); this may represent a regional cooling age. The initial [sup 87]/Sr[sup 86]Sr ratio of the gneiss is 0.7098, i.e., much more evolved than the surrounding plutons which have [sup 87]Sr/[sup 86]Sr ratios near 0.706. Sr model ages indicate that the protolith of the gneiss is considerably older than 90 Ma, one such calculation suggests an age of approximately 250 Ma. Single crystals of zircon have been isolated from the gneiss for U-Pb dating, and analytical work on the zircons is presently on-going.

  19. Evidence and dynamics for the change of strike-slip direction of the Changle Nanao ductile shear zone, southeastern China

    NASA Astrophysics Data System (ADS)

    Zhihong, Wang; Huafu, Lu

    1997-12-01

    The Changle-Nanao ductile shear zone was developed from a suture zone. The evidence from the ductile fabrics and mylonitic microstructures indicates that the strike-slip was sinistral during pre-collision. It became dominantly dextral in the syn-collision stage in late Early Cretaceous. The dextral strike-slip movement continued in the post-collision stage with extension as the dominant process. The strike-slip movement of the zone was strictly controlled by dynamics of collision between the Fujian (Min)-Taiwan (Tai) microcontinent and the Fujian (Min)-Zhejiang (Zhe) Mesozoic volcanic arc during the time interval of 100-120 Ma. The Min-Tai microcontinent in which the ductile shear zone developed might have been located originally to the south of its present position. The northward migration of the microcontinent had contributed to a few hundred kilometers of drift rather than a shear displacement. The real shear displacement is small due to the change of strike-slip direction from sinistral to dextral.

  20. Differential Magnetic Field Shear in an Active Region

    NASA Technical Reports Server (NTRS)

    Schmeider, B.; DeMoulin, P.; Aulanier, G.; Golub, Leon

    1997-01-01

    The three-dimensional extrapolation of magnetic field lines from a magnetogram obtained at Kitt Peak allows us to understand the global structure of the NOAA active region 6718, as observed in X-rays with the Normal Incidence X-ray Telescope (NIXT) and in Ha with the Multichannel Subtractive Double Pass spectrograph (MSDP) in Meudon on 1991 July 11. This active region was in a quiet stage. Bright X-ray loops connect plages having field strengths of approx. 300 G, while H-alpha fibriles connect penumbrae having strong spot fields to the surrounding network. Small, intense X-ray features in the moat region around a large spot, which could be called X-ray-bright points, are due mainly to the emergence of magnetic flux and merging of these fields with surrounding ones. A set of large-scale, sheared X-ray loops is observed in the central part of the active region. Based on the fit between the observed coronal structure and the field configurations (and assuming a linear force-free field), we propose a differential magnetic field shear model for this active region. The decreasing shear in outer portions of the active region may indicate a continual relaxation of the magnetic field to a lower energy state in the progressively older portions of the AR.

  1. Modeling Spatial Structure of Rock Fracture Surfaces Before and After Shear Test: A Method for Estimating Morphology of Damaged Zones

    NASA Astrophysics Data System (ADS)

    Babanouri, Nima; Karimi Nasab, Saeed

    2015-05-01

    This paper deals with the structural analysis of rock fracture roughness, and accordingly, a method is developed for estimating/predicting the post-shearing 3D geometry of the fracture surface. For this purpose, surfaces of three natural rock fractures were digitized and studied before and after the direct shear test. The variogram analysis of the surfaces indicated a strong non-linear trend in the topography data. Hence, the spatial variability of the rock fracture surfaces was decomposed to: one deterministic component, characterized by a high-order polynomial representing the large-scale undulations, and one stochastic component, described by the variogram of residuals representing the small-scale roughness. Using an image-processing technique, a total of 343 damage zones with different sizes, shapes, initial roughness characteristics, local stress fields, and/or asperity strength values were spatially located and clustered. In order to characterize the overall spatial structure of the degraded zones, the concept of the `pseudo-zonal variogram' was introduced. The results showed that the spatial continuity at the damage zones increases due to the asperity degradation. The increase in the variogram range is anisotropic and tends to be higher along the shearing. Consequently, the direction of maximum continuity rotates towards the shear direction. After modeling the evolution of the spatial structure with shearing and detecting boundaries of the degraded areas, a methodology was presented to provide a regression-kriging estimate of the morphology of sheared surfaces. The proposed method can be considered as a cost-free and reasonably accurate alternative to expensive techniques of scanning the rock fracture surface after the shear test.

  2. Metagabbro associated with the shear zone on Prins Karls Forland (Svalbard, Arctic)

    NASA Astrophysics Data System (ADS)

    Maraszewska, Maria; Manecki, Maciej; Czerny, Jerzy; Schneider, David; Myhre, Per Inge; Faehnrich, Karol; Barnes, Christopher

    2016-04-01

    Prins Karls Forland (PKF) is a N-S elongated island situated west of Spitsbergen in the Svalbard archipelago, High Arctic. The northern part of the island is dominated by siliciclastic metasediments regionally metamorphosed to greenshist facies assemblages during one distinct stage of tectonism. Amphibolite facies garnet-mica schists, mica schists, quartzites and carbonate-silicate rocks exhibiting evidence of at least two distinct, strong deformation episodes (including mylonitization) locally outcrop on the east coast of PKF, termed the Pinkie Unit. A ~1 km wide shear zone containing ductile to brittle structures and distinct outcrops of greenstones (metagabbros and greenschists), associated with magnetite ore, separates these two contrasting tectonic units. Ten samples of greenstones were collected on the slopes of Lauratzonfjellet and Boureefjellet for petrologic and geochemical analyses. Despite intense localized shearing, the metagabbros are undeformed and preserve coarse crystalline, magmatic texture, which is locally poikilitic. The primary magmatic assemblage consists of brown hornblende, plagioclase, biotite and opaque minerals, with accessory apatite and titanite. No relicts of pyroxenes are preserved. Formation of secondary uralite, sericite and chlorite is observed. Metamorphic assemblage consists of actinolite pseudomorhs after hornblende, epidote, and second generation biotite. Blue amphibole is observed in one sample from Boureefjellet; greenschists from Boureefjellet also contain fibrous blue amphibole, as well as garnets, actinolite, epidote and biotite. Some rocks sampled on Boureefjellet are more strongly deformed and exhibit probably two stages of metamorphism: amphibolite facies metamorphism resulting in blue amphibole-garnet assemblage followed by greenschist facies metamorphism resulting in actinolite-epidote-biotite paragenesis. Parallel and overlapping patterns on chondrite-normalized REE diagrams and spider diagrams indicate that these

  3. A review of porosity-generating mechanisms in crustal shear zones

    NASA Astrophysics Data System (ADS)

    Fusseis, F.; Regenauer-Lieb, K.; Revets, S.

    2009-04-01

    Knowledge of the spatiotemporal characteristics of permeability is critical for the understanding of fluid migration in rocks. In diagenetic and metamorphic rocks different porosity-generating mechanisms contribute to permeability and so influence fluid migration and fluid/rock interaction. However, little is known about their relative contributions to the porosity architecture of a rock in a tectono-metamorphic environment. This presentation reviews porosity-generating mechanisms that affect fluid migration in shear zones, the most important crustal fluid conduits, in the context of the tectonometamorphic evolution of rocks. Mechanisms that generate porosity can be classified in a) those that involve the direct action of a fluid, b) processes in which a fluid partakes or that are supported by a fluid or c) mechanism that do not involve a fluid. a) Hydraulic fracturing, where it happens through the formation of tensile fractures, occurs where pore fluid pressures equalize the combined lithostatic pressure and strength of the rock (Etheridge et al., 1984, Cox & Etheridge, 1989, Oliver, 1996). Here an internally released (devolatilisation reactions, e.g., Rumble, 1994, Hacker, 1997, Yardley, 1997 and references therein) or externally derived (infiltrating from metamorphic, magmatic or meteoric sources, Baumgartner et al., 1997, Jamtveit et al., 1997, Thompson, 1997, Gleeson et al., 2003) fluid directly causes the mechanical failure of a rock. Where a fluid is in chemical disequilibrium with a rock (undersaturated with regard to a chemical species) minerals will be dissolved, generating dissolution porosity. Rocks ‘leached' by the removal of chemical components by vast amounts of fluid are reported to lose up to 60% of their original volume (e.g., Kerrich et al., 1984, McCaig 1988). Dissolution porosity is probably an underrated porosity-generating mechanism. It can be expected along the entire metamorphic evolution, including diagenesis (Higgs et al., 2007) and

  4. Investigation of Mantle Kinematics beneath Hellenic-Subduction Zone by using Teleseismic Direct Shear Waves

    NASA Astrophysics Data System (ADS)

    Confal, Judith; Eken, Tuna; Tilmann, Frederik; Yolsal-evikbilen, Seda; Çubuk, Yeşim; Saygin, Erdinc; Taymaz, Tuncay

    2016-04-01

    Direct shear-wave splitting measurements based on the Reference Station Technique in the southern Aegean Sea revealed significant seismic anisotropy. The technique overcomes possible contamination from the source-side anisotropy on direct S-wave signals recorded at a station pair by maximizing the correlation between the seismic traces at reference and target stations after correcting the reference stations for known receiver-side anisotropy and the target stations for arbitrary splitting parameters probed via a grid search. We initially determined receiver-side anisotropy derived from SKS splitting measurements performed at four broadband stations. Following the bootstrap approach, in which only these four stations with well-constrained SKS splitting parameters are used as seeds to determine the splitting parameters of seismic stations of the EGELADOS temporary network in an iterative manner, we obtained splitting parameters at 35 stations with good-quality S-wave signals extracted from 82 teleseismic events. The fast polarization directions (φ) show a general trend of NNE-SSW orientation that ranges from 5.8° to 51.8°. Two stations in the west close to the Hellenic Trench and one in the east show N-S oriented fast polarizations. In the back-arc region three stations exhibit NE-SW orientation. Split time delays (δt) vary between 1.0s and 1.6s. Employing direct S-waves enabled more stable and reliable splitting measurements, with an average of 46 individual measurements. The overall fast polarization variations tend to be similar to those obtained from previous SKS splitting studies in the region but indicate a more consistent pattern. Splitting analyses on direct shear waves resulted in larger split time delays compared to the previous studies, possibly because they travel along a longer path in the same anisotropic structure. Observed differences between direct shear waves-derived (this study) and previous SKS splitting measurements could be due to the fact

  5. The lateral boundary of a metamorphic core complex: The Moutsounas shear zone on Naxos, Cyclades, Greece☆

    PubMed Central

    Cao, Shuyun; Neubauer, Franz; Bernroider, Manfred; Liu, Junlai

    2013-01-01

    We describe the structure, microstructures, texture and paleopiezometry of quartz-rich phyllites and marbles along N-trending Moutsounas shear zone at the eastern margin of the Naxos metamorphic core complex (MCC). Fabrics consistently indicate a top-to-the-NNE non-coaxial shear and formed during the main stage of updoming and exhumation between ca. 14 and 11 Ma of the Naxos MCC. The main stage of exhumation postdates the deposition of overlying Miocene sedimentary successions and predates the overlying Upper Miocene/Pliocene conglomerates. Detailed microstructural and textural analysis reveals that the movement along the Moutsounas shear zone is associated with a retrograde greenschist to subgreenschist facies overprint of the early higher-temperature rocks. Paleopiezometry on recrystallized quartz and calcite yields differential stresses of 20–77 MPa and a strain rate of 10−15–10−13 s−1 at 350 °C for quartz and ca. 300 °C for calcite. Chlorite geothermometry of the shear zone yields two temperature regimes, 300–360 °C, and 200–250 °C. The lower temperature group is interpreted to result from late-stage hydrothermal overprint. PMID:26523079

  6. Simulations of a stretching bar using a plasticity model from the shear transformation zone theory

    SciTech Connect

    Rycroft, Chris H.; Gibou, Frederic

    2010-06-05

    An Eulerian simulation is developed to study an elastoplastic model of amorphous materials that is based upon the shear transformation zone theory developed by Langer and coworkers. In this theory, plastic deformation is controlled by an effective temperature that measures the amount of configurational disorder in the material. The simulation is used to model ductile fracture in a stretching bar that initially contains a small notch, and the effects of many of the model parameters are examined. The simulation tracks the shape of the bar using the level set method. Within the bar, a finite difference discretization is employed that makes use of the essentially non-oscillatory (ENO) scheme. The system of equations is moderately stiff due to the presence of large elastic constants, and one of the key numerical challenges is to accurately track the level set and construct extrapolated field values for use in boundary conditions. A new approach to field extrapolation is discussed that is second order accurate and requires a constant amount of work per gridpoint.

  7. The Eastern California Shear Zone as the northward extension of the southern San Andreas Fault

    NASA Astrophysics Data System (ADS)

    Thatcher, W.; Savage, J. C.; Simpson, R. W.

    2016-04-01

    Cluster analysis offers an agnostic way to organize and explore features of the current GPS velocity field without reference to geologic information or physical models using information only contained in the velocity field itself. We have used cluster analysis of the Southern California Global Positioning System (GPS) velocity field to determine the partitioning of Pacific-North America relative motion onto major regional faults. Our results indicate the large-scale kinematics of the region is best described with two boundaries of high velocity gradient, one centered on the Coachella section of the San Andreas Fault and the Eastern California Shear Zone and the other defined by the San Jacinto Fault south of Cajon Pass and the San Andreas Fault farther north. The ~120 km long strand of the San Andreas between Cajon Pass and Coachella Valley (often termed the San Bernardino and San Gorgonio sections) is thus currently of secondary importance and carries lesser amounts of slip over most or all of its length. We show these first order results are present in maps of the smoothed GPS velocity field itself. They are also generally consistent with currently available, loosely bounded geologic and geodetic fault slip rate estimates that alone do not provide useful constraints on the large-scale partitioning we show here. Our analysis does not preclude the existence of smaller blocks and more block boundaries in Southern California. However, attempts to identify smaller blocks along and adjacent to the San Gorgonio section were not successful.

  8. Simulations of a stretching bar using a plasticity model from the shear transformation zone theory

    NASA Astrophysics Data System (ADS)

    Rycroft, Chris H.; Gibou, Frédéric

    2012-03-01

    An Eulerian simulation framework is developed to study an elastoplastic model of amorphous materials that is based upon the shear transformation zone (STZ) theory developed by Langer and coworkers [1]. In this theory, plastic deformation is controlled by an effective temperature that measures the amount of configurational disorder in the material. The simulation is used to model ductile fracture in a stretching bar that initially contains a small notch, and the effects of many of the model parameters are examined. The simulation tracks the shape of the bar using the level set method. Within the bar, a finite difference discretization is employed that makes use of the essentially non-oscillatory (ENO) scheme. The system of equations is moderately stiff due to the presence of large elastic constants, and one of the key numerical challenges is to accurately track the level set and construct extrapolated field values for use in boundary conditions. A new approach to field extrapolation is discussed that is second-order accurate and requires a constant amount of work per grid point.

  9. Structural evolution of the Irtysh Shear Zone: implication for the Late Paleozoic amalgamation of multiple arc systems in Central Asia

    NASA Astrophysics Data System (ADS)

    Li, Pengfei; Sun, Min; Rosenbaum, Gideon

    2015-04-01

    The NW-SE Irtysh Shear Zone represents a major tectonic boundary in the Central Asian Orogenic Belt, recording the amalgamation history between the peri-Siberian orogenic system and the Kazakhstan orogenic system. The structural evolution and geodynamics of this shear zone is still poorly documented. Here we present new structural data complemented by chronological data in an attempt to unravel the geodynamic significance of the Irtysh Shear Zone in the context of accretion history of the Central Asian Orogenic Belt. Our results show three episodes of deformation for the shear zone. D1 foliation is locally recognized in low strain area and recorded by garnet inclusions, whereas D2 is represented by a sub-horizontal fabric and related NW-SE lineation. D3 is characterized by a transpersonal deformation event, to form a series of NW-SE mylonitic belts with sinistral kinematics, and to overprint D2 fabric forming regional-scale NW-SE upright folds. A paragneiss sample from the shear zone yielded the youngest detrital zircon peaks in the late Carboniferous, placing a maximum age constraint on the deformation, which overlaps in time with the late Paleozoic collision between the Chinese Altai and the intraoceanic arc system of the East Junggar and West Junggar. We interpret three episodes of deformation to represent orogenic thickening (D1), collapse (D2) and thickening (D3) in response to this collisional event. Sinistral shearing (D3) together with the coeval dextral shearing in the Tianshan accommodate eastward extrusion of the Kazakhstan orogenic system during the late Paleozoic amalgamation of the Central Asian Orogenic Belt. Acknowledgements: This study was financially supported by the Major Basic Research Project of the Ministry of Science and Technology of China (Grant: 2014CB440801), Hong Kong Research Grant Council (HKU705311P and HKU704712P), National Science Foundation of China (41273048, 41273012) and a HKU CRCG grant. The work is a contribution of the Joint

  10. Kinematic evolution of the Fodjomekwet-Fotouni Shear Zone (West-Cameroon): Implications for emplacement of the Fomopéa and Bandja plutons

    NASA Astrophysics Data System (ADS)

    Tcheumenak Kouémo, Jules; Njanko, Théophile; Kwékam, Maurice; Naba, Séta; Bella Nké, Bertille E.; Yakeu Sandjo, Angeline F.; Fozing, Eric M.; Njonfang, Emmanuel

    2014-11-01

    The Fodjomekwet-Fotouni shear zone (FFSZ) belongs to the western part of the Central Cameroon Shear Zone (CCSZ). From the border to the core, the FFSZ is made of: (i) protomylonites, (ii) garnet-sillimanite mylonites and (iii) garnet-sillimanite ultramylonites. Structural data indicate (1) a N40°E mylonitic foliation trend (strongly dipping toward NE or SW) with strike varying between N34°E-78°NW at Fotouni and N52°E-61°SE at Fodjomekwet; (2) a stretching lineation (plunging toward NE or SW) with best line varying between 224/4 at Fotouni and 232/10 at Fodjomekwet; (3) sinistral (N-S) and dextral (NE-SW) kinematic markers, respectively correlated to the D2 and D3 of the CCSZ deformation phases. The presence of relic sinistral shear markers wrapped by dominant dextral shear markers rendered difficult the understanding of the FFSZ evolution. Anisotropy of magnetic susceptibility in the FFSZ shows that magnetic fabrics are characterized by dominant NE-SW and locally N-S to NNE-SSW trending foliation and lineation. These directions show that mylonitisation was initiated in the N-S direction during the sinistral syn-D2 shearing, evolved and reoriented toward NNE-SSW and NE-SW between 613 and 590 Ma. The sinistral shearing event was thereafter followed by a NE-SW trend dextral syn-D3 reactivation of the N50°E fault between 580 and 552 Ma. The sinistral phase seems to have favoured the evolution of fractures in pull-apart structures. The pull-apart opening of fracture enables the upwelling of magma and the emplacement of the Fomopéa and the Bandja plutons (FBP). This synkinematic emplacement is witnessed by the elongated shape and the NE-SW trending direction of these massifs. Kinematic emplacement of FBP controlled by active shear zone is comparable to syntectonic plutonism in eastern Nigeria and NE Brazil.

  11. Symmetric and near-symmetric objects in ductile shear zones- examples from Higher Himalaya, Bhagirathi section, India

    NASA Astrophysics Data System (ADS)

    Mukherjee, Soumyajit

    2013-04-01

    Ductile shear zones contain numerous asymmetric fabrics/structures/clasts/ objects of three overall geometries viz. sigmoidal, lenticular and parallelogram-shaped (Mukherjee, 2011). These indicate the shear sense of the rock, and have been reviewed from time to time by other authors and myself. By contrast, there has been no concise study or review of symmetric and near symmetric objects within ductile shear zones. This work studies morphologies of such symmetric objects from the Bhagirathi river section in the Higher Himalaya (HH), India. The HH in this section began top-to~SW compressional ductile shear at ~ 25 Ma. This was followed by intervals of extrusion by channel flow alternating with critical taper spreading from~ 18 Ma onwards. During its two pulses/phases of channel flow, two sub-zones of extensional ductile top-to~NE shear developed (Mukherjee, in press). Mesoscopic near symmetric objects in the HH are of the following types. (A) Isolated objects- most often lenticular/elliptical, but rarely sub-circular, rectangular or rhombic. (B) Gently curved quartz veins. (C) Warped NE dipping main foliations. (D) Nearly uniformly thin quartzofeldspathic foliations that bulge locally into sub-circular or sub-elliptical shapes. Most of the fractures inside clasts are irregular and their geometries do not conclusively indicate any shear sense. In general these fractures are confined to within the (harder?) clasts, very gently curved, sub-parallel to each other, and are at high angle with ~ NE dipping main foliations defined within mylonitized host rocks. This probably indicates (local?) brittle-ductile extension parallel to the main foliation. Notice that (a) one common explanation for the evolution of S-fabrics is that they rotate towards parallelism with the C-planes as the intensity of shear increases; and (b) lenticular clasts in mylonitized gneiss have classically been called 'augens'. In both these cases, a low-angle S-fabric and 'augen' may not indicate a

  12. Formation of left-lateral fractures within the Summit Ridge shear zone, 1989 Loma Prieta, California, earthquake

    SciTech Connect

    Johnson, A.M.; Fleming, R.W. |

    1993-12-01

    The 1989 Loma Prieta, California, earthquake is characterized by the lack of major, throughgoing, coseismic, right-lateral faulting along strands of the San Andreas fault zone in the epicentral area. Instead, throughout the Summit Ridge area there are zones of tension cracks and left-lateral fracture zones oriented about N45 deg W, that is, roughly parallel to the San Andreas fault in this area. The left-lateral fractures zones are enigmatic because their left-lateral slip is opposite to the right-lateral sense of the relative motion between the Pacific and North American plates. We suggest that the enigmatic fractures can be understood if we assume that coesiesmic deformation was by right-lateral shear across a broad zone, about 0.5 km wide and 4 km long, beneath Summit Ridge. Contrary to most previous reports on the Loma Prieta earthquake, which assert that coseismic, right-lateral ground rupture was restricted to considerable (greater than 4 km) depths in the epicentral area, we find that nearly all the right-lateral offset is represented at the ground surface by the Summit Ridge shear zone.

  13. A review of porosity-generating mechanisms in crustal shear zones

    NASA Astrophysics Data System (ADS)

    Fusseis, F.; Regenauer-Lieb, K.; Revets, S.

    2009-04-01

    Knowledge of the spatiotemporal characteristics of permeability is critical for the understanding of fluid migration in rocks. In diagenetic and metamorphic rocks different porosity-generating mechanisms contribute to permeability and so influence fluid migration and fluid/rock interaction. However, little is known about their relative contributions to the porosity architecture of a rock in a tectono-metamorphic environment. This presentation reviews porosity-generating mechanisms that affect fluid migration in shear zones, the most important crustal fluid conduits, in the context of the tectonometamorphic evolution of rocks. Mechanisms that generate porosity can be classified in a) those that involve the direct action of a fluid, b) processes in which a fluid partakes or that are supported by a fluid or c) mechanism that do not involve a fluid. a) Hydraulic fracturing, where it happens through the formation of tensile fractures, occurs where pore fluid pressures equalize the combined lithostatic pressure and strength of the rock (Etheridge et al., 1984, Cox & Etheridge, 1989, Oliver, 1996). Here an internally released (devolatilisation reactions, e.g., Rumble, 1994, Hacker, 1997, Yardley, 1997 and references therein) or externally derived (infiltrating from metamorphic, magmatic or meteoric sources, Baumgartner et al., 1997, Jamtveit et al., 1997, Thompson, 1997, Gleeson et al., 2003) fluid directly causes the mechanical failure of a rock. Where a fluid is in chemical disequilibrium with a rock (undersaturated with regard to a chemical species) minerals will be dissolved, generating dissolution porosity. Rocks ‘leached' by the removal of chemical components by vast amounts of fluid are reported to lose up to 60% of their original volume (e.g., Kerrich et al., 1984, McCaig 1988). Dissolution porosity is probably an underrated porosity-generating mechanism. It can be expected along the entire metamorphic evolution, including diagenesis (Higgs et al., 2007) and

  14. Slip rate of the Calico fault: Implications for anomalous geodetic strain accumulation across the Eastern California shear zone

    NASA Astrophysics Data System (ADS)

    Oskin, M.; Perg, L.; Blumentritt, D.; Mukhopadhyay, S.; Iriondo, A.

    2004-12-01

    Recent earthquake activity and high geodetically derived fault-slip rates across the Eastern California shear zone motivate comparisons with long-term geologic deformation rates to test for transient strain accumulation. We report new geologic slip-rate results from a transect at 34.8° N across the central Mojave Desert where six dextral faults (Helendale, Lenwood, Camp Rock, Calico, Pisgah-Bullion, and Ludlow) accommodate all late Quaternary right-lateral displacement. High-resolution LIDAR topography data have been successfully acquired across all six faults as part of a project to measure a complete budget of long-term geologic fault slip rates. Field investigations of the northern Rodman Mountains conducted with the aid of the new topography data identified several surfaces dextrally offset by the Calico fault. A preliminary slip rate of 1.3±0.3 mm/yr is calculated from an 800± 200 m offset of alluvial fan deposits containing clasts of the ca. 600 ka Pipkin basalt flow. Cosmogenic surface exposure age dating of offset geomorphic surfaces and refined Ar/Ar dating of the basalt flow, in progress, will provide multiple constraints of this fault slip rate. The slip rate of the Calico fault is more than twice that of the Blackwater fault, located on strike with the Calico fault in the northwest Mojave Desert. This discrepancy supports that strain is transferred away from the Calico fault and other adjacent northwest-striking dextral faults onto domino-style rotating blocks bounded by sinistral faults in the Fort Irwin region. A newly identified active thrust fault and fault-related fold bounding the northern Rodman mountains accommodates shortening east of the Calico fault that may be caused by space problems at the intersection of these conjugate fault systems. Overall, slip rate on the Calico fault, together with existing paleoseismic histories on adjacent faults, does not account for more than 5 mm/yr of strain accumulation across the Eastern California shear

  15. The Western Tauern Window (Eastern Alps): Timing and Interplay of Folds and Sinistral Shear Zones as Result of South-Alpine Indentation

    NASA Astrophysics Data System (ADS)

    Schneider, Susanne; Rosenberg, Claudio; Hammerschmidt, Konrad

    2010-05-01

    Zentralgneiss to the Upper Austroalpine units. Within the Upper Penninic nappes N-side up kinematic indicators occur, in addition to the sinistral ones. Newly formed biotites of Zentralgneiss rocks have been dated with the Rb/Sr technique (Kitzig et al. 2009), yielding 18-20Ma for their formation during sinistral deformation. Fine-grained phengites from the axial plane foliation of the upright folds were dated with the K/Ar method, yielding 14-17Ma. Ar/Ar in-situ LA analyses of sinistral mylonites (Ahorn, Olperer and Greiner) yield formation ages of syn-kinematic phengites between 24-12Ma. These grains are overgrown by post-kinematic phengites of 12-9Ma. Northeast of the western TW, sinistral shear is accommodated by the brittle sinistral SEMP Fault system, whose activity has been dated to 17Ma (Peresson & Decker 1997). Several sinistral shear zones (Ahorn, Greiner, Ahrntal) of the western TW may coalesce into the SEMP Fault (e.g., Linzer et al., 2002). In the west, the Ahorn Shear Zone terminates nearly 10km east of the Brenner Fault, into a NW-striking fold belt. The Ahrntal Fault continues into the Jaufen Fault, which merges with the brittle sinistral Giudicarie Fault. Motion along the Giudicarie Fault initiated in the Miocene (Stipp et al., 2004), or already in the Oligocene (Müller et al 2001). Based on these results, a temporal, kinematic and geometric continuity between sinistral shearing along the Giudicarie Fault, along the SEMP Fault, and throughout the western TW, can be assessed. The sinistral shear zones of the western TW are kinematically linked to upright folds, hence to crustal thickening. Upright folding and sinistral shearing were active since 24Ma and terminated at 12Ma. In summary, the sinistral displacements of the Giudicarie System appear to be partitioned into upright folds and sinistral, transpressive shear zones in the western Tauern Window, both of which contribute to its exhumation. The coalescence of the sinistral shear zones into the SEMP Fault

  16. Evaluation of Shear-Induced Platelet Activation Models Under Constant and Dynamic Shear Stress Loading Conditions Relevant to Devices

    PubMed Central

    Sheriff, Jawaad; Soares, João Silva; Xenos, Michalis; Jesty, Jolyon; Bluestein, Danny

    2013-01-01

    The advent of implantable blood-recirculating devices such as left ventricular assist devices and prosthetic heart valves provides a viable therapy for patients with end-stage heart failure and valvular disease. However, device-generated pathological flow patterns result in thromboembolic complications that require complex and lifelong anticoagulant therapy, which entails hemorrhagic risks and is not appropriate for certain patients. Optimizing the thrombogenic performance of such devices utilizing numerical simulations requires the development of predictive platelet activation models that account for variations in shear-loading rates characterizing blood flow through such devices. Platelets were exposed in vitro to both dynamic and constant shear stress conditions emulating those found in blood-recirculating devices in order to determine their shear-induced activation and sensitization response. Both these behaviors were found to be dependent on the shear loading rates, in addition to shear stress magnitude and exposure time. We then critically examined several current models and evaluated their predictive capabilities using these results. Shear loading rate terms were then included to account for dynamic aspects that are either ignored or partially considered by these models, and model parameters were optimized. Independent optimization for each of the two types of shear stress exposure conditions tested resulted in different sets of best-fit constants, indicating that universal optimization may not be possible. Inherent limitations of the current models require a paradigm shift from these integral-based discretized power law models to better address the dynamic conditions encountered in blood-recirculating devices. PMID:23400312

  17. Exhumation of the Greater Himalayan Sequence Along the Zanskar Shear Zone, NW India

    NASA Astrophysics Data System (ADS)

    Basta, S.; Beck, E.; Burlick, T.

    2013-12-01

    The Zanskar Shear Zone (ZSZ), the western extent of the South Tibetan Detachment System, exposes high-grade metamorphic rocks of the Greater Himalayan Sequence (GHS) in its footwall. Granites and metapelites collected along the the ZSZ in the Suru River valley provide how and when the GHS rocks exhumed and were deformed. There are two suits of Paleozoic granites deformed within the ZSZ: Pan-African Cambrian-Ordovician granites at the cores of gneiss domes and Mississippian-Permian granites related to Panjal Traps magmatism. Age of Himalayan granites indicates 28-16 Ma which is concurrent with anatectic leucogranite crystallization. The metamorphic mineral assemblage indicates increasing metamorhic grade from NE to SW and comprises Qtz × Kfs + Pl + Bt × Ms × Sil × Ky × Grt × St × Chl × Tur × Rt. In addition to macroscopic evidence, strongly deformed quartz grains, deformation twins, pressure shadows, and kink bands have been observed to demonstrate micro-tectonics evidence. There are two different method to explain exhumation and deformation of the GHS metapelites: Electron backscatter diffraction (EBSD) crystallographic mapping and pseudosection modeling. While EBSD indicates the potential temperature of deformation, pseudosection modeling with Perple_X specifically presents the exhumation path of the GHS rocks. Pseudosection modeling with Perple_X , based on whole-rock geochemical analysis, is set pressure and temperature to 0.4-1.2 GPa and 300-900°C, and uses specific solution models, Bio(TCC), Chl(HP), St(HP), feldspar, Mica(CHA), Gt(HP), and hCrd, namely. These two methods are used by combining with geo/thermochronology data from U-Pb, 40Ar/39Ar, and (U-Th)/He, constraining the age of metamorphism, the cooling and exhumation time of the GHS rocks, and the end of shearing of deformation, respectively. 40Ar/39Ar dating on muscovite and biotite constrains cooling and exhumation ages of the GHS as ~20-19 Ma and 15 Ma, respectively. A metamorphic pressure

  18. Deformation and metamorphism in an eclogite-bearing shear zone within the Sveconorwegian Orogen, Sweden

    NASA Astrophysics Data System (ADS)

    Tual, Lorraine; Möller, Charlotte; Pinan-Llamas, Aranzazu

    2013-04-01

    The Eastern Segment is the counterpart to the Parautochthonous Belt in the Grenville Province, and represents a part of the Fennoscandian Shield that was thoroughly reworked during late stages of the Sveconorwegian orogeny (~1.1-0.9 Ga). Relics of eclogite occur as lenses in high-grade gneisses in a part of the southern Eastern Segment. These rocks constitute evidence of a high-pressure event at c. 0.98 Ga, followed by regional deformation and metamorphism in the granulite and upper amphibolite facies. Structural and petrological data combined with airborne magnetic anomalies have been used to define the structure and the relationship between deformation and metamorphism related to the Sveconorwegian orogeny. Along the southern and intensely deformed boundary of the eclogite domain, the Ullared Zone, three main deformation phases (D1-D3) have been identified. D1 is identified as a lithotectonic and gneissic layering with locally preserved isoclinal folds. These structures were overprinted by a major deformation phase (D2), which affected heterogeneously the entire southern Eastern Segment. D2 is characterized by asymmetric tight to isoclinal folds, commonly with a well-developed axial planar mineral fabric. The folds are associated with shear sub-parallel to their axial planes in areas where the deformation was intense. They are interpreted as the consequence of tectonic extrusion of eclogitic-bearing crust into mid-crustal levels, resulting in decompression and retrogression of eclogite under high-pressure granulite and upper amphibolite conditions. This deformation resulted in strong E-W to WNW-ESE stretching, associated with systematic top-the-east sense of shear. Late open upright folding (D3) with predominant NNE-SSW axes is superimposed on D2. Ongoing studies aim at constraining the P-T-t path of the eclogite-bearing unit and surrounding units by multiequilibrium thermobarometry and pseudosections. The metamorphic evolution will be linked to the structural

  19. Using Local Second Gradient Model and Shear Strain Localisation to Model the Excavation Damaged Zone in Unsaturated Claystone

    NASA Astrophysics Data System (ADS)

    Pardoen, Benoît; Levasseur, Séverine; Collin, Frédéric

    2015-03-01

    The drilling of galleries induces damage propagation in the surrounding medium and creates, around them, the excavation damaged zone (EDZ). The prediction of the extension and fracture structure of this zone remains a major issue, especially in the context of underground nuclear waste storage. Experimental studies on geomaterials indicate that localised deformation in shear band mode usually appears prior to fractures. Thus, the excavation damaged zone can be modelled by considering the development of shear strain localisation bands. In the classical finite element framework, strain localisation suffers a mesh-dependency problem. Therefore, an enhanced model with a regularisation method is required to correctly model the strain localisation behaviour. Among the existing methods, we choose the coupled local second gradient model. We extend it to unsaturated conditions and we include the solid grain compressibility. Furthermore, air ventilation inside underground galleries engenders a rock-atmosphere interaction that could influence the damaged zone. This interaction has to be investigated in order to predict the damaged zone behaviour. Finally, a hydro-mechanical modelling of a gallery excavation in claystone is presented and leads to a fairly good representation of the EDZ. The main objectives of this study are to model the fractures by considering shear strain localisation bands, and to investigate if an isotropic model accurately reproduces the in situ measurements. The numerical results provide information about the damaged zone extension, structure and behaviour that are in very good agreement with in situ measurements and observations. For instance, the strain localisation bands that develop in chevron pattern during the excavation and rock desaturation, due to air ventilation, are observed close to the gallery.

  20. Jurassic Cordilleran dike swarm-shear zones: Implications for the Nevadan orogeny and North American plate motion

    SciTech Connect

    Wolf, M.B.; Saleeby, J.B. )

    1992-08-01

    A cogenetic and coeval tonalitic and mafic dike swarm has been identified within a southern fragment (the Owens Mountain area) of the western Foothills terrane (California). The dikes were mylonitized and transposed (rotated into subparallel orientation) during emplacement, from 155 to 148 Ma (U-Pb zircon data), which coincides in time with the Nevadan orogeny. Steeply southeast-plunging fold axes and S-fold geometries indicate a sinistral-sense of shear, possibly with some dip-slip motion as well. This shear zone may be the southern and possibly deeper extension of the Bear Mountains fault zone. This and other Late Jurassic Cordilleran dike swarms record a complex pattern of sinistral-sense transtension-transpression that developed at the apparent-polar-wander J2 cusp ([approximately] 150 Ma) and during subsequent, rapid, northwestward acceleration of North America. The Late Jurassic Nevadan orogeny is a manifestation of these dramatic changes in magnitude and direction of North American motion.

  1. Sutures, Sinkers, Shear zones and Sills: Cryptic Targets and Explicit Strategies for EARTHSCOPE FlexArray in the East

    NASA Astrophysics Data System (ADS)

    Brown, L. D.

