CONSTRAINING THE TIMING AND KINEMATICS OF DEFORMATION IN THE ADIRONDACKS: MICROSTRUCTURAL ANALYSIS OF THE COLTON-CARTHAGE SHEAR ZONE, NORTHWEST ADIRONDACKS

Fabrics of the East Pernambuco shear zone (EPSZ) were studied via microstructural analysis, mineral chemistry and isochemical phase diagram modelling to constrain the pressure and temperature conditions of deformation during shearing. Granitic mylonites show fractured feldspar porphyroclasts embedded in a fine-grained, recrystallized quartzo-feldspathic matrix. These mylonites grade laterally into banded ultramylonites characterized by stretched feldspar clasts alternated with recrystallized quartz bands. Fractures in these ultramylonites are filled by phyllosilicates. The mineral chemistry of the feldspars points to systematic changes between porphyroclasts, grains within fractures and fine-grained mixtures. Quartz crystallographic fabrics in the mylonites suggest activation of prism slip, while the ultramylonites show the activation of both rhomb and basal slip systems. Thermodynamic modelling suggests that the mylonites were formed at 4.75 ± 0.25 kbar and 526 ± 9°C, while the ultramylonites yield conditions of 5.9 ± 1 kbar and 437 ± 17°C. These observations suggest that the EPSZ records a heterogeneous path of strain accommodation, marked by decreasing temperature from its western sector to its eastern termination. The differences in metamorphic conditions are consistent with a transitional, brittle–ductile strain regime. Such characteristics indicate that the EPSZ is a Neoproterozoic shear belt nucleated and heterogeneously exhumed at the brittle–ductile transition, possibly in an intracontinental setting.Supplementary Material: EPMA analysis of feldspars in Caruaru and Gravatá domains and T-X(O2) pseudosections are available at https://doi.org/10.6084/m9.figshare.c.5125957

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Fluid-mediated, brittle–ductile deformation at seismogenic depth – Part 2: Stress history and fluid pressure variations in a shear zone in a nuclear waste repository (Olkiluoto Island, Finland)

Solid Earth ◽

10.5194/se-11-489-2020 ◽

2020 ◽

Vol 11 (2) ◽

pp. 489-511 ◽

Cited By ~ 2

Author(s):

Francesca Prando ◽

Luca Menegon ◽

Mark Anderson ◽

Barbara Marchesini ◽

Jussi Mattila ◽

...

Keyword(s):

Shear Zone ◽

Nuclear Waste ◽

Strain Localization ◽

Microstructural Analysis ◽

Fluid Pressure ◽

Rheological Parameters ◽

Nuclear Waste Repository ◽

Brittle Deformation ◽

Differential Stress ◽

Brittle Ductile Transition

Abstract. The microstructural record of fault rocks active at the brittle–ductile transition zone (BDTZ) may retain information on the rheological parameters driving the switch in deformation mode and on the role of stress and fluid pressure in controlling different fault slip behaviours. In this study we analysed the deformation microstructures of the strike-slip fault zone BFZ045 in Olkiluoto (SW Finland), located in the site of a deep geological repository for nuclear waste. We combined microstructural analysis, electron backscatter diffraction (EBSD), and mineral chemistry data to reconstruct the variations in pressure, temperature, fluid pressure, and differential stress that mediated deformation and strain localization along BFZ045 across the BDTZ. BFZ045 exhibits a mixed ductile–brittle deformation, with a narrow (<20 cm thick) brittle fault core with cataclasites and pseudotachylytes that overprint a wider (60–100 cm thick) quartz-rich mylonite. Mylonitic deformation took place at 400–500 ∘C and 3–4 kbar, typical of the greenschist facies metamorphism at the base of the seismogenic crust. We used the recrystallized grain size piezometry for quartz to document a progressive increase in differential stress, from ca. 50 to ca. 120 MPa, towards the shear zone centre during mylonitization and strain localization. Syn-kinematic quartz veins formed along the mylonitic foliation due to transiently high pore fluid pressure (up to lithostatic value). The overprint of the veins by dynamic recrystallization and mylonitic creep is further evidence of the occurrence of brittle events under overall ductile conditions. We propose a conceptual model in which the ductile–brittle deformation cycle was controlled by transient oscillations in fluid pressure and progressively higher differential stress, possibly occurring in a narrowing shear zone deforming towards the peak strength of the crust at the BDTZ.

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Deformation history of the Marmara Granitoid and implications for a dextral shear zone in NW Anatolia

10.5194/egusphere-egu2020-9231 ◽

2020 ◽

Author(s):

Salim Birkan Bayrak ◽

Işıl Nur Güraslan ◽

Alp Ünal ◽

Ömer Kamacı ◽

Şafak Altunkaynak ◽

...