    2006-05-01

    Given the 3D framework represented by EarthScope's USArray as it scans eastward, the strategic challenge falls to defining cost-effective deployments of FlexArray to address specific lithospheric targets. Previous deep geophysical surveys (e.g. COCORP, USGS, GLIMPCE, et al.) provide guidance not only in framing the geological issues involved, but in designing field experiments that overcome the limitations of previous work. Opportunities highlighted by these precursor studies include: a) Collisional sutures (e.g. Brunswick Anomaly/Suwannee terrane) which lie buried beneath overthrust terranes/ younger sedimentary covers. Signal penetration in previous controlled source surveys has been insufficient. High resolution passive surveys designed to map intralithospheric detachments, Moho, and mantle subduction scars is needed to validate the extrapolations of the existing upper crustal information; b) Intracratonic basins and domes (e.g. Michigan Basin, Adirondack Dome) are perhaps the greatest geological mystery hosted in the east. Previous geophysical studies have lacked the resolution or penetration needed to identify the buoyancy drivers presumed to be responsible for such structures. It is likely that these drivers lie in the upper mantle and will require detailed velocity imaging to recognized. c) Distributed shear fabrics are a defining characteristic of the deep crust in many deformation zones (e.g. Grenville Front). Detailed mapping of crustal anisotropy associated with such shear zones should help delineate ductile flow directions associated with the orogenies that accreted the eastern U.S. 3 component, 3D active+passive surveys are needed to obtain definitive remote measures of such vector characteristics in the deep crust. d) Extensive reflectors in the central U.S. may mark important buried Precambrian basins and/or sill complexes. If the latter, the magmatic roots of those systems remain unrecognized, as does their volumetric contribution to crustal growth

  2. Characterization of Quartz and Feldspar Deformation in the Mid-crust: Insights from the Cordillera Blanca Shear Zone, Peru

    NASA Astrophysics Data System (ADS)

    Hughes, C. A.; Jessup, M. J.; Shaw, C. A.

    2014-12-01

    Deformation mechanisms within shear zones from various crustal levels must be characterized to develop accurate models of lithospheric rheology. The Cordillera Blanca Shear Zone (CBSZ) in the central Peruvian Andes records changes in temperature, microstructures, and deformation mechanisms that occurred during exhumation through the brittle-ductile-transition during normal-sense slip over the last ~5 m.y. The 100-500-m-thick mylonitic shear zone occupies the footwall of a 200-km-long normal detachment fault, marking the western boundary of the 8 Ma, leucogranodiorite Cordillera Blanca Batholith. Though local variations do occur, including recrystallized quartz veins and local, decimeter- to meter- scale shear zones, the CBSZ follows a general trend of increasing strain towards the detachment. Structurally lowest positions are weakly deformed and transition to protomylonite, mylonite, and ultramylonite at higher positions, truncating at a cataclasite nearest the detachment. We characterize strain using EBSD analyses of quartz lattice preferred orientations and deformation temperatures using quartz and feldspar textures and two-feldspar thermometry of asymmetric strain-induced myrmekite. At the deepest structural positions, feldspar grains record a complex history characterized by bulging recrystallization, myrmekite formation, and brittle fracture, while quartz exhibits dominant grain-boundary migration recrystallization (T> 500 °C) and prism slip. Intermediate samples exhibit more prevalent strain-induced myrmekite, brittle fracture in feldspar, and reaction-associated recrystallization of K-feldspar to mica; quartz records mainly subgrain-rotation recrystallization (400-500 °C) and dominant prism slip with a rhomb component. Shallower positions preserve fewer, smaller, and more rounded feldspar porphyroclasts with no myrmekite, and dominant bulging recrystallization (280-400 °C) in quartz that records prism , rhomb , and some basal slip.

  3. Detachment shear zone of the Atlantis Massif core complex, Mid-Atlantic Ridge, 30°N

    NASA Astrophysics Data System (ADS)

    Karson, J. A.; Früh-Green, G. L.; Kelley, D. S.; Williams, E. A.; Yoerger, D. R.; Jakuba, M.

    2006-06-01

    Near-bottom investigations of the cross section of the Atlantis Massif exposed in a major tectonic escarpment provide an unprecedented view of the internal structure of the footwall domain of this oceanic core complex. Integrated direct observations, sampling, photogeology, and imaging define a mylonitic, low-angle detachment shear zone (DSZ) along the crest of the massif. The shear zone may project beneath the nearby, corrugated upper surface of the massif. The DSZ and related structures are inferred to be responsible for the unroofing of upper mantle peridotites and lower crustal gabbroic rocks by extreme, localized tectonic extension during seafloor spreading over the past 2 m.y. The DSZ is characterized by strongly foliated to mylonitic serpentinites and talc-amphibole schists. It is about 100 m thick and can be traced continuously for at least 3 km in the tectonic transport direction. The DSZ foliation arches over the top of the massif in a convex-upward trajectory mimicking the morphology of the top of the massif. Kinematic indicators show consistent top-to-east (toward the MAR axis) tectonic transport directions. Foliated DSZ rocks grade structurally downward into more massive basement rocks that lack a pervasive outcrop-scale foliation. The DSZ and underlying basement rocks are cut by discrete, anastomosing, normal-slip, shear zones. Widely spaced, steeply dipping, normal faults cut all the older structures and localize serpentinization-driven hydrothermal outflow at the Lost City Hydrothermal Field. A thin (few meters) sequence of sedimentary breccias grading upward into pelagic limestones directly overlies the DSZ and may record a history of progressive rotation of the shear zone from a moderately dipping attitude into its present, gently dipping orientation during lateral spreading and uplift.

  4. Neotectonic reactivation of shear zones and implications for faulting style and geometry in the continental margin of NE Brazil

    NASA Astrophysics Data System (ADS)

    Bezerra, F. H. R.; Rossetti, D. F.; Oliveira, R. G.; Medeiros, W. E.; Neves, B. B. Brito; Balsamo, F.; Nogueira, F. C. C.; Dantas, E. L.; Andrades Filho, C.; Góes, A. M.

    2014-02-01

    The eastern continental margin of South America comprises a series of rift basins developed during the breakup of Pangea in the Jurassic-Cretaceous. We integrated high resolution aeromagnetic, structural and stratigraphic data in order to evaluate the role of reactivation of ductile, Neoproterozoic shear zones in the deposition and deformation of post-rift sedimentary deposits in one of these basins, the Paraíba Basin in northeastern Brazil. This basin corresponds to the last part of the South American continent to be separated from Africa during the Pangea breakup. Sediment deposition in this basin occurred in the Albian-Maastrichtian, Eocene-Miocene, and in the late Quaternary. However, our investigation concentrates on the Miocene-Quaternary, which we consider the neotectonic period because it encompasses the last stress field. This consisted of an E-W-oriented compression and a N-S-oriented extension. The basement of the basin forms a slightly seaward-tilted ramp capped by a late Cretaceous to Quaternary sedimentary cover ~ 100-400 m thick. Aeromagnetic lineaments mark the major steeply-dipping, ductile E-W- to NE-striking shear zones in this basement. The ductile shear zones mainly reactivated as strike-slip, normal and oblique-slip faults, resulting in a series of Miocene-Quaternary depocenters controlled by NE-, E-W-, and a few NW-striking faults. Faulting produced subsidence and uplift that are largely responsible for the present-day morphology of the valleys and tablelands in this margin. We conclude that Precambrian shear zone reactivation controlled geometry and orientation, as well as deformation of sedimentary deposits, until the Neogene-Quaternary.

  5. Weakly sheared active suspensions: hydrodynamics, stability, and rheology.

    PubMed

    Cui, Zhenlu

    2011-03-01

    We present a kinetic model for flowing active suspensions and analyze the behavior of a suspension subjected to a weak steady shear. Asymptotic solutions are sought in Deborah number expansions. At the leading order, we explore the steady states and perform their stability analysis. We predict the rheology of active systems including an activity thickening or thinning behavior of the apparent viscosity and a negative apparent viscosity depending on the particle type, flow alignment, and the anchoring conditions, which can be tested on bacterial suspensions. We find remarkable dualities that show that flow-aligning rodlike contractile (extensile) particles are dynamically and rheologically equivalent to flow-aligning discoid extensile (contractile) particles for both tangential and homeotropic anchoring conditions. Another key prediction of this work is the role of the concentration of active suspensions in controlling the rheological behavior: the apparent viscosity may decrease with the increase of the concentration. PMID:21517529

  6. Weakly sheared active suspensions: Hydrodynamics, stability, and rheology

    NASA Astrophysics Data System (ADS)

    Cui, Zhenlu

    2011-03-01

    We present a kinetic model for flowing active suspensions and analyze the behavior of a suspension subjected to a weak steady shear. Asymptotic solutions are sought in Deborah number expansions. At the leading order, we explore the steady states and perform their stability analysis. We predict the rheology of active systems including an activity thickening or thinning behavior of the apparent viscosity and a negative apparent viscosity depending on the particle type, flow alignment, and the anchoring conditions, which can be tested on bacterial suspensions. We find remarkable dualities that show that flow-aligning rodlike contractile (extensile) particles are dynamically and rheologically equivalent to flow-aligning discoid extensile (contractile) particles for both tangential and homeotropic anchoring conditions. Another key prediction of this work is the role of the concentration of active suspensions in controlling the rheological behavior: The apparent viscosity may decrease with the increase of the concentration.

  7. Simultaneous Clockwise and Counterclockwise Rotation Along the Eastern California Shear Zone at the Coso Range, California

    NASA Astrophysics Data System (ADS)

    Pluhar, C. J.; Coe, R. S.; Monastero, F. C.

    2005-12-01

    The Coso Range, California lies in a releasing step-over of the westernmost strand of the dextral Eastern California Shear Zone (ECSZ). Previous paleomagnetic studies on c.a. 3 Ma lavas of Wild Horse Mesa, within the Coso Range, demonstrate small-magntiude (12.0°±4.6°) clockwise vertical-axis rotations since emplacement of these rocks. Seismogenic, slickenline, and GPS strain data in the same region indicate a present-day rotation rate consistent with lava paleomagnetism results. However, paleomagnetism of lake beds of the White Hills, south of Wild Horse Mesa, yields a (reversed-polarity) mean direction of declination = 171.8°, inclination = -27.8°, κ = 12.3, and α95 = 7.5° for n = 34 samples. Thus, White Hills have rotated counterclockwise 8.2°±8.2° since the sediments were deposited around 1 Ma. Although not significant at 95% confidence, this rotation is significant at any lesser confidence level and therefore indicates counterclockwise rotation. The opposite sense of rotation between White Hills and Wild Horse Mesa is understandable by the differing fault orientation and deformation styles between the two locales. The White Hills are an anticline within the larger-scale releasing bend of the ECSZ. Faults in the White Hills are generally oriented north-south to northwest-southeast. On the other hand the Wild Horse Mesa forms a transtentional basin with widespread north-south to northeast-southwest oriented normal faults. These differing fault orientations, probably intimately related to the local transpression and transtension within the two areas, respectively, results in simultaneous rotation of opposite sense due to an invariant far-field strain.

  8. Constitutive modeling of large inelastic deformation of amorphous polymers: Free volume and shear transformation zone dynamics

    NASA Astrophysics Data System (ADS)

    Voyiadjis, George Z.; Samadi-Dooki, Aref

    2016-06-01

    Due to the lack of the long-range order in their molecular structure, amorphous polymers possess a considerable free volume content in their inter-molecular space. During finite deformation, these free volume holes serve as the potential sites for localized permanent plastic deformation inclusions which are called shear transformation zones (STZs). While the free volume content has been experimentally shown to increase during the course of plastic straining in glassy polymers, thermal analysis of stored energy due to the deformation shows that the STZ nucleation energy decreases at large plastic strains. The evolution of the free volume, and the STZs number density and nucleation energy during the finite straining are formulated in this paper in order to investigate the uniaxial post-yield softening-hardening behavior of the glassy polymers. This study shows that the reduction of the STZ nucleation energy, which is correlated with the free volume increase, brings about the post-yield primary softening of the amorphous polymers up to the steady-state strain value; and the secondary hardening is a result of the increased number density of the STZs, which is required for large plastic strains, while their nucleation energy is stabilized beyond the steady-state strain. The evolutions of the free volume content and STZ nucleation energy are also used to demonstrate the effect of the strain rate, temperature, and thermal history of the sample on its post-yield behavior. The obtained results from the model are compared with the experimental observations on poly(methyl methacrylate) which show a satisfactory consonance.

  9. Determination of the stress conditions of the ductile-to-brittle regime along the Asuke Shear Zone, SW Japan

    NASA Astrophysics Data System (ADS)

    Kanai, Takuto; Takagi, Hideo

    2016-04-01

    The stress conditions of the ductile-to-brittle regime have been assessed along the Asuke Shear Zone (ASZ), which strikes NE-SW in the Cretaceous Ryoke granite terrain in SW Japan. Along the ASZ, pseudotachylyte and mylonitized pseudotachylyte are locally developed together with cataclasite. The simultaneous operation of dislocation creep and grain-size-sensitive creep, as indicated by the coexistence of the Z-maximum and relatively random c-axis lattice preferred orientations as well as the sizes of dynamically recrystallized quartz grains (6.40-7.79 μm) in the mylonitized pseudotachylyte, suggest differential stresses of 110-130 MPa at ∼300 °C. The e-twin morphology, twinning ratio, and distribution of the glide direction on the e-twin plane of the twinned calcite in the amygdules of the pseudotachylyte suggest the stress conditions of the σ1 and σ3 axes trend 228° and 320° and plunge 55° and 1°, respectively, and indicate differential stresses of 40-80 MPa at 150-200 °C. Based on kinematic indicators in the fault rocks, the stress conditions estimated from calcite twins, and the cooling history of the granitic protolith, the ASZ is inferred to have been activated under a stress state that caused sinistral normal movements before and after pseudotachylyte formation at 70-50 Ma.

  10. Neoproterozoic Structural Evolution of the NE-trending 620-540 Ma Ad-Damm Shear Zone, Arabian Shield, Saudi Arabia

    NASA Astrophysics Data System (ADS)

    Hamimi, Z.; El-Sawy, E. K.; El-Fakharan, A. S.; Shujoon, A.; Matsah, M.; El-Shafei, M.

    2012-04-01

    Ad-Damm Shear Zone (ASZ) is a NE-trending fault zone separating Jeddah and Asir tectonostratigraphic terranes in the Neoproterozoic juvenile Arabian Shield. ASZ extends ~380 km, with an average width ~2-3 km, from the eye-catching Ruwah Fault Zone in the eastern shield to the Red Sea Coastal plain. It was believed to be one of the conjugate shears of the NW- to NNW- trending sinistral Najd Shear System based on noteworthy dextral shear criteria recorded within the 620 Ma sheared granites of Numan Complex, as well as right-lateral offsets within quartz veins and dikes transected by the shear zone. The present study is an integrated field-based structural analysis and remote sensing. We utilized the ASTER data for lithologic discrimination and automatic structural lineament extraction and analysis of the Neoproterozoic basement lithologies encountered along and within the vicinity of ASZ. Various false color composite images were generated and evaluated for lithological mapping and structural lineaments. The obtained map was analyzed using GIS techniques to interpret the behavior of the existing lineaments and their spatial distribution. Based on the results of the ASTER data, two significant areas; around Bir Ad-Damm and to the south of Wadi Numan, are selected for detailed field investigation. Shear-sense indicators and overprinting relations clearly show a complicated Neoproterozoic history of ASZ, involving at least three deformations: (1) an early attenuated NE-SW sinistral shearing; followed by (2) a SE-directed thrusting phase resulted in the formation SE-verging thrusts and associated thrust-related folds; and (3) late NE-SW intensive dextral transcurrent shearing played a significant role in the creation of mesoscopic shear-zone related folds, particularly in the area near Bir Ad-Damm. Such deformation history demonstrates the same episode of Neoproterozoic deformation exhibited in the NE-trending shear zones in the Arabian-Nubian Shield (ANS).

  11. Kinematics of the Tengchong Terrane in SE Tibet from the late Eocene to early Miocene: Insights from coeval mid-crustal detachments and strike-slip shear zones

    NASA Astrophysics Data System (ADS)

    Xu, Zhiqin; Wang, Qin; Cai, Zhihui; Dong, Hanwen; Li, Huaqi; Chen, Xijie; Duan, Xiangdong; Cao, Hui; Li, Jing; Burg, Jean-Pierre

    2015-12-01

    It is generally believed that the extrusion of SE Tibet was bounded by the dextral Gaoligong and the sinistral Ailaoshan-Red River strike-slip shear zones from the Oligocene to early Miocene. This study integrates field mapping, structural analysis and geochronology in western Yunnan (China), where foliated Precambrian basement rocks and late Cretaceous to early Eocene plutons are exposed to the west of the Gaoligong shear zone. We found that late Eocene to early Miocene flat-lying ductile shear zones were kinematically related to steeply dipping strike-slip shear zones. Four elongated gneiss domes (Donghe, Guyong, Yingjiang and Sudian) are cored by high-grade metamorphic rocks and pre-kinematic granite plutons, and bounded by top-to-NE detachments and NE-trending dextral strike-slip shear zones. Zircon U-Pb ages from LA-ICP-MS analysis and 40Ar/39Ar ages of micas and hornblende demonstrate that the flat-lying Donghe Detachment (> 35-15 Ma) and the Nabang dextral strike-slip shear zone (41-19 Ma) were sites of prolonged, mostly coeval ductile deformation from amphibolite to greenschist facies metamorphism. The Gaoligong shear zone experienced dextral shearing under similar metamorphic conditions between 32 and 10 Ma. Consistent 40Ar/39Ar ages of hornblende from the three shear zones indicate their contemporaneity at mid-crustal depth, causing the rapid exhumation and SW-ward extrusion of the Tengchong Terrane. The strain geometry and shear zone kinematics in the Tengchong Terrane are interpreted with folding of the anisotropic lithosphere around a vertical axis, i.e., the northeast corner of the Indian Plate since 41 Ma. The newly discovered NE-trending Sudian, Yingjiang, and Lianghe strike-slip shear zones are subordinate ductile faults accommodating the initially rapid clockwise rotation of the Tengchong Terrane. The detachments caused mid-crustal decoupling and faster SW-ward extrusion below the sedimentary cover, whereas the strike-slip shear zones accommodated

  12. Infiltration of meteoric fluids in an extensional detachment shear zone (Kettle dome, WA, USA): How quartz dynamic recrystallization relates to fluid-rock interaction

    NASA Astrophysics Data System (ADS)

    Quilichini, Antoine; Siebenaller, Luc; Nachlas, William O.; Teyssier, Christian; Vennemann, Torsten W.; Heizler, Matthew T.; Mulch, Andreas

    2015-02-01

    We document the interplay between meteoric fluid flow and deformation processes in quartzite-dominated lithologies within a ductile shear zone in the footwall of a Cordilleran extensional fault (Kettle detachment system, Washington, USA). Across 150 m of shear zone section, hydrogen isotope ratios (δD) from synkinematic muscovite fish are constant (δD ∼ -130‰) and consistent with a meteoric fluid source. Quartz-muscovite oxygen isotope thermometry indicates equilibrium fractionation temperatures of ∼365 ± 30 °C in the lower part of the section, where grain-scale quartz deformation was dominated by grain boundary migration recrystallization. In the upper part of the section, muscovite shows increasing intragrain compositional zoning, and quartz microstructures reflect bulging recrystallization, solution-precipitation, and microcracking that developed during progressive cooling and exhumation. The preserved microstructural characteristics and hydrogen isotope fingerprints of meteoric fluids developed over a short time interval as indicated by consistent mica 40Ar/39Ar ages ranging between 51 and 50 Ma over the entire section. Pervasive fluid flow became increasingly channelized during detachment activity, leading to microstructural heterogeneity and large shifts in quartz δ18O values on a meter scale. Ductile deformation ended when brittle motion on the detachment fault rapidly exhumed the mylonitic footwall.

  13. Clast Rotation and Nature of Strain Localization in Thick Ultramylonites: the El Pichao Shear Zone (Sierra de Quilmes), NW Argentina.

    NASA Astrophysics Data System (ADS)

    Hasalova, Pavlina; Hunter, Nicholas James; Weinberg, Roberto; Finch, Melanie

    2013-04-01

    Ultramylonite formation is integral to understanding the accommodation of high strain in ductile shear zones, mountain building and crustal movement. The El Pichao Shear Zone (PSZ) is 3-7km thick ductile thrust zone in the Sierra de Quilmes, NW Argentina. Sinistral thrusting along the PSZ has placed granulite facies migmatites of the Tolombón Complex on top of amphibolite metasedimentary rocks of the Agua del Sapo Complex, separated by a sheared granitic body intruded by pegmatites. The fabric varies from protomylonite to ultramylonite. Ultramylonites in the core of the shear zone reach ~1km in thickness. Ultramylonites of this thickness are extremely rare, and thus the El Pichao Shear Zone provides a unique opportunity to investigate the origin of such high strain rocks. We used microstructural and quantitative textural analysis, quartz crystallographic preferred orientation (CPO), clast vorticity and geochemical data to investigate the origin of the thick ultramylonites, and variable strain accommodation associated with the mylonitization process. The mylonitic rocks have granitic composition and consist of a matrix of Bt+Qtz+Ms+Pl+Kfs, Qtz ribbons, mica bands and feldspar porphyroclasts. Feldspar clasts have been variably rotated and their deformation behaviour varies between brittle faulting and partial to complete dynamic recrystallisation. In the ultramylonite Qtz ribbons or strong S-C fabrics are lacking and the matrix tends to be homogeneous with only weak foliation defined by the preferred orientation of micas. There is also a systematic decrease in matrix grain size and mica connectivity towards ultramylonite. Quartz CPO suggests changes in deformation mechanisms associated with strain increase. The transition between mylonite and ultramylonite in the PSZ occurred due to a switch from dominant dislocation creep to dominant diffusion creep. Major and trace element data show no geochemical variation between samples, indicating that the mylonite

  14. Strain localization on different scales and the importance of brittle precursors during deformation in the lower crust (Davenport Shear Zone, Central Australia)

    NASA Astrophysics Data System (ADS)

    Hawemann, Friedrich; Mancktelow, Neil; Wex, Sebastian; Camacho, Alfredo; Pennacchioni, Giorgio

    2014-05-01

    High strain rocks in the Musgrave Ranges (Central Australia) provide a rather unique insight into the development of lower crustal shear zones during the 550 Ma Petermann Orogeny, allowing common models for lower crustal deformation to be critically evaluated. The observed structures in the study area are, from south to north: (1) The Mann Fault, which is poorly exposed but evident on airborne geomagnetic maps. This regional scale fault with a component of dextral shear shows a step-over resulting in the formation of a pull-apart basin. (2) The Davenport Shear Zone, accommodating the horizontal extension in a 7 km wide WNW-ESE-trending mylonitic zone developed under subeclogitic, lower crustal conditions. This high strain zone is bounded to the north by a more than 50 km long, continuous, sheared dolerite dyke. North of this dyke, the ~1200 Ma Musgravian fabric is still preserved, only slightly rotated and typically N-S trending. (3) The Woodroffe Thrust, marking the northern boundary of the Musgrave Ranges, brings these lower crustal rocks on top of amphibolite facies units, with a top-to-north sense of movement. Strain in the Davenport Shear Zone is very heterogeneously distributed, with localization and partitioning from the kilometre down to the millimetre scale. Pseudotachylyte is commonly associated with dykes, especially on the boundaries, and is often sheared. The orientation of sheared dykes and localized shear zones is typically at a high angle to either side of the shortening direction, resulting in a variable sense of shear and a major component of flattening, with a nearly horizontal extension direction. Detailed outcrop-scale mapping shows that compositional inhomogeneities, such as quartz veins, are generally not exploited, even when favourably oriented for shear reactivation. Ultramylonitic shear zones are sometimes only a few millimetres wide but extend for several metres and are generally oblique to the background foliation. Pseudotachylyte often

  15. Metamorphic and structural evidence for significant vertical displacement along the Ross Lake fault zone, a major orogen-parallel shear zone in the Cordillera of western North America

    USGS Publications Warehouse

    Baldwin, J.A.; Whitney, D.L.; Hurlow, H.A.

    1997-01-01

    Results of an investigation of the petrology and structure of the Skymo complex and adjacent terranes constrain the amount, timing, and sense of motion on a segment of the > 600-km-long Late Cretaceous - early Tertiary Ross Lake fault zone (RLFZ), a major orogen-parallel shear zone in the Cordillera of western North America. In the study area in the North Cascades, Washington state, the RLFZ accommodated significant pre-middle Eocene vertical displacement, and it juxtaposes the Skymo complex with upper amphibolite facies (650??-690??C and 6-7 kbar) Skagit Gneiss of the North Cascades crystalline core to the SW and andalusite-bearing phyllite of the Little Jack terrane (Intermontane superterrane) to the NE. The two main lithologic units of the Skymo complex, a primitive mafic intrusion and a fault-bounded block of granulite facies metasedimentary rocks, are unique in the North Cascades. Granulite facies conditions were attained during high-temperature (> 800??C), low pressure (??? 4 kbar) contact metamorphism associated with intrusion of the mafic magma. P-T estimates and reaction textures in garnet-orthopyroxene gneiss suggest that contact metamorphism followed earlier, higher pressure regional metamorphism. There is no evidence that the Skagit Gneiss experienced high-T - low-P contact metamorphism. In the Little Jack terrane, however, texturally late cordierite ?? spinel and partial replacement of andalusite by sillimanite near the terrane's fault contact with Skymo gabbro suggest that the Little Jack terrane experienced high-T (??? 600??C) - low-P (??? 4 kbar) contact metamorphism following earlier low-grade regional metamorphism. Similarities in the protoliths of metasedimentary rocks in the Skymo and Little Jack indicate that they may be part of the same terrane. Differences in pressure estimates for the Little Jack versus Skymo for regional metamorphism that preceded contact metamorphism indicate vertical displacement of ??? 10 km (west side up) on the strand

  16. Timing of deformation in the Sarandí del Yí Shear Zone, Uruguay: Implications for the amalgamation of western Gondwana during the Neoproterozoic Brasiliano-Pan-African Orogeny

    NASA Astrophysics Data System (ADS)

    Oriolo, Sebastián.; Oyhantçabal, Pedro; Wemmer, Klaus; Basei, Miguel A. S.; Benowitz, Jeffrey; Pfänder, Jörg; Hannich, Felix; Siegesmund, Siegfried

    2016-03-01

    U-Pb and Hf zircon (sensitive high-resolution ion microprobe -SHRIMP- and laser ablation-inductively coupled plasma-mass spectrometry -LA-ICP-MS-), Ar/Ar hornblende and muscovite, and Rb-Sr whole rock-muscovite isochron data from the mylonites of the Sarandí del Yí Shear Zone, Uruguay, were obtained in order to assess the tectonothermal evolution of this crustal-scale structure. Integration of these results with available kinematic, structural, and microstructural data of the shear zone as well as with geochronological data from the adjacent blocks allowed to constrain the onset of deformation along the shear zone at 630-625 Ma during the collision of the Nico Pérez Terrane and the Río de la Plata Craton. The shear zone underwent dextral shearing up to 596 Ma under upper to middle amphibolite facies conditions, which was succeeded by sinistral shearing under lower amphibolite to upper greenschist facies conditions until at least 584 Ma. After emplacement of the Cerro Caperuza granite at 570 Ma, the shear zone underwent only cataclastic deformation between the late Ediacaran and the Cambrian. The Sarandí del Yí Shear Zone is thus related to the syncollisional to postcollisional evolution of the amalgamation of the Río de la Plata Craton and the Nico Pérez Terrane. Furthermore, the obtained data reveal that strain partitioning and localization with time, magmatism emplacement, and fluid circulation are key processes affecting the isotopic systems in mylonitic belts, revealing the complexity in assessing the age of deformation of long-lived shear zones.

  17. Synaptic Vesicle Proteins and Active Zone Plasticity

    PubMed Central

    Kittel, Robert J.; Heckmann, Manfred

    2016-01-01

    Neurotransmitter is released from synaptic vesicles at the highly specialized presynaptic active zone (AZ). The complex molecular architecture of AZs mediates the speed, precision and plasticity of synaptic transmission. Importantly, structural and functional properties of AZs vary significantly, even for a given connection. Thus, there appear to be distinct AZ states, which fundamentally influence neuronal communication by controlling the positioning and release of synaptic vesicles. Vice versa, recent evidence has revealed that synaptic vesicle components also modulate organizational states of the AZ. The protein-rich cytomatrix at the active zone (CAZ) provides a structural platform for molecular interactions guiding vesicle exocytosis. Studies in Drosophila have now demonstrated that the vesicle proteins Synaptotagmin-1 (Syt1) and Rab3 also regulate glutamate release by shaping differentiation of the CAZ ultrastructure. We review these unexpected findings and discuss mechanistic interpretations of the reciprocal relationship between synaptic vesicles and AZ states, which has heretofore received little attention. PMID:27148040

  18. Differential Activation and Inhibition of RhoA by Fluid Flow Induced Shear Stress in Chondrocytes

    PubMed Central

    Wan, Qiaoqiao; Kim, Seung joon; Yokota, Hiroki; Na, Sungsoo

    2013-01-01

    Physical force environment is a major factor that influences cellular homeostasis and remodeling. It is not well understood, however, as a potential role of force intensities in the induction of cellular mechanotransduction. Using a fluorescence resonance energy transfer (FRET)-based approach, we asked whether activities of GTPase RhoA in chondrocytes are dependent on intensities of flow induced shear stress. We hypothesized that RhoA activities can be either elevated or reduced by selecting different levels of shear stress intensities. The result indicate that C28/I2 chondrocytes have increased RhoA activities in response to high shear stress (10 or 20 dyn/cm2), whereas a decrease in activity was seen with an intermediate shear stress of 5 dyn/cm2. No changes were seen under low shear stress (2 dyn/cm2). The observed 2-level switch of RhoA activities is closely linked to the shear stress-induced alterations in actin cytoskeleton and traction forces. In the presence of constitutively active RhoA (RhoA-V14), intermediate shear stress suppressed RhoA activities, while high shear stress failed to activate them. In chondrocytes, expression of various metalloproteinases is, in part, regulated by shear and normal stresses through a network of GTPases. Collectively, the data suggest that intensities of shear stress are critical in differential activation and inhibition of RhoA activities in chondrocytes. PMID:23408748

  19. Two-zone heterogeneous structure within shear bands of a bulk metallic glass

    SciTech Connect

    Shao, Yang; Yao, Kefu; Liu, Xue; Li, Mo

    2013-10-21

    Shear bands, the main plastic strain carrier in metallic glasses, are severely deformed regions often considered as disordered and featureless. Here we report the observations of a sandwich-like heterogeneous structure inside shear bands in Pd{sub 40.5}Ni{sub 40.5}P{sub 19} metallic glass sample after plastic deformation by high-resolution transmission electron microscopy. The experimental results suggest a two-step plastic deformation mechanism with corresponding microstructure evolution at atomic scale, which may intimately connected to the stability of the shear band propagation and the overall plastic deformability.

  20. Seismic Anisotropy of the Mexican Subduction Zone Based on the Surface Waves, Shear Wave Splitting, and Higher Modes.

    NASA Astrophysics Data System (ADS)

    Stubailo, I.; Davis, P. M.

    2014-12-01

    The Mexico subduction zone is characterized by both steep and flat subduction, and a volcanic arc that appears to be oblique to the trench. It has excellent seismic data coverage due to the 2005-2007 Middle America Subduction Experiment (MASE) and the permanent Mexican stations. Here, we study the anisotropy of the region using Surface waves, shear-wave splitting measurements, and higher modes. Our goal is to verify and complement the three-dimensional model of shear-wave velocity and anisotropy in the region constructed using Rayleigh wave phase velocity dispersion measurements (Stubailo et al., JGR, 2012) and constrain the depth of the shear-wave splitting anisotropy with the help of the n1-3 overtones. The 3D model contains lateral variations in shear wave velocity consistent with the presence of flat and steep subduction, as well as variations in azimuthal anisotropy, that suggest a tear between the flat and steep portions of the slab. Shear-wave splitting is effective for studying mantle anisotropy beneath the receivers and has a better lateral resolution than the Rayleigh wave phase velocity dispersion measurements, although it suffers from a poor depth resolution. To better resolve the anisotropy at depth, we also calculate the anisotropy based on the higher mode surface waves of different overtones for Mexican stations using least-squares fitting of the synthetic higher mode seismograms to the data collected from the deep earthquakes. The three methods allow us to separate the anisotropy and its strength at different depths. We will report on our shear-wave splitting and higher mode results, and their comparison, and present evidence that anisotropy under Mexico is of deep origin.

  1. The Cora Lake Shear Zone, an Exhumed Deep Crustal Lithotectonic Discontinuity, Western Churchill Province, Canada

    NASA Astrophysics Data System (ADS)

    Regan, S.; Leslie, S.; Holland, M. E.; Williams, M. L.; Mahan, K. H.; Jercinovic, M. J.

    2011-12-01

    Deep crustal flow is a fundamental tectonic process that may serve to reduce topographic gradients, especially in overthickened collisional orogens. Recent studies have utilized numerical models and seismic interpretations, but generally in two dimensions. Although useful, two dimensional models can not fully characterize lower crustal flow or coupling of crustal layers because they cannot fully incorporate lateral heterogeneity in the flow field. The Athabasca Granulite terrane, in northern Saskatchewan, is an exposed deep crustal terrane that underwent granulite grade deformation during the Neoarchean (ca. 2.55), then cooled isobarically for 600 m.y., and then was reactivated during the Paleoproterozoic (ca. 1.9 Ga). Regional exhumation occurred at roughly 1.85 Ga. This exposure, is a field laboratory for studying lower crustal flow, stabilization, and reactivation. Recent work suggests that the northwestern domain, dominated by the multiphase, opx-bearing, Mary batholith, underwent top-to-the-east lower crustal flow during the Neoarchean. The Chipman domain, to the SE , is primarily underlain by the 3.2 Ga, Chipman tonalite straight gneiss, which was likely restitic, and rheologically strong during the 2.6 Ga flow event. The Cora Lake shear zone (CLsz), which divides the two domains, is interpreted to represent a lithotectonic, compositional, and rheologic boundary within the deep crust. Recent mapping of the western gradient of the CLsz has provided insight into the role and evolution of the rheologic discontinuity and its relationship to crustal flow. The Mary granite (gneiss) contains excellent assemblages for P-T and pseudosection analysis. Interlayered felsic granulite contain abundant monazite for in-situ geochronology. An intense subhorizontal tectonic fabric (S1), interpreted to be the product of crustal flow, is present in both units. This early fabric was locally crenulated, folded, and transposed, by a sub-vertical S2 fabric. Current work involves

  2. TTGs in the making: Natural evidence from Inyoni shear zone (Barberton, South Africa)

    NASA Astrophysics Data System (ADS)

    Nédélec, A.; Chevrel, M. O.; Moyen, J. F.; Ganne, J.; Fabre, S.

    2012-11-01

    Despite the consensus that TTGs, the main constituents of the Archaean continental crust worldwide, originated by partial melting of garnet-bearing amphibolites, natural evidence is scarce. A large variety of Archaean amphibole-rich rocks, including migmatitic amphibolites and hornblende-rich cumulates, was exhumed as a tectonic melange in the Inyoni shear zone (ISZ) near Barberton (South Africa), likely at the time of emplacement of the 3.2 Ga Nelshoogte-Badplaas plutons. This unique collection provides the opportunity to investigate partial melting of garnet-amphibolites, as well as the differentiation processes occurring in the TTG magmas en route to the surface. The ISZ migmatitic amphibolites are characterized by quartz-plagioclase leucosomes in equilibrium with garnet, amphibole, titanite ± epidote. Garnet compositions are characterized by high almandine and grossular contents. Actually, the leucosomes and neosomes likely lost part of their melt component and mesosomes may have been also modified towards restite-rich compositions due to melt segregation. Restite-bearing rocks are very iron-rich according to their high contents in garnet. Besides, the Mg-rich compositions of the hornblende cumulates and their REE distribution patterns resemble those of the nearby TT (tonalite-trondhjemite) plutons. Thermobarometric estimates from the ISZ migmatitic rocks cluster in the range 720-800 °C and 1.1-1.2 GPa for the melting reaction and the derived geothermal gradient is ca 17-22 °C/km. These conditions are consistent with either water-present melting or, more likely, water-absent epidote-dehydration melting. Such a geothermal gradient is therefore assigned to the genesis of large volumes of medium-pressure TTG magmas, that will likely form at greater depth. Unmelted garnet-bearing amphibolites correspond to slightly lower conditions. The magmatic cumulates crystallized at ca 0.6 GPa. The cumulate rocks evidence that the TTG parental magmas en route to the surface

  3. Surface temperature estimation in Singhbhum Shear Zone of India using Landsat-7 ETM+ thermal infrared data

    NASA Astrophysics Data System (ADS)

    Srivastava, P. K.; Majumdar, T. J.; Bhattacharya, Amit K.

    2009-05-01

    Land surface temperature (LST) is an important factor in global change studies, heat balance and as control for climate change. A comparative study of LST over parts of the Singhbhum Shear Zone in India was undertaken using various emissivity and temperature retrieval algorithms applied on visible and near infrared (VNIR), and thermal infrared (TIR) bands of high resolution Landsat-7 ETM+ imagery. LST results obtained from satellite data of October 26, 2001 and November 2, 2001 through various algorithms were validated with ground measurements collected during satellite overpass. In addition, LST products of MODIS and ASTER were compared with Landsat-7 ETM+ and ground truth data to explore the possibility of using multi-sensor approach in LST monitoring. An image-based dark object subtraction (DOS3) algorithm, which is yet to be tested for LST retrieval, was applied on VNIR bands to obtain atmospheric corrected surface reflectance images. Normalized difference vegetation index (NDVI) was estimated from VNIR reflectance image. Various surface emissivity retrieval algorithms based on NDVI and vegetation proportion were applied to ascertain emissivities of the various land cover categories in the study area in the spectral range of 10.4-12.5 μm. A minimum emissivity value of about 0.95 was observed over the reflective rock body with a maximum of about 0.99 over dense forest. A strong correlation was established between Landsat ETM+ reflectance band 3 and emissivity. Single channel based algorithms were adopted for surface radiance and brightness temperature. Finally, emissivity correction was applied on 'brightness temperature' to obtain LST. Estimated LST values obtained from various algorithms were compared with field ground measurements for different land cover categories. LST values obtained after using Valor's emissivity and single channel equations were best correlated with ground truth temperature. Minimum LST is observed over dense forest as about 26 °C and

  4. Block and shear-zone architecture of the Minnesota River Valley subprovince: Implications for late Archean accretionary tectonics

    USGS Publications Warehouse

    Southwick, D.L.; Chandler, V.W.

    1996-01-01

    The Minnesota River Valley subprovince of the Superior Province is an Archean gneiss terrane composed internally of four crustal blocks bounded by three zones of east-northeast-trending linear geophysical anomalies. Two of the block-bounding zones are verified regional-scale shears. The geological nature of the third boundary has not been established. Potential-field geophysical models portray the boundary zones as moderately north-dipping surfaces or thin slabs similar in strike and dip to the Morris fault segment of the Great Lakes tectonic zone at the north margin of the subprovince. The central two blocks of the subprovince (Morton and Montevideo) are predominantly high-grade quartzofeldspathic gneiss, some as old as 3.6 Ga, and late-tectonic granite. The northern and southern blocks (Benson and Jeffers, respectively) are judged to contain less gneiss than the central blocks and a larger diversity of syntectonic and late-tectonic plutons. A belt of moderately metamorphosed mafic and ultramafic rocks having some attributes of a dismembered ophiolite is partly within the boundary zone between the Morton and Montevideo blocks. This and the other block boundaries are interpreted as late Archean structures that were reactivated in the Early Proterozoic. The Minnesota River Valley subprovince is interpreted as a late accretionary addition to the Superior Province. Because it was continental crust, it was not subductible when it impinged on the convergent southern margin of the Superior Craton in late Archean time, and it may have accommodated to convergent-margin stresses by dividing into blocks and shear zones capable of independent movement.

  5. Analecta of structures formed during the 28 June 1992 Landers-Big Bear, California earthquake sequence (including maps of shear zones, belts of shear zones, tectonic ridge, duplex en echelon fault, fault elements, and thrusts in restraining steps)

    SciTech Connect

    Johnson, A.M.; Johnson, N.A.; Johnson, K.M.; Wei, W.; Fleming, R.W.; Cruikshank, K.M.; Martosudarmo, S.Y.

    1997-12-31

    The June 28, 1992, M{sub s} 7.5 earthquake at Landers, California, which occurred about 10 km north of the community of Yucca Valley, California, produced spectacular ground rupturing more than 80 km in length (Hough and others, 1993). The ground rupturing, which was dominated by right-lateral shearing, extended along at least four distinct faults arranged broadly en echelon. The faults were connected through wide transfer zones by stepovers, consisting of right-lateral fault zones and tension cracks. The Landers earthquakes occurred in the desert of southeastern California, where details of ruptures were well preserved, and patterns of rupturing were generally unaffected by urbanization. The structures were varied and well-displayed and, because the differential displacements were so large, spectacular. The scarcity of vegetation, the aridity of the area, the compactness of the alluvium and bedrock, and the relative isotropy and brittleness of surficial materials collaborated to provide a marvelous visual record of the character of the deformation zones. The authors present a series of analecta -- that is, verbal clips or snippets -- dealing with a variety of structures, including belts of shear zones, segmentation of ruptures, rotating fault block, en echelon fault zones, releasing duplex structures, spines, and ramps. All of these structures are documented with detailed maps in text figures or in plates (in pocket). The purpose is to describe the structures and to present an understanding of the mechanics of their formation. Hence, most descriptions focus on structures where the authors have information on differential displacements as well as spatial data on the position and orientation of fractures.

  6. Brittle grain-size reduction of feldspar, phase mixing and strain localization in granitoids at mid-crustal conditions (Pernambuco shear zone, NE Brazil)

    NASA Astrophysics Data System (ADS)

    Viegas, Gustavo; Menegon, Luca; Archanjo, Carlos

    2016-03-01

    The Pernambuco shear zone (northeastern Brazil) is a large-scale strike-slip fault that, in its eastern segment, deforms granitoids at mid-crustal conditions. Initially coarse-grained (> 50 µm) feldspar porphyroclasts are intensively fractured and reduced to an ultrafine-grained mixture consisting of plagioclase and K-feldspar grains (< 15 µm) localized in C' shear bands. Detailed microstructural observations and electron backscatter diffraction (EBSD) analysis do not show evidence of intracrystalline plasticity in feldspar porphyroclasts and/or fluid-assisted replacement reactions. Quartz occurs either as thick (˜ 1-2 mm) monomineralic veins transposed along the shear zone foliation or as thin ribbons ( ≤ 25 µm width) dispersed in the feldspathic mixture. The microstructure and c axis crystallographic-preferred orientation are similar in the thick monomineralic veins and in the thin ribbons, and they suggest dominant subgrain rotation recrystallization and activity of prism < a > and rhomb < a > slip systems. However, the grain size in monophase recrystallized domains decreases when moving from the quartz monomineralic veins to the thin ribbons embedded in the feldspathic C' bands (14 µm vs. 5 µm respectively). The fine-grained feldspar mixture has a weak crystallographic-preferred orientation interpreted as the result of shear zone parallel-oriented growth during diffusion creep, as well as the same composition as the fractured porphyroclasts, suggesting that it generated by mechanical fragmentation of rigid porphyroclasts with a negligible role of chemical disequilibrium. Once C' shear bands were generated and underwent viscous deformation at constant stress conditions, the polyphase feldspathic aggregate would have deformed at a strain rate 1 order of magnitude faster than the monophase quartz monomineralic veins, as evidenced by applying experimentally and theoretically calibrated flow laws for dislocation creep in quartz and diffusion creep in

  7. A mega shear zone in the Central Range of Taiwan and it's implication for the Late Mesozoic subduction of the paleo-Pacific plate

    NASA Astrophysics Data System (ADS)

    Yi, D. C.; Lin, C. W.

    2012-04-01

    The metamorphic basement "Tananao Complex" exposed in the eastern flank of the Central Range of Taiwan. The ancient Asian continental margin deposited a thick sequence of sandstone, shale, limestone and volcanic rocks that was the protolith of Tananao Complex. In Late Mesozoic Era, the thick sequence of rocks was subjected to several phases of metamorphism and deformation to form the pair metamorphic belts which were the western Tailuko Belt and the eastern Yuli Belt. The Tailuko belt is composed of phyllite, quartzite, quartz-mica schist, meta-conglomerate, gneiss, meta-basite, amphibolite, serpentinite, marble and meta-chert, etc. The Yuli belt is composed of a monotonous assemblage of quartz-mica schist, subordinate meta-basite and serpentinite, etc. It is believed that the boundary of the Tailuko belt and the Yuli belt is a large fault, but the field evidence of the fault has never been found. In this study, meso-scale field investigation of the lithologies and rock fabrics indicate that a mega shear zone, called "The Daguan shear zone", separated the Tailuko belt from the Yuli belt. The Daguan shear zone is a NNE trending and west dipping mega shear zone which is mainly composed of mylonitic dark gray quartz-mica schist and mica schist, intercalated with 1 to 2 centimeters thick of elongated meta-conglomerate band. The shear zone is composed of numerous meso-scale ductile shear zones. Additionally, the shaer zone is characterized by abundant varied quartz veins that have been refolded to lenticular or pod shape and nearly parallel to S2 cleavage. Compaed to the existing geological information of the Central Range, we believe that the Daguan shear zone played a role as the boundary of the subduction zone which the paleo-Pacific Plate subducted into the Eurasian Plate in Late Mesozoic Era.

  8. Fe-Ti-oxide textures and microstructures in shear zones from oceanic gabbros at Atlantis Bank, Southwest Indian Ridge

    NASA Astrophysics Data System (ADS)

    Till, Jessica; Morales, Luiz F. G.; Rybacki, Erik

    2016-04-01

    Ocean drilling expeditions at several oceanic core complexes formed at slow- and ultra-slow-spreading ridges have recovered cores containing numerous zones of oxide-rich gabbros containing ilmenite and magnetite. In these cores, high modal concentrations of Fe-Ti-oxides are systematically associated with high-temperature plastic deformation features in silicates. We present observations of Fe-Ti-oxide mineral structures and textural characteristics from a series of oxide-rich shear zones from Atlantis Bank (ODP Site 735B) on the Southwest Indian Ridge aimed at determining how oxide mineral abundances relate to strain localization. Fe-Ti-oxide minerals in undeformed oxide gabbros and in highly deformed samples from natural shear zones generally have morphologies characteristic of crystallized melt, including highly cuspate grains and low dihedral angles. Anisotropy of magnetic susceptibility in oxide-rich shear zones is very strong, with fabrics mainly characterized by strong magnetic foliations parallel to the macroscopic foliation. Crystallographic preferred orientations (CPO) in magnetite are generally weak, with occasionally well-defined textures. Ilmenite typically displays well-developed CPOs, however, the melt-like ilmenite grain shapes indicate that at least part of the crystallographic texture results from oriented ilmenite growth during post-deformation crystallization. The oxides are hypothesized to have initially been present as isolated pockets of trapped melt (intercumulus liquid) in a load-bearing silicate framework. Progressive plastic deformation of silicate phases at high-temperature mainly produced two features: (i) elongated melt pockets, which crystallized to form strings of opaque minerals and (ii), interconnected networks of melt regions. The latter lead to intense strain localization of the rock, which appears as oxide-rich mylonites in the samples. In some samples, abundant low-angle grain boundaries in both magnetite and ilmenite suggest

  9. Right-lateral shear across Iran and kinematic change in the Arabia-Eurasia collision zone

    NASA Astrophysics Data System (ADS)

    Allen, Mark B.; Kheirkhah, Monireh; Emami, Mohammad H.; Jones, Stuart J.

    2011-02-01

    New offset determinations for right-lateral strike-slip faults in Iran revise the kinematics of the Arabia-Eurasia collision, by indicating along-strike lengthening of the collision zone before a change to the present kinematic regime at ˜5 Ma. A series of right-lateral strike-slip faults is present across the Turkish-Iranian plateau between 48°E and 57°E. Fault strikes vary between NW-SE and NNW-SSE. Several of the faults are seismically active and/or have geomorphic evidence for Holocene slip. None of the faults affects the GPS-derived regional velocity field, indicating active slip rates are ≤2 mm yr-1. We estimate total offsets for these faults from displaced geological and geomorphic markers, based on observations from satellite imagery, digital topography, geology maps and our own fieldwork observations, and combine these results with published estimates for fault displacement. Total right-lateral offset of the Dehu, Anar, Deh Shir, Kashan, Ab-Shirin-Shurab, Kousht Nousrat, Qom, Bid Hand, Indes, Soltanieh and Takab faults is ˜250 km. Other faults (North Zanjan, Saveh, Jorjafk, Rafsanjan, Kuh Banan and Behabad) have unknown or highly uncertain amounts of slip. Collectively, these faults are inferred to have accommodated part of the Arabia-Eurasia convergence. Three roles are possible, which are not mutually exclusive: (1) shortening via anticlockwise, vertical axis rotations; (2) northward movement of Iranian crust with respect to stable Afghanistan to the east; (3) combination with coeval NW-SE thrusts in the Turkish-Iranian plateau, to produce north-south plate convergence (`strain partitioning'). This strike-slip faulting across Iran requires along-strike lengthening of the collision zone. This was possible until the Pliocene (≤ 5 Ma), when the Afghan crust collided with the western margin of the Indian plate, thereby sealing off a free face at the eastern side of the Arabia-Eurasia collision zone. Continuing Arabia-Eurasia plate convergence had to

  10. Role of mineralogical and chemical changes on shear zone nucleation: an example from the Neves area (Tauern Window, Eastern Alps, Italy)

    NASA Astrophysics Data System (ADS)

    Link, Gaetan; Goncalves, Philippe; Lanari, Pierre; Oliot, Emilien

    2015-04-01

    Several studies have proposed a model for shear zone nucleation in granitoids, in which an initial anisotropy, frequently a brittle structure or a compositional heterogeneity, is required to localize and initiate the ductile shearing in granitoids. This model is referred as an inherited-localization model. This brittle precursor is also the loci of intense fluid-rock interactions that induce mineral and chemical transformations that control the behavior of shear zone. However, Oliot et al. (2010, 2014) suggest that brittle precursors are not necessary required to initiate the ductile shearing in granites. Indeed, the early recrystallization of metastable phases (like plagioclase) into a fine-grained metamorphic assemblage, stable at the P-T-fluid conditions of the deformation, might induce local weakening required for strain localization and shear zone nucleation. We will define this model as the metamorphic-localization model. In both models, shear zone nucleation appears to be associated with metamorphic reactions, fluid flow and mass transfer. The goal of this contribution is to document and quantify the mineralogical and chemical changes involved during the process of shear zone nucleation in granitoids, and to discuss their role. Answering these questions requires to study shear zones in which petrological and microstructural evidences of the deformation have not been obliterated by subsequent ductile deformation. One of the best example is located in the Neves area (Tauern Window, Eastern Alps, Italy) where a Variscan granodiorite is affected by an Alpine amphibolite facies deformation strongly localized on precursor fractures. The studied samples are mm to cm-wide shear zones with distinct mineralogical evolution. In the undeformed granodiorite, the mineralogical assemblage consists of quartz, K-feldspar, biotite and a fine-grained assemblage of albite, epidote and white mica developed at the expense of the metastable magmatic plagioclase. In addition to the

  11. Evidence for nearly orthogonal, Oligocene crustal flow beneath the coeval mylonitic shear zone of the Ruby Mountains core complex, Nevada

    SciTech Connect

    Maccready, T.; Snoke, A.W. . Dept. of Geology and Geophysics); Wright, J.E. . Dept. of Geology and Geophysics)

    1993-04-01

    Structural analysis and new geochronologic data indicate a nearly orthogonal, Oligocene flow pattern in migmatitic infrastructure immediately beneath the kilometer-scale mylonitic shear zone of the Ruby Mountains metamorphic core complex, Nevada. Three zircon fractions from a biotite monzogranitic orthogneiss yielded a crystallization age of 30 [+-] 1 Ma. Single monazite fractions from two leucocratic granitic orthogneisses indicated a preliminary crystallization age of 37 Ma. These three samples all exhibit a penetrative, approximately north-south-trending elongation lineation. The elongation lineation is interpreted as the slip line in the flow plane of the migmatitic, nonmylonitic infrastructural core of the northern Ruby Mountains. This flow pattern is coeval with the well-documented, west-northwest sense-of-slip in the structurally overlying kilometer-scale mylonitic shear zone suggesting fundamental plastic decoupling between structural levels in this core complex. Furthermore, the study area is characterized by overlapping, oppositely verging fold-nappes which are rooted to the east and west. The penetrative, elongation lineation is subparallel to parasitic folds developed synchronous with the fold-nappes suggesting a kinematically related evolution. One possible model is that late Eocene-early Oligocene granitic magmas invaded a migmatitic core zone undergoing a complex flow pattern related to large-scale crustal extension. The local flow pattern in the migmatitic infrastructure may be a manifestation of mid-crustal migration of low viscosity crustal material into an area beneath a domain of highly extended upper crustal rocks.

  12. Geodetic slip rate for the eastern California shear zone and the recurrence time of Mojave desert earthquakes

    USGS Publications Warehouse

    Sauber, J.; Thatcher, W.; Solomon, S.C.; Lisowski, M.

    1994-01-01

    Where the San Andreas fault passes along the southwestern margin of the Mojave desert, it exhibits a large change in trend, and the deformation associated with the Pacific/North American plate boundary is distributed broadly over a complex shear zone. The importance of understanding the partitioning of strain across this region, especially to the east of the Mojave segment of the San Andreas in a region known as the eastern California shear zone (ECSZ), was highlighted by the occurrence (on 28 June 1992) of the magnitude 7.3 Landers earthquake in this zone. Here we use geodetic observations in the central Mojave desert to obtain new estimates for the rate and distribution of strain across a segment of the ECSZ, and to determine a coseismic strain drop of ~770 ??rad for the Landers earthquake. From these results we infer a strain energy recharge time of 3,500-5,000 yr for a Landers-type earthquake and a slip rate of ~12 mm yr-1 across the faults of the central Mojave. The latter estimate implies that a greater fraction of plate motion than heretofore inferred from geodetic data is accommodated across the ECSZ.

  13. Field and Microstructure Study of Transpressive Jogdadi shear zone near Ambaji, Aravalli- Delhi Mobile Belt, NW India and its tectonic implication on the exhumation of granulites.

    NASA Astrophysics Data System (ADS)

    Tiwari, Sudheer Kumar; Biswal, Tapas Kumar

    2016-04-01

    Aravalli- Delhi mobile belt is situated in the northwestern part of Indian shield. It comprises tectono- magmatic histories from Archean to Neoproterozoic age. It possesses three tectono- magmatic metamorphic belts namely Bhilwara Supergroup (3000 Ma), Aravalli Supergorup (1800 Ma) and Delhi Supergroup (1100 -750Ma). The Delhi Supergroup is divided in two parts North Delhi and South Delhi; North Delhi (1100 Ma to 850 Ma) is older than South Delhi (850 Ma to 750 Ma). The study area falls in the South Delhi terrane; BKSK granulites are the major unit in this terrane. BKSK granulites comprise gabbro- norite-basic granulite, pelitic granulite, calcareous granulite and occur within the surrounding of low grade rocks as meta- rhyolite, quartzite, mica schist and amphibolites. The high grade and low grade terranes share a sheared margin. Granulites have undergone three phases of folding, intruded by three phases of granites and traversed by many shear zones. One of the shear zones is Jogdadi shear zone which consists of granitic mylonites and other sheared rocks. Jogdadi shear zone carries the evidence of both ductile as well as brittle shearing. It strikes NW- SE; the mylonitic foliation dip moderately to SW or NE and stretching lineations are oblique towards SE. The shear zone is folded and gabbro- norite - basic granulite occurs at the core. One limb of fold passes over coarse grained granite while other limb occurs over gabbro- norite- basic granulite. Presence of mylonitic foliation, asymmetric folding, S-C fabrics, porphyroclasts, mica fishes and book shelf- gliding are indicative of ductile deformation. Most of the porphyroclasts are sigmoidal and delta types but there are also some theta and phi type porphyroclasts. Book shelf-gliding structures are at low angle to the C plane. The shear zone successively shows protomylonite, mylonite and ultramylonites from margin to the centre. As the mylonitization increases recrystallized quartz grains appear. Porphyroclasts

  14. The Eidsfjord shear zone, Lofoten-Vesterålen, north Norway: An Early Devonian, paleoseismogenic low-angle normal fault

    NASA Astrophysics Data System (ADS)

    Steltenpohl, Mark G.; Moecher, David; Andresen, Arild; Ball, Jacob; Mager, Stephanie; Hames, Willis E.

    2011-05-01

    We report structural and 40Ar/39Ar isotopic information on the Eidsfjord shear zone that document it to be a seismogenic, tops-west (hinterland directed), Devonian ductile low angle (25-30° dip, shallowing locally) normal detachment fault. Anorthosite/migmatitic gneiss in the detachment's upper plate, mangerite in the lower plate, and detachment mylonites are all cut by generations of abundant pseudotachylyte occurring over approximately 150 km2. The mean of four laser 40Ar/39Ar plateau ages for single crystals of recrystallized muscovite from mylonites defining the Eidsfjord shear zone indicates an age of 403.6 ± 1.1 Ma (2σ) for deformation and recrystallization. 40Ar/39Ar step-heating analyses are reported for muscovite from mylonitized rocks of the Fiskefjord shear zone, a nearby tops-east Caledonian thrust that was reactivated as a tops-west normal fault, documenting cooling of the upper plate through the ˜350 °C isotherm at ˜457 Ma. Together with Middle-Ordovician tectonothermal relics found farther west in Lofoten, tops-down-to-the-west normal-slip movement on these extensional shear zones explains maintenance of high-crustal levels throughout the Siluro-Devonian Scandian event. Potassium feldspar 40Ar/39Ar results document a pulse, or multiple pulses, of uplift and cooling between ca. 235 Ma and 185 Ma, consistent with formation of Triassic-Jurassic rift basins flanking the Lofoten Ridge. The Eidsfjord detachment appears to mark the northern terminus of the Early Devonian detachment system traceable 800 km southward to the Nordfjord-Sogn detachment and westward across the North Atlantic to detachments of roughly the same age on the conjugate side of the orogen in East Greenland. The timing, geometry, kinematics, and rheological development of Eidsfjord detachment are grossly similar to the Nordfjord-Sogn detachment but the former contrasts in that it presently lacks exposed deposits of Devonian sedimentary rocks, has smaller magnitudes of displacement

  15. Distribution of radionuclides in surface soils, Singhbhum Shear Zone, India and associated dose.

    PubMed

    Patra, A C; Sahoo, S K; Tripathi, R M; Puranik, V D

    2013-09-01

    Gamma emitters were estimated in surface soils from a mineralized zone in Eastern India using high purity Germanium detector-based high resolution gamma spectrometry system. Activities of (238)U, (226)Ra, (232)Th, (235)U, (227)Th, (234 m)Pa, (210)Pb, (40)K, and (137)Cs were 79 ± 50, 81 ± 53, 65 ± 23, 4 ± 2, 5 ± 4, 92 ± 50, 97 ± 45, 517 ± 201, and 4 ± 2 Bq/kg, respectively. Most radionuclides were observed to follow log-normal distribution. The correlation between physicochemical properties of the samples, like pH, organic matter content, particle size, and moisture content were also studied. Activity ratios of (226)Ra/(238)U, (210)Pb/(226)Ra, and (227)Th/(235)U indicated deviation from secular equilibrium in some samples. The associated annual effective dose ranged from 0.07 to 0.24 mSv and the mean was calculated to be 0.12 ± 0.04 mSv for this region, indicating it to be one of normal natural background radiation. PMID:23456273

  16. Heterogeneity within a deep crustal strike-slip shear zone with implications for lower crustal flow, Athabasca granulite terrane, western Canadian Shield

    NASA Astrophysics Data System (ADS)

    Leslie, S. R.; Mahan, K. H.; Regan, S.; Williams, M. L.

    2011-12-01

    Deep crustal strike-slip shear zones play a fundamental role in lower crustal flow. Although commonly modeled in two-dimensions, regional considerations suggest that large-scale crustal flow is a heterogeneous, three-dimensional process. The Athabasca granulite terrane, western Canadian Shield, exposes a large region of high-pressure tectonite (>20,000 km2) that provides a natural example of ancient lower crustal flow and an analog for similar processes active today in other regions. Regional heterogeneous deformation permits preservation of Neoarchean deformation fabrics and metamorphic textures. The Cora Lake shear zone (CLsz) is a NW-dipping km-scale mylonite to ultramylonite zone that forms a discrete tectonic discontinuity between two rheologically distinct Neoarchean lower-crustal domains. Northwest of the CLsz, the domain is primarily underlain by ~2.6 Ga felsic to mafic metaplutonic gneisses and interlayered ~2.55 Ga felsic granulite. Lithologies here preserve Neoarchean granulite-facies metamorphism coupled with partial melting and synkinematic melt-enhanced ESE-directed subhorizontal flow at ~0.9 GPa (~30 km paleodepths). Southeast of the CLsz, the Chipman domain is underlain by ~3.2 Ga metatonalite gneiss, an extensive ~1.9 Ga mafic dike swarm, and generally minor ~2.55 Ga mafic and felsic granulite. In contrast to the northwest, lithologies of the western Chipman domain document higher pressures at ~1.3 GPa (~40 km paleodepths) synchronous with development of a gently dipping Neoarchean gneissic fabric. Strong, anhydrous Chipman domain lithologies and melt-weakened lithologies to the northwest are juxtaposed by sinistral to sinistral-normal oblique shear along the CLsz, consistent with higher pressures (deeper paleodepths) documented in the footwall Chipman domain. A notable and pervasive feature along strike of the CLsz in the western Chipman domain is the marked increase in abundance of m-scale layers of mafic and felsic granulite westward with

  17. Geometry and subsidence history of the Dead Sea basin: A case for fluid-induced mid-crustal shear zone?

    USGS Publications Warehouse

    ten Brink, U.S.; Flores, C.H.

    2012-01-01

    Pull-apart basins are narrow zones of crustal extension bounded by strike-slip faults that can serve as analogs to the early stages of crustal rifting. We use seismic tomography, 2-D ray tracing, gravity modeling, and subsidence analysis to study crustal extension of the Dead Sea basin (DSB), a large and long-lived pull-apart basin along the Dead Sea transform (DST). The basin gradually shallows southward for 50 km from the only significant transverse normal fault. Stratigraphic relationships there indicate basin elongation with time. The basin is deepest (8-8.5 km) and widest (???15 km) under the Lisan about 40 km north of the transverse fault. Farther north, basin depth is ambiguous, but is 3 km deep immediately north of the lake. The underlying pre-basin sedimentary layer thickens gradually from 2 to 3 km under the southern edge of the DSB to 3-4 km under the northern end of the lake and 5-6 km farther north. Crystalline basement is ???11 km deep under the deepest part of the basin. The upper crust under the basin has lower P wave velocity than in the surrounding regions, which is interpreted to reflect elevated pore fluids there. Within data resolution, the lower crust below ???18 km and the Moho are not affected by basin development. The subsidence rate was several hundreds of m/m.y. since the development of the DST ???17 Ma, similar to other basins along the DST, but subsidence rate has accelerated by an order of magnitude during the Pleistocene, which allowed the accumulation of 4 km of sediment. We propose that the rapid subsidence and perhaps elongation of the DSB are due to the development of inter-connected mid-crustal ductile shear zones caused by alteration of feldspar to muscovite in the presence of pore fluids. This alteration resulted in a significant strength decrease and viscous creep. We propose a similar cause to the enigmatic rapid subsidence of the North Sea at the onset the North Atlantic mantle plume. Thus, we propose that aqueous fluid flux

  18. Slip rate of the Calico fault: Implications for geologic versus geodetic rate discrepancy in the Eastern California Shear Zone

    NASA Astrophysics Data System (ADS)

    Oskin, Michael; Perg, Lesley; Blumentritt, Dylan; Mukhopadhyay, Sujoy; Iriondo, Alexander

    2007-03-01

    Long-term (105 years) fault slip rates test the scale of discrepancy between infrequent paleoseismicity and relatively rapid geodetic rates of dextral shear in the Eastern California Shear Zone (ECSZ). The Calico fault is one of a family of dextral faults that traverse the Mojave Desert portion of the ECSZ. Its slip rate is determined from matching and dating incised Pleistocene alluvial fan deposits and surfaces displaced by fault slip. A high-resolution topographic base acquired via airborne laser swath mapping aids in identification and mapping of deformed geomorphic features. The oldest geomorphically preserved alluvial fan, unit B, is displaced 900 ± 200 m from its source at Sheep Springs Wash in the northern Rodman Mountains. This fan deposit contains the first preserved occurrence of basalt clasts derived from the Pipkin lava field and overlies Quaternary conglomerate deposits lacking these clasts. The 40Ar/39Ar dating of two flows from this field yields consistent ages of 770 ± 40 ka and 735 ± 9 ka. An age of 650 ± 100 ka is assigned to this fan deposit based on these ages and on the oldest cosmogenic 3He exposure date of 653 ± 20 ka on a basalt boulder from the surface of unit B. This assigned age and offset together yield a mid-Pleistocene to present average slip rate of 1.4 ± 0.4 mm/yr. A younger fan surface, unit K, records 100 ± 10 m of dextral displacement and preserves original depositional morphology of its surface. Granitic boulders and pavement samples from this surface yield an average age of 56.4 ± 7.7 ka after taking into account minimal cosmogenic inheritance of granitic clasts. The displaced and dated K fans yield a slip rate of 1.8 ± 0.3 mm/yr. Distributed deformation of the region surrounding the fault trace, if active, could increase the overall displacement rate to 2.1 ± 0.5 mm/yr. Acceleration of slip rate from an average of 1.4 mm/yr prior to ˜50 ka to 1.8 mm/yr since ˜50 ka is possible, though a single time-averaged slip

  19. Role of brittle deformation during the initiation of ductile HP-LT shear zone in a metarhyolite (Suretta nappe, Eastern Central Alps).

    NASA Astrophysics Data System (ADS)

    Poilvet, Jean-Charles; Goncalves, Philippe; Oliot, Emilien; Marquer, Didier

    2014-05-01

    Although ductile shear zones are common deformation structures in the middle to deep continental lithosphere, the initiation of such structures among homogeneous and isotropic protolith (e.g. granitoid bodies) is still a matter of debate. Indeed, the lack of consensus concerns the presence and the nature of a preexisting heterogeneity (structural or compositional, such as dykes, joints or cracks). This is mainly due to the lack of observation of preserved precursors, which, if they were present initially, are generally obliterated by subsequent intense deformation. Different conceptual models require a structural precursor, which enables fluids to flow and promotes metamorphic and metasomatic reactions via fluid-rock interactions. Those fluid-rock interactions are commonly presented as a key factor controlling strain localization or lateral propagation. The main goal of this contribution is to present the first observations, to our knowledge, of a preserved brittle precursor of a millimeter scale shear zone under blueschist facies metamorphic conditions. This work provides new evidences into how ductile shear zones occurring within homogeneous and isotropic protolith nucleate and develop. The present study exposes shear zones from the Roffna metarhyolite, a subvolcanic intrusion representing most of the northern part of the Suretta nappe (Penninic domain, Eastern Central Alps). This early Permian massif intruded an older basement and was affected only by Tertiary Alpine tectonics. The heterogeneous strain pattern consists, at all scales of anastomosing shear zones surrounding lenses of nearly undeformed rocks The investigated outcrop is characterized by the presence of a shear zone network from millimetric to plurimetric scale developed under blueschist facies conditions related to continental subduction of the European plate. A combined study including field observations, EBSD analysis, SEM-CL and conventional imaging together with thermodynamic modeling of phase

  20. Are quartz LPOs predictably oriented with respect to the shear zone boundary?: A test from the Alpine Fault mylonites, New Zealand

    NASA Astrophysics Data System (ADS)

    Little, Timothy A.; Prior, David J.; Toy, Virginia G.

    2016-03-01

    The Alpine fault self-exhumes its own ductile shear zone roots and has a known slip kinematics. Within ˜1 km of the fault, the mylonitic foliation is subparallel to the boundary of the amphibolite-facies ductile shear zone in which it formed. Using EBSD, we analyzed quartz Lattice Preferred Orientations [LPOs) of mylonites along a central part of the Alpine Fault. All LPOs feature a strongest girdle of [c]-axes that is forward-inclined ˜28 ± 4° away from the pole to the fault. A maximum of axes is inclined at the same angle relative the fault. The [c]-axis girdle is perpendicular to extensional (C') shear bands and the maximum is parallel to their slip direction. [c]-axis girdles do not form perpendicular to the SZB. Schmid factor analysis suggests that σ1 was arranged at 60-80° to the Alpine Fault. These observations indicate ductile transpression in the shear zone. The inclined arrangement of [c]-axis girdles, axes, and C' planes relative to the fault can be explained by their alignment relative to planes of maximum shear-strain-rate in a general shear zone, a significant new insight regarding shear zones and how LPO fabrics may generally develop within them. For the Alpine mylonite zone, our data imply a kinematic vorticity number (Wk) of ˜0.7 to ˜0.85. Inversions of seismic focal mechanisms in the brittle crust of the Southern Alps indicate that σ1 is oriented ˜60° to the Alpine Fault; that shear bands form at ˜30° to this direction, and that σ2 and σ3 flip positions between the brittle and ductile parts of the crust.

  1. Linking deformation and chemical re-equilibration: new results from the Cretaceous Vinschgau shear zone (Southern Tyrol, Italy)

    NASA Astrophysics Data System (ADS)

    Eberlei, Tobias; Habler, Gerlinde; Grasemann, Bernhard; Abart, Rainer

    2013-04-01

    The Austroalpine Matsch Unit in the European Eastern Alps preserves evidence for at least three tectonometamorphic cycles, including a Variscan amphibolite-facies metamorphism, a Permian high-T/low-P event related to the intrusion of pegmatites and a Cretaceous metamorphism at the greenschist to amphibolite-facies transition. The southern tectonic boundary of the Matsch unit is formed by the about 2 km thick Vinschgau shear zone, which was active during the Cretaceous and involves metapelites, orthogneisses and metapegmatites (Schmid & Haas 1989). A characteristic mylonitic foliation with an E-W trending stretching lineation, S-C and S-C-Ć fabrics with general top W non-coaxial shear kinematics developed. Remarkable strain-gradients occur at the cm- to m-scale, partly associated with lithological heterogeneities. In this contribution we focus on the relationship between deformation and microstructural- and chemical re-equilibration of white mica and feldspar in the Permian metapegmatites. Based on detailed structural characterization during field work, we used optical microscopy and scanning electron microscopy (SEM) as well as electron backscatter diffraction (EBSD) for microstructural and microtextural characterization. An electron microprobe (EPMA) was used for analyzing the major element compositions of microstructurally different phase generations. In the Permian metapegmatites metamorphic white mica, plagioclase feldspar (ab87 - ab99) and K-feldspar replaced the primary pegmatite assemblages. Magmatic white mica with near-end member composition of muscovite is strongly deformed, showing kinks, undulose extinction and cracks. A new fine-grained white mica generation with elevated celadonite-component (SiIV = 3.05-3.25 cations/11O) not only predominates the mylonitic foliation and compressional quadrants of mm-sized albite and muscovite clasts but also compositionally altered zones within primary magmatic white mica along kink planes and cleavage planes. The

  2. Metamorphic conditions in the Nilgiri Granulite Terrane and the adjacent Moyar and Bhavani Shear Zones: A reevaluation

    NASA Technical Reports Server (NTRS)

    Raith, M.; Hengst, C.; Nagel, B.; Bhattacharya, A.; Srikantappa, C.

    1988-01-01

    Data were presented on pressure and temperature determinations from the Nilgiri Hills. About 70 samples were analyzed by probe and several calibrations of garnet-pyroxene thermometry and barometry applied. Most calibrations gave considerable scatter; however, a new calibration by Bhattacharya, Raith, Lal, and others, accounting for nonideality in both garnet and orthopyroxene, gave consistent results of 754 + or - 52 C and 9.2 + or - 0.7 kbar. On the regional scale, a pressure increase of 6.5 to 7 kbar in the SW to 11 kbar in the NE was related to block tilting. A continuous pressure gradient into the Moyar shear zone suggests that the zone is not a suture juxtaposing unrelated blocks.

  3. Cell-matrix adhesion characterization using multiple shear stress zones in single stepwise microchannel

    NASA Astrophysics Data System (ADS)

    Kim, Min-Ji; Doh, Il; Bae, Gab-Yong; Cha, Hyuk-Jin; Cho, Young-Ho

    2014-08-01

    This paper presents a cell chip capable to characterize cell-matrix adhesion by monitoring cell detachment rate. The proposed cell chip can supply multiple levels of shear stress in single stepwise microchannel. As epithelial-mesenchymal transition (EMT), one of hallmarks of cancer metastasis is closely associated to the interaction with extracelluar matrix (ECM), we took advantage of two lung cancer cell models with different adhesion properties to ECM depending their epithelial or mesenchymal properties, including the pair of lung cancer cells with (A549sh) or without E-cadherin expression (A549sh-Ecad), which would be optimal model to examine the alteration of adhesion properties after EMT induction. The cell-matrix adhesion resisting to shear stress appeared to be remarkably differed between lung cancer cells. The detachment rate of epithelial-like H358 and mesenchymal-like H460 cells was 53%-80% and 25%-66% in the shear stress range of 34-60 dyn/cm2, respectively. A549sh-Ecad cells exhibits lower detachment rate (5%-9%) compared to A549sh cells (14%-40%). By direct comparison of adhesion between A549sh and A549sh-Ecad, we demonstrated that A549shE-cad to mimic EMT were more favorable to the ECM attachment under the various levels of shear stress. The present method can be applied to quantitative analysis of tumor cell-ECM adhesion.