Keyword(s):

Shear Zone ◽

Boundary Migration ◽

Microstructural Analysis ◽

Structural Data ◽

Ductile Deformation ◽

Brittle Deformation ◽

Deformation History ◽

Shear Sense ◽

History Of ◽

Dextral Shear

Marmara granitoid (47 Ma) is a representative example of the Eocene post-collisional magmatism which produced several granitic plutons in NW Anatolia, Turkey. It is a W-E trending sill-like magmatic body which was concordantly emplaced into the metamorphic basement rocks of Erdek Complex and Saraylar Marble. The granitoid is represented by deformed granodiorite which displays well-developed lineation and foliation in meso-scale defined by the elongation of mica and feldspar crystals and recrystallization of quartz however, in some places, magmatic textures are preserved. Deformed granodiorite is broadly cut by aplitic and pegmatitic dikes and contains mafic enclaves which display the same deformation indicators with the main granitoid.Microstructural analysis shows that the solid-state deformation of the Marmara granitoid is classified as ductile deformation with high temperatures and ductile-to-brittle deformation with relatively lower temperatures. Evidence for the ductile deformation of the granitoid is represented by chessboard extinction of quartz, grain boundary migration (GBM) and subgrain rotation recrystallisation (SGR) which exhibits that the deformation temperature changed from 600 oC to 400oC. Bulging recrystallization (BLG), grain size reduction of amphibole, biotite and plagioclases and microcracks on plagioclases were considered as overlying ductile-to-brittle deformation signatures which develop between 300-<250 oC temperatures.All of these field and micro-structural data collectively suggest that the shear sense indicators such as micafish structures and &#948; type mantled porphyroclasts displayed stair-steppings pointing out to a right lateral movement, indicating that the structural evolution and deformation history of Marmara granitoid was controlled by a dextral shear zone.

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Formation of Ultramylonites in an Upper Mantle Shear Zone, Erro-Tobbio, Italy

Minerals ◽

10.3390/min11101036 ◽

2021 ◽

Vol 11 (10) ◽

pp. 1036

Author(s):

Jolien Linckens ◽

Sören Tholen

Keyword(s):

Grain Size ◽

Shear Zone ◽

Upper Mantle ◽

Microstructural Analysis ◽

Shear Zones ◽

Geodynamic Setting ◽

Mixing Processes ◽

Phase Mixing ◽

Fine Grained ◽

Order Of Magnitude

Deformation in the upper mantle is localized in shear zones. In order to localize strain, weakening has to occur, which can be achieved by a reduction in grain size. In order for grains to remain small and preserve shear zones, phases have to mix. Phase mixing leads to dragging or pinning of grain boundaries which slows down or halts grain growth. Multiple phase mixing processes have been suggested to be important during shear zone evolution. The importance of a phase mixing process depends on the geodynamic setting. This study presents detailed microstructural analysis of spinel bearing shear zones from the Erro-Tobbio peridotite (Italy) that formed during pre-alpine rifting. The first stage of deformation occurred under melt-free conditions, during which clinopyroxene and olivine porphyroclasts dynamically recrystallized. With ongoing extension, silica-undersaturated melt percolated through the shear zones and reacted with the clinopyroxene neoblasts, forming olivine–clinopyroxene layers. Furthermore, the melt reacted with orthopyroxene porphyroclasts, forming fine-grained polymineralic layers (ultramylonites) adjacent to the porphyroclasts. Strain rates in these layers are estimated to be about an order of magnitude faster than within the olivine-rich matrix. This study demonstrates the importance of melt-rock reactions for grain size reduction, phase mixing and strain localization in these shear zones.

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Deformation conditions during syn-convergent extension along the Cordillera Blanca shear zone, Peru

Geosphere ◽

10.1130/ges02040.1 ◽

2019 ◽

Vol 15 (4) ◽

pp. 1342-1367 ◽

Cited By ~ 1

Author(s):

Cameron A. Hughes ◽

Micah J. Jessup ◽

Colin A. Shaw ◽

Dennis L. Newell

Keyword(s):