  4. Fault valve action and vein development during strike slip faulting: An example from the Ribeira Shear Zone, Southeastern Brazil

    NASA Astrophysics Data System (ADS)

    Faleiros, Frederico Meira; Campanha, Ginaldo Ademar da Cruz; Bello, Rosa Maria da Silveira; Fuzikawa, Kazuo

    2007-06-01

    Fluid inclusion microthermometry and structural data are presented for quartz vein systems of a major dextral transcurrent shear zone of Neoproterozoic-Cambrian age in the Ribeira River Valley area, southeastern Brazil. Geometric and microstructural constraints indicate that foliation-parallel and extensional veins were formed during dextral strike-slip faulting. Both vein systems are formed essentially by quartz and lesser contents of sulfides and carbonates, and were crystallized in the presence of CO 2-CH 4 and H 2O-CO 2-CH 4-NaCl immiscible fluids following unmixing from a homogeneous parental fluid. Contrasting fluid entrapment conditions indicate that the two vein systems were formed in different structural levels. Foliation-parallel veins were precipitated beneath the seismogenic zone under pressure fluctuating from moderately sublithostatic to moderately subhydrostatic values (319-397 °C and 47-215 MPa), which is compatible with predicted fluid pressure cycle curves derived from fault-valve action. Growth of extensional veins occurred in shallower structural levels, under pressure fluctuating from near hydrostatic to moderately subhydrostatic values (207-218 °C and 18-74 MPa), which indicate that precipitation occurred within the near surface hydrostatically pressured seismogenic zone. Fluid immiscibility and precipitation of quartz in foliation-parallel veins resulted from fluid pressure drop immediately after earthquake rupture. Fluid immiscibility following a local pressure drop during extensional veining occurred in pre-seismic stages in response to the development of fracture porosity in the dilatant zone. Late stages of fluid circulation within the fault zone are represented dominantly by low to high salinity (0.2 to 44 wt.% equivalent NaCl) H 2O-NaCl-CaCl 2 fluid inclusions trapped in healed fractures mainly in foliation-parallel veins, which also exhibit subordinate H 2O-NaCl-CaCl 2, CO 2-(CH 4) and H 2O-CO 2-(CH 4)-NaCl fluid inclusions trapped

  5. Aggregation efficiency of activated normal or fixed platelets in a simple shear field: effect of shear and fibrinogen occupancy.

    PubMed Central

    Xia, Z; Frojmovic, M M

    1994-01-01

    Shear rate can affect protein adsorption and platelet aggregation by regulating both the collision frequency and the capture efficiency (alpha). These effects were evaluated in well defined shear field in a micro-couette for shear rate G = 10 - 1000 s-1. The rate of protein binding was independent of G, shown for adsorption of albumin to latex beads and PAC1 to activated platelets. The initial aggregation rate for ADP-activated platelets in citrated platelet-rich plasma followed second order kinetics at the initial platelet concentrations between 20,000 and 60,000/microliters. alpha values, which dropped nearly fivefold for a 10-fold increase in G, were approximately proportional to G-1, contrary to a minor drop predicted by the theory that includes protein cross-bridging. Varying ADP concentration did not change alpha of maximally activated platelet subpopulations, suggesting that aggregation between unactivated and activated platelets is negligible. Directly blocking the unoccupied but activated GPIIb-IIIa receptors without affecting pre-bound Fg on "RGD"-activated, fixed platelets (AFP) by GRGDSP or Ro 43-5054 eliminated aggregation, suggesting that cross-bridging of GPIIb-IIIa on adjacent platelets by fibrinogen mediates aggregation. Alpha for AFP remained maximal (approximately 0.24) over 25-75% Fg occupancy, otherwise decreasing rapidly, with a half-maximum occurring at around 2% occupancy, suggesting that very few bound Fg were required to cause significant aggregation. Images FIGURE 1 PMID:8075353

  6. Elevated shear strength of sediments on active margins: Evidence for seismic strengthening

    NASA Astrophysics Data System (ADS)

    Sawyer, Derek E.; DeVore, Joshua R.

    2015-12-01

    Earthquakes are a primary trigger of submarine landslides, yet some of the most seismically active areas on Earth show a surprisingly low frequency of submarine landslides. Here we show that within the uppermost 100 m below seafloor (mbsf) in previously unfailed sediment, active margins have elevated shear strength by a factor of 2-3 relative to the same interval on passive margins. The elevated shear strength is seen in a global survey of undrained shear strength with depth as well as a normalized analysis that accounts for lithology and stress state. The enhanced shear strength is highest within the uppermost 10 mbsf. These results indicate that large areas of modern day slopes on active margins have enhanced slope stability, which may explain the relative paucity of landslides. These findings lend support to the seismic strengthening hypothesis that the repeated exposure to earthquake energy gradually increases shear strength by shear-induced compaction.

  7. Contrasts in sillimanite deformation in felsic tectonites from anhydrous granulite- and hydrous amphibolite-facies shear zones, western Canadian Shield

    NASA Astrophysics Data System (ADS)

    Leslie, S. R.; Mahan, K. H.; Regan, S.; Williams, M. L.; Dumond, G.

    2015-02-01

    The deformation behavior of crustal materials in variably hydrated metamorphic environments can significantly influence the rheological and seismic properties of continental crust. Optical observations and electron backscatter diffraction (EBSD) analyses are used to characterize sillimanite deformation behavior in felsic tectonites from two deformation settings in the Athabasca granulite terrane, western Canadian Shield. Under estimated conditions of 0.8-1.0 GPa, 725-850 °C in the Cora Lake shear zone, the data suggest that sillimanite deformed by dislocation creep with slip in the [001] direction accompanied by subgrain rotation recrystallization. Where sillimanite locally remained undeformed, strain was concentrated in surrounding weaker phases. Under hydrated conditions of 0.4-0.6 GPa, 550-650 °C in the Grease River shear zone, textures and cathodoluminescence imaging point to dissolution-precipitation creep as the major deformation mechanism for sillimanite, resulting in synkinematic growth of foliation-parallel euhedral sillimanite in a preferred orientation with [001] parallel to the lineation. The results suggest that temperature, fluid content, and modal mineralogy of the surrounding phases may all have significant influence on sillimanite deformation but that preferential alignment of sillimanite [001] parallel to the lineation persists regardless of contrasts in the conditions or mechanisms of deformation.

  8. Review of the Senegalo-Malian shear zone system - Timing, kinematics and implications for possible Au mineralisation styles

    NASA Astrophysics Data System (ADS)

    Diene, M.; Fullgraf, T.; Diatta, F.; Gloaguen, E.; Gueye, M.; Ndiaye, P. M.

    2015-12-01

    The Kédougou Kénieba Inlier (KKI) of eastern Senegal forms a typical Paleoproterozoic greenstone belt characterised by low-metamorphic sequences of volcanic rocks and volcano-sediments that have been intruded at various stages by gabbroic suites and calc-alkaline granitoids. The main structures of the KKI comprise two anastomosing structures about N-S trending shear belts that are distinguished into the western Main Transcurrent Zone (MTZ) and the eastern Senegalo-Malian shear zone system (SMSZ). These shear belts are taken to define the limits between the western Mako, the central Diale-Kéniebandi and the eastern Daléma-Kofi domains even though transitions exist between their characteristic deposition sequences. Remote sensing analysis of airborne geophysics and satellite data (Landsat, ENVI, SRTM) in combination with geological field mapping, structural analysis and geochronology suggests that the SMSZ represents a Birimian structure that records a main stage of deformation that could characterise the major transcurrent Eburnean tectonics (D2) dated from 2105 Ma (Feybesse et al., 2006a-e). This sinistral transpressive deformation marked by a major constraint oriented NNW-SSE, is accompanied by a NNE-SSW extension leading to the opening of dilational areas such as small pull-apart basins marked by local calk-alkaline volcanic sequences and several coeval intrusions of the Boboti Suite dated 2080 ± 0.9 Ma (Hirdes and Davis, 2002). A post-Birimian to pre-Neoproterozoic deformation post dates the transcurrent phase and is marked by regional N-S extension. Review of the existing Au-mineralisation models in combination with the spatial analysis of soil geochemical data suggest seven possible mineralisation styles that are related to the transpressional tectonics and coeval magmatism.

  9. The structural controls of gold mineralisation within the Bardoc Tectonic Zone, Eastern Goldfields Province, Western Australia: implications for gold endowment in shear systems

    NASA Astrophysics Data System (ADS)

    Morey, Anthony A.; Weinberg, Roberto F.; Bierlein, Frank P.

    2007-08-01

    The Bardoc Tectonic Zone (BTZ) of the late Archaean Eastern Goldfields Province, Yilgarn Craton, Western Australia, is physically linked along strike to the Boulder-Lefroy Shear Zone (BLSZ), one of the richest orogenic gold shear systems in the world. However, gold production in the BTZ has only been one order of magnitude smaller than that of the BLSZ (˜100 t Au vs >1,500 t Au). The reasons for this difference can be found in the relative timing, distribution and style(s) of deformation that controlled gold deposition in the two shear systems. Deformation within the BTZ was relatively simple and is associated with tight to iso-clinal folding and reverse to transpressive shear zones over a <12-km-wide area of high straining, where lithological contacts have been rotated towards the plane of maximum shortening. These structures control gold mineralisation and also correspond to the second major shortening phase of the province (D2). In contrast, shearing within the BLSZ is concentrated to narrow shear zones (<2 km wide) cutting through rocks at a range of orientations that underwent more complex dip- and strike-slip deformation, possibly developed throughout the different deformation phases recorded in the region (D1-D4). Independent of other physico-chemical factors, these differences provided for effective fluid localisation to host units with greater competency contrasts during a prolonged mineralisation process in the BLSZ as compared to the more simple structural history of the BTZ.

  10. Structural control and metamorphic setting of the shear zone-related Au vein mineralization of the Adola Belt (southern Ethiopia) and its tectono-genetic development

    NASA Astrophysics Data System (ADS)

    Worku, H.

    1996-10-01

    Structural study of the Adola Belt shows that most of its known and potential Au deposits occur in quartz veins which are localised within shear contacts between lithological units, and along major shear zones that divide the Adola Belt into different lithostructural domains. Analysis of the shear zone-related ore bodies and their host volcano-sedimentary succession and gneisses indicates that Au mineralization in the Adola Belt is pre-dated by two stages of deformation and a regional prograde metamorphism. The first deformation event (D 1) is a fold-and-thrust event which is characterised by low-angle thrusts, associated recumbent folds and axial planar S 1 foliation, and is related to nappe-style deformation. The second event (D 2) has folded and/or reactivated the thrust-related structures and formed upright folds and high-angle reverse shear zones and is related to the collision event. Gold mineralization occurred over a prolonged deformation history but is closely related to alteration, retrograde greenschistfacies assemblages and brittle-ductile deformation of late D 2 and D 3 transpressional shear zones that accommodate regional shortening both by crustal thickening and lateral displacement. The mineralization occurs in associated dilational jogs or bends that might have formed during the lateral or vertical expulsion. The Au-hosting shear zones are characterised by extensive development of heterogeneous mylonitic fault rocks which reveals that the accompanied deformation is characterised by processes that can increase the porosity and permeability of the rocks within the shear zones. This gave rise to further extensive dilatancy within the major dilational jogs and produced a suitable structural regime for vein-hosted Au mineralization. This implies that the Au mineralization is epigenetic in origin and that it resulted from precipitation from metamorphic hydrothermal fluids circulating through major shear zones and associated structures late during the

  11. Spinel + quartz assemblage in granulites from the Achankovil Shear Zone, southern India: Implications for ultrahigh-temperature metamorphism

    NASA Astrophysics Data System (ADS)

    Shimizu, Hisako; Tsunogae, Toshiaki; Santosh, M.

    2009-09-01

    We report the finding of equilibrium spinel + quartz assemblage enclosed within garnet in garnet-orthopyroxene-cordierite granulites from Pakkandom within the Achankovil Shear Zone, a region which is considered as the trace of an accretionary suture in recent tectonic models on southern India. The spinel + quartz bearing granulites are composed of poikiloblastic garnet and subidioblastic orthopyroxene in the matrix of quartz, plagioclase, biotite, cordierite, and Fe-Ti oxides. Garnet contains numerous inclusions of sillimanite and biotite as well as spinel and quartz. The spinel in direct contact with quartz has moderate XMg (= Mg/(Fe 2+ + Mg) = 0.44-0.47), and is Zn and Fe 3+ poor ( XZn = Zn/(Fe 2+ + Mg + Zn) = 0.027-0.036, Fe 3+/(Fe 2+ + Fe 3+) = 0.12-0.17). Spinel is also present in the matrix surrounded by magnetite, but the matrix spinel contains more Zn( XZn = 0.067-0.072) and does not show any contact relationship with quartz. Such Zn- and Fe 3+-poor spinel in direct contact with quartz has been regarded as a diagnostic evidence of ultrahigh-temperature (UHT) metamorphism. The high-temperature stability of the spinel + quartz is also supported by the results of geothermobarometric calculation of garnet-orthopyroxene assemblages that provides robust evidence for peak UHT metamorphism at 920-980 °C and 8-10 kbar, which was further confirmed by Al-in-Opx and magnetite-ilmenite geothermometers (900-950 °C and ˜1000 °C, respectively). The peak UHT event was followed by decompression down to 4.0-4.2 kbar and 640-670 °C toward the stability of cordierite along a clockwise P-T path. Similar spinel + quartz assemblage enclosed in poikiloblastic garnet has also been reported from the Palghat-Cauvery Shear Zone system, the trace of a major suture zone within the Gondwana amalgam with evidence for prograde high-pressure ( P up to 20 kbar) metamorphism followed by UHT event. The fine-grained spinel + quartz may thus indicate decompression from higher pressure

  12. The integrated analyses of digital field mapping techniques and traditional field methods: implications from the Burdur-Fethiye Shear Zone, SW Turkey as a case-study

    NASA Astrophysics Data System (ADS)

    Elitez, İrem; Yaltırak, Cenk; Zabcı, Cengiz; Şahin, Murat

    2015-04-01

    The precise geological mapping is one of the most important issues in geological studies. Documenting the spatial distribution of geological bodies and their contacts play a crucial role on interpreting the tectonic evolution of any region. Although the traditional field techniques are still accepted to be the most fundamental tools in construction of geological maps, we suggest that the integration of digital technologies to the classical methods significantly increases the resolution and the quality of such products. We simply follow the following steps in integration of the digital data with the traditional field observations. First, we create the digital elevation model (DEM) of the region of interest by interpolating the digital contours of 1:25000 scale topographic maps to 10 m of ground pixel resolution. The non-commercial Google Earth satellite imagery and geological maps of previous studies are draped over the interpolated DEMs in the second stage. The integration of all spatial data is done by using the market leading GIS software, ESRI ArcGIS. We make the preliminary interpretation of major structures as tectonic lineaments and stratigraphic contacts. These preliminary maps are controlled and precisely coordinated during the field studies by using mobile tablets and/or phablets with GPS receivers. The same devices are also used in measuring and recording the geologic structures of the study region. Finally, all digitally collected measurements and observations are added to the GIS database and we finalise our geological map with all available information. We applied this integrated method to map the Burdur-Fethiye Shear Zone (BFSZ) in the southwest Turkey. The BFSZ is an active sinistral 60-to-90 km-wide shear zone, which prolongs about 300 km-long between Suhut-Cay in the northeast and Köyceğiz Lake-Kalkan in the southwest on land. The numerous studies suggest contradictory models not only about the evolution but also about the fault geometry of this

  13. Paleoseismologic evidence for late Holocene earthquakes on the Southern Panamint Valley fault zone: Implications for earthquake clustering in the Eastern California Shear Zone north of the Garlock fault

    NASA Astrophysics Data System (ADS)

    McAuliffe, L. J.; Dolan, J. F.; Kirby, E.; Haravitch, B.; Alm, S.

    2010-12-01

    New paleoseismological data from two trenches excavated across the southern end of the Panamint Valley fault (PVF), the most active of the three major faults in the eastern California shear zone (ECSZ) north of the Garlock fault, reveal the occurrence of at least two, and probably three, surface ruptures during the late Holocene. These trenches were designed to test the hypothesis that the earthquake clusters and intervening seismic lulls observed in the Mojave section of the ECSZ (Rockwell et al. 2000, Ganev et al. 2010) at 8-9.5 ka, 5-6 ka and during the past ~1-1.5 ka, also involved the fault systems of the ECSZ north of the Garlock fault. Well stratified playa sands, silts and clays exposed in the trench allowed precise identification of two event horizons; a likely third event horizon occurred during a period of soil development across the playa. Calibrated radiocarbon dates from 25 charcoal samples constrain the dates of the most recent event (MRE) to ~1450-1500 AD and the ante-penultimate event at 3.2-3.6 ka. The penultimate event occurred during a period of soil development spanning ~350-1400 AD. The presence of large blocks of soil in what appears to be scarp-derived colluvium in a large fissure opened during this event require that it occurred late during soil development, probably only a few hundred years before the MRE. The timing of the three events indicate that the southern PVF has ruptured at least once, and probably twice during the ongoing seismic cluster in the Mojave region. The PVF earthquakes also are similar in age to the 1872 Owens Valley earthquakes and the geomorphically youthful, but undated MRE in central Death Valley. Although we were unable to excavate deeply enough at this site to expose mid-to lower - Holocene playa strata, the timing of the ante-penultimate earthquake at our site shows that the PVF has ruptured at least once during the well-defined 2-5 ka seismic lull in the Mojave section of the ECSZ. Interestingly the 3.2-3.6 ka

  14. Geometrical analysis of the Punta del Pedrón shear zone (Asturias, Spain): Implications related to exploration of Salave Gold-type mineralization

    NASA Astrophysics Data System (ADS)

    Gumiel, Pablo; Martín-Izard, Agustín; Arias, Mónica; Rodríguez-Terente, Luis

    2008-03-01

    The Salave Gold deposit is located at the northern end of the Oscos Gold Belt which forms part of the Asturian-Leonese Zone of the Variscan Massif. The Gold Belt and the Salave deposit are genetically related to the emplacement of post-kinematic granitoids which were largely controlled by Variscan fracture systems. A geometrical analysis following the McCoss [McCoss, A.M., 1986. Simple construction for deformation in transpression/transtension zones. Journal of Structural Geology 8(6), 715-718] method has been utilized to discriminate the transpressional, transtensional, and simple shear components, and the displacement vectors (S-vectors) of the Punta del Pedrón shear zone. A contour map of the S-vector orientations is consistent with the contour map of the calculated extension of the whole area, and shows those shear zones where extensional and contractional components have been determined. Extension is higher ( E = 15%) towards the east where transtension is predominant near the Salave gold deposit. This deposit is located at the intersection of the extensional sinistral shear zones striking between 130° and 140°, which promoted dilation and the emplacement of the Salave granodiorite, with the dextral shear zones striking between 070° and 080°. Extension was transferred to the dextral shear zones, and associated 030° pinnate veins, in a transtensional stage which promoted connected vein systems and localized fluid-flow. This gave rise to the alteration patterns of the deposit and ore deposition in an area previously affected by the Mondoñedo thrust system. The use of this geometrical method, with limitations and several assumptions discussed in the text, and the analysis of the scaling properties of the vein sets in traverses can be utilized in the field to aid in mineral exploration.

  15. The VLF EM Method Used for Verification of Fracture/ Shear Zone Aquifers in the Hyper-arid Eastern Desert, Egypt

    NASA Astrophysics Data System (ADS)

    Sauck, W. A.; Sultan, M.; Wagdy, A.; Roouf, O. A.

    2007-05-01

    An integrated program using Landsat remote sensing and ground follow-up with the Very Low Frequency (VLF) geophysical method was applied to the basement rocks of the Red Sea Hills (Eastern Desert) to locate fracture and shear-system aquifers. This part of the Nubian Shield was formed by accretion of a complex of ensimatic and ensialic island arcs and interleaving oceanic basins that were later accreted against the old African continent. Hence, melange and ophiolite sequences are common. This basement complex was intensely fractured (630- 530 Mybp) by the Najd transcurrent shear system (NSS) along a NW-SE trend that is up to 350 km wide, and finally the ocean-arc complex was intruded (~550 Mybp) by anorogenic K-granites. A false-color composite image was created, from Landsat thematic mapper band ratio images that are sensitive to the Fe-bearing aluminosilicate, hydroxyl, and opaque phase content of rocks. On these images mafic rocks (e.g., gabbro and mafic volcanics) rich in Fe-bearing aluminosilicates appear in shades of blue, ultramafics (e.g., serpentinites) rich in hydroxyl-bearing phases and opaque phases appear in shades of red, and granitoid rock units poor in the above phases show as green areas. Using this base map (effectively a pseudo geologic map) and a co-registered DEM, locations of potential shallow water occurrence were plotted based on the following criteria: 1) intersection of the NSS system with transverse faults defining wadis, 2) intersection of two or more fault zones, 3) within highly deformed melange units, especially their internal lithologic contacts and their crossings of wadis, 4) relatively unfractured younger dikes and their intersections with wadis. The VLF instrument was first used to make profiles at a number of existing water wells located at the structural intersections described above, to verify that sub-vertical sheet-like electrical conductors (water-filled fissures) could be successfully located with this instrument. Then

  16. Entrainment-Zone Restratification and Flow Structures in Stratified Shear Turbulence

    NASA Technical Reports Server (NTRS)

    Reif, B. Anders Pettersson; Werne, Joseph; Andreassen, Oyvind; Meyer, Christian; Davis-Mansour, Melissa

    2002-01-01

    Late-time dynamics and morphology of a stratified turbulent shear layer are examined using 1) Reynolds-stress and heat-flux budgets, 2) the single-point structure tensors introduced by Kassinos et al. (2001), and 3) flow visualization via 3D volume rendering. Flux reversal is observed during restratification in the edges of the turbulent layer. We present a first attempt to quantify the turbulence-mean-flow interaction and to characterize the predominant flow structures. Future work will extend this analysis to earlier times and different values of the Reynolds and Richardson numbers.

  17. Ultra Low Velocity Zone existence in the high shear velocity region beneath Cocos Plate, Central America, and the Caribbean

    NASA Astrophysics Data System (ADS)

    Yu, S.; Garnero, E.; Shim, S. H. D.; Zhao, C.

    2014-12-01

    The lowermost mantle beneath subduction is typically characterized by higher than average shear wave speeds, often with the presence of one or more D" discontinuities. These regions are considered the cooler parts of the convective cycle, in contrast to warmer zones of convective return flow, namely, the vicinity of large low shear velocity provinces (LLSVPs). Ultra-low velocity zones (ULVZs) have been long characterized as related to elevated temperature (and/or chemistry) of LLSVP regions. However, some past work has suggested evidence for ULVZ in the presumed cooler regions. In this study we investigate the region beneath the Cocos Plate, Central America, and the Carribbean for ULVZ using high quality broadband Transportable Array data from EarthScope's USArray for the presence of ULVZs. We utilize an ScS-stripping technique that combines a precursor and postcursor to ScS that arise from ULVZ structure, if present. The precursor is a reflection off the top of the ULVZ, while the postcursor is a core-reflection with an added reverberation between the ULVZ top and the core-mantle boundary (CMB). We collected data from deep South American earthquakes recorded in North America and stack data in geographic bins. We find clear evidence for a ULVZ beneath the Gulf of Mexico, but the rest of the study area appears to lack any significant structure. The structure we find is of the order of 100 km wide. The ULVZ properties will be constrained by comparison to predictions from synthetic seismograms. We explore hypotheses for the origin of a ULVZ in a high shear velocity region. These include mineralogical heterogeneities that convective currents have collected; notable possibilities are accumulated melts from subducted materials, such as ocean crust basalts and banded-iron formation. If water can be transported by subducted slabs to the deep mantle, it can significantly decrease the melting temperature of mantle materials and cause such anomalies. A ULVZ a relatively cold

  18. Geology of Precambrian rocks and isotope geochemistry of shear zones in the Big Narrows area, northern Front Range, Colorado

    USGS Publications Warehouse

    Abbott, Jeffrey T.

    1970-01-01

    Rocks within the Big Narrows and Poudre Park quadrangles located in the northern Front Range of Colorado are Precambrian metasedimentary and metaigneous schists and gneisses and plutonic igneous rocks. These are locally mantled by extensive late Tertiary and Quaternary fluvial gravels. The southern boundary of the Log Cabin batholith lies within the area studied. A detailed chronology of polyphase deformation, metamorphism and plutonism has been established. Early isoclinal folding (F1) was followed by a major period of plastic deformation (F2), sillimanite-microcline grade regional metamorphism, migmatization and synkinematic Boulder Creek granodiorite plutonism (1.7 b.y.). Macroscopic doubly plunging antiformal and synformal structures were developed. P-T conditions at the peak of metamorphism were probably about 670?C and 4.5 Kb. Water pressures may locally have differed from load pressures. The 1.4 b.y. Silver Plume granite plutonism was post kinematic and on the basis of petrographic and field criteria can be divided into three facies. Emplacement was by forcible injection and assimilation. Microscopic and mesoscopic folds which postdate the formation of the characteristic mineral phases during the 1.7 b.y. metamorphism are correlated with the emplacement of the Silver Plume Log Cabin batholith. Extensive retrograde metamorphism was associated with this event. A major period of mylonitization postdates Silver Plume plutonism and produced large E-W and NE trending shear zones. A detailed study of the Rb/Sr isotope geochemistry of the layered mylonites demonstrated that the mylonitization and associated re- crystallization homogenized the Rb87/Sr 86 ratios. Whole-rock dating techniques applied to the layered mylonites indicate a probable age of 1.2 b.y. Petrographic studies suggest that the mylonitization-recrystallization process produced hornfels facies assemblages in the adjacent metasediments. Minor Laramide faulting, mineralization and igneous activity

  19. Brittle grain size reduction of feldspar, phase mixing and strain localization in granitoids at mid-crustal conditions (Pernambuco shear zone, NE Brazil)

    NASA Astrophysics Data System (ADS)

    Viegas, Gustavo; Menegon, Luca; Archanjo, Carlos

    2016-04-01

    The Pernambuco shear zone (northeastern Brazil) is a large-scale strike-slip fault that, in its eastern segment, deforms granitoids at mid-crustal conditions. Initially coarse (> 50 μm) grained feldspar porphyroclasts are intensively fractured and reduced to an ultrafine-grained mixture consisting of plagioclase and K-feldspar grains (< 15 μm in size) localized in C' shear bands. Detailed microstructural observations and EBSD analysis do not show evidence of intracrystalline plasticity in feldspar porphyroclasts and/or fluid-assisted replacement reactions. Quartz occurs either as thick (˜ 1-2 mm) monomineralic bands or as thin ribbons dispersed in the feldspathic mixture. The microstructure and c-axis crystallographic preferred orientation are similar in the thick monomineralic band and in the thin ribbons, and suggest dominant subgrain rotation recrystallization and activity of prism and rhomb slip systems. However, the grain size in monophase recrystallized domains decreases when moving from the monomineralic veins to the thin ribbons embedded in the feldspathic C' bands (14 μm vs 5 μm, respectively).The fine-grained feldspar mixture has a weak crystallographic preferred orientation interpreted as the result of oriented growth during diffusion creep, as well as the same composition as the fractured porphyroclasts, suggesting that it generated by mechanical fragmentation of rigid porphyroclasts with a negligible role of chemical disequilibrium. Assuming that the C' shear bands deformed under constant stress conditions, the polyphase feldspathic aggregate would have deformed at a strain rate one order of magnitude faster than the monophase quartz ribbons. Overall, our dataset indicates that feldspar underwent a brittle-viscous transition while quartz was deforming via crystalline plasticity. The resulting rock microstructure consists of a two-phase rheological mixture (fine-grained feldspars and recrystallized quartz) in which the feldspathic material

  20. Application of strain analysis to estimate pressure solution processes in regional shear zones

    NASA Astrophysics Data System (ADS)

    Voytenko, V. N.; Khlebalin, I. Yu.; Senotrusov, V. A.

    2016-01-01

    This paper considers the basic principles of the strain analysis method based on the analysis of antitaxial regeneration fibrous fringes around linear rigid inclusions in a low-viscosity rock matrix. This method has been developed for pressure shadows composed of fibrous minerals, whose orientation is controlled by the major elongation direction rather than the orientation of rigid inclusions. This approach is applicable only for rocks exposed to uniform coaxial straining. The strain ellipse is calculated in two ways: for three variably oriented strain markers, it is calculated using Mohr's circles, and for numerous strain markers by average body ellipse. The strain ellipsoid is calculated using the parameters of a few strain ellipses calculated with three and more non-parallel planes. This paper provides the data on the method testing in reference sites of Dora-Pil' ore field in the Upper Indigirka district and Vangash area in the Yenisei Range. Regeneration fibrous fringes around fragments of fern fossils and linear rutile metacrystals were used as markers. The results of strain analysis obtained for the reference sites in the Upper Indigirka district made it possible to describe the signs of variable strain stages of developing strike-slip zones making up the Adycha-Taryn Fault Zone. Sublatitudinal ore-bearing strike-slip zones are characterized by a subvertical orientation of the elongation axes X of elongated strain ellipsoids, which are subperpendicular to quartz-carbonate veins and slope kink zones. NW-trending strike-slip zones are characterized by subhorizontal orientation of the Z shortening axes of flattened strain ellipsoids, which are subparallel to the normals of quartz-carbonate veins and veinlets. The results of strain analysis obtained for reference sites in the Vangash area made it possible to describe the thrust strain environment following the metamorphism stage and to reveal specific features in the formation of the strain textures of ore

  1. Strain localization in brittle-ductile shear zones: fluid-abundant vs. fluid-limited conditions (an example from Wyangala area, Australia)

    NASA Astrophysics Data System (ADS)

    Spruzeniece, L.; Piazolo, S.

    2015-07-01

    This study focuses on physiochemical processes occurring in a brittle-ductile shear zone at both fluid-present and fluid-limited conditions. In the studied shear zone (Wyangala, SE Australia), a coarse-grained two-feldspar-quartz-biotite granite is transformed into a medium-grained orthogneiss at the shear zone margins and a fine-grained quartz-muscovite phyllonite in the central parts. The orthogneiss displays cataclasis of feldspar and crystal-plastic deformation of quartz. Quartz accommodates most of the deformation and is extensively recrystallized, showing distinct crystallographic preferred orientation (CPO). Feldspar-to-muscovite, biotite-to-muscovite and albitization reactions occur locally at porphyroclasts' fracture surfaces and margins. However, the bulk rock composition shows very little change in respect to the wall rock composition. In contrast, in the shear zone centre quartz occurs as large, weakly deformed porphyroclasts in sizes similar to that in the wall rock, suggesting that it has undergone little deformation. Feldspars and biotite are almost completely reacted to muscovite, which is arranged in a fine-grained interconnected matrix. Muscovite-rich layers contain significant amounts of fine-grained intermixed quartz with random CPO. These domains are interpreted to have accommodated most of the strain. Bulk rock chemistry data show a significant increase in SiO2 and depletion in NaO content compared to the wall rock composition. We suggest that the high- and low-strain microstructures in the shear zone represent markedly different scenarios and cannot be interpreted as a simple sequential development with respect to strain. Instead, we propose that the microstructural and mineralogical changes in the shear zone centre arise from a local metasomatic alteration around a brittle precursor. When the weaker fine-grained microstructure is established, the further flow is controlled by transient porosity created at (i) grain boundaries in fine

  2. Mitigation of atmospheric phase delays in InSAR data, with application to the eastern California shear zone

    NASA Astrophysics Data System (ADS)

    Tymofyeyeva, Ekaterina; Fialko, Yuri

    2015-08-01

    We present a method for estimating radar phase delays due to propagation through the troposphere and the ionosphere based on the averaging of redundant interferograms that share a common scene. Estimated atmospheric contributions can then be subtracted from the radar interferograms to improve measurements of surface deformation. Inversions using synthetic data demonstrate that this procedure can considerably reduce scatter in the time series of the line-of-sight displacements. We demonstrate the feasibility of this method by comparing the interferometric synthetic aperture radar (InSAR) time series derived from ERS-1/2 and Envisat data to continuous Global Positioning System data from eastern California. We also present results from several sites in the eastern California shear zone where anomalous deformation has been reported by previous studies, including the Blackwater fault, the Hunter Mountain fault, and the Coso geothermal plant.

  3. Representation and Management of the Knowledge of Brittle Deformation in Shear Zones Using Microstructural Data From the SAFOD Core Samples

    NASA Astrophysics Data System (ADS)

    Babaie, H. A.; Broda, C. M.; Kumar, A.; Hadizadeh, J.

    2010-12-01

    Web access to data that represent knowledge acquired by investigators studying the microstructures in the core samples of the SAFOD (San Andreas Observatory at Depth) project can help scientists efficiently integrate and share knowledge, query the data, and update the knowledge base on the Web. To achieve this, we have used OWL (Web Ontology Language) to build the brittle deformation ontology for the microstructures observed in the SAFOD core samples, by explicitly formalizing the knowledge about deformational processes, geological objects undergoing deformation, and the underlying mechanical and environmental conditions in brittle shear zones. The developed Web-based ‘SAFOD Brittle Microstructure and Mechanics Knowledge base’ (SAFOD BM2KB), which instantiates this ontology and is available at http://codd.cs.gsu.edu:9999/safod/index.jsp, will host and serve data that pertains to spatial objects, such as microstructure, gouge, fault, and SEM image, acquired by the SAFOD investigators through the studies of the SAFOD core samples. Deformation in shear zones involves complex brittle and ductile processes that alter, create, and/or destroy a wide variety of one- to three-dimensional, multi-scale spatial entities such as rocks and their constituent minerals and structure. These processes occur through a series of sub-processes that happen in different time intervals, and affect the spatial objects at granular to regional scales within shear zones. The processes bring about qualitative change to the spatial entities over time intervals that start and end with events. Processes, such as mylonitization and cataclastic flow, change the spatial location, distribution, dimension, size, shape, and orientation of some objects through translation, rotation and strain. These processes may also result in newly formed entities, such as a new mineral, gouge, vein, or fault, during one or more phases of deformation. Deformation processes may also destroy entities, such as a

  4. Shear stress activation of SREBP1 in endothelial cells is mediated by integrins.

    PubMed

    Liu, Yi; Chen, Benjamin P-C; Lu, Min; Zhu, Yi; Stemerman, Michael B; Chien, Shu; Shyy, John Y-J

    2002-01-01

    We investigated the effect of shear stress on the sterol regulatory element-binding protein 1 (SREBP1) in vascular endothelial cells (ECs) and the mechanotransduction mechanism involved. Application of a shear stress (12 dyn/cm(2)) caused the proteolytic cleavage of SREBP1 and the ensuing translocation of its transcription factor domain into the nucleus. As a result, shear stress increased the mRNAs encoding the low density lipoprotein receptor (LDLR), as well as the binding of (125)I-LDL. Using a step flow channel, we showed that SREBP1 activation in ECs under laminar flow is transient, but disturbed flow causes sustained activation. In studying the shear stress-elicited molecular signaling that activates SREBP1, we found that blocking the beta(1)-integrin with the AIIB2 blocking-type monoclonal antibody inhibited SREBP1 activation induced by shear stress. EC attachment to fibronectin or the activation of beta(1)-integrin in the suspended ECs by the TS2/16 monoclonal antibody was sufficient for SREBP1 activation. Furthermore, transient transfection assays showed that dominant-negative mutants of focal adhesion kinase and c-Src attenuated the shear stress-increased LDLR promoter activity. These results demonstrate that integrin signaling plays a critical role in the modulation of SREBP in ECs in response to shear stress. PMID:11788464

  5. Constitutive relations between dynamic physical parameters near a tip of the propagating slip zone during stick-slip shear failure

    NASA Astrophysics Data System (ADS)

    Ohnaka, Mitiyasu; Kuwahara, Yasuto; Yamamoto, Kiyohiko

    1987-12-01

    Constitutive relations between physical parameters in the cohesive zone during stick-slip shear failure are experimentally investigated. Stick-slip was generated along a 40 cm long precut fault in Tsukuba granite samples using a servocontrolled biaxial loading apparatus. Dynamic behavior during local breakdown processes near a tip of the slipping zone is revealed; the slip velocity and acceleration are given as a function of the slip displacement and the cohesive (or breakdown) shear stress as a function of the slip velocity. A cycle of the breakdown and restrengthening process of stick-slip is composed of five phases characterized in terms of the cohesive strength and the slip velocity. The cohesive strength can degrade regardless of the slip velocity during slip instabilities. The maximum slip acceleration ümax and the maximum slip velocity u˙max are obtained experimentally as: ümax= {2}/{u cu˙max2}andu˙max= {Δτ b}/{G}v where u c is the critical displacement, Δτb the breakdown stress drop, G the rigidity and v the rupture velocity. These relations are consistent with Ida's theoretical estimation based on the cohesive zone model. The above formula gives good estimates for the maximum slip acceleration of actual earthquakes. The cutoff frequency ƒ maxof the power spectral density of the slip acceleration increases with increasing normal stress; in particular, ƒ maxis found to be directly proportional to the normal stress σn within the normal stress range less than 17 MPa as: ƒ max(kHz) = 4.0σ n(MPa) σn<17(MPa) ƒ maxincrease with an increase in u˙max or ümax. All these results lead to the conclusion that ümax, u˙max and ƒ max increase with increasing normal stress. This is consistent with a previous observation that τb increases with increasing normal stress. The above empirical linear relation between ƒ max and σn can be explained by a linear dependence of Δτb on σn. The size-scale dependence of physical parameters is discussed, and such

  6. Recovery of secular deformation field of Mojave Shear Zone in Southern California from historical terrestrial and GPS measurements

    NASA Astrophysics Data System (ADS)

    Liu, Shaozhuo; Shen, Zheng-Kang; Bürgmann, Roland

    2015-05-01

    The 1992 Mw 7.3 Landers and 1999 Mw 7.1 Hector Mine earthquakes struck the Eastern California Shear Zone (ECSZ) in the Mojave Desert, Southern California. Coseismic and postseismic deformation from these events affect efforts to use Global Positioning System (GPS) observations collected since these events to establish a secular surface velocity field, especially in the near field of the coseismic ruptures. We devise block motion models constrained by both historical pre-Landers triangulation and trilateration observations and post-Landers GPS measurements to recover the secular deformation field and differentiate the postseismic transients in the Mojave region. Postseismic transients are found to remain in the Southern California Earthquake Center Crustal Motion Map Version 4, Plate Boundary Observatory, and Scripps Orbit and Permanent Array Center GPS velocity solutions in the form of 2-3 mm/yr excess right-lateral shear across the Landers and Hector Mine coseismic ruptures. The cumulative deformation rate across the Mojave ECSZ is 13.2-14.4 mm/yr, at least twice the geologic rate since the late Pleistocene (≤6.2 ± 1.9 mm/yr). Postseismic GPS time series based on our secular velocity field reveal enduring late-stage transient motions in the near field of the coseismic ruptures that provide new constraints on the rheological structure of the lower crust and upper mantle.

  7. Transient behavior and stability analyses of halite shear zones with an empirical rate-and-state friction to flow law

    NASA Astrophysics Data System (ADS)

    Noda, Hiroyuki; Shimamoto, Toshihiko

    2012-05-01

    Generation of large earthquakes involves with behaviors of whole plate boundaries or faults from brittle to ductile regimes. This paper reports stability analyses of halite shear zones using a recently developed rate-and-state friction to flow law with an emphasis on the behaviors across the brittle-ductile transition. The law smoothly connects the friction law with pressure-insensitive flow law without any additional constitutive parameter. Behavior upon a velocity step is characterized by an instantaneous change in shear resistance followed by transient behavior toward a steady-state. These transient behaviors are in opposite directions between friction and flow regimes, resulting in variable transient behaviors across the brittle-ductile transition. Linear stability analyses of a spring-slider system around steady-state solutions predict pressure and temperature conditions for unstable fault motion that are consistent with experimental results. The condition for potential instability is not equal to, but includes that for rate-weakening. A nonlinear analysis at the stable-unstable boundary has revealed that a sub-critical Hopf bifurcation takes place and thus a permanently sustained oscillation around a destabilized steady-state solution does not exist although experimental results suggest it. This issue deserves further study including the investigation of the friction law and construction of a physical model for brittle-ductile transition.

  8. Multi-scale structural and kinematic analysis of a Neoarchean shear zone in northeastern Minnesota: Implications for assembly of the southern Superior Province

    NASA Astrophysics Data System (ADS)

    Dyess, Jonathan

    This dissertation is a multi-scale structural and kinematic analysis of the Shagawa Lake shear zone in northeastern Minnesota (USA). The Neoarchean Shagawa Lake shear zone is an ~70 km long ~7 km wide subvertical package of L-S tectonites located within the Wawa Subprovince of the Archean Superior Province. In this dissertation, I (1) discuss a new method for mapping regional tectonic fabrics using high-resolution LiDAR altimetry data; (2) examine the geometric relationships between metamorphic foliation, elongation lineation, vorticity, and non-coaxial shear direction within individual L-S tectonites; and (3) incorporate LiDAR, field, and microstructural data sets into a comprehensive structural and kinematic analysis of the Western Shagawa Lake shear zone. Lastly, I discuss implications for assembly of the southern Superior Province. In Chapter one I examine an Archean granite-greenstone terrane in NE Minnesota to illustrate the application of high-resolution LiDAR altimetry to mapping regional tectonic fabrics in forested, glaciated areas. I describe the recognition of lineaments and distinguishing between tectonic and glacial lineament fabrics. I use a 1-m posted LiDAR derived bare-earth digital elevation model (DEM) to construct multiple shaded-relief images for lineament mapping with sun elevation of 45˚ and varying sun azimuth in 45˚ intervals. Two suites of lineaments are apparent. Suite A has a unimodal orientation, mean trend of 035, and consists of short (> 2 km long) lineaments within sediment deposits and bedrock. Suite B lineaments, which are longer (1-30 km) than those of suite A, have a quasi-bimodal orientation distribution, with maximum trends of 065 and 090. Only one lineament suite is visible in areas where suites A and B are parallel. I interpret suite A as a surficial geomorphologic fabric related to recent glaciation, and suite B as a proxy for the regional tectonic fabric. In Chapter two I present a detailed kinematic study of seven

  9. Shallow seismic reflection profiling over a Mylonitic Shear Zone, Ruby Mountains-East Humboldt Range Metamorphic Core Complex, NE Nevada

    NASA Astrophysics Data System (ADS)

    Hawman, Robert B.; Ahmed, Hishameldin O.

    Seismic reflection profiling carried out with a sledgehammer source has imaged Tertiary extensional structures over a depth range of 45-500 m within lower plate rocks of the Ruby Mountains-East Humboldt Range metamorphic core complex. The 400-m CMP profile straddles an exposed contact between tectonic slices of dolomitic marble and metaquartzite emplaced by low-angle ductile-brittle normal faulting. Subhorizontal reflections from layering within the tectonic slices give way at 160 ms (160-220 m depth) to reflections that dip 15-45° to the east, in contrast with dips indicated in a poorly imaged segment of a coincident regional seismic line but in agreement with dips of foliation mapped for nearby up-plunge exposures of a late Proterozoic - early Cambrian sequence of metaquartzites, marbles, schists, and granitic rocks that forms the bulk of the underlying shear zone. Differences with the regional profile are attributed to the higher frequencies (30-100 Hz) generated by the smaller hammer source and the enhanced lateral resolution provided by the straighter profile and much smaller shot-receiver offsets (46-157 m) contributing to the stack for each CMP. The results suggest that the near-surface, east-dipping component of the anastomozing shear zone extends at least 2 km farther east than previously interpreted. Rough estimates of interval velocities (1500-4500 m/s) inferred from stacking velocities are consistent with velocities of mylonitic rocks measured perpendicular to foliation at low confining pressures when the effects of macroscopic fractures and joints are taken into account. Peaks in amplitude spectra of stacked traces suggest long-wavelength components of layering resolved at scales from 5-8 m (depth: 50 m) to 15-25 m (depth: 500 m).

  10. Strike-slip accomodation during the development of the Cantabrian and Central-Iberian oroclines: 40Ar*/39Ar geochronological ages of major shear zones.

    NASA Astrophysics Data System (ADS)

    Gutierrez-Alonso, Gabriel; Pastor-Galán, Daniel; Collins, Alan S.

    2013-04-01

    One of the most striking features found in the West European Variscan Belt is a large strikeslip shear zone/fault system, characterized as "Late-Variscan", that runs parallel to the broad structural trends around the Iberian Armorican Arc. 40Ar*-39Ar ages of micas grown during fabric development in five shear zones of this system (Traguntia-Juzbado; Porto-Tomar; Malpica-Tuy, Punta Langosteira and Ricobayo, both dextral and left lateral, have yielded ages that, within error, cluster at 307 Ma, suggesting that their development took place within the time frame of oroclinal bending constrained by paleomagnetism and structural data, that is to say, coeval with the formation of the Ibero-Armorican Arc. According to our new data and other data from the literature, we interpret the development of the strike-slip shear zone system and the origin of the magmatic pulse at ca 307 Ma as being related to the initiation of the orocline development. These new ages constrain deformation in the outer arc to be penecontemporaneous with thrust-sheet rotations in the inner arc Cantabrian Zone. The 307 Ma strike-slip shear-zones are inferred to have accommodated the vertical axis crustal or lithospheric-block rotations needed to accommodate oroclinal bending. Coeval granitoid ages, clustering at 307 Ma and located in Cantabrian orocline outer arc represent decompressive melting during the mechanical thinning of the mantle lithosphere below the outer arc during bending.

  11. The Sundance fault: A newly recognized shear zone at Yucca Mountain, Nevada

    SciTech Connect

    Spengler, R.W.; Braun, C.A.; Martin, L.G.; Weisenberg, C.W.

    1994-12-31

    Ongoing detailed mapping at a scale of 1:240 of structural features within the potential repository area indicates the presence of several previously unrecognized structural features. Minor north-trending west-side-down faults occur east and west of the Ghost Dance fault and suggest a total width of the Ghost Dance fault system of nearly 366 m (1200 ft). A zone of near-vertical N30{degrees}-40{degrees} W-trending faults, at least 274 m (900 ft) wide, has been identified in the northern part of our study area and may traverse across the potential repository area. On the basis of a preliminary analysis of available data, we propose to name this zone the {open_quotes}Sundance fault system{close_quotes} and the dominant structure, occurring near the middle of the zone, the {open_quotes}Sundance fault{close_quotes}. Some field relations suggest left-stepping deflections of north-trending faults along a pre-existing northwest-trending structural fabric. Other field observations suggest that the {open_quotes}Sundance fault system{close_quotes} offsets the Ghost Dance fault system in an apparent right lateral sense by at least 52 m (170 ft). Additional detailed field studies are needed to better understand structural complexities at Yucca Mountain.

  12. The Sundance fault: A newly recognized shear zone at Yucca Mountain, Nevada

    SciTech Connect

    Spengler, R.W.; Braun, C.A.; Martin, L.G.; Weisenberg, C.W.

    1994-04-01

    Ongoing detailed mapping at a scale of 1:240 of structural features within the potential repository area indicates the presence of several previously unrecognized structural features. Minor north-trending west-side-down faults occur east and west of the Ghost Dance fault and suggest a total width of the Ghost Dance fault system of nearly 366 m (1200 ft). A zone of near-vertical N30{degrees} {minus} 40{degrees}W {minus} trending faults, at least 274 m (900 ft) wide, has been identified in the northern part of our study area and may traverse across the proposed repository area. On the basis of a preliminary analysis of available data, we propose to name this zone the ``Sundance fault system`` and the dominant structure, occurring near the middle of the zone, the ``Sundance fault.`` Some field relations suggest left-stepping deflections of north-trending faults along a preexisting northwest-trending structural fabric. Other field observations suggest that the ``Sundance fault system`` offsets the Ghost Dance fault system in an apparent right lateral sense by at least 52 m (170 ft). Additional detailed field studies, however, are needed to better understand structural complexities at Yucca Mountain.

  13. Structures, kinematic analysis, rheological parameters and temperature-pressure estimate of the Mesozoic Xingcheng-Taili ductile shear zone in the North China Craton

    NASA Astrophysics Data System (ADS)

    Liang, Chenyue; Liu, Yongjiang; Neubauer, Franz; Bernroider, Manfred; Jin, Wei; Li, Weimin; Zeng, Zuoxun; Wen, Quanbo; Zhao, Yingli

    2015-09-01

    The NE to ENE trending Mesozoic Xingcheng-Taili ductile shear zone of the northeastern North China Craton was shaped by three phases of deformation. Deformation phase D1 is characterized by a steep, generally E-W striking gneissosity. It was then overprinted by deformation phase D2 with NE-sinistral shear with K-feldspar porphyroclasts forming a subhorizontal low-angle stretching lineation on a steep foliation. During deformation phase D3, lateral motion accommodated by ENE sinistral strike-slip shear zones dominated. Associated fabrics developed at upper greenschist metamorphic facies conditions and show the deformation characteristics of middle- to shallow crustal levels. In some parts, the older structures have been in turn overprinted by late-stage sinistral D3 shearing. Finite strain and kinematic vorticity in all deformed granitic rocks indicate a prolate ellipsoid (L-S tectonites) near plane strain. Simple shear-dominated general shear during D3 deformation is probably of general significance. The quartz c-axis textures indicate prism-gliding with a dominant rhomb slip and basal slip system formed mainly at low-middle temperatures. Mineral deformation behavior, quartz c-axis textures, quartz grain size and the Kruhl thermometer demonstrate that the ductile shear zone developed under greenschist facies metamorphic conditions at deformation temperatures ranging from 400 to 500 °C. Dislocation creep is the main deformation mechanism at a shallow crustal level. Fractal analysis showed that the boundaries of recrystallized quartz grains had statistically self-similarities. Differential stresses deduced from dynamically recrystallized quartz grain size are at around 20-39 MPa, and strain rates in the order of 10-12 to 10-14 s-1. This indicates deformation of granitic rocks in the Xingcheng-Taili ductile shear zone at low strain rates, which is consistent with most other ductile shear zones. Hornblende-plagioclase thermometer and white mica barometer

  14. Effect of length-scale on localization of shear zones along precursor fractures and layers during deformation of middle to lower crustal rocks

    NASA Astrophysics Data System (ADS)

    Mancktelow, Neil; Pennacchioni, Giorgio; Hawemann, Friedrich; Wex, Sebastian; Camacho, Alfredo

    2016-04-01

    Deformation of high grade rocks at middle to lower crustal levels involves both distributed and more highly localized ductile strain, with localized shear zones developing on elongate near-planar rheological precursors. These planar heterogeneities may be compositional layers (e.g. dykes) or pre-existing or newly developed fractures, with or without pseudotachylyte. Usual rheological models for viscous rock deformation are scale independent. The geometry of developing localized shear zones should therefore be scalable and depend only on the pre-existing geometry and imposed boundary conditions, as shown in numerical and analogue models. However, this is not what is observed in natural examples. Shear zones preferentially or exclusively develop on long fractures and dykes, typically on the scales of many (tens of) metres to (tens of) kilometres, whereas smaller-scale healed fractures, basic enclaves and short layers or inclusions are less prone to reactivation and locally may be largely ignored. Preferential localization of strain on these longer structures means that the intervening rock volumes remain low-strain domains, so that the smaller-scale planar heterogeneities are effectively shielded during progressive deformation. Any localized deformation of these intervening low-strain domains requires the formation of new elongate fractures acting as a necessary precursor for subsequent localization. These field observations suggest that ductile shear zone localization is more effective with increasing length of the approximately planar precursor. Localized shear zones do not develop by propagation away from an initial small heterogeneity. Instead, their length is largely predetermined by the length of the controlling precursor structure and in-plane propagation of the tips appears to be very limited. Preferential shear reactivation of longer precursors introduces a length-scale dependence from the very initiation of localized "viscous" or "ductile" shear zones

  15. Multi-surface Earthquake Rupture Recorded in Pseudotachylyte Vein Geometries, Norumbega Shear Zone, southern Maine

    NASA Astrophysics Data System (ADS)

    Ross, C.; Rowe, C. D.; Pollock, S. G.; Swanson, M.; Tarling, M.; Backeberg, N. R.; Coulson, S.; Barshi, N.; Bate, C.; Dascher-Cousineau, K.; Scibek, J.; Harrichhausen, N.; Timofeev, A.; Rakoczy, P.; Nisbet, H.; Castro, A.; Smith, H.

    2015-12-01

    Earthquake rupture surfaces are typically treated as single rupture planes. However, the observation of four linked, non-parallel to sub-parallel slip surfaces on a mining induced earthquake in 2004 shows that rupture geometries may be more complicated (Heesakkers et al., 2011). Multiple pseudotachylyte-bearing fault surfaces are exposed within a 1.1 km wide mylonite zone of the Paleozoic Norumbega fault system. The pseudotachylytes are present in two juxtaposed mylonite zones: the Ray Corner mylonite and a mylonite derived from Scarboro Formation metavolcanics. The Ray Corner mylonite crosscuts pelitic schists of the Cape Elizabeth Formation, at upper greenschist-facies conditions (quartz + feldspar + chlorite + muscovite ± titanite ± pyrite). The pseudotachylyte veins formed late in the deformational history, during a period of predominantly brittle dextral offset. The pseudotachylytes are cryptocrystalline and have rounded porphyroclasts of quartz and feldspar. Microstructural observations show evidence for static and dynamic recrystallization overprinting the primary quench textures, suggesting that previous generations of rupture surfaces have been recycled into the mylonitic fabric (Price et al., 2012). Many of the pseudotachylyte veins have a sharp boundary on one side and are poorly defined on the other, providing insight to the propagation direction. This confirms that the paleo-earthquake ruptures occurred at conditions where quartz and feldspar were able to deform plastically, near the base of the seismogenic zone. Using differential GPS, we mapped the geometry of pseudotachylyte fault veins, injection veins, and slip surface intersections. At Ray Corner, there are 7 layer-parallel pseudotachylytes in a 4 m wide zone with linking and subsequent oblique pseudotachylytes. Some intersections between pseudotachylytes are dilational, depending on the intersection angle and relative displacement on the two faults. At these sites, pseudotachylyte melt sourced

  16. Shear stress reduces protease activated receptor-1 expression in human endothelial cells

    NASA Technical Reports Server (NTRS)

    Nguyen, K. T.; Eskin, S. G.; Patterson, C.; Runge, M. S.; McIntire, L. V.

    2001-01-01

    Shear stress has been shown to regulate several genes involved in the thrombotic and proliferative functions of endothelial cells. Thrombin receptor (protease-activated receptor-1: PAR-1) increases at sites of vascular injury, which suggests an important role for PAR-1 in vascular diseases. However, the effect of shear stress on PAR-1 expression has not been previously studied. This work investigates effects of shear stress on PAR-1 gene expression in both human umbilical vein endothelial cells (HUVECs) and microvascular endothelial cells (HMECs). Cells were exposed to different shear stresses using a parallel plate flow system. Northern blot and flow cytometry analysis showed that shear stress down-regulated PAR-1 messenger RNA (mRNA) and protein levels in both HUVECs and HMECs but with different thresholds. Furthermore, shear-reduced PAR-1 mRNA was due to a decrease of transcription rate, not increased mRNA degradation. Postshear stress release of endothelin-1 in response to thrombin was reduced in HUVECs and HMECs. Moreover, inhibitors of potential signaling pathways applied during shear stress indicated mediation of the shear-decreased PAR-1 expression by protein kinases. In conclusion, shear stress exposure reduces PAR-1 gene expression in HMECs and HUVECs through a mechanism dependent in part on protein kinases, leading to altered endothelial cell functional responses to thrombin.

  17. P-T-t-d History of the Greater Himalaya Sequence metapelites in the Zanskar Shear Zone, NW India

    NASA Astrophysics Data System (ADS)

    Beck, E.; Leech, M. L.; Basta, S.

    2013-12-01

    Greater Himalaya Sequence (GHS) metapelites deformed in the Zanskar Shear Zone (ZSZ) record geochemical and structural evidence of a complex history. This study applies a multi-component approach to understanding the metamorphic and deformational evolution of the high-grade metapelites in western Himalaya. Samples collected from NE to SW from the ZSZ, along Malung Takpo, record increasing metamorphic grade and decreasing mylonitization with increased distance from the shear zone. Microscopic evidence for variable degrees of deformation includes: change in crystal size, foliation development, pressure shadows, and kink bands. The dominant mineralogy is Qz+Kfs+Pl+Bt+Ms+Grt+Ky×Sil×St+opaques. Advanced isochemical phase diagrams (pseudosections) are calculated in Perple_X using whole-rock geochemical (XRF) data for six samples. The pseudosection conditions range 3-15 kbar and 300-800°C and use the solution models Bio(TCC), Chl(HP), St(HP), hCrd, feldspar, Mica(CHA), and Gt(HP) with modifications made to increase the models' accuracy. The generated phase equilibria diagrams, in conjunction with observed mineral growth relationships, are used to determine the P-T paths and illustrate peak and retrograde metamorphic events. Three dating techniques [U-Pb, 40Ar/39Ar, and (U-Th)/He] are incorporated to constrain timing along the P-T paths. U-Pb SHRIMP dating of monazite constrains the timing of regional metamorphism to ~27 Ma. 40Ar/39Ar dating of muscovite and biotite yields ages of ~20-19 Ma and 15 Ma, respectively, for cooling and exhumation through the middle crust. Dates acquired from (U-Th)/He analyses of monazite and zircon will indicate the timing of the end of movement along the ZSZ. Electron backscatter diffraction (EBSD) maps crystallographic orientation of minerals and is used to determine their responses to deformation. Crystallographic responses in quartz and feldspar are used to constrain the conditions during deformation. This will be supplemented by

  18. Structural evolution of the Irtysh Shear Zone (northwestern China) and implications for the amalgamation of arc systems in the Central Asian Orogenic Belt

    NASA Astrophysics Data System (ADS)

    Li, Pengfei; Sun, Min; Rosenbaum, Gideon; Cai, Keda; Yu, Yang

    2015-11-01

    The NW-SE Irtysh Shear Zone is a major tectonic boundary in the Central Asian Orogenic Belt (CAOB), which supposedly records the amalgamation history between the peri-Siberian orogenic system and the Kazakhstan/south Mongolia orogenic system. However, the tectonic evolution of the Irtysh Shear Zone is not fully understood. Here we present new structural and geochronological data, which together with other constraints on the timing of deformation suggests that the Irtysh Shear Zone was subjected to three phases of deformation in the late Paleozoic. D1 is locally recognized as folded foliations in low strain areas and as an internal fabric within garnet porphyroblasts. D2 is represented by a shallowly dipping fabric and related ˜ NW-SE stretching lineations oriented sub-parallel to the strike of the orogen. D2 foliations are folded by ˜ NW-SE folds (F3) that are bounded by a series of mylonite zones with evidence for sinistral/reverse kinematics. These fold and shear structures are kinematically compatible, and thus interpreted to result from a transpressional deformation phase (D3). Two samples of mica schists yielded youngest detrital zircon peaks at ˜322 Ma, placing a maximum constraint on the timing of D1-D3 deformation. A ˜ NE-SW granitic dyke swarm (˜252 Ma) crosscuts D3 fold structures and mylonitic fabrics in the central part of the shear zone, but is displaced by a mylonite zone that represents the southern boundary of the Irtysh Shear Zone. This observation indicates that the major phase of D3 transpressional deformation took place prior to ˜252 Ma, although later phases of reactivation in the Mesozoic and Cenozoic are likely. The late Paleozoic deformation (D1-D3 at ˜322-252 Ma) overlaps in time with the collision between the Chinese Altai and the intra-oceanic arc system of the East Junggar. We therefore interpret that three episodes of late Paleozoic deformation represent orogenic thickening (D1), collapse (D2), and transpressional deformation (D3

  19. Activation and shedding of platelet glycoprotein IIb/IIIa under non-physiological shear stress.

    PubMed

    Chen, Zengsheng; Mondal, Nandan K; Ding, Jun; Koenig, Steven C; Slaughter, Mark S; Griffith, Bartley P; Wu, Zhongjun J

    2015-11-01

    The purpose of this study was to investigate the influence of non-physiological high shear stress on activation and shedding of platelet GP IIb/IIIa receptors. The healthy donor blood was exposed to three levels of high shear stresses (25, 75, 125 Pa) from the physiological to non-physiological status with three short exposure time (0.05, 0.5, 1.5 s), created by a specific blood shearing system. The activation and shedding of the platelet GPIIb/IIIa were analyzed using flow cytometry and enzyme-linked immunosorbent assay. In addition, platelet P-selectin expression of sheared blood, which is a marker for activated platelets, was also analyzed. The results from the present study showed that the number of activated platelets, as indicated by the surface GPIIb/IIIa activation and P-selectin expression, increased with increasing the shear stress level and exposure time. However, the mean fluorescence of GPIIb/IIIa on the platelet surface, decreased with increasing the shear stress level and exposure time. The reduction of GPIIb/IIIa on the platelet surface was further proved by the reduction of further activated platelet GPIIb/IIIa surface expression induced by ADP and the increase in GPIIb/IIIa concentration in microparticle-free plasma with increasing the applied shear stress and exposure time. It is clear that non-physiological shear stress induce a paradoxical phenomenon, in which both activation and shedding of the GPIIb/IIIa on the platelet surface occur simultaneously. This study may offer a new perspective to explain the reason of both increased thrombosis and bleeding events in patients implanted with high shear blood-contacting medical devices. PMID:26160282

  20. Preliminary Investigation to Resolve the Shear Velocity Structure of the Mantle Transition Zone beneath the Caroline Plate, Equatorial Western Pacific

    NASA Astrophysics Data System (ADS)

    Konishi, K.; Kawai, K.; Fuji, N.; Lee, S.; Geller, R. J.

    2013-12-01

    The Mantle Transition Zone (MTZ), which lies in the depth range from 410-660 km, is considered to be a region capable of carrying a large amount of water and other volatiles. A unique feature of the MTZ beneath the northwest Pacific rim is the stagnant slab which lies below much of the West Philippine Basin and extends laterally over a distance of thousands of kilometers beneath Korea and northeast China. In recent years, suggestions have been made that explain the seismicity and intra-plate volcanism in this region in terms of hydrous magmatic plumes rising from the MTZ. However, the exact mechanism remains under debate. An equally important, but less well-known, observation is that a stagnant slab appears to exist beneath much of the Caroline Plate in the equatorial western Pacific as well. If a stagnant slab does exist here, it is most likely a result of the long northward migration of the Australian Plate and subduction since its breakaway from the Antarctic. However, due to tectonic complexity and the lack of seismic stations, the structure and properties of the stagnant slab and the MTZ beneath the Caroline plate are not well understood. Also it is unclear if the large volcanic outflows around the Caroline Plate such as the Eurpik Rise can be explained by a hydrous magmatic plume stemming from the MTZ. To understand the shear-wave velocity structure of the MTZ beneath the Caroline Plate, we employ a body wave waveform inversion technique. Fuji et al. (PEPI, 2010) conducted body wave waveform inversion for the mantle transition zone beneath Japan. In this study we present preliminary results for an application of their methods to infer upper mantle and MTZ structure beneath the Caroline plate. We also estimate the resolving power of full-waveform inversion for a dataset obtained from the IRIS (Incorporated Research Institutions for Seismology) network for shear velocity structure in the upper mantle, especially for the mantle transition zone beneath the

  1. Microstructural evolution and rheology of quartz in a mid-crustal shear zone

    NASA Astrophysics Data System (ADS)

    Rahl, Jeffrey M.; Skemer, Philip

    2016-06-01

    We present microstructural and crystallographic preferred orientation (CPO) data on quartz deformed in the middle crust to explore the interaction and feedback between dynamic recrystallization, deformation processes, and CPO evolution. The sample investigated here is a moderately deformed quartz-rich mylonite from the Blue Ridge in Virginia. We have created high-resolution crystallographic orientation maps using electron backscatter diffraction (EBSD) of 51 isolated quartz porphyroclasts with recrystallized grain fractions ranging from 10 to 100%. Recrystallized grains are internally undeformed and display crystallographic orientations dispersed around the orientation of the associated parent porphyroclast. We document a systematic decrease in fabric intensity with recrystallization, suggesting that progressive deformation of the recrystallized domains involves processes that can weaken a pre-existing CPO. Relationships between recrystallization fraction and shear strain suggest that complete microstructural re-equilibration requires strains in excess of γ = 5. Variation in the degree of recrystallization implies that strain was accumulated heterogeneously, and that a steady-state microstructure and rheology were not achieved.

  2. Dynamics of flexible active Brownian dumbbells in the absence and the presence of shear flow.

    PubMed

    Winkler, Roland G

    2016-04-20

    The dynamical properties of a flexible dumbbell composed of active Brownian particles are analytically analyzed. The dumbbell is considered as a simplified description of a linear active polymer. The two beads are independently propelled in directions which change in a diffusive manner. The relaxation behavior of the internal degree of freedom is tightly coupled to the dumbbell activity. The latter dominates the dynamics for strong propulsion. As is shown, limitations in bond stretching strongly influence the relaxation behavior. Similarly, under shear flow, activity determines the relaxation and tumbling behavior at strong propulsion. Moreover, shear leads to a preferred alignment and consequently to shear thinning. Thereby, a different power-law dependence on the shear rate compared to passive dumbbells under flow is found. PMID:26980630

  3. Integrated Analysis of Airborne Geophysical Data to Understand the Extent, Kinematics and Tectonic Evolution of the Precambrian Aswa Shear Zone in East Africa.

    NASA Astrophysics Data System (ADS)

    Katumwehe, A. B.; Atekwana, E. A.; Abdelsalam, M. G.; Laó-Dávila, D. A.

    2014-12-01

    The Aswa Shear zone (ASZ) is a Precambrian lithospheric structure which forms the western margin of the East African Orogeny (EAO) that influenced the evolution of many tectonic events in Eastern Africa including the East African Rift System. It separates the cratonic entities of Saharan Metacraton in the northeast from the Congo craton and the Tanzanian craton and the Kibaran orogenic belt to the southwest. However little is known about its kinematics and the extent and tectonic origin are not fully understood. We developed a new technique based on the tilt method to extract kinematic information from high-resolution airborne magnetic data. We also used radiometric data over Uganda integrated with Shuttle Radar Topography Mission (SRTM) Digital Elevation Model (DEM) in South Sudan to understand the extent, kinematics and define the tectonic origin of ASZ. (1) Our results suggest that the ASZ extends in a NW-SE for ~550 km in Uganda and South Sudan. (2) The airborne magnetic and radiometric data revealed a much wider (~50 km) deformation belt than the mapped 5-10 km of exposed surface expression of the ASZ. The deformation belt associated with the shear is defined by three NW-trending sinistral strike-slip shear zones bounding structural domains with magnetic fabrics showing splays of secondary shear zones and shear-related folds. These folds are tighter close to the discrete shear zones with their axial traces becoming sub-parallel to the shear zones. Similar fold patterns are observed from South Sudan in the SRTM DEM. We interpret these folds as due to ENE-WSW shortening associated with the sinistral strike-slip movement. (3) To the northeast of the shear zone, the magnetic patterns suggest a series of W-verging nappes indicative of strong E-W oriented shortening. Based on the above observations, we relate the evolution of the ASZ to Neoproterozoic E-W collision between East and West Gondwana. This collision produced E-W contraction resulting in W-verging thrusts

  4. Thermal evolution of the Sisters shear zone, southern New Zealand; Formation of the Great South Basin and onset of Pacific-Antarctic spreading

    NASA Astrophysics Data System (ADS)

    Kula, Joseph; Tulloch, Andy J.; Spell, Terry L.; Wells, Michael L.; Zanetti, Kathleen A.

    2009-10-01

    The separation of Zealandia from West Antarctica was the final stage in the Cretaceous breakup of the Gondwana Pacific margin. Continental extension resulting in formation of the Great South Basin and thinning of the Campbell Plateau leading to development of the Pacific-Antarctic spreading ridge was partially accommodated along the Sisters shear zone. This east-northeast striking brittle-ductile structure exposed along the southeast coast of Stewart Island, New Zealand, is a greenschist facies extensional shear zone that separates a hanging wall of chloritic, brecciated granites, and undeformed conglomerate from a footwall of mylonitic Carboniferous and Early Cretaceous granites. This complex structure exhibits bivergent kinematics and can be subdivided into a northern and southern segment. The 40Ar/39Ar thermochronology indicates that cooling of the shear zone footwall began at ˜94 Ma with accelerated cooling over the interval ˜89-82 Ma. Structural and thermochronological data indicate a spatial and temporal link between the Sisters shear zone, initial sedimentation within the offshore Great South Basin, extension of the Campbell Plateau, and initiation of the Pacific-Antarctic spreading ridge.

  5. Molecular Mechanism of Active Zone Organization at Vertebrate Neuromuscular Junctions

    PubMed Central

    Nishimune, Hiroshi

    2013-01-01

    Organization of presynaptic active zones is essential for development, plasticity, and pathology of the nervous system. Recent studies indicate a trans-synaptic molecular mechanism that organizes the active zones by connecting the pre- and the postsynaptic specialization. The presynaptic component of this trans-synaptic mechanism is comprised of cytosolic active zone proteins bound to the cytosolic domains of voltage-dependent calcium channels (P/Q-, N-, and L-type) on the presynaptic membrane. The postsynaptic component of this mechanism is the synapse organizer (laminin β2) that is expressed by the postsynaptic cell and accumulates specifically on top of the postsynaptic specialization. The pre- and the postsynaptic components interact directly between the extracellular domains of calcium channels and laminin β2 to anchor the presynaptic protein complex in front of the postsynaptic specialization. Hence, the presynaptic calcium channel functions as a scaffolding protein for active zone organization and as an ion-conducting channel for synaptic transmission. In contrast to the requirement of calcium influx for synaptic transmission, the formation of the active zone does not require the calcium influx through the calcium channels. Importantly, the active zones of adult synapses are not stable structures and require maintenance for their integrity. Furthermore, aging or diseases of the central and peripheral nervous system impair the active zones. This review will focus on the molecular mechanisms that organize the presynaptic active zones and summarize recent findings at the neuromuscular junctions and other synapses. PMID:22135013

  6. Mechanisms of shear localization in the continental lithosphere: inference from the deformation microstructures of peridotites from the Ivrea zone, northwestern Italy

    NASA Astrophysics Data System (ADS)

    Jin, Denghui; Karato, Shun-ichiro; Obata, Masaaki

    1998-03-01

    The ultramafic massif of Balmuccia, northwestern Italy, shows a variety of deformation fabrics including some localized shear zones that resulted in nearly complete melting (pseudotachylyte). A series of peridotite specimens were collected near one of the pseudotachylyte fault veins to investigate the mechanisms of shear localization. The microstructural analyses show at least three deformation stages. The first (stage I) is nearly homogeneous deformation at low stress (~ 3 MPa) and high temperature (~ 1300-1500 K), followed by localized deformation (stage II) at a higher stress (~ 60 MPa) and moderate temperature (~ 1000-1150 K) and, finally, semi-brittle deformation (stage III) at a very high stress (~ 400 MPa) that resulted in the formation of pseudotachylyte. The stage II deformation resulted in relatively small strains in most areas but significant localized deformation leading to dynamic recrystallization occurred, the degree of which increases systematically toward the fault zone. Based on this observation, together with the observation that the brittle deformation post-dated dynamic recrystallization, we conclude that the shear localization in this locality occurred as a result of ductile deformation rather than brittle deformation. Various mechanisms of shear localization in the ductile regime are examined and we conclude that the grain-size reduction due to dynamic recrystallization at relatively high stresses and moderate temperatures is a probable mechanism of shear localization. The intermediate regime between dislocation and diffusion creep, where rheology is grain-size-sensitive yet continuous recrystallization occurs due to dislocation creep, is suggested to play an important role in shear localization in the upper mantle. The conditions of shear localization due to this mechanism are examined based on the laboratory data on creep and dynamic recrystallization. It is shown that shear localization occurs at relatively low temperatures and/or high

  7. The Vincent Fault in the San Gabriel Mtns, southern California, USA: a modified plate boundary shear zone

    NASA Astrophysics Data System (ADS)

    Xia, Haoran; Platt, John

    2015-04-01

    The Vincent Fault in southern California separates the ocean-affiliated Pelona schist of Late Cretaceous age in the footwall from a Meso-Proterozoic gneiss complex and Mesozoic granitoid rocks in the hanging wall. The Vincent fault has been regarded as the original megathrust formed during Laramide flat-slab subduction. Our new pressure, temperature and geochronologic data from the rocks in the hanging wall and the footwall indicate that the Vincent fault has undergone post-subduction modification. The Pelona schist in the San Gabriel Mtns was metamorphosed up to high-pressure greenschist facies. The peak metamorphic temperature given by laser Raman spectroscopy of carbonaceous material is 518.9 ± 19.6°C, consistent with the temperature range of 515-550°C from the quartz c-axis opening-angle thermometer. The peak pressure yielded by Si-in-muscovite barometry is 10.5 ± 1 kbar. The upper ~50 m of the Pelona schist was then mylonitized together with the lower 500-800 m of the hanging wall, which overprinted the pre-existing texture. Mylonitization produced a strong ESE-trending lineation in both rock units, with a consistent ESE sense of shear: opposite to what would be predicted by E-directed subduction. Pressure and temperature of mylonitization of the Pelona schist and the lower part of the hangingwall mylonite zone constrained by the Ti-in-quartz thermobarometer and Si-in-muscovite barometer is around ~4.7 kbar and 372 to 425°C; whereas the upper part of the mylonite zone was equilibrated at ~2.4 kbar and ~365°C. The quartz c-axis fabric opening-angle thermometer also gives a temperature range from 360 to 420°C in the entire mylonite zone. Mylonitization therefore took place during exhumation and cooling of the Pelona Schist. Fission track ages of detrital zircons from both the footwall and the hanging wall of the Vincent fault cluster around 46.7 ± 5.9 Ma, indicating that both footwall and hanging wall had cooled to ~200°C by that time. No other major

  8. The Fluid Evolution of a Ductile-To Dextral Shear Zone in the Central Sierra Nevada, California

    NASA Astrophysics Data System (ADS)

    Holk, G. J.; Lojasiewicz, I.; Hartman, S. M.; Compton, K.; Paterson, S. R.; Kirkpatrick, J. D.