Shear Zone ◽

Microstructural Analysis ◽

Convergent Extension ◽

Brittle Deformation ◽

Cordillera Blanca ◽

Peruvian Andes ◽

Fine Grained ◽

Brittle Ductile Transition ◽

Tectonically Active ◽

Lower Temperature

AbstractStrain localization across the brittle-ductile transition is a fundamental process in accommodating tectonic movement in the mid-crust. The tectonically active Cordillera Blanca shear zone (CBSZ), a ∼200-km-long normal-sense shear zone situated within the footwall of a discrete syn-convergent extensional fault in the Peruvian Andes, is an excellent field laboratory to explore this transition. Field and microscopic observations indicate consistent top-down-to-the-southwest sense of shear and a sequence of tectonites ranging from undeformed granodiorite through mylonite and ultimately fault breccia along the detachment.Using microstructural analysis, two-feldspar and Ti-in-quartz (TitaniQ) thermometry, recrystallized quartz paleopiezometry, and analysis of quartz crystallographic preferred orientations, we evaluate the deformation conditions and mechanisms in quartz and feldspar across the CBSZ. Deformation temperatures derived from asymmetric strain-induced myrmekite in a subset of tectonite samples are 410 ± 30 to 470 ± 36 °C, consistent with TitaniQ temperatures of 450 ± 60 to 490 ± 33 °C and temperatures >400 °C estimated from microstructural criteria. Brittle fabrics overprint ductile fabrics within ∼150 m of the detachment that indicate that deformation continued to lower-temperature (∼280–400 °C) and/or higher-strain-rate conditions prior to the onset of pervasive brittle deformation. Initial deformation occurred via high-temperature fracturing and dissolution-precipitation in feldspar. Continued subsolidus deformation resulted in either layering of mylonites into monophase quartz and fine-grained polyphase domains oriented subparallel to macroscopic foliation or the interconnection of recrystallized quartz networks oriented obliquely to macroscopic foliation. The transition to quartz-controlled rheology occurred at temperatures near ∼500 °C and at a differential stress of ∼16.5 MPa. Deformation within the CBSZ occurred predominantly above ∼400 °C and at stresses up to ∼71.4 MPa prior to the onset of brittle deformation.

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Microstructures and deformation temperature of carbonate mylonites in Shajigami shear zone at eastern margin of Abukuma Mountains, Northeastern Japan

10.5194/egusphere-egu21-14195 ◽

2021 ◽

Author(s):

Hiroaki Yokoyama ◽

Jun Muto ◽

Hiroyuki Nagahama

Keyword(s):

Grain Size ◽

Shear Zone ◽

Deformation Temperature ◽

Microstructural Analysis ◽

Shear Plane ◽

Strike Slip ◽

Eastern Margin ◽

Northeastern Japan ◽

Shear Sense ◽

Shear Component

&#12288;&#12288;Microstructural analysis is essential for estimating the deformation conditions of plastically deformed rocks. In this study, we analyze the microstructures of carbonate mylonites and deformation conditions in natural shear zone to reconstruct tectonics. Carbonate mylonites originated from late Carboniferous Tateishi Formation and mylonitized in middle Cretaceous by the strike-slip motion of Shajigami shear zone in the eastern margin of the Abukuma Mountain, Northeastern Japan. &#12288;&#12288;Microstructural analysis was carried out by optical microscope and electron backscattered diffraction (EBSD) mapping to determine grain size, aspect ratio, shape preferred orientation (SPO) and crystallographic preferred orientation (CPO) of calcite aggregates. &#12288;&#12288;Pervasive deformation twins and dynamically recrystallized grains are observed. Although most porphyroclasts show symmetric structure, some show asymmetric structure that indicates dextral shear sense. Mean dynamically recrystallized grain size is 16-67 &#181;m, and it decreases close to the shear zone. CPOs show that c-axes concentrate normal to the shear plane or slightly rotate to the shear sense. The strong CPOs suggest that the dominant deformation mechanism is dislocation creep. SPOs show the foliation which is slightly oblique or almost parallel to the shear plane. However, we observed the SPOs parallel to the shear plane at the location 150 m away from the shear zone. &#160;The 3D dynamically recrystallized grain shapes are between plane-strain ellipsoid and oblate ellipsoid. The grain shapes tend to be relatively polygonal close to the shear zone, while more elongated further away from the shear zone. The distribution of the carbonate mylonite originated from same Tateishi Formation is known to be about 5 km apart from the Shajigami shear zone (Tateishi location). However, based on many aspects of differences in microstructures among both locations such as SPOs of recrystallized grains, we infer that the deformation of Shajigami shear zone was not related to one at Tateishi location. The pervasive dynamic recrystallization suggests that the deformation temperature was at least 200&#176;C. Observed type &#8545; and type &#8546; twin morphologies (Burkhard, 1993) of calcite grains suggest deformation temperature below 300&#176;C.&#160; &#12288;&#12288;These results indicate that the deformation of the Shajigami shear zone was in the range from 200 to 300&#8451; and deformation was stronger near the shear zone. In addition, the polygonal grain shape close to the shear zone suggests that the deformation temperature is higher close to the shear zone. Furthermore, SPOs show that pure shear component is larger than simple shear component in terms of SPOs that almost parallel to the shear plane away from the shear zone. This study including several additional results will provide the microstructural development of carbonate mylonites in natural strike-slip shear zones deformed near the brittle-ductile condition of the upper crust.