    2015-12-01

    The application of stable isotopes in conjunction with detailed mapping of the dextral Steelhead Lake shear zone (SLSZ) in the central Sierra Nevada reveals a complex fluid history as the system transitioned from ductile-to-brittle behavior at shallow crustal depth in response to cooling of the adjacent Tuolumne Batholith (TB) at ~85 Ma. This system has a multi-stage alteration history that includes: (1) ductiley deformed tourmaline-bearing leucogranite dikes, (2) ductile-to-brittle quartz+tourmaline veins, (3) brittle calc-silicate-rich leach zones, and (4) 1-10-m-thick quartz veins with crack-seal textures. High and variable quartz δ18O values (> +13‰) from metasedimentary rocks belonging to the Saddlebag pendant outside the SLSZ indicate metamorphic fluids at low water/rock ratio. Leucogranite dikes associated with the TB have δ18O (plagioclase = +9.5±0.3‰) and δD (~ -80‰) values consistent with magmatic fluids. Quartz+tourmaline veins that record the ductile-to-brittle transition display a range of δ18O values (quartz: +3.4 to +16.4‰; tourmaline: +8.0‰) that indicate a complex fluid system involving magmatic, metamorphic, and meteoric-hydrothermal fluids. Variable mineral δ18O (-1.5 to +11.3‰) and δD (-140 to -77‰) values from the Sawmill Sequence calc-silicates and Koip Sequence metavolcanics are the product of a fluid system with magmatic (TB) and meteoric-hydrothermal end-member sources, with lowest values found within a pull-apart zone and the highest values are outside this zone. Quartz δ18O (-3.2 to +14.5‰) and fluid inclusion δD values (-137 to -79‰) define a mixing line with magmatic (TB) and meteoric-hydrothermal water end members for the crack-seal veins. Detailed studies of one 10-m-thick vein reveal a trend of decreasing isotope values from the margin to the median plane. Most mineral pairs failed to reach isotopic equilibrium, indicating a very complex and short-lived fluid evolution for this system. This study documents

  9. Ti distribution in quartz across a heterogeneous shear zone within a granodiorite: The effect of deformation mechanism and strain on Ti resetting

    NASA Astrophysics Data System (ADS)

    Bestmann, Michel; Pennacchioni, Giorgio

    2015-06-01

    The study of a heterogeneous ductile shear zone that developed at ~ 500 °C and 0.2 GPa during post-magmatic cooling of a granodiorite has allowed the effect of strain and recrystallization on Ti re-equilibration of quartz to be assessed. Understanding this effect is critical for applying Ti-in-quartz thermobarometry to mylonites. Differently strained quartz across the shear zone shows a heterogeneous distribution of Ti concentrations ([Ti]) (measured by Secondary Ion Mass Spectrometry, SIMS) ranging between 2 and 45 ppm. Quartz cathodoluminescence (CL) is proven by spectral analysis to be correlated with [Ti], allowing CL images to be calibrated as Ti maps using SIMS measurements. Coarse-grained weakly deformed domains consist of magmatic quartz extensively recrystallized by grain boundary migration (GBM) and mostly (65-75% area) contain 20-38 ppm Ti. Resetting to lower [Ti] occurred locally: (i) in haloes surrounding titanite and biotite inclusions ([Ti] as low as 6 ppm); (ii) along grain boundaries; and (iii) towards the interface of quartz domains with other mineral domains. With increasing strain, quartz underwent progressive grain size reduction and developed a bimodal microstructure with elongate grains (> 100's μm long) surrounded by mantles of new grains (10-30 μm in size) recrystallized by subgrain rotation (SGR). Dynamic recrystallization by SGR, associated with prism < a > slip, became increasingly dominant over GBM as strain increased towards the shear zone core. Significant resetting of Ti in quartz only occurred in high strain domains (at shear strain γ probably >> 10) in the shear zone core where fine recrystallization amounts to 50-60% by area and coarser cores are strongly sub-structured. These domains are not compositionally homogeneous and still show a range of [Ti] mostly between 2 and 10 ppm. In all strain facies of the shear zone quartz-filled pressure shadows associated with feldspar show an almost constant [Ti] of ~ 2 ppm. The pristine

  10. Ti distribution in quartz across a heterogeneous shear zone within a granodiorite: the effect of deformation mechanism and strain on Ti resetting

    NASA Astrophysics Data System (ADS)

    Bestmann, Michel; Pennacchioni, Giorgio

    2015-04-01

    The study of a heterogeneous ductile shear zone developed at ca. 500 °C and 0.2 GPa during post-magmatic cooling of a granodiorite allows the effect of strain and recrystallization on Ti re-equilibration of quartz to be assessed. Understanding this effect is critical for applying Ti-in-quartz (TitaniQ) thermobarometry to mylonites. Differently strained quartz across the shear zone shows a heterogeneous distribution of Ti (measured by SIMS) with overall Ti range between 2 and 45 ppm. Quartz cathodoluminescence (CL) is proved by spectral analysis to be univocally correlated to Ti content and CL images were calibrated as Ti maps using SIMS measurements. Coarse grained weakly deformed domains consist of magmatic quartz extensively recrystallized by grain boundary migration (GBM) and mostly (65-75% area) contain 20-38 ppm Ti. Ti resetting to lower amounts occurred locally: (i) in haloes surrounding titanite and biotite inclusions (Ti as low as 6 ppm); (ii) along grain boundaries and healed microfractures; and (iii) towards the quartz domain boundary. With increasing strain quartz underwent progressive grainsize reduction and developed a bimodal microstructure with elongate grains (>100's µm long) surrounded by mantles of new grains (10-30 µm in size) recrystallized by subgrain rotation (SR). Dynamic recrystallization by SGR, associated with prism slip, became increasingly important over GBM as strain increased towards the shear zone core. Relevant resetting of Ti in quartz only occurred in high strain domains (shear strain gamma ≥ 10) in the shear zone core where fine recrystallization amounts at 50-60% area and coarser cores are strongly substructured. These domains are not compositionally homogeneous and still show a range of Ti content between 2 and 10 ppm. In all strain facies of the shear zone quartz-filled pressure shadows associated with feldspar show an almost constant Ti of ~ 2ppm. Therefore the pristine Ti content of the magmatic quartz mylonitized in

  11. Paleozoic development of the Qilian orogen: Insights from a ductile right-slip shear zone and monazite geochronology in the Central Qilian Shan, northeastern Tibetan Plateau

    NASA Astrophysics Data System (ADS)

    Zuza, A. V.; Yin, A.; Dong, S.; Liu, W.; Reith, R. C.; Zhang, J.; Wu, C.; Wu, L.; Gong, J.

    2012-12-01

    The Cenozoic Qilian Shan-Nan Shan thrust belt in the northern Tibetan Plateau was constructed in a region that had experienced a complex history involving Precambrian basement rocks, early Paleozoic orogeny, and several Mesozoic-Cenozoic deformational events. The Paleozoic development of the Qilian orogen remains enigmatic, with models invoking single or multiple arcs in the south colliding against the North China craton in the north. The boundary between the Qilian arc complex and North China is expressed in the central Qilian mountains as a discontinuously exposed ductile shear zone (~10 km x 350 km) in Precambrian basement juxtaposed against a low-grade metamorphosed mafic to ultramafic Ordovician mélange and ophiolite complex. This complex is made up of meta-volcanics, meta-sediments, pillow basalts, ultramafic rocks, and gabbros. Detailed mapping (~1:100,000) was conducted in two valleys (around 38°30' N and 98°45' E) in the central Qilian mountains. In order to unravel the nature and history of the Qilian orogen, it is necessary to also understand the role of later deformation in redistributing earlier tectonic features. Cretaceous extension--inferred by the formation of east-trending basins displaying internal extensional growth strata--was followed by Cenozoic contraction related to the India-Asia collision. This results in NW-SE and W-E trending thrust faults and folds within Permian-Cretaceous sedimentary strata. Slices of the Precambrian basement, shear zone, and Ordovician complex are also duplicated, with local thrusting of the basement onto younger sedimentary strata. The unknown magnitude of Cenozoic thrusting leads to uncertainty as to the actual regional extent of these older units and features. This study is ultimately focused on interpreting the role of the ductile shear zone within the Paleozoic tectonic evolution of the central Qilian Shan. High-grade metamorphic rocks in the Central Qilian belt consist of quartzo-feldspathic gneiss, mica

  12. Petrological and geochemical studies of ultramafic-mafic rocks from the North Puruliya Shear Zone (eastern India)

    NASA Astrophysics Data System (ADS)

    Mandal, Aditi; Ray, Arijit

    2015-12-01

    Ultramafic and mafic rocks occur within a linear belt, trending nearly E-W along North Puruliya Shear Zone of the Chhotanagpur Gneissic Complex (CGC). These rocks are classified as gabbro, norite, gabbro-norite, dolerite, diorite, olivine-websterite and lherzolite. Mafic rocks (Group 1) often occur in association with ultramafic variants (Group 2) and sometimes in isolation. A genetic link has been established between these mafic and ultramafic rocks using disposition of ultramafic and mafic rocks in the outcrop, systematic variation in modal mineralogy, co-linearity of plots in biaxial chemical variation diagram. Chemical composition of biotite and clinopyroxene reveal calc-alkaline nature and arc signature in these mafic-ultramafic rocks and whole rock geochemical characters indicate similarity with arc magma in subduction zone setting. The high values of Mg no. (47-81) and Al 2 O 3 (5.5-17.9) of mafic rocks indicate primitive, aluminous nature of the parental melt and presence of amphibole and biotite indicate its hydrous nature. The parent mafic melt evolved through fractionation of olivine, spinel, clinopyroxene and plagioclase. The crystal cumulates gave rise to the ultramafic rocks and the associated mafic rocks formed from residual melt. Crustal contamination played an important role in magmatic evolution as evident from variation in abundance of Rb in different lithomembers. Mafic-ultramafic rocks of the present study have been compared with intra-cratonic layered complexes, mafic-ultramafic rocks of high grade terrain, Alaskan type ultramafic-mafic complex and ophiolites. It is observed that the ultramafic-mafic rocks of present study have similarity with Alaskan type complex.

  13. Thermal effects in the shear-transformation-zone theory of amorphous plasticity: comparisons to metallic glass data.

    PubMed

    Falk, M L; Langer, J S; Pechenik, L

    2004-07-01

    We extend our earlier shear-transformation-zone theory of amorphous plasticity to include the effects of thermally assisted molecular rearrangements. This version of our theory is a substantial revision and generalization of conventional theories of flow in noncrystalline solids. As in our earlier work, it predicts a dynamic transition between jammed and flowing states at a yield stress. Below that yield stress, it now describes thermally assisted creep. We show that this theory accounts for the experimentally observed strain-rate dependence of the viscosity of metallic glasses, and that it also captures many of the details of the transient stress-strain behavior of those materials during loading. In particular, it explains the apparent onset of superplasticity at sufficiently high stress as a transition between creep at low stresses and plastic flow near the yield stress. We also argue that there are internal inconsistencies in the conventional theories of these deformation processes, and suggest ways in which further experimentation as well as theoretical analysis may lead to better understanding of a broad range of nonequilibrium phenomena. PMID:15324056

  14. Fabric-related velocity anisotropy and shear wave splitting in rocks from the Santa Rosa Mylonite Zone, California

    SciTech Connect

    Kern, H. ); Wenk, H.R. )

    1990-07-10

    The directional dependence of P and S wave velocities have been measured at pressures (up to 600 MPa) and temperatures (up to 700C) in rocks from the Santa Rosa Mylonite Zone (southern California). During tectonism, these were progressively deformed from granodiorite protolith to mylonite and ultimately phyllonite. The mineralogical and chemical composition of protolith and mylonite is nearly identical. Thus these rocks provide excellent material for documenting the effect of microstructural and textural changes on rock anisotropy. Velocity anisotropy increases significantly with the degree of deformation, whereas average velocities and densities do not change. At low pressure (50 MPa) the velocity anisotropy ranges from 1.7% in granodiorite up to 19% in phyllonite and is due to both oriented microfractures and crystallographic preferred orientation. At high pressure (600 MPa), the residual anisotropy up to 12% is mainly due to preferred mineral orientation, in particular of biotite. Significant shear wave splitting is measured parallel to the foliation plane and shows a good correlation with the biotite texture. These observations confirm that oriented microcracks and preferred orientation of minerals should be taken into account in the interpretation of seismic reflection and refraction data in terranes with deformed rocks.

  15. Accumulation of Exogenous Activated TGF-β in the Superficial Zone of Articular Cartilage

    PubMed Central

    Albro, Michael B.; Nims, Robert J.; Cigan, Alexander D.; Yeroushalmi, Kevin J.; Alliston, Tamara; Hung, Clark T.; Ateshian, Gerard A.

    2013-01-01

    It was recently demonstrated that mechanical shearing of synovial fluid (SF), induced during joint motion, rapidly activates latent transforming growth factor β (TGF-β). This discovery raised the possibility of a physiological process consisting of latent TGF-β supply to SF, activation via shearing, and transport of TGF-β into the cartilage matrix. Therefore, the two primary objectives of this investigation were to characterize the secretion rate of latent TGF-β into SF, and the transport of active TGF-β across the articular surface and into the cartilage layer. Experiments on tissue explants demonstrate that high levels of latent TGF-β1 are secreted from both the synovium and all three articular cartilage zones (superficial, middle, and deep), suggesting that these tissues are capable of continuously replenishing latent TGF-β to SF. Furthermore, upon exposure of cartilage to active TGF-β1, the peptide accumulates in the superficial zone (SZ) due to the presence of an overwhelming concentration of nonspecific TGF-β binding sites in the extracellular matrix. Although this response leads to high levels of active TGF-β in the SZ, the active peptide is unable to penetrate deeper into the middle and deep zones of cartilage. These results provide strong evidence for a sequential physiologic mechanism through which SZ chondrocytes gain access to active TGF-β: the synovium and articular cartilage secrete latent TGF-β into the SF and, upon activation, TGF-β transports back into the cartilage layer, binding exclusively to the SZ. PMID:23601326

  16. Amplitude analysis of active source seismic data from the grounding zone of Whillans Ice Stream

    NASA Astrophysics Data System (ADS)

    Horgan, Huw; Anandakrishnan, Sridhar; Alley, Richard; Christianson, Knut

    2015-04-01

    Amplitude analysis of active source seismic data is often used to estimate acoustic properties and thereby infer the lithology of the substrate beneath glaciers and ice streams. The substrate beneath the ice streams of West Antarctica is of particular interest as here subglacial sediment deformation results in the rapid flow of the overriding ice. At the grounding zone, where the grounded ice sheet transitions to the floating ice shelf, this substrate is thought to stiffen due to tidal compaction resulting in a zone of higher basal shear stress which is manifest in the buckling of the internal layering in the overriding ice. Here we investigate these processes by estimating subglacial properties using active source seismic data acquired across the grounding zone of Whillans Ice Stream. Perhaps uniquely, we are able to test our methodology due to the survey crossing from an ice overlying sediment interface into a known ice overlying water interface. Our analysis indicates that lithological variations within the grounding zone are below the resolution of our methodology with the exception of a body of water trapped by a hydropotential reversal upstream of the grounding zone.

  17. Constraints on strain rate and fabric partitioning in ductilely deformed black quartzites (Badajoz-Córdoba Shear Zone, Iberian Massif)

    NASA Astrophysics Data System (ADS)

    Puelles, Pablo; Ábalos, Benito; Fernández-Armas, Sergio

    2013-04-01

    The Badajoz-Córdoba Shear Zone is a is 30-40 km wide and 400 km long, NW-SE trending structure located at the boundary between the Ossa-Morena and Central-Iberian Zones of the Iberian Massif. Two elongated domains can be differentiated inside: the Obejo-Valsequillo domain to the NE and the Ductile Shear Belt (DSB) to the SW. The former exhibits Precambrian to Cambrian volcano-sedimentary rocks unconformably overlaying a Neoproterozoic basement formed by the "Serie Negra". The latter, 5-15 km wide, is composed mainly of metamorphic tectonites including the "Serie Negra" and other units located structurally under it. The petrofabric of "Serie Negra" black quartzites from the DSB is analyzed in this study with the Electron Back-Scattered Diffraction technique (EBSD). Black quartzites represent originally siliceous, chemical-biochemical shallow-water marine deposits, currently composed almost exclusively of quartz and graphite. Macroscopically they exhibit an outstanding planolinear tectonic fabric. Petrographically, coarse- and fine-grained dynamically recrystallized quartz bands alternate. The former contain quartz grains with irregular shapes, mica inclusions and "pinning" grain boundaries. Oriented mica grains and graphite particles constrain irregular quartz grain shapes. Quartz ribbons with chessboard microstructures also occur, indicating recrystallization under elevated temperatures coeval with extreme stretching. Fine-grained recrystallized quartz bands are dominated by quartz grains with straight boundaries, triple junctions, a scarcer evidence of bulging, and a higher concentration of dispersed, minute graphite grains. Quartz lattice-preferred orientation (LPO) patterns permit to identify two well-developed maxima for [c] axes: one close to the Y structural direction and the other one around Z, and -axes girdles normal to Y and Z. Although both [c] axis maxima appear in the coarse- and fine-grained bands, subsets can be isolated with grain cluster

  18. Polyphase evolution of a crustal-scale shear zone during progressive exhumation from ductile to brittle behaviour: a case study from Calabria, Italy

    NASA Astrophysics Data System (ADS)

    Fazio, E.; Ortolano, G.; Cirrincione, R.; Pezzino, A.; Visalli, R.

    2015-03-01

    Mylonitic rocks involved within a polyphase crustal-scale shear zone, cropping out in the Aspromonte Massif (Calabria, Italy), has been investigated to reveal the meso- and micro-structural evolution (from ductile- to brittle-type deformation) occurred during exhumation trajectory. A relatively small area (about 4 km2) has been selected in the central-eastern part of the massif to constrain the sequence of the structural features from the earliest ones (Hercynian in age), almost totally obliterated by a pervasive mylonitic foliation (plastic regime), up to recent ones, consisting of various sets of veins typical of semibrittle to brittle regime. The former ductile evolution was followed by a compressive thin-skinned thrusting stage developed during the Apennine phase of the Alpine Orogeny, interested by a second brittle stage, consistent with the switching from compressive to extensional tectonics. This last stage accompanied the final exhumation process causing the activation of regional scale normal faults, which partly disarticulated previous mylonitic microstructures. A suite of oriented specimens were collected and analyzed to complete the deformational history already recognized in the field. Quartz c axis orientation patterns confirm the greenschist facies conditions of the former ductile exhumation stage with a dominant top-to-NE sense of shear. Microstructural investigations highlighted the progressive development from plastic- to brittle-type structures, allowing to constrain each step of the multistage exhumation history, and to establish the relative timing of the stress field variation causing thrusting and subsequent normal faulting. Obtained results support a continue compressional exhumation of this sector since the opening of Tyrrhenian basin (10 Ma).

  19. Cosmogenic 10Be and 36Cl geochronology of offset alluvial fans along the northern Death Valley fault zone: Implications for transient strain in the eastern California shear zone

    USGS Publications Warehouse

    Frankel, K.L.; Brantley, K.S.; Dolan, J.F.; Finkel, R.C.; Klinger, R.E.; Knott, J.R.; Machette, M.N.; Owen, L.A.; Phillips, F.M.; Slate, J.L.; Wernicke, B.P.

    2007-01-01

    The northern Death Valley fault zone (NDVFZ) has long been recognized as a major right-lateral strike-slip fault in the eastern California shear zone (ECSZ). However, its geologic slip rate has been difficult to determine. Using high-resolution digital topographic imagery and terrestrial cosmogenic nuclide dating, we present the first geochronologically determined slip rate for the NDVFZ. Our study focuses on the Red Wall Canyon alluvial fan, which exposes clean dextral offsets of seven channels. Analysis of airborne laser swath mapping data indicates ???297 ?? 9 m of right-lateral displacement on the fault system since the late Pleistocene. In situ terrestrial cosmogenic 10Be and 36C1 geochronology was used to date the Red Wall Canyon fan and a second, correlative fan also cut by the fault. Beryllium 10 dates from large cobbles and boulders provide a maximum age of 70 +22/-20 ka for the offset landforms. The minimum age of the alluvial fan deposits based on 36Cl depth profiles is 63 ?? 8 ka. Combining the offset measurement with the cosmogenic 10Be date yields a geologic fault slip rate of 4.2 +1.9/-1.1 mm yr-1, whereas the 36Cl data indicate 4.7 +0.9/-0.6 mm yr-1 of slip. Summing these slip rates with known rates on the Owens Valley, Hunter Mountain, and Stateline faults at similar latitudes suggests a total geologic slip rate across the northern ECSZ of ???8.5 to 10 mm yr-1. This rate is commensurate with the overall geodetic rate and implies that the apparent discrepancy between geologic and geodetic data observed in the Mojave section of the ECSZ does not extend north of the Garlock fault. Although the overall geodetic rates are similar, the best estimates based on geology predict higher strain rates in the eastern part of the ECSZ than to the west, whereas the observed geodetic strain is relatively constant. Copyright 2007 by the American Geophysical Union.

  20. Orbital fluid shear stress promotes osteoblast metabolism, proliferation and alkaline phosphates activity in vitro.

    PubMed

    Aisha, M D; Nor-Ashikin, M N K; Sharaniza, A B R; Nawawi, H; Froemming, G R A

    2015-09-10

    Prolonged disuse of the musculoskeletal system is associated with reduced mechanical loading and lack of anabolic stimulus. As a form of mechanical signal, the multidirectional orbital fluid shear stress transmits anabolic signal to bone forming cells in promoting cell differentiation, metabolism and proliferation. Signals are channeled through the cytoskeleton framework, directly modifying gene and protein expression. For that reason, we aimed to study the organization of Normal Human Osteoblast (NHOst) cytoskeleton with regards to orbital fluid shear (OFS) stress. Of special interest were the consequences of cytoskeletal reorganization on NHOst metabolism, proliferation, and osteogenic functional markers. Cells stimulated at 250 RPM in a shaking incubator resulted in the rearrangement of actin and tubulin fibers after 72 h. Orbital shear stress increased NHOst mitochondrial metabolism and proliferation, simultaneously preventing apoptosis. The ratio of RANKL/OPG was reduced, suggesting that orbital shear stress has the potential to inhibit osteoclastogenesis and osteoclast activity. Increase in ALP activity and OCN protein production suggests that stimulation retained osteoblast function. Shear stress possibly generated through actin seemed to hold an anabolic response as osteoblast metabolism and functional markers were enhanced. We hypothesize that by applying orbital shear stress with suitable magnitude and duration as a non-drug anabolic treatment can help improve bone regeneration in prolonged disuse cases. PMID:26163894

  1. Alternations in burial and exhumation along the Selimiye (Kayabükü) shear zone in the Menderes Massif from detailed garnet pressure-temperature paths

    NASA Astrophysics Data System (ADS)

    Kelly, E. D.; Atakturk, K. R.; Catlos, E. J.; Lizzadro-McPherson, D. J.; Cemen, I.; Lovera, O. M.

    2015-12-01

    Pressure-temperature (P-T) paths derived from garnet chemical zoning and supported by thermal modeling record alternating burial and exhumation during Main Menderes Metamorphism in western Turkey. We studied six rocks along the Selimiye (Kayabükü) shear zone, three from the footwall (Çine nappe) and three from the hanging wall (Selimiye nappe). The shear zone bounds the southern Menderes Massif metamorphic core complex and has been suggested to record compression followed by extension. The rocks are lower-amphibolite facies garnet-bearing metapelites with nearly identical mineral suites. Retrograde overprinting hinders classical thermobarometry; to overcome this, preserved chemical zoning in garnet combined with a G-minimization approach was used to construct detailed P-T paths (e.g., 50 points in some paths). During continuous temperature increase, the Çine nappe paths show increasing, decreasing, and then increasing pressure (an N-shaped path) ending at 7-8 kbar and ~565-590 °C. The Selimiye nappe paths show a single increase in P-T ending at ~7.3 kbar and ~580 °C. Similar bulk-rock compositions in all samples and the separation by the shear zone suggest that garnets grew during distinct events in each nappe. The timing of garnet growth, and thus the P-T paths, is currently undetermined, as monazite inclusions in garnet appear secondary and complicated by excess common Pb. The Çine nappe N-shaped path describes alternations in burial and exhumation, possibly due to thrust motion along the shear zone. To demonstrate the physical plausibility of the P-T paths, a 2-D finite difference solution to the diffusion-advection equation was applied. The results of the thermal modeling suggest that thrusting, denudation, and renewed thrusting would produce similar changes in P-T to the N-shaped path. Thus, the Çine nappe N-shaped P-T path appears to record a gap in thrust motion along the Selimiye (Kayabükü) shear zone prior to ultimate unroofing of the massif.

  2. Frictional Properties of Experimentally Sheared Gouges from the 2011 Mw 9.0 Tohoku-Oki Earthquake Fault Zone

    NASA Astrophysics Data System (ADS)

    Mittempergher, S.; Smith, S. A. F.; Remitti, F.; Gualtieri, A.; Di Toro, G.

    2014-12-01

    Smectite-rich fault gouge recovered during IODP exp. 343 (J-FAST project) from the plate-boundary slip zone of the 2011 Mw 9.0 Tohoku-oki earthquake was deformed at slip velocities of 10 -5 - 3 m s -1 with theSlow to High Velocity Apparatus (SHIVA) at INGV, Rome. Experiments were performed "room-dry" (40-60% humidity, 8.5 -12.5 MPa normal stress) or "water-dampened" (0.5 ml distilled water, 3.5 MPa normal stress), with displacements up to 1m. Mineralogy and microstructures of pre and post-experiment material was investigated by quantitative X Ray Powder Diffraction (XRPD), Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM). The starting material is composed of smectite (beidellite, 55.8 wt.%), illite (17.2 wt.%), quartz (8.5 wt.%), plagioclase (7.4 wt.%), K-feldspar (7.1 wt.%) and kaolinite (5 wt.%). At all investigated slip velocities, water-dampened gouges have peak and steady state frictional strengths (0.04<μ<0.1) lower than room dry gouges and are velocity-neutral to velocity-weakening. Under room-dry conditions, the gouges are velocity-strengthening at intermediate velocities (0.001 - 0.1 m s -1 , 0.25<μ<0.35) and strongly velocity-weakening at slip velocities > 0.1 m s -1 (μ<0.1). A detectable amount of amorphous material formed in room-dry experiments at low and high-slip velocities, likely by comminution and disordering of smectite.Room-dry gouges deformed at low slip velocities are foliated (P foliation) and cut by a series of shear bands lying either sub-parallel (Y) or at low angles to gouge layer boundaries (R). At high slip velocities, room-dry gouges contain a weak P foliation and a single, prominent Y shear. Deformed water-dampened gouges display homogeneous internal texture, lacking foliation or systematically organized fracture sets. In room-dry gouges, velocity strengthening at intermediate slip velocities and a pronounced peak friction at high slip velocities, represent an energy barrier to seismic rupture

  3. Differentiation of Secular and Postseismic Deformation in the Mojave Shear Zone in Southern California and Inference of Lithospheric Rheology

    NASA Astrophysics Data System (ADS)

    Shen, Z.; Liu, S.; Burgmann, R.

    2015-12-01

    The 1992 Mw 7.3 Landers and 1999 Mw7.1 Hector Mine earthquakes struck the Eastern California Shear Zone (ECSZ) in the Mojave Desert, Southern California. Coseismic and postseismic deformation from these events affect efforts to use Global Positioning System (GPS) observations collected since these events to establish a secular surface velocity field, especially in the near field of the coseismic ruptures. We devise block motion models constrained by both historical pre-Landers triangulation and trilateration observations and post-Landers GPS measurements to recover the secular deformation field and differentiate the postseismic transients in the Mojave region. Postseismic transients are found to remain in various "interseismic" GPS velocity solutions in the form of 2-3 mm/yr excess right-lateral shear across the Landers and Hector Mine coseismic ruptures [Liu et al., 2015 JGR]. Postseismic GPS time series differentiated from the secular velocity field reveal enduring late-stage transient motions in the near field of the coseismic ruptures. Using the postseismic time series data as model constraints, we develop postseismic deformation model invoking afterlip on faults and viscoelastic relaxation in the lower crust and upper mantle. A Burgers body material and a Maxwell material are assumed for the lower crust and upper mantle respectively. Our preliminary modeling result, constrained using GPS time series data from the SCEC Crustal Motion Map 4.0 (covering the time period of 1992-2004), reveals that both the long-term viscosities for the lower crust and upper mantle are on the order of e+19 Pa-s. This finding differs significantly from the "Crème Brulee" model predictions about the rheological structure of the lower crust and upper mantle, in which the lower crust has a substantially higher viscosity. We are incorporating more GPS time series data into our model, particularly the ones from continuous sites of the Plate Boundary Observatory network with post-2004

  4. Late Neoproterozoic metamorphic assemblages along the Pan-African Hamisana Shear Zone, southeastern Egypt: Metamorphism, geochemistry and petrogenesis

    NASA Astrophysics Data System (ADS)

    Ali-Bik, Mohamed W.; Sadek, Mohamed F.; Ghabrial, Doris Sadek

    2014-11-01

    A variety of Late Neoproterozoic gneisses and amphibolites are distributed along the N-S trending Hamisana Shear Zone (HSZ), in southeastern Egypt. The HSZ originated after the accretion of the Arabian-Nubian Shield (ANS) and covers an area of about 1500 km2 in southeastern Egypt and northeastern Sudan. The architecture of the northern part of the HSZ is best explained as a tectono-stratigraphic column, in which allochthonous ophiolitic mélange was thrusted onto metamorphosed island-arc assemblages (gneisses and amphibolites). The latter rock units were generally subjected to two successive phases of amphibolite facies metamorphism, followed by a thermal phase and retrograde overprint. The early penetrative, low- to medium-pressure metamorphism (M1) was synchronous with D1-gneissosity and N-S trending lineation, demarcating the high strain HSZ. The mineral assemblages formed during the M1 phase include quartz + andesine + hornblende (I) + biotite (I) in hornblende-biotite gneiss, quartz + andesine + pargasitic hornblende (I) + ferroan pargasitic hornblende (I) + edenitic hornblende (I) in hornblende-schist, quartz + plagioclase + biotite + muscovite in psammopelitic gneiss, and diopside + tremolite + calcite + sphene ± garnet in calc-silicates, being characteristic for amphibolite facies with metamorphic conditions of 600 ± 50 °C and 5-6.5 kbar. The second metamorphic phase (M2) is related to the crystallization of biotite and/or hornblende in S2 foliation demarcating the NE-SW trending dextral shear deformation (D2). The calculated temperature for this M2 phase is about 592 °C. Subsequent thermal events are documented by growth of spinel and scapolite in calc-silicate rocks and of cordierite in psammopelitic gneiss in response to uplift, decomposition and heat provided by the nearby late-formed igneous intrusions. Finally, the rocks reached a temperature of about 530 °C during the cooling retrogressive stage. Based on geological, petrological and geochemical

  5. Crystallographically controlled crystal-plastic deformation of zircon in shear zones

    NASA Astrophysics Data System (ADS)

    Kovaleva, Elizaveta; Klötzli, Urs

    2014-05-01

    Plastically-deformed zircons from various types of strained natural metamorphic rocks have been investigated in-situ by electron backscatter diffraction analysis (EBSD), allowing crystallographic orientation mapping at high spatial resolution. Plastic deformation often forms under the control of grain-internal heterogeneities. At the crystal structure scale deformation is controlled by the physical anisotropy of the lattice. Three most common slip systems in zircon are [100]{010}, [010]{001} and [001]{010} (Leroux et. al., 1999; Reddy et. al., 2007). They are genetically connected with the main zircon crystallographic directions: [001] (c-axis), [100] and [010] (a and b axes). Atomic models show weak planes normal to these directions that preferably evolve to glide planes in the deforming crystal. The visualization of seismic (elastic) properties of zircon with the MATLAB toolbox MTEX shows a similar pattern. The slowest S-wave velocities are observed in directions parallel to [100], [010] and [001] crystallographic directions. The highest Young's modulus values lie in the same directions. In natural zircon grains, the common slip systems are preferably activated when zircon is hosted by rheologically comparatively weaker phases or a fine-grained matrix. In these cases zircon behaves as a rigid clast. During progressive deformation high deviatoric stresses together with high strain rates concentrate at crystal tips, as shown by numerical modeling. Softer host phases allow more degrees of freedom for zircon to be deformed according to its crystallographic and internal properties. These conclusions are supported by the misorientation axes density distribution maps, derived with MTEX. Deformed zircon hosted by a relatively soft phase (mostly biotite) develops a crystallographic preferred orientation (CPO), which has not been documented for zircon before. At the same time deformation of zircon hosted by a rheologically stronger matrix causes the activation of less

  6. Brittle grain size reduction of feldspar, phase mixing and strain localization in granitoids at mid-crustal conditions (Pernambuco shear zone, NE Brazil)

    NASA Astrophysics Data System (ADS)

    Viegas, G.; Menegon, L.; Archanjo, C. J.

    2015-10-01

    The Pernambuco shear zone (northeastern Brazil) is a large-scale strike-slip fault that, in its eastern segment, deforms granitoids at mid-crustal conditions. Initially coarse (> 50 μm) grained feldspar porphyroclasts are intensively fractured and reduced to an ultrafine-grained mixture consisting of plagioclase and K-feldspar grains (~ < 15 μm in size) localized in C' shear bands. Detailed microstructural observations and EBSD analysis do not show evidence of intracrystalline plasticity in feldspar porphyroclasts and/or fluid-assisted replacement reactions. Quartz occurs either as thick (~ 1-2 mm) monomineralic bands or as thin ribbons dispersed in the feldspathic mixture. The microstructure and c axis crystallographic preferred orientation are similar in the thick monomineralic band and in the thin ribbons, and suggest dominant subgrain rotation recrystallization and activity of prism ⟨a⟩ and rhomb ⟨a⟩ slip systems. However, the grain size in monophase recrystallized domains decreases when moving from the transposed veins to the thin ribbons embedded in the feldspathic C' bands (14 μm vs. 5 μm, respectively). The fine-grained feldspar mixture has a weak crystallographic preferred orientation interpreted as the result of oriented growth during diffusion creep, as well as the same composition as the fractured porphyroclasts, suggesting that it generated by mechanical fragmentation of rigid porphyroclasts with a negligible role of chemical disequilibrium. Assuming that the C' shear bands deformed under constant stress conditions, the polyphase feldspathic aggregate would have deformed at a strain rate one order of magnitude faster than the monophase quartz ribbons. Overall, our dataset indicates that feldspar underwent a brittle-viscous transition while quartz was deforming via crystal plasticity. The resulting rock microstructure consists of a two-phase rheological mixture (fine-grained feldspars and recrystallized quartz) in which the polyphase

  7. Effects of Fusion Zone Size and Failure Mode on Peak Load and Energy Absorption of Advanced High Strength Steel Spot Welds under Lap Shear Loading Conditions

    SciTech Connect

    Sun, Xin; Stephens, Elizabeth V.; Khaleel, Mohammad A.

    2008-06-01

    This paper examines the effects of fusion zone size on failure modes, static strength and energy absorption of resistance spot welds (RSW) of advanced high strength steels (AHSS) under lap shear loading condition. DP800 and TRIP800 spot welds are considered. The main failure modes for spot welds are nugget pullout and interfacial fracture. Partial interfacial fracture is also observed. Static weld strength tests using lap shear samples were performed on the joint populations with various fusion zone sizes. The resulted peak load and energy absorption levels associated with each failure mode were studied for all the weld populations using statistical data analysis tools. The results in this study show that AHSS spot welds with conventionally required fusion zone size of can not produce nugget pullout mode for both the DP800 and TRIP800 welds under lap shear loading. Moreover, failure mode has strong influence on weld peak load and energy absorption for all the DP800 welds and the TRIP800 small welds: welds failed in pullout mode have statistically higher strength and energy absorption than those failed in interfacial fracture mode. For TRIP800 welds above the critical fusion zone level, the influence of weld failure modes on peak load and energy absorption diminishes. Scatter plots of peak load and energy absorption versus weld fusion zone size were then constructed, and the results indicate that fusion zone size is the most critical factor in weld quality in terms of peak load and energy absorption for both DP800 and TRIP800 spot welds.

  8. Geochronology and geochemistry of a dyke host rock association and implications for the formation of the Bavarian Pfahl shear zone, Bohemian Massif

    NASA Astrophysics Data System (ADS)

    Siebel, Wolfgang; Blaha, Ulrich; Chen, Fukun; Rohrmüller, Johann

    2005-02-01

    To place constraints on the formation and deformation history of the major Variscan shear zone in the Bavarian Forest, Bavarian Pfahl zone, SW Bohemian Massif, granitic dykes and their feldspar-phyric massive host rock (so-called “palite”), zircons were dated by the U Pb isotope dilution and Pb-evaporation methods. The dated samples comprise two host rocks and four dykes from a K-rich calc-alkaline complex adjoining the SW part of the Bavarian Pfahl shear zone. The palites, which appear to be the oldest magmatic rocks emplaced in the shear zone, yield ages of 334±3, 334.5±1.1 Ma (average 207Pb/206Pb-evaporation zircon ages) and 327 342 Ma (range of U/Pb zircon ages) suggesting a Lower Carboniferous age for the initiation of the Pfahl zone. Absence of inherited older cores in all investigated zircons indicates that incorporation of crustal zircon material has played virtually no role or that the melting temperature was very high. Determination of the dyke emplacement age is complicated by partial Pb-loss in most of the fractions analysed. This Pb-loss can be ascribed to higher U content of the dyke zircons compared to those from host rock. Upper discordia intercept ages of the different dykes range from 322±5 to 331±9 Ma. The dykes are pre- to synkinematic with respect to penetrative regional mylonitisation along the Pfahl zone, and the upper intercept ages provide a maximum age for this tectonic event.

  9. New structural and seismological evidence and interpretation of a lithospheric-scale shear zone at the southern edge of the Ionian subduction system (central-eastern Sicily, Italy)

    NASA Astrophysics Data System (ADS)

    Barreca, G.; Scarfı, L.; Cannavò, F.; Koulakov, I.; Monaco, C.

    2016-06-01

    Geological, gravimetric, and seismological data from the central-eastern Sicily (Italy) provide evidences of a NW-SE oriented shear zone at the southern edge of the Ionian subduction system. This structure consists of a near 100 km long lithospheric-scale structural and seismic boundary. In the near-surface, it shows Plio-Pleistocene vertical-axis structural rotations, kilometer-scale topographic imprint, progressive wrenching, and large down-faulting. All these features, together with its location south-west of the subduction system, allow us to interpret the shear zone as the upper plate expression of an abandoned Subduction Transform Edge Propagator fault, working before slab detachment, currently reactivated by elastic rebound or mantle upwelling mechanism triggered by slab detachment, to form an incipient transform belt separating compartments characterized by different motion in the modern context of Africa-Europe convergence.

  10. Active zones of mammalian neuromuscular junctions: formation, density, and aging

    PubMed Central

    Nishimune, Hiroshi

    2012-01-01

    Presynaptic active zones are synaptic vesicle release sites that playessential roles in the function and pathology of mammalian neuromuscular junctions (NMJs). The molecular mechanisms of active zone organization utilize presynaptic voltage-dependent calcium channels (VDCCs) in NMJs as scaffolding proteins. VDCCs interact extracellularly with the muscle-derived synapse organizer, laminin β2, and interact intracellularly with active zone-specific proteins, such as Bassoon, CAST/Erc2/ELKS2alpha, ELKS, Piccolo, and RIMs. These molecular mechanisms are supported by studies in P/Q- and N-type VDCCs double-knockout mice, and they are consistent with the pathological conditions of Lambert-Eaton myasthenic syndrome and Pierson syndrome, which are caused by autoantibodies against VDCCs or by a laminin β2 mutation. During normal postnatal maturation, NMJs maintain the density of active zones, while NMJs triple their size. However, active zones become impaired during aging. Propitiously, muscle exercise ameliorates the active zone impairment in aged NMJs, which suggests the potential for therapeutic strategies. PMID:23252894

  11. Temporal patterns of slip rate on the Little Lake fault, eastern California shear zone, from terrestrial lidar, cosmogenic radionuclides, and InSAR analysis (Invited)

    NASA Astrophysics Data System (ADS)

    Amos, C. B.; Burgmann, R.; Jayko, A. S.; Fisher, G., III; Rood, D. H.

    2010-12-01

    An ever-expanding inventory of fault slip-rates spanning various time intervals reveals contrasting spatial and temporal patterns of strain for active faults within the Eastern California shear zone (ECSZ). Comparison of data derived from geologic, geomorphic, and paleoseismic records with geodetic estimates of fault loading affords unparalleled opportunity to investigate the dynamics of earthquake processes and the evolution of an intracontinental plate boundary fault system. We focus here on the Little Lake fault, which lies along the western margin of the ECSZ between the Sierra Nevada and the Coso Range in east-central California. The fault accommodates between 10-20% of the total dextral motion within the ECSZ at this latitude and a smaller fraction of the relative motion between the Pacific and North American plates. Relatively high rates of decadal fault loading previously described for the Little Lake fault zone from GPS measurements and InSAR data suggest potential discrepancies with longer, late Quaternary records of fault slip. To address this inconsistency, we targeted a series of fluvial terraces related to overtopping and outflow from pluvial Owens Lake that cross the fault and record dextral offset since Late-Pleistocene time. High-resolution topographic surveying using a tripod-mounted laser scanner suggests between 33 and 38 m of reconstructed right-lateral displacement of two individual terrace risers over this time period. Coupled with cosmogenic 10Be dating of intact, meter-scale outwash boulders preserved on terrace treads bounding each riser, detailed characterization of these geomorphic features provides a robust geologic slip-rate estimate for the Little Lake fault zone at the ~104 year timescale. Preliminary correlation of terrace surfaces with available exposure dating of fluvially scoured basalt upstream at Fossil Falls suggests a right-lateral displacement-rate on the order of ~2 mm/yr. Although more than double the previously reported

  12. Evolution of metamorphic fluids in shear zones: The record from the emeralds of Habachtal, Tauern Window, Austria

    NASA Astrophysics Data System (ADS)

    Nwe, Y. Y.; Grundmann, G.

    1990-11-01

    Fluid inclusions in emeralds from the Habachtal, Central Tauern Window, have been studied by microthermometry. Results allow a detailed reconstruction of trapping history and evolution of the metamorphic fluids during the Middle Alpine Tauernkristallisation metamorphic event and some of the subsequent cooling period. Five different types of fluid inclusions, corresponding to at least five trapping periods, have been distinguished. In general, the earliest primary (type 1) inclusions, which occur as negative crystals or thin long tubes, are represented by low salinity ( < 10 wt. % NaCl equivalent) aqueous fluids with or without CO 2 with up to XCO 2 ≈ 0.04. Later primary type 2 inclusions are distinguished by different morphologies and distribution patterns. Lower salinity CO 2-free brines and CO 2-bearing denser inclusions with higher CO 2 contents (up to XCO 2 ≈ 0.11) are characteristic of this stage. The type 2 inclusions may also occur as pseudosecondary arrays. The effects of necking have been studied, and found to be considerable in the type 1 primary inclusions. This mechanism has occasionally resulted in the appearance of almost pure CO 2 fluids. The possibility of fluid immiscibility has been examined, and rejected, for the apparent "coexistence" of primary brine and CO 2-bearing inclusions. Instead, mixing of fluids which fluctuated between two different compositions is proposed. The fluctuation was probably due to the sequence of hydration reactions during the Tauernkristallisation. Maximum trapping pressures (3.6 kbar) obtained for stage 1 of the Tauernkristallisation are thought to represent a situation where sublithostatic fluid pressures exested in shear zones during the crystallisation period of many of the emerald cores and coexisting biotite and actinolite. Maximum fluid pressures of 7 kbar were obtained from the type 2 inclusions. This is similar to pressure estimates obtained from mineral equilibria. At least four phases of deformation are

  13. Rock mechanics observations pertinent to the rheology of the continental lithosphere and the localization of strain along shear zones

    USGS Publications Warehouse

    Kirby, S.H.

    1985-01-01

    the general phenomenon of ductile faulting in which ductile strains are localized into shear zones. Ductile faults have been produced in experiments of five different rock types and is generally expressed as strain softening in constant-strain-rate tests or as an accelerating-creep-rate stage at constant differential stress. A number of physical mechanisms have been identified that may be responsible for ductile faulting, including the onset of dynamic recrystallization, phase changes, hydrothermal alteration and hydrolytic weakening. Microscopic evidence for these processes as well as larger-scale geological and geophysical observations suggest that ductile faulting in the middle to lower crust and upper mantle may greatly influence the distribution and magnitudes of differential stresses and the style of deformation in the overlying upper continental lithosphere. ?? 1985.

  14. Caveolin-1 regulates shear stress-dependent activation of extracellular signal-regulated kinase

    NASA Technical Reports Server (NTRS)

    Park, H.; Go, Y. M.; Darji, R.; Choi, J. W.; Lisanti, M. P.; Maland, M. C.; Jo, H.

    2000-01-01

    Fluid shear stress activates a member of the mitogen-activated protein (MAP) kinase family, extracellular signal-regulated kinase (ERK), by mechanisms dependent on cholesterol in the plasma membrane in bovine aortic endothelial cells (BAEC). Caveolae are microdomains of the plasma membrane that are enriched with cholesterol, caveolin, and signaling molecules. We hypothesized that caveolin-1 regulates shear activation of ERK. Because caveolin-1 is not exposed to the outside, cells were minimally permeabilized by Triton X-100 (0.01%) to deliver a neutralizing, polyclonal caveolin-1 antibody (pCav-1) inside the cells. pCav-1 then bound to caveolin-1 and inhibited shear activation of ERK but not c-Jun NH(2)-terminal kinase. Epitope mapping studies showed that pCav-1 binds to caveolin-1 at two regions (residues 1-21 and 61-101). When the recombinant proteins containing the epitopes fused to glutathione-S-transferase (GST-Cav(1-21) or GST-Cav(61-101)) were preincubated with pCav-1, only GST-Cav(61-101) reversed the inhibitory effect of the antibody on shear activation of ERK. Other antibodies, including m2234, which binds to caveolin-1 residues 1-21, had no effect on shear activation of ERK. Caveolin-1 residues 61-101 contain the scaffolding and oligomerization domains, suggesting that binding of pCav-1 to these regions likely disrupts the clustering of caveolin-1 or its interaction with signaling molecules involved in the shear-sensitive ERK pathway. We suggest that caveolae-like domains play a critical role in the mechanosensing and/or mechanosignal transduction of the ERK pathway.

  15. Pressure solution in rocks: focused ion beam/transmission electron microscopy study on orthogneiss from South Armorican Shear Zone, France

    NASA Astrophysics Data System (ADS)

    Bukovská, Zita; Wirth, Richard; Morales, Luiz F. G.

    2015-09-01

    In order to characterize the µm-to-nm structures related to operation of pressure solution on phase boundaries in naturally deformed rocks, we have performed a detailed focused ion beam/transmission electron microscopy study in ultramylonite samples from South Armorican Shear Zone (France) that focused on grain boundary scale. We have studied phase boundaries between quartz, K-feldspar and white mica in both 2D and 3D and compare our evidences with theoretical dissolution precipitation models in the current literature. The dissolution (re)precipitation processes lead to the development of different features at different phase boundaries. In both quartz-white mica and quartz-K-feldspar phase boundaries, voids were ubiquitously observed. These voids have different shapes, and the development of some of them is crystallographically controlled. In addition, part of these voids might be filled with vermiculite. Amorphous leached layers with kaolinite composition were observed at the boundaries of K-feldspar-quartz and K-feldspar-white mica. The development of different features along the phase boundaries is mainly controlled by the crystallography of the phases sharing a common interface, together with the presence of fluids that either leaches or directly dissolve the mineral phases. In addition, the local dislocation density in quartz may play an important role during pressure solution. We suggest that the nanoscale observations of the quartz-white mica phase boundaries show direct evidence for operation of island-and-channel model as described in Wassmann and Stockhert (Tectonophysics 608:1-29, 2013), while K-feldspar-quartz phase boundaries represents amorphous layers formed via interface-coupled dissolution reprecipitation as described by Hellmann et al. (Chem Geol 294-295:203-216, 2012).

  16. Latest Quaternary paleoseismology and evidence of distributed dextral shear along the Mohawk Valley fault zone, northern Walker Lane, California

    NASA Astrophysics Data System (ADS)

    Gold, Ryan D.; Briggs, Richard W.; Personius, Stephen F.; Crone, Anthony J.; Mahan, Shannon A.; Angster, Stephen J.

    2014-06-01

    The dextral-slip Mohawk Valley fault zone (MVFZ) strikes northwestward along the eastern margin of the Sierra Nevada in the northern Walker Lane. Geodetic block modeling indicates that the MVFZ may accommodate ~3 mm/yr of regional dextral strain, implying that it is the highest slip-rate strike-slip fault in the region; however, only limited geologic data are available to constrain the system's slip rate and earthquake history. We mapped the MVFZ using airborne lidar data and field observations and identified a site near Sulphur Creek for paleoseismic investigation. At this site, oblique dextral-normal faulting on the steep valley margin has created a closed depression that floods annually during spring snowmelt to form an ephemeral pond. We excavated three fault-perpendicular trenches at the site and exposed pond sediment that interfingers with multiple colluvial packages eroded from the scarp that bounds the eastern side of the pond. We documented evidence for four surface-rupturing earthquakes on this strand of the MVFZ. OxCal modeling of radiocarbon and luminescence ages indicates that these earthquakes occurred at 14.0 ka, 12.8 ka, 5.7 ka, and 1.9 ka. The mean ~4 kyr recurrence interval is inconsistent with slip rates of ~3 mm/yr; these rates imply surface ruptures of more than 10 m per event, which is geologically implausible for the subdued geomorphic expression and 60 km length of the MVFZ. We propose that unidentified structures not yet incorporated into geodetic models may accommodate significant dextral shear across the northern Walker Lane, highlighting the role of distributed deformation in this region.

  17. Catalase-only nanoparticles prepared by shear alone: Characteristics, activity and stability evaluation.

    PubMed

    Huang, Xiao-Nan; Du, Xin-Ying; Xing, Jin-Feng; Ge, Zhi-Qiang

    2016-09-01

    Catalase is a promising therapeutic enzyme; however, it carries risks of inactivation and rapid degradation when it is used in practical bioprocess, such as delivery in vivo. To overcome the issue, we made catalase-only nanoparticles using shear stress alone at a moderate shear rate of 217s(-1) in a coaxial cylinder flow cell. Properties of nanoparticles, including particle size, polydispersity index and zeta potential, were characterized. The conformational changes of pre- and post-sheared catalase were determined using spectroscopy techniques. The results indicated that the conformational changes of catalase and reduction in α-helical content caused by shear alone were less significant than that by desolvation method. Catalase-only nanoparticles prepared by single shear retained over 90% of its initial activity when compared with the native catalase. Catalase nanoparticles lost only 20% of the activity when stored in phosphate buffer solution for 72h at 4°C, whereas native catalase lost 53% under the same condition. Especially, the activity of nanogranulated catalase was decreased only slightly in the simulated intestinal fluid containing α-chymotrypsin during 4h incubation at 37°C, implying that the catalase nanoparticle was more resistant to the degradation of proteases than native catalase molecules. Overall, catalase-only nanoparticles offered a great potential to stabilize enzymes for various pharmaceutical applications. PMID:26318217

  18. Neutral-Line Magnetic Shear and Enhanced Coronal Heating in Solar Active Regions

    NASA Technical Reports Server (NTRS)

    Falconer, D. A.; Moore, R. L.; Porter, J. G.; Gary, G. A.; Shimizu, T.

    1997-01-01

    By examining the magnetic structure at sites in the bright coronal interiors of active regions that are not flaring but exhibit persistent strong coronal heating, we establish some new characteristics of the magnetic origins of this heating. We have examined the magnetic structure of these sites in five active regions, each of which was well observed by both the Yohkoh SXT and the Marshall Space Flight Center Vector Magnetograph and showed strong shear in its magnetic field along part of at least one neutral line (polarity inversion). Thus, we can assess whether this form of nonpotential field structure in active regions is a characteristic of the enhanced coronal heating and vice versa. From 27 orbits' worth of Yohkoh SXT images of the five active regions, we have obtained a sample of 94 persistently bright coronal features (bright in all images from a given orbit), 40 long (greater than or approximately equals 20,000 km) neutral-line segments having strong magnetic shear throughout (shear angle greater than 45 deg), and 39 long neutral-line segments having weak magnetic shear throughout (shear angle less than 45 deg). From this sample, we find that: (1) all of our persistently bright coronal features are rooted in magnetic fields that are stronger than 150 G; (2) nearly all (95%) of these enhanced coronal features are rooted near neutral lines (closer than 10,000 km); (3) a great majority (80%) of the bright features are rooted near strong-shear portions of neutral lines; (4) a great majority (85%) of long strong-shear segments of neutral lines have persistently bright coronal features rooted near them; (5) a large minority (40%) of long weak-shear segments of neutral lines have persistently bright coronal features rooted near them; and (6) the brightness of a persistently bright Coronal feature often changes greatly over a few hours. From these results, we conclude that most persistent enhanced heating of coronal loops in active regions: (1) requires the

  19. Inferences about Shear Zone Flow Pathways between CFM 06.002i2 and Pinkel from Tracer Tests 10-01 to 12-02

    SciTech Connect

    Reimus, Paul W.

    2012-06-26

    This presentation provides an analysis of several tracer tests conducted at the Grimsel Test Site, Switzerland, between 2010 and early 2012, with the objective of testing a conceptual model of flow through the shear zone in which the tracer tests were conducted. The analysis includes predictions of tracer residence times in each of two flow pathways in the shear zone as a function of injection and extraction flow rates in the tracer tests. Conclusions are: (1) Separation of shear zone flow between CFM 06.002i2 and Pinkel into two predominant flow pathways seems reasonable; (2) Conceptual model is that travel time in pathway 1 is dependent on injection flow rate, and travel time in pathway 2 is dependent on extraction flow rate; (3) Predict residence time (in hours) in Pathway 1 equal to {approx}9.9/(Injection Flow Rate, ml/min), provided injection interval flow is greater than about 0.15 ml/min (which is not reliably achieved under natural flow/dilution conditions after installation of CFM 11.00X holes); and (4) Predict residence time of {approx}8 hrs in Pathway 2 with extraction flow rate of 25 ml/min.

  20. Along strike variation of tremor activities and thermal structures in various subduction zones

    NASA Astrophysics Data System (ADS)

    Yabe, S.; Ide, S.; Yoshioka, S.

    2012-12-01

    A family of slow earthquakes, e.g., deep low frequency tremors, low frequency earthquakes (LFEs), very low frequency earthquakes (VLFs) and slow slip events (SSEs), are observed in various subduction zones. These phenomena represent shear slip on the plate interface, and they are thought to be related to brittle-ductile transition behavior on the plate interface because they are often located near the transition zones of interplate coupling estimated from GPS data. Such slip behavior along the plate interface would be controlled by temperature. Furthermore, tremors are considered to be related to fluid dehydrated from the subducting slab, through temperature dependent chemical reactions. Therefore, tremors occurrences are expected to be influenced by temperature, though some studies have questioned about the relationship between tremor activity and temperature. Here we investigate the source locations of deep tremor using an envelope correlation method and compare them with the temperature and shear strength profiles along the plate interface calculated using a numerical model (Yoshioka and Sanshadokoro, 2002). The study areas include New Zealand, southern Chile, and Mexico, where tremor behavior changes significantly along the strike of the plate interface. Investigating such along-strike variation in individual subduction zone may clarify the temperature dependence of tremor because environmental conditions affecting tremor occurrence are similar, unlike the comparison between different subduction zones. In the Hikurangi subduction zone beneath the North Island, New Zealand, the depth of SSE are quite different along the strike, e.g., deeper in the central region and shallower in the northern region (e.g. Wallace and Beavan, 2010). We reanalyze tremors detected by previous studies (Kim et al., 2011; Ide, 2012) to estimate their absolute depth and confirm that tremors in North Island are on the plate interface in both the central and the northern regions. Thermal

  1. Spatial Evolution of Neogene Normal Faults, Northern Owens Valley: Constraints on Oblique-slip Partioning Within the Eastern California Shear Zone.

    NASA Astrophysics Data System (ADS)

    Sheehan, T. P.; Dawers, N.

    2005-05-01

    Simple geometric constraints can be used to predict fault interaction at depth. Such interaction within crustal scale fault populations plays an important role in the tectonic evolution of extensional tectonic settings. Here we use a theoretical relationship between fault dip, horizontal fault spacing, and depth to the base of the seismogenic zone to explain the late Cenozoic temporal and spatial evolution of faulting within the Eastern California shear zone, including the northern extent of Owens Valley, California. Our results show that during its evolution, the east-dipping Sierra Nevada frontal fault in northern Owens Valley became inactive due to intersection with the larger west-dipping range-bounding fault of the White Mountains. The horizontal spacing of 10 km between these two conjugate faults is such that they intersect within the brittle seismic layer resulting in the locking of this segment of Sierra Nevada frontal fault. Continued accumulation of normal displacement along the White Mountains fault zone has since resulted in the present-day half-graben basement structure of northern Owens Valley. This down-dropping along the eastern margin of the valley imposes a flexural tension across the surface of the Coyote Warp, which can be considered a large relay zone between the Sierra Nevada frontal fault and the Round Valley fault further west. It is suggested that this tension is responsible for the formation of west-dipping antithetic normal faults that are distributed locally around the Coyote Warp. This extensional fault geometry has imposed a kinematic restraint on the development and distribution of right-lateral shear within this part of the Eastern California shear zone, including northeastward transfer of right-lateral slip from the Owens Valley fault to the White Mountains fault.

  2. High-resolution shear-wave seismics across the Carlsberg Fault zone south of Copenhagen - Implications for linking Mesozoic and late Pleistocene structures

    NASA Astrophysics Data System (ADS)

    Kammann, Janina; Hübscher, Christian; Boldreel, Lars Ole; Nielsen, Lars

    2016-07-01

    The Carlsberg Fault zone (CFZ) is a NNW-SSE striking structure close to the transition zone between the Danish Basin and the Baltic Shield. We examine the fault evolution by combining very-high-resolution onshore shear-wave seismic data, one conventional onshore seismic profile and marine reflection seismic profiles. The faulting geometry indicates a strong influence of Triassic subsidence and rifting in the Central European Basin System. Growth strata within the CFZ surrounding Höllviken Graben reveal syntectonic sedimentation in the Lower Triassic, indicating the opening to be a result of Triassic rifting. In the Upper Cretaceous growth faulting documents continued rifting. These findings contrast the Late Cretaceous to Paleogene inversion tectonics in neighboring structures, such as the Tornquist Zone. The high-resolution shear-wave seismic method was used to image faulting in Quaternary and Danian layers in the CFZ. The portable compact vibrator source ElViS III S8 was used to acquire a 1150 m long seismic section on the island Amager, south of Copenhagen. The shallow subsurface in the investigation area is dominated by Quaternary glacial till deposits in the upper 5-11 m and Danian limestone below. In the shear-wave profile, we imaged the uppermost 30 m of the western part of CFZ. The complex fault zone comprises normal block faults and one reverse block fault. The observed faults cut through the Danian as well as the Quaternary overburden. Hence, there are strong indicators for ongoing faulting, like mapped faulting in Quaternary sediments and ongoing subsidence of the eastern block of the CFZ as interpreted by other authors. The lack of earthquakes localized in the fault zone implies that either the frequency of occurring earthquakes is too small to be recorded in the observation time-span, or that the movement of the shallow sub-surface layers may be due to other sources than purely tectonic processes.

  3. The internal deformation of the Peridotite Nappe of New Caledonia: A structural study of serpentine-bearing faults and shear zones in the Koniambo Massif

    NASA Astrophysics Data System (ADS)

    Quesnel, Benoît; Gautier, Pierre; Cathelineau, Michel; Boulvais, Philippe; Couteau, Clément; Drouillet, Maxime

    2016-04-01

    We present a structural analysis of serpentine-bearing faults and shear zones in the Koniambo Massif, one of the klippes of the Peridotite Nappe of New Caledonia. Three structural levels are recognized. The upper level is characterized by a dense network of fractures. Antigorite and polygonal serpentine form slickenfibers along fault planes with distinct kinematics. As a result, the upper level keeps the record of at least two deformation events, the first associated with the growth of antigorite (WNW-ESE extension), the second with the growth of polygonal serpentine (NW-SE compression). The lower level coincides with the 'serpentine sole' of the nappe, which consists of massive tectonic breccias overlying a layer of mylonitic serpentinites. The sole records pervasive tangential shear with top-to-SW kinematics and represents a décollement at the base of the nappe. The intermediate level is characterized by the presence of several meters-thick conjugate shear zones accommodating NE-SW shortening. Like the sole, these shear zones involve polygonal serpentine and magnesite as the main syn-kinematic mineral phases. The shear zones likely root into the basal décollement, either along its roof or, occasionally, around its base. Compared to top-to-SW shearing along the sole, the two deformation events recorded in the upper level are older. The three structural levels correlate well with previously recognized spatial variations in the degree of serpentinization. It is therefore tempting to consider that the intensity of serpentinization played a major role in the way deformation has been distributed across the Peridotite Nappe. However, even the least altered peridotites, in the upper level, contain so much serpentine that, according to theoretical and experimental work, they should be nearly as weak as pure serpentinite. Hence, no strong vertical gradient in strength due to variations in the degree of serpentinization is expected within the exposed part of the nappe

  4. Triggered tremors beneath the seismogenic zone of an active fault zone, Kyushu, Japan

    NASA Astrophysics Data System (ADS)

    Miyazaki, Masahiro; Matsumoto, Satoshi; Shimizu, Hiroshi

    2015-11-01

    Non-volcanic tremors were induced by the surface waves of the 2012 Sumatra earthquake around the Hinagu fault zone in Kyushu, Japan. We inferred from dense seismic observation data that the hypocenters of these tremors were located beneath the seismogenic zone of the Hinagu fault. Focal mechanisms of the tremors were estimated using S-wave polarization angles. The estimated focal mechanisms show similarities to those of shallow earthquakes in this region. In addition, one of the nodal planes of the focal mechanisms is almost parallel to the strike direction of the Hinagu fault. These observations suggest that the tremors were triggered at the deeper extension of the active fault zone under stress conditions similar to those in the shallower seismogenic region. A low-velocity anomaly beneath the hypocentral area of the tremors might be related to the tremor activity.

  5. The Black Mountain tectonic zone--a reactivated northeast-trending crustal shear zone in the Yukon-Tanana Upland of east-central Alaska: Chapter D in Recent U.S. Geological Survey studies in the Tintina Gold Province, Alaska, United States, and Yukon, Canada--results of a 5-year project

    USGS Publications Warehouse

    O'Neill, J. Michael; Day, Warren C.; Alienikoff, John N.; Saltus, Richard W.

    2007-01-01

    The Black Mountain tectonic zone in the YukonTanana terrane of east-central Alaska is a belt of diverse northeast-trending geologic features that can been traced across Black Mountain in the southeast corner of the Big Delta 1°×3° degree quadrangle. Geologic mapping in the larger scale B1 quadrangle of the Big Delta quadrangle, in which Black Mountain is the principal physiographic feature, has revealed a continuous zone of normal and left-lateral strikeslip high-angle faults and shear zones, some of which have late Tertiary to Quaternary displacement histories. The tectonic zone includes complexly intruded wall rocks and intermingled apophyses of the contiguous mid-Cretaceous Goodpaster and Mount Harper granodioritic plutons, mafic to intermediate composite dike swarms, precious metal mineralization, early Tertiary volcanic activity and Quaternary fault scarps. These structures define a zone as much as 6 to 13 kilometers (km) wide and more than 40 km long that can be traced diagonally across the B1 quadrangle into the adjacent Eagle 1°×3° quadrangle to the east. Recurrent activity along the tectonic zone, from at least mid-Cretaceous to Quaternary, suggests the presence of a buried, fundamental tectonic feature beneath the zone that has influenced the tectonic development of this part of the Yukon-Tanana terrane. The tectonic zone, centered on Black Mountain, lies directly above a profound northeast-trending aeromagnetic anomaly between the Denali and Tintina fault systems. The anomaly separates moderate to strongly magnetic terrane on the northwest from a huge, weakly magnetic terrane on the southeast. The tectonic zone is parallel to the similarly oriented left-lateral, strike-slip Shaw Creek fault zone 85 km to the west.

  6. Differential regulation of protease activated receptor-1 and tissue plasminogen activator expression by shear stress in vascular smooth muscle cells

    NASA Technical Reports Server (NTRS)

    Papadaki, M.; Ruef, J.; Nguyen, K. T.; Li, F.; Patterson, C.; Eskin, S. G.; McIntire, L. V.; Runge, M. S.

    1998-01-01

    Recent studies have demonstrated that vascular smooth muscle cells are responsive to changes in their local hemodynamic environment. The effects of shear stress on the expression of human protease activated receptor-1 (PAR-1) and tissue plasminogen activator (tPA) mRNA and protein were investigated in human aortic smooth muscle cells (HASMCs). Under conditions of low shear stress (5 dyn/cm2), PAR-1 mRNA expression was increased transiently at 2 hours compared with stationary control values, whereas at high shear stress (25 dyn/cm2), mRNA expression was decreased (to 29% of stationary control; P<0.05) at all examined time points (2 to 24 hours). mRNA half-life studies showed that this response was not due to increased mRNA instability. tPA mRNA expression was decreased (to 10% of stationary control; P<0.05) by low shear stress after 12 hours of exposure and was increased (to 250% of stationary control; P<0.05) after 24 hours at high shear stress. The same trends in PAR-1 mRNA levels were observed in rat smooth muscle cells, indicating that the effects of shear stress on human PAR-1 were not species-specific. Flow cytometry and ELISA techniques using rat smooth muscle cells and HASMCs, respectively, provided evidence that shear stress exerted similar effects on cell surface-associated PAR-1 and tPA protein released into the conditioned media. The decrease in PAR-1 mRNA and protein had functional consequences for HASMCs, such as inhibition of [Ca2+] mobilization in response to thrombin stimulation. These data indicate that human PAR-1 and tPA gene expression are regulated differentially by shear stress, in a pattern consistent with their putative roles in several arterial vascular pathologies.

  7. Decreased platelet function in aortic valve stenosis: high shear platelet activation then inactivation.

    PubMed Central

    O'Brien, J. R.; Etherington, M. D.; Brant, J.; Watkins, J.

    1995-01-01

    OBJECTIVE--To elucidate the mechanism of the bleeding tendency observed in patients with aortic valve stenosis. DESIGN--A prospective study of high and low shear platelet function tests in vitro in normal controls compared with that in patients with severe aortic valve stenosis with a mean (SD) systolic gradient by Doppler of 75 (18) mm Hg before and at least 4 months after aortic valve replacement. SETTING--District general hospital. RESULTS--The patients showed reduced retention in the high shear platelet function tests. (a) Platelet retention in the filter test was 53.6 (12.6)% in patients with aortic valve stenosis and 84.8 (9.6)% in the controls (P < 0.001). (b) Retention in the glass bead column test was 49.8 (19.2) in the patients and 87.4 (8.7) in the controls (P < 0.001). (c) The standard bleeding time was longer in the patients (P < 0.06). Results of the high shear tests (a, b, and c) after aortic valve replacement were within the normal range. The platelet count was low but within the normal range before surgery and increased postoperatively (P < 0.01). There were no differences in the results of standard clotting tests, plasma and intraplatelet von Willebrand's factor, or in 15 platelet aggregation tests using five agonists between patients with aortic valve stenosis and controls. CONCLUSIONS--The high shear haemodynamics of aortic valve stenosis modify platelet function in vivo predisposing to a bleeding tendency. This abnormality of platelet function is detectable only in vitro using high shear tests. The abnormal function is reversed by aortic valve replacement. High shear forces in vitro activate and then inactivate platelets. By the same mechanisms aortic valve stenosis seems to lead to high shear damage in vivo, resulting in a clinically important bleeding tendency in some patients. PMID:8541170

  8. Late Quaternary faulting in Clayton Valley, Nevada: Implications for distributed deformation in the eastern California shear zone-Walker Lane

    NASA Astrophysics Data System (ADS)

    Foy, T. A.; Lifton, Z. M.; Frankel, K. L.; Johnson, C.

    2010-12-01

    The Walker Lane is a key component of the Pacific-North America plate boundary. This transtensional region of right lateral strike-slip and normal faulting accommodates ~25% of the total relative motion between the Pacific and North American plates. Recent studies indicate a discrepancy between short- and long-term rates of right-lateral shear in the Walker Lane with geodetic measurements of ~9.3 mm/yr being more than double the late Pleistocene geologic rate of <3.5 mm/yr. The Walker Lane, therefore, provides an excellent natural laboratory in which to address one of the major unresolved questions in active tectonics: are rates of strain accumulation and release along active plate boundaries constant through space and time or are significant temporal and spatial variations common? The Clayton Valley fault system of the Silver Peak-Lone Mountain extensional complex (SPLM) is a prime candidate to account for part of the “missing” strain in the Walker Lane due to its dominantly down-to-the-NW orientation, which ultimately accommodates right lateral shear in the region. The distribution of late Quaternary faults and alluvial fan deposits in Clayton Valley was determined through detailed geologic mapping (1:10,000 scale), which resulted in eight individual units that are consistent with the well-established western U.S. alluvial stratigraphy. Differential GPS was used to survey the prominent normal fault scarps displacing the fan deposits and cosmogenic nuclide (TCN) geochronology depth profile samples were collected from four units (Q2b, Q2c, Q2d, and Q3a). Displacement measured from the scarp profiles combined with TCN ages of the deformed fans will allow us to determine extension rates over multiple late Pleistocene time scales. Analysis of the Clayton Valley extension rates coupled with those from nearby structures, such as the Lone Mountain and Lida faults, will help determine if slip along these extensional faults can account for the observed discrepancy

  9. Fault Slip Partitioning in the Eastern California Shear Zone-Walker Lane Belt: Pliocene to Late Pleistocene Contraction Across the Mina Deflection

    NASA Astrophysics Data System (ADS)

    Lee, J.; Stockli, D.; Gosse, J.

    2007-12-01

    Two different mechanisms have been proposed for fault slip transfer between the subparallel NW-striking dextral- slip faults that dominant the Eastern California Shear Zone (ECSZ)-Walker Lane Belt (WLB). In the northern WLB, domains of sinistral-slip along NE-striking faults and clockwise block rotation within a zone of distributed deformation accommodated NW-dextral shear. A somewhat modified version of this mechanism was also proposed for the Mina deflection, southern WLB, whereby NE-striking sinistral faults formed as conjugate faults to the primary zone of NW-dextral shear; clockwise rotation of the blocks bounding the sinistral faults accommodated dextral slip. In contrast, in the northern ECSZ and Mina deflection, domains of NE-striking pure dip-slip normal faults, bounded by NW-striking dextral-slip faults, exhibited no rotation; the proposed mechanism of slip transfer was one of right-stepping, high angle normal faults in which the magnitude of extension was proportional to the amount of strike-slip motion transferred. New geologic mapping, tectonic geomorphologic, and geochronologic data from the Queen Valley area, southern Mina deflection constrain Pliocene to late Quaternary fault geometries, slip orientations, slip magnitudes, and slip rates that bear on the mechanism of fault slip transfer from the relatively narrow northern ECSZ to the broad deformation zone that defines the Mina deflection. Four different fault types and orientations cut across the Queen Valley area: (1) The NE-striking normal-slip Queen Valley fault; (2) NE-striking sinistral faults; (3) the NW-striking dextral Coyote Springs fault, which merges into (4) a set of EW-striking thrust faults. (U-Th)/He apatite and cosmogenic radionuclide data, combined with magnitude of fault offset measurements, indicate a Pliocene to late Pleistocene horizontal extension rate of 0.2-0.3 mm/yr across the Queen Valley fault. Our results, combined with published slip rates for the dextral White Mountain

  10. Measuring the activities of higher organisms in activated sludge by means of mechanical shearing pretreatment and oxygen uptake rate.