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Macro- and microstructural analysis of the North Tea Lake Mylonite Zone: an extensional shear zone in the Central Gneiss Belt, Grenville Province, Ontario

Canadian Journal of Earth Sciences ◽

10.1139/cjes-2015-0009 ◽

2015 ◽

Vol 52 (11) ◽

pp. 1027-1044 ◽

Cited By ~ 4

Author(s):

Nicholas Culshaw ◽

Christopher Gerbi ◽

Laura Ratcliffe

Keyword(s):

Shear Zone ◽

Microstructural Analysis ◽

High Stress ◽

Shear Zones ◽

Grain Boundary Sliding ◽

Dislocation Creep ◽

Diffusion Creep ◽

The North ◽

Mylonite Zone ◽

Central Gneiss Belt

The North Tea Lake Mylonite Zone is a late extensional ductile fault that is concordant with and has reworked fabrics of the North Tea Lake Shear Zone, the frontal thrust shear zone of the upper amphibolite–granulite facies Kiosk domain within the interior of the Central Gneiss Belt. North Tea Lake Mylonite Zone fabric is an anomalously fine-grained mylonite compared to Central Gneiss Belt gneisses, and consists of three microstructural domains that display progressive recrystallization and grain size refinement of the protolith granitoid. On the basis of petrography and electron backscatter diffraction, these microdomains are inferred to represent a transition from dominantly dislocation creep to diffusion creep and diffusion-accommodated grain boundary sliding at elevated stress (>100 MPa), low fluid activity, and temperatures ∼500 °C. The North Tea Lake Mylonite Zone is interpreted to mark a step in the progressive transition in deformation mode during late- to post-Ottawan extension and cooling of the Grenville orogen from weak, wide, wet, and warm shear zones to Rigolet-phase cooler, narrow, ultrafine, high-stress shear zones reworking dry protoliths.

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Meso- and Microstructural Analysis of Coseismic Shear Zone of the 1999 MW 7.6 Chi-Chi Earthquake, Taiwan

Bulletin of the Seismological Society of America ◽

10.1785/0120040015 ◽

2005 ◽

Vol 95 (2) ◽

pp. 486-501 ◽

Cited By ~ 10

Author(s):

A. Lin

Keyword(s):

Shear Zone ◽

Microstructural Analysis

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Supplemental Material: Macro- and microstructural analysis of the Zhujiafang ductile shear zone, Hengshan Complex: Tectonic nature and geodynamic implications of the evolution of Trans–North China orogen

10.1130/gsab.s.12946967 ◽

2020 ◽

Author(s):

Lingchao He ◽

Jian Zhang ◽

et al.

Keyword(s):

Shear Zone ◽

Inductively Coupled Plasma ◽

Mineral Content ◽

North China ◽

Microstructural Analysis ◽

Ductile Shear Zone ◽

Inductively Coupled ◽

High Pressure Granulite ◽

Plasma Mass Spectrometry ◽

Ductile Shear

Table S1: Checked mineral content and grain size of EBSD samples; Table S2: Representative compositional analyses of minerals; Table S3: Zircon U-Pb isoplot data obtained by laser ablation–inductively coupled plasma–mass spectrometry for samples 17HS06-1 and 14630-1 from Zhujiafang; Table S4: Summary of isotopic ages for the high-pressure granulite of the Trans–North China orogen and Zhujiafang ductile shear zone.

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Supplemental Material: Macro- and microstructural analysis of the Zhujiafang ductile shear zone, Hengshan Complex: Tectonic nature and geodynamic implications of the evolution of Trans–North China orogen

10.1130/gsab.s.12946967.v1 ◽

2020 ◽

Author(s):

Lingchao He ◽

Jian Zhang ◽

et al.

Keyword(s):

Shear Zone ◽

Inductively Coupled Plasma ◽

Mineral Content ◽

North China ◽

Microstructural Analysis ◽

Ductile Shear Zone ◽

Inductively Coupled ◽

High Pressure Granulite ◽

Plasma Mass Spectrometry ◽

Ductile Shear

Table S1: Checked mineral content and grain size of EBSD samples; Table S2: Representative compositional analyses of minerals; Table S3: Zircon U-Pb isoplot data obtained by laser ablation–inductively coupled plasma–mass spectrometry for samples 17HS06-1 and 14630-1 from Zhujiafang; Table S4: Summary of isotopic ages for the high-pressure granulite of the Trans–North China orogen and Zhujiafang ductile shear zone.

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