    PubMed

    Hao, Xiaodi; Wang, Qilin; Cao, Yali; van Loosdrecht, Mark C M

    2010-07-01

    A pretreatment method was developed to assess the activities of higher organisms. The method is based on mechanical shearing to damage the large cells of the protozoan and metazoan community in activated sludge. The procedure was confirmed through experimentation to be effective in determining the activities of higher organisms by comparing oxygen uptake rates (OURs) before and after the higher organisms were eradicated. Shearing led to disintegration of flocs, which could be effectively reconstituted by centrifugation. The reconstitution of the sludge flocs was essential since otherwise the activity of the floc mass would be too high due to lack of diffusion limitation. Mechanical shearing had no influence on the morphology, quantity and specific activity of yeasts, and it was inferred that bacteria smaller than yeasts in size would also not be influenced by the applied shearing procedure. Moreover, the effect of filamentous organisms on the measured activities of higher organisms was experimentally demonstrated and analyzed, and determined to be so weak that it could be ignored. Based on these tests, five typical activated sludge processes were selected to measure the contribution of higher organisms to the original OUR. The measured activities of higher organisms ranged from 9.4 to 25.0% of the original OURs. PMID:20605188

  11. Indirect estimation of the tectonic evolution of magnetic structures along the Indiavaí-Lucialva Shear Zone, Mato Grosso, Brazil

    NASA Astrophysics Data System (ADS)

    Louro, V. H.; Ribeiro, V. B.; Mantovani, M. S.; Geolit Team

    2013-05-01

    The Indiavaí-Lucialva Shear Zone (ILSZ) has a notorious cinematic standard, moving from SW to NE, juxtaposing the Santa Helena Granitic Batholith to the metavolcanosedimentary sets and orthogneisses from the Jauru Domain basement. Along the ILSZ, a sequence of magnetic anomalies of high interference, with each other, and varied polarities occurs, what suggests the presence of different lithologies or times of (re)crystallization of the ferromagnetic minerals from these magnetic structures. In its southernmost portion, the sequence of magnetic anomalies splits in two directions, SW and SE, with the first invading the limits of the Santa Helena batholith and, the latest, accompanying the ILSZ. This study aimed for the comprehension of complex tectonic setting of this region. It analyzed the set of anomalies estimating their lateral limits, depths and directions of total magnetization, with the Enhanced Horizontal Derivatives (EHD), its extrapolation for depth estimative (EHD-Depth), and through an iterative reduction to the magnetic pole, respectively. This procedure allowed the composition of initial models for further inversions of magnetic data which, results, indicate contrasts of magnetic susceptibility in sub-surface. Once known the approximated 3-D shape of the magnetic structures along the ILSZ, the total magnetization intensity of each anomaly was recovered, what consequently allowed, by vector subtraction, to estimate their individual remnant magnetization. The remnant magnetization's inclinations and declinations of the anomalies sources and their latitudes and longitudes permitted the calculus of their respective virtual magnetic paleopoles. When confronted with the South American paleopole wander path and the datings linked to this path, available in the literature, it was possible to have an indirect approximation of the age of (re)crystallization of each magnetic structure near the ILSZ. This procedure indicated an increasing of the ages of the

  12. Active microrheology and simultaneous visualization of sheared phospholipid monolayers.

    PubMed

    Choi, S Q; Steltenkamp, S; Zasadzinski, J A; Squires, T M

    2011-01-01

    Two-dimensional films of surface-active agents-from phospholipids and proteins to nanoparticles and colloids-stabilize fluid interfaces, which are essential to the science, technology and engineering of everyday life. The 2D nature of interfaces present unique challenges and opportunities: coupling between the 2D films and the bulk fluids complicates the measurement of surface dynamic properties, but allows the interfacial microstructure to be directly visualized during deformation. Here we present a novel technique that combines active microrheology with fluorescence microscopy to visualize fluid interfaces as they deform under applied stress, allowing structure and rheology to be correlated on the micron-scale in monolayer films. We show that even simple, single-component lipid monolayers can exhibit viscoelasticity, history dependence, a yield stress and hours-long time scales for elastic recoil and aging. Simultaneous visualization of the monolayer under stress shows that the rich dynamical response results from the cooperative dynamics and deformation of liquid-crystalline domains and their boundaries. PMID:21587229

  13. Active microrheology and simultaneous visualization of sheared phospholipid monolayers

    PubMed Central

    Choi, S.Q.; Steltenkamp, S.; Zasadzinski, J.A.; Squires, T.M.

    2011-01-01

    Two-dimensional films of surface-active agents—from phospholipids and proteins to nanoparticles and colloids—stabilize fluid interfaces, which are essential to the science, technology and engineering of everyday life. The 2D nature of interfaces present unique challenges and opportunities: coupling between the 2D films and the bulk fluids complicates the measurement of surface dynamic properties, but allows the interfacial microstructure to be directly visualized during deformation. Here we present a novel technique that combines active microrheology with fluorescence microscopy to visualize fluid interfaces as they deform under applied stress, allowing structure and rheology to be correlated on the micron-scale in monolayer films. We show that even simple, single-component lipid monolayers can exhibit viscoelasticity, history dependence, a yield stress and hours-long time scales for elastic recoil and aging. Simultaneous visualization of the monolayer under stress shows that the rich dynamical response results from the cooperative dynamics and deformation of liquid-crystalline domains and their boundaries. PMID:21587229

  14. Orbital fluid shear stress promotes osteoblast metabolism, proliferation and alkaline phosphates activity in vitro

    SciTech Connect

    Aisha, M.D.; Nor-Ashikin, M.N.K.; Sharaniza, A.B.R.; Nawawi, H.; Froemming, G.R.A.

    2015-09-10

    Prolonged disuse of the musculoskeletal system is associated with reduced mechanical loading and lack of anabolic stimulus. As a form of mechanical signal, the multidirectional orbital fluid shear stress transmits anabolic signal to bone forming cells in promoting cell differentiation, metabolism and proliferation. Signals are channeled through the cytoskeleton framework, directly modifying gene and protein expression. For that reason, we aimed to study the organization of Normal Human Osteoblast (NHOst) cytoskeleton with regards to orbital fluid shear (OFS) stress. Of special interest were the consequences of cytoskeletal reorganization on NHOst metabolism, proliferation, and osteogenic functional markers. Cells stimulated at 250 RPM in a shaking incubator resulted in the rearrangement of actin and tubulin fibers after 72 h. Orbital shear stress increased NHOst mitochondrial metabolism and proliferation, simultaneously preventing apoptosis. The ratio of RANKL/OPG was reduced, suggesting that orbital shear stress has the potential to inhibit osteoclastogenesis and osteoclast activity. Increase in ALP activity and OCN protein production suggests that stimulation retained osteoblast function. Shear stress possibly generated through actin seemed to hold an anabolic response as osteoblast metabolism and functional markers were enhanced. We hypothesize that by applying orbital shear stress with suitable magnitude and duration as a non-drug anabolic treatment can help improve bone regeneration in prolonged disuse cases. - Highlights: • OFS stress transmits anabolic signals to osteoblasts. • Actin and tubulin fibers are rearranged under OFS stress. • OFS stress increases mitochondrial metabolism and proliferation. • Reduced RANKL/OPG ratio in response to OFS inhibits osteoclastogenesis. • OFS stress prevents apoptosis and stimulates ALP and OCN.

  15. Diabatic heating profiles over the continental convergence zone during the monsoon active spells

    NASA Astrophysics Data System (ADS)

    Chattopadhyay, Rajib; Sur, Sharmila; Joseph, Susmitha; Sahai, A. K.

    2013-07-01

    The present paper aims to bring out the robust common aspects of spatio-temporal evolution of diabatic heating during the monsoon intraseasonal active phases over the continental tropical convergence zone (CTCZ). The robustness of spatio-temporal features is determined by comparing the two state-of-the art reanalyses: NCEP Climate Forecast System reanalysis and Modern ERA Retrospective Analysis. The inter-comparison is based on a study period of 26 years (1984-2009). The study confirms the development of deep heating over the CTCZ region during the active phase and is consistent between the two datasets. However, the detailed temporal evolution of the vertical structure (e.g., vertical tilts) of heating differs at times. The most important common feature from both the datasets is the significant vertical redistribution of heating with the development of shallow (low level) heating and circulation over the CTCZ region 3-7 days after the peak active phase. The shallow circulation is found to be associated with increased vertical shear and relative vorticity over certain regions in the subcontinent. This increased vertical shear and relative vorticity in the lower levels could be crucial in the sustenance of rainfall after the peak active phase. Model experiments with linear dynamics affirm the role of shallow convection in increasing the lower level circulation as observed.

  16. Integrating GIS-based geologic mapping, LiDAR-based lineament analysis and site specific rock slope data to delineate a zone of existing and potential rock slope instability located along the grandfather mountain window-Linville Falls shear zone contact, Southern Appalachian Mountains, Watauga County, North Carolina

    USGS Publications Warehouse

    Gillon, K.A.; Wooten, R.M.; Latham, R.L.; Witt, A.W.; Douglas, T.J.; Bauer, J.B.; Fuemmeler, S.J.

    2009-01-01

    Landslide hazard maps of Watauga County identify >2200 landslides, model debris flow susceptibility, and evaluate a 14km x 0.5km zone of existing and potential rock slope instability (ZEPRSI) near the Town of Boone. The ZEPRSI encompasses west-northwest trending (WNWT) topographic ridges where 14 active/past-active rock/weathered rock slides occur mainly in rocks of the Grandfather Mountain Window (GMW). The north side of this ridgeline is the GMW / Linville Falls Fault (LFF) contact. Sheared rocks of the Linville Falls Shear Zone (LFSZ) occur along the ridge and locally in the valley north of the contact. The valley is underlain principally by layered granitic gneiss comprising the Linville Falls/Beech Mountain/Stone Mountain Thrust Sheet. The integration of ArcGIS??? - format digital geologic and lineament mapping on a 6m LiDAR (Light Detecting and Ranging) digital elevation model (DEM) base, and kinematic analyses of site specific rock slope data (e.g., presence and degree of ductile and brittle deformation fabrics, rock type, rock weathering state) indicate: WNWT lineaments are expressions of a regionally extensive zone of fractures and faults; and ZEPRSI rock slope failures concentrate along excavated, north-facing LFF/LFSZ slopes where brittle fabrics overprint older metamorphic foliations, and other fractures create side and back release surfaces. Copyright 2009 ARMA, American Rock Mechanics Association.

  17. Microstructural and mineral analysis on the fault gouge in the coseismic shear zone of the 2008 M w 7.9 Wenchuan earthquake

    NASA Astrophysics Data System (ADS)

    Yuan, Ren-mao; Zhang, Bing-liang; Xu, Xi-wei; Lin, Chuan-yong; Han, Zhu-jun

    2015-07-01

    The 2008 M w 7.9 Wenchuan earthquake formed two coseismic surface rupture zones with the trend of N35°E, known as the Beichuan-Yingxiu rupture and the Pengguan rupture. The Beichuan-Yingxiu rupture is the principle one with abundant fault gouge development along its length. In the exploratory trench at the Saba village along the Beichuan-Yingxiu rupture, the new fault gouge zone is only ~3 mm wide, which suggests that fault slip was constrained in a very narrow zone. In this study, we thus carried out detailed microstructural and mineral component analysis on the oriented fault gouge samples from the Saba exploratory trench to understand their features and geological implication. The results show that different microstructures of localized brittle deformation can be observed in the fault gouges, including Y-shear, R1-shear, R2-shear, P-shear as well as tension fracture, bookshelf glided structure and so on. These microstructures are commonly recognized as the product of seismic fault slipping. Furthermore, within the area between two parallel Y-shears of the fault gouge, a few of microstructures of distributed ductile deformations were developed, such as P-foliation, elongation and asymmetrical trailing structure of detrital particles. The microstructure features of fault gouges implicate the thrust movement of the fault during the Wenchuan earthquake. In addition, the fault gouge has less quartz and feldspar and more clay than the surrounding rocks, which indicates that some quartz and feldspar in the surrounding rocks were transformed into clay, whereas the fault gouge has more illite and less illite/montmorillonite mixed layers than the surrounding rocks, which shows that the illite/montmorillonite mixed layer was partly converted into illite due to temperature increasing induced by coseismic fault slipping friction (also being affected partly by the chemical action of solutions). Such microstructures features and mineral component changes recorded the

  18. Pseudotachylyte in the Bench Canyon Shear Zone, central Sierra Nevada, California: Frictional melting in the brittle and semi-brittle fields

    SciTech Connect

    McNulty, B.A. )

    1993-04-01

    Many aspects of pseudotachylyte are controversial, particularly whether it is the product of intense comminution (e.g. ultracataclasite'') or frictional melting. Ubiquitous exposures of pseudotachylyte in the Bench Canyon shear Zone (BCSZ), central Sierra Nevada, California, provide an excellent opportunity for further study. Scanning electron microscopy (SEM) reveals vesicles, amygdules, crystallites and embayments of microxenocrysts, textures which are supportive of a melt origin for pseudotachylyte in the BCAZ. EDS and microprobe analyses indicate strong compositional contrasts between pseudotachylyte and granodiorite host; one explanation for this is preferential melting in order of individual mineral melting points.

  19. Structural Analysis of Active North Bozgush Fault Zone (NW Iran)

    NASA Astrophysics Data System (ADS)

    Saber, R.; Isik, V.; Caglayan, A.

    2013-12-01

    NW Iran is one of the seismically active regions between Zagros Thrust Belt at the south and Caucasus at the north. Not only large magnitude historical earthquakes (Ms>7), but also 1987 Bozgush, 1997 Ardebil (Mw 6.1) and 2012 Ahar-Varzagan (Mw 6.4) earthquakes reveal that the region is seismically active. The North Bozgush Fault Zone (NBFZ) in this region has tens of kilometers in length and hundreds of meters in width. The zone has produced some large and destructive earthquakes (1593 M:6.1 and 1883 M:6.2). The NBFZ affects the Cenozoic units and along this zone Eocene units thrusted over Miocene and/or Plio-Quaternary sedimentary units. Together with morphologic features (stream offsets and alluvial fan movements) affecting the young unites reveal that the zone is active. The zone is mainly characterized by strike-slip faults with reverse component and reverse faults. Reverse faults striking N55°-85°E and dip of 40°-50° to the SW while strike-slip faults show right lateral slip with N60°-85°W and N60°-80°E directions. Our structural data analysis in NBFZ indicates that the axis direction of σ2 principal stress is vertical and the stress ratio (R) is 0.12. These results suggest that the tectonic regime along the North Bozgush Fault Zone is transpressive. Obtained other principal stresses (σ1, σ3) results are compatible with stress directions and GPS velocity suggested for NW Iran.

  20. Endothelial cell activation by hemodynamic shear stress derived from arteriovenous fistula for hemodialysis access.

    PubMed

    Franzoni, Marco; Cattaneo, Irene; Longaretti, Lorena; Figliuzzi, Marina; Ene-Iordache, Bogdan; Remuzzi, Andrea

    2016-01-01

    Intimal hyperplasia (IH) is the first cause of failure of an arteriovenous fistula (AVF). The aim of the present study was to investigate the effects on endothelial cells (ECs) of shear stress waveforms derived from AVF areas prone to develop IH. We used a cone-and-plate device to obtain real-time control of shear stress acting on EC cultures. We exposed human umbilical vein ECs for 48 h to different shear stimulations calculated in a side-to-end AVF model. Pulsatile unidirectional flow, representative of low-risk stenosis areas, induced alignment of ECs and actin fiber orientation with flow. Shear stress patterns of reciprocating flow, derived from high-risk stenosis areas, did not affect EC shape or cytoskeleton organization, which remained similar to static cultures. We also evaluated flow-induced EC expression of genes known to be involved in cytoskeletal remodeling and expression of cell adhesion molecules. Unidirectional flow induced a significant increase in Kruppel-like factor 2 mRNA expression, whereas it significantly reduced phospholipase D1, α4-integrin, and Ras p21 protein activator 1 mRNA expression. Reciprocating flow did not increase Kruppel-like factor 2 mRNA expression compared with static controls but significantly increased mRNA expression of phospholipase D1, α4-integrin, and Ras p21 protein activator 1. Reciprocating flow selectively increased monocyte chemoattractant protein-1 and IL-8 production. Furthermore, culture medium conditioned by ECs exposed to reciprocating flows selectively increased smooth muscle cell proliferation compared with unidirectional flow. Our results indicate that protective vascular effects induced in ECs by unidirectional pulsatile flow are not induced by reciprocating shear forces, suggesting a mechanism by which oscillating flow conditions may induce the development of IH in AVF and vascular access dysfunction. PMID:26497959

  1. Preliminary Interpretation of a Radionuclide and Colloid Tracer Test in a Granodiorite Shear Zone at the Grimsel Test Site, Switzerland

    SciTech Connect

    Reimus, Paul W.

    2012-08-30

    In February and March 2012, a tracer test involving the injection of a radionuclide-colloid cocktail was conducted in the MI shear zone at the Grimsel Test Site, Switzerland, as part of the Colloids Formation and Migration (CFM) project. The colloids were derived from FEBEX bentonite, which is mined in Spain and is being considered as a potential waste package backfill in a Spanish nuclear waste repository. The tracer test, designated test 12-02 (second test in 2012), involved the injection of the tracer cocktail into borehole CFM 06.002i2 and extraction from the Pinkel surface packer at the main access tunnel wall approximately 6.1 m from the injection interval. The test configuration is depicted in Figure 1. This configuration has been used in several conservative tracer tests and two colloid-homologue tracer tests since 2007, and it is will be employed in an upcoming test involving the emplacement of a radionuclide-doped bentonite plug into CFM 06.002i2 to evaluate the swelling and erosion of the bentonite and the transport of bentonite colloids and radionuclides from the source to the extraction point at the tunnel wall. Interpretive analyses of several of the previous tracer tests, from 09-01 through 12-02 were provided in two previous Used Fuel Disposition Program milestone reports (Arnold et al., 2011; Kersting et al., 2012). However, only the data for the conservative tracer Amino-G Acid was previously analyzed from test 12-02 because the other tracer data from this test were not available at the time. This report documents the first attempt to quantitatively analyze the radionuclide and colloid breakthrough curves from CFM test 12-02. This report was originally intended to also include an experimental assessment of colloid-facilitated transport of uranium by bentonite colloids in the Grimsel system, but this assessment was not conducted because it was reported by German collaborators at the Karlsruhe Institute of Technology (KIT) that neither uranium nor

  2. Strain-dependent evolution of garnets in a high pressure ductile shear zone using Synchroton x-ray microtomography

    NASA Astrophysics Data System (ADS)

    Macente, Alice; Fusseis, Florian; Menegon, Luca; John, Timm

    2016-04-01

    Synkinematic reaction microfabrics carry important information on the kinetics, timing and rheology of tectonometamorphic processes. Despite being routinely interpreted in metamorphic and structural studies, reaction and deformation microfabrics are usually described in two dimensions. We applied Synchrotron-based x-ray microtomography to document the evolution of a pristine olivine gabbro into a deformed omphacite-garnet eclogite in 3D. In the investigated samples, which cover a strain gradient into a shear zone from the Western Gneiss Region (Norway) previously described by John et al., (2009), we focused on the spatial transformation of garnet coronas into elongated clusters of garnets. Our microtomographic data allowed us to quantify changes to the garnet volume, their shapes and their spatial arrangement. We combined microtomographic observations with light microscope- and backscatter electron images as well as electron microprobe- (EMPA) and electron backscatter diffraction (EBSD) analyses to correlate mineral composition and orientation data with the x-ray absorption signal of the same mineral grains. This allowed us to extrapolate our interpretation of the metamorphic microfabric evolution to the third dimension, effectively yielding a 4-dimensional dataset. We found that: - The x-ray absorption contrast between individual mineral phases in our microtomographic data is sufficient to allow the same petrographic observations than in light- and electron microscopy, but extended to 3D. - Amongst the major constituents of the synkinematic reactions, garnet is the only phase that can be segmented confidently from the microtomographic data. - With increasing deformation, the garnet volume increases from about 9% to 25%. - Garnet coronas in the gabbros never completely encapsulate olivine grains. This may indicate that the reaction progressed preferentially in some directions, but also leaves pathways for element transport to and from the olivines that are

  3. Increase in Mechanical Resistance to Force in a Shear-Activated Protein

    NASA Astrophysics Data System (ADS)

    Botello, Eric; Harris, Nolan; Choi, Huiwan; Zhou, Zhou; Bergeron, Angela; Dong, Jing-Fei; Kiang, Ching-Hwa

    2009-03-01

    von Willebrand factor (VWF) is the largest multimeric adhesion ligand found in human blood. Plasma VWF (pVWF) must be exposed to shear stress, like at sites of vascular injury, to be activated to bind platelets to induce blood clotting. In addition, adhesion activity of VWF is related to its polymer size, with the ultra-large form of VWF (ULVWF) being hyper-active, and forming fibers even without exposure to shear stress. We used the AFM to stretch pVWF, sheared VWF (sVWF) and ULVWF, and monitor the forces as a function of molecular extension. We showed a similar increase in force resistance to unfolding for sVWF and ULVWF when compared to pVWF. The increase in force is reduced when other molecules that are known to disrupt their fibril formation are present. Our results provide evidence that the common higher order structure of sVWF and ULVWF may affect the domain structure that causes difference in their adhesion activity compared to pVWF.

  4. Development of discrete aggregates of recrystallization along micro-shear zones in quartz ribbons during multistage ductile evolution of a quartz vein

    NASA Astrophysics Data System (ADS)

    Ceccato, Alberto; Pennacchioni, Giorgio; Bestmann, Michel

    2016-04-01

    The post-magmatic ductile deformation of the Rieserferner pluton (Eastern Alps) includes localized ductile shear zones exploiting a set of joint-filling quartz veins. These deformed veins show different stages of evolution, from coarse grained vein quartz to the fine grained recrystallized aggregates of ultramylonites, locally recorded in different domains of heterogeneously sheared veins. The microstructural evolution includes, with increasing strain: (i) Development of ribbon mylonites consisting of elongated grains, oblique to the shear zone boundary, derived from different quartz veins crystals. The individual ribbons have different crystallographic orientations and aspect ratios. (ii) Dismantling of ribbons along a fracture-like network of fine grained recrystallized quartz aggregates, that commonly represent micro-shear zones (μSZ). These discrete recrystallization zones are preferentially developed in ribbons whose crystallographic axis is oriented either parallel or normal to ribbon elongation. (iii) Extensive dynamic recrystallization to fine-grained (10-20 μm) aggregates leading to quartz ultramylonites. Typically ultramylonites show a layered texture with bands having different crystallographic preferred orientation (CPO) that probably reflect the original heterogeneity in crystallographic orientations of the vein. Electron backscattered diffraction analysis indicates that the μSZ within quartz ribbons are mainly parallel to {r} or {z} planes of the host grain, and the new grain inside μSZ show a weak CPO with their basal plane parallel to the μSZ boundary. There is no systematic relationships between the Dauphiné twinning and the μSZ. Misorientation analysis suggests that in the host grain dislocation creep is dominant on {m} slip system, whereas it is probably a minor mechanism within μSZ. Subgrains and low-angle boundaries (LAB) are heterogeneously developed at the border of the μSZ, and more commonly occur around the tips of

  5. Plane constrained shear of single crystal strip with two active slip systems

    NASA Astrophysics Data System (ADS)

    Le, K. C.; Sembiring, P.

    Within continuum dislocation theory the plane constrained shear of a single crystal strip with two active slip systems is considered. An analytical solution is found for symmetric double slip which exhibits the energetic and dissipative thresholds for dislocation nucleation, the Bauschinger translational work hardening, and the size effects. Comparison with discrete dislocation simulations shows good agreement between the discrete and continuum approaches. Numerical procedures in the general case of non-symmetric double slip are proposed.

  6. Earthquake mechanisms and active tectonics of the Hellenic subduction zone

    NASA Astrophysics Data System (ADS)

    Shaw, Beth; Jackson, James

    2010-05-01

    sense. In the west, the subduction zone terminates in a distributed zone of parallel NE-SW strike-slip faults, of which the most prominent is the Kefalonia Transform Fault (KTF). A flexural gravity anomaly confirms that the deep bathymetric escarpment of the KTF is a lateral ramp, formed as the Ionian islands are emplaced SW onto the Apulian lithosphere, and enhanced by minor thrust faulting with slip vectors perpendicular to the scarp. Distributed parallel strike-slip faults both SW and NE of mainland central Greece terminate in E-W graben in central Greece, which accommodate the overall NE-SW shear by clockwise block rotation. Central Greece therefore acts as a relay zone between the strike-slip faulting of the NE Aegean and the Ionian Islands-western Peloponnese.

  7. Finite strain analysis of metavolcanics and metapyroclastics in gold-bearing shear zone of the Dungash area, Central Eastern Desert, Egypt

    NASA Astrophysics Data System (ADS)

    Kassem, Osama M. K.; Abd El Rahim, Said H.

    2014-11-01

    The Dungash gold mine area is situated in an EW-trending quartz vein along a shear zone in metavolcanic and metasedimentary host rocks in the Eastern Desert of Egypt. These rocks are associated with the major geologic structures, which are attributed to various deformational stages of the Neoproterozoic basement rocks. Field geology, finite strain and microstructural analyses were carried out and the relation-ships between the lithological contacts and major/minor structures have been studied. The R f/ϕ and Fry methods were applied on the metavolcano-sedimentary and metapyroclastic samples from 5 quartz veins samples, 7 metavolcanics samples, 3 metasedimentary samples and 4 metapyroclastic samples in Dungash area. Finite-strain data show that a low to moderate range of deformation of the metavolcano-sedimentary samples and axial ratios in the XZ section range from 1.70 to 4.80 for the R f/ϕ method and from 1.65 to 4.50 for the Fry method. We conclude that finite strain in the deformed rocks is of the same order of magnitude for all units of metavolcano-sedimentary rocks. Furthermore, the contact between principal rock units is sheared in the Dungash area under brittle to semi-ductile deformation conditions. In this case, the accumulated finite strain is associated with the deformation during thrusting to assemble nappe structure. It indicates that the sheared contacts have been formed during the accumulation of finite strain.

  8. 78 FR 4155 - Agency Information Collection Activities: Application for Foreign Trade Zone and/or Status...

    Federal Register 2010, 2011, 2012, 2013, 2014

    2013-01-18

    ... Foreign Trade Zone and/or Status Designation, and Application for Foreign Trade Zone Activity Permit... Application for Foreign Trade Zone Admission and/or Status Designation, and Application for Foreign Trade Zone... Foreign Trade Zone Admission and/or Status Designation, and Application for Foreign Trade......

  9. Chemical and sulphur isotope compositions of pyrite in the Jaduguda U (-Cu-Fe) deposit, Singhbhum shear zone, eastern India: Implications for sulphide mineralization

    NASA Astrophysics Data System (ADS)

    Pal, Dipak C.; Sarkar, Surajit; Mishra, Biswajit; Sarangi, A. K.

    2011-06-01

    The Jaduguda U (-Cu-Fe) deposit in the Singhbhum shear zone has been the most productive uranium deposit in India. Pyrite occurs as disseminated grains or in sulphide stringers and veins in the ore zone. Veins, both concordant and discordant to the pervasive foliation, are mineralogically either simple comprising pyrite ± chalcopyrite or complex comprising pyrite + chalcopyrite + pentlandite + millerite. Nickel-sulphide minerals, though fairly common in concordant veins, are very rare in the discordant veins. Pyrite in Ni-sulphide association is commonly replaced by pentlandite at the grain boundary or along micro-cracks. Based on concentrations of Co and Ni, pyrite is classified as: type-A - high Co (up to 30800 ppm), no/low Ni; type-B - moderate Co (up to 16500 ppm) and moderate to high Ni (up to 32700 ppm); type-C - no/low Co and high Ni (up to 43000 ppm); type-D - neither Co nor Ni. Textural and compositional data of pyrites suggest that the hydrothermal fluid responsible for pre-/early-shearing mineralization evolved from Co-rich to Ni-rich and the late-/post-shearing fluid was largely depleted in minor elements. Sulphur isotope compositions of pyrite mostly furnish positive values ranging between -0.33 and 12.06‰. Composite samples of pyrites with only type-A compositions and mixed samples of type-A and type-B are consistently positive. However, pyrite with mixed type-A and type-C and pyrite with type-D compositions have negative values but close to 0‰. By integrating minor element and sulphur isotope compositions of pyrite in conjunction with other published data on the Jaduguda deposit, it is proposed that reduced sulphur for the precipitation of most pyrites (type-A, type-B) was likely derived from isotopically heavy modified seawater. However, some later sulphur might be magmatic in origin remobilized from existing sulphides in the mafic volcanic rocks in the shear zone.

  10. Shear-induced Endothelial NOS Activation and Remodeling via Heparan Sulfate, Glypican-1, and Syndecan-1

    PubMed Central

    Ebong, Eno E; Lopez-Quintero, Sandra V; Rizzo, Victor; Spray, David C; Tarbell, John M

    2014-01-01

    Mammalian epithelial cells are coated with a multifunctional surface glycocalyx (GCX). On vascular endothelial cells (EC), intact GCX is atheroprotective. It is degraded in many vascular diseases. GCX heparan sulfate (HS) is essential for healthy flow-induced EC nitric oxide (NO) release, elongation, and alignment. The HS core protein mechanisms involved in these processes are unknown. We hypothesized that the glypican-1 (GPC1) HS core protein mediates flow-induced EC NO synthase (eNOS) activation because GPC1 is anchored to caveolae where eNOS resides. We also hyphothesized that the HS core protein syndecan-1 (SDC1) mediates flow-induced EC elongation and alignment because SDC1 is linked to the cytoskeleton which impacts cell shape. We tested our hypotheses by exposing EC monolayers treated with HS degrading heparinase III (HepIII), and monolayers with RNA-silenced GPC1, or SDC1, to 3 to 24 hours of physiological shear stress. Shear-conditioned EC with intact GCX exhibited characteristic eNOS activation in short-term flow conditions. After long-term exposure, EC with intact GCX were elongated and aligned in the direction of flow. HS removal and GPC1 inhibition, not SDC1 reduction, blocked shear-induced eNOS activation. EC remodeling in response to flow was attenuated by HS degradation and in the absence of SDC1, but preserved with GPC1 knockdown. These findings clearly demonstrate that HS is involved in both centralized and decentralized GCX-mediated mechanotransduction mechanisms, with GPC1 acting as a centralized mechanotransmission agent and SDC1 functioning in decentralized mechanotransmission. This foundational work demonstrates how EC can transform fluid shear forces into diverse biomolecular and biomechanical responses. PMID:24480876

  11. Off-fault shear failure potential enhanced by high-stiff/low-permeable damage zone during fluid injection in porous reservoirs

    NASA Astrophysics Data System (ADS)

    Rohmer, J.; Nguyen, T. K.; Torabi, A.

    2015-09-01

    Several studies have focused on the role of damage zone (DZ) on the hydromechanical behaviour of faults by assuming a fractured DZ (i.e. low stiffness/high permeability). Yet, this vision may not be valid in all geological settings, in particular, in high-porosity reservoirs as targeted by several underground exploitations. We investigate the impact of a high-stiff/low-permeable DZ on the shear reactivation of a blind, undetectable normal fault (1 km long, ≤10 m offset), with a 0.5 m thick low-porosity/permeability fault core during fluid injection into a high-porosity reservoir. The spatial distribution of effective properties (elastic moduli, Biot's coefficients and permeability) of DZ including deformation bands (DB; elliptic inclusions) and intact rock were derived using upscaling analytical expressions. The influence of DZ on the hydromechanical behaviour of the fault zone was numerically explored using 2-D plane-strain finite-element simulations within the framework of fully saturated isothermal porous media by accounting for an orthotropic elastic rheology. The numerical results showed that the presence of DB plays a protective role by reducing the potential for shear reactivation inside the fault core. On the other hand, they favour shear failure in the vicinity of the fault core (off-fault damage) by accelerating the decrease of the minimum principal effective stress while limiting the decrease of the maximum one. This behaviour is strongly enhanced by the fault-parallel DZ effective stiffness, but limited by the combined effect of fault-normal DZ effective permeability and of the Biot's coefficients. This can have implications for the location and size of aftershocks during fault reactivation.

  12. Shear bond strength to enamel after power bleaching activated by different sources.

    PubMed

    Can-Karabulut, Deniz C; Karabulut, Baris

    2010-01-01

    The purpose of the present study was to evaluate enamel bond strength of a composite resin material after hydrogen peroxide bleaching, activated by a diode laser (LaserSmile), an ozone device (HealOzone), a light-emitting diode (BT Cool whitening system), and a quartz-Plus. Fifty extracted caries-free permanent incisors were used in this study. Thirty-eight percent hydrogen peroxidegel was applied to sound, flattened labial enamel surfaces and activated by different sources. Enamel surfaces that had received no treatment were used as control samples. Bonding agent was applied according to the manufacturer's instructions and the adhesion test was performed according to ISO/TS 11405. Statistical analysis showed significant influence of the different activation technique of hydrogen peroxide on shear bond strength to enamel (ANOVA, LSD, P < 0.05). The data in this vitro explorative study suggest the activation of hydrogen peroxide by different sources may further affect the shear bond strength of subsequent composite resin restoration to enamel. Within the limitations of this in vitro study, further studies examining the structural changes of activated hydrogen peroxide-treated enamel are needed. Due to the different activation methods; duration of light irradiation effects, longer time periods may be needed before application of adhesive restorations to enamel, compared with non-activated bleaching. PMID:21069109

  13. Mineralization stages of the unique shear zone-hosted "Felsit-type" Sn-polymetallic mineralization in the eastern Erzgebirge, Germany

    NASA Astrophysics Data System (ADS)

    Járóka, Tom; Seifert, Thomas

    2014-05-01

    The polymetallic cassiterite-bearing mineralization of the "Felsit-type" is located in the NW of the Freiberg mining district within the Großschirma area. The mineralization is hosted by metamorphic rocks of the Preßnitzer Group unit that form the most northeastern part of the Erzgebirge metamorphic core complex. This geological unit is in the Großschirma area predominantly composed of two-mica gneisses and mica schists, whereas intercalations of muscovite-gneiss ("red gneiss"), amphibolites and metacarbonates occur less commonly. These metamorphic rocks were deformed by several NE-SW striking fault zones. The hydrothermal Sn-bearing fluids migrated within shear zones that developed primarily at the contact between different lithotypes. The shear zones are characterized by strong pervasive metasomatic alterations which were triggered by small chlorite-cassiterite-quartz-sulfide-veins. The rock-forming minerals are strongly corroded and displaced by the ore and gangue minerals within the shear zones. The bulk geochemistry of selected drill core samples feature grades up to 0.28 wt. % Sn, 0.15 wt. % Cu, 300 ppm Pb, 140 ppm Zn, 1.1 wt. % F, 250 ppm Li, 820 ppm Rb, 90 ppm Cs, and 130 ppm W. Microscopic and geochemical studies of the samples show that the Sn-polymetallic mineralization of the "Felsit-type" can be distinguished into three different mineralization stages. The first one is dominated by chlorite and quartz. Cassiterite probably appears in two generations with different grain shapes: acicular (< 1 - 100 µm) and isometric cassiterite (< 10 - 650 µm). Smaller amounts of fluorite, rutile, apatite, and scheelite are also associated with the first stage. The second mineralization stage is dominated by pyrite which is the most abundant ore mineral of this paragenesis, while marcasite, chalcopyrite, pyrrhotite, galena, sphalerite, arsenopyrite, bismuthinite, and magnetite appear only subordinately. The third stage is dominated by carbonates that are often

  14. Neurotransmitters couple brain activity to subventricular zone neurogenesis

    PubMed Central

    Young, Stephanie Z.; Taylor, M. Morgan; Bordey, Angélique

    2011-01-01

    Adult neurogenesis occurs in two privileged microenvironments, the hippocampal subgranular zone of the dentate gyrus and the subventricular zone (SVZ) along the lateral ventricle. This review focuses on accumulating evidence suggesting that the activity of specific brain regions or bodily states influences SVZ cell proliferation and neurogenesis. Neuromodulators such as dopamine and serotonin have been shown to have long-range effects through neuronal projections into the SVZ. Local GABA and glutamate signaling have demonstrated effects on SVZ proliferation and neurogenesis, but an extra-niche source of these neurotransmitters remains to be explored and options will be discussed. There is also accumulating evidence that diseases and bodily states such as Alzheimer's disease, seizures, sleep, and pregnancy influence SVZ cell proliferation. With such complex behavior and environmentally-driven factors that control subregion-specific activity, it will become necessary to account for overlapping roles of multiple neurotransmitter systems on neurogenesis when developing cell therapies or drug treatments. PMID:21395856

  15. Bond slip detection of concrete-encased composite structure using shear wave based active sensing approach

    NASA Astrophysics Data System (ADS)

    Zeng, Lei; Parvasi, Seyed Mohammad; Kong, Qingzhao; Huo, Linsheng; Lim, Ing; Li, Mo; Song, Gangbing

    2015-12-01

    Concrete-encased composite structure exhibits improved strength, ductility and fire resistance compared to traditional reinforced concrete, by incorporating the advantages of both steel and concrete materials. A major drawback of this type of structure is the bond slip introduced between steel and concrete, which directly reduces the load capacity of the structure. In this paper, an active sensing approach using shear waves to provide monitoring and early warning of the development of bond slip in the concrete-encased composite structure is proposed. A specimen of concrete-encased composite structure was investigated. In this active sensing approach, shear mode smart aggregates (SAs) embedded in the concrete act as actuators and generate desired shear stress waves. Distributed piezoceramic transducers installed in the cavities of steel plates act as sensors and detect the wave response from shear mode SAs. Bond slip acts as a form of stress relief and attenuates the wave propagation energy. Experimental results from the time domain analysis clearly indicate that the amplitudes of received signal by lead zirconate titanate sensors decreased when bond slip occurred. In addition, a wavelet packet-based analysis was developed to compute the received signal energy values, which can be u