The Black Bay Shear Zone, northwestern Saskatchewan, lies within the Rae Province of the Canadian Shield. It is one of several major, northeast trending, steeply dipping shear zones that were active during the Paleoproterozoic Trans-­Hudson or Thelon Orogenies. For part of its exposed length of approximately 50 km it separates Archean supracrustal gneisses of the Tazin Group to the northwest, from the unconformably overlying Paleoproterozoic, red-bed sequence of the Martin Group to the southeast. Deformation fabrics along the Black Bay Shear Zone indicate the sequential development of early ductile to late brittle episodes of movement. The early ductile episode (D_{<font size=1>1</font>}, pre-Martin Gp) is characterized by a 1 km-wide mylonite belt in the Tazin gneisses, with mylonitic lamination (C-fabric), dextral ä-asymmetric winged porphyroclasts, stretch lineation (L<ub><font size=1>1</font></sub>) and rare sheath folds. This was followed by a ductile to brittle transition (D_{<font size=1>2</font>}, also pre-Martin Gp) characterized by small asymmetric folds in C, and small post-C compressional and extensional shear bands (SB) ranging from thin ductile shear zones to brittle fractures commonly vein filled. The late brittle phase (D_{<font size=1>3</font>}, post-Martin Gp), to which uranium vein mineralization is related, included the formation of a major brittle fault zone along the southeastern side of the mylonitic shear zone, and several sets of vein filled joints. Movement directions during D_{<font size=1>1</font>} to D_{<font size=1>3</font>} are reflected by various shear sense indicators. The D_{<font size=1>1</font>} dextral ä-asymmetrical winged porphyroclasts, combined with the gently NE and SW plunging L_{<font size=1>1</font>}, stretch lineation, indicate sub-horizontal dextral displacement during the ductile phase of deformation. Predominantly NE-verging and steeply dipping SW plunging minor D_{<font size=1>2</font>} folds, along with the predominantly dextral-verging, post-C shear bands, indicate that oblique dextral SE-side-up slip occurred during the ductile to brittle transition. Brittle movement (D_{<font size=1>3</font>})resulted in the preservation of an approximately 8-km thick succession of Martin Group on the SE-side of the fault, indicating a SE-side-down vertical throw of at least 8 km. The local presence of poorly preserved, down-dip slickenlineations suggests that at least some of this movement was dip slip. Time constraints on deformation are poor, tentatively all of the Black Bay Structure deformational history took place between 2300 Ma and 1700 Ma.

The Åreskutan nappe complex consists of the partly molten rock migmatite, which originates from the subduction formed by the collision of continents Baltica and Laurentia. It is a so-called hot nappe, which has been deeply buried in the subduction zone, based on findings of high-pressure minerals in the migmatitic gneiss. As the nappe returned to shallower depths the rock was partially molten during the subsequent exhumation as the lithostatic pressure decreased. Tectonic forces led to thrusting of the nappe towards the east and the building of mount Åreskutan. It is generally accepted that the shear zone between the migmatite of the Åreskutan Nappe and the underlying Lower Seve Nappe is a mylonitic shear zone, but the question of whether similar shear zones can be found at other sites in the migmatite complex has now been raised. In this project two major shear zones have been identified and shear direction has been determined after detailed geological mapping. Many small shear zones have also been identified, but their sense of shear direction was more difficult to determine. The two major shear zones identified have been labelled the Eastern Major Shear Zone and Western Major Shear Zone. In these shear zones the original migmatite appearing on Åreskutan is deformed and sheared with a top to the east sense of shear. The strongest evidence for determining the shear sense are garnets found mantled by micas in a sigma-type shear microstructures, found during microscope analysis. A grade of mylonitization can be seen in the mineral microstructures, with the most fine-grained matrix in the centre of the shear zones. It indicates that ductile deformation dominates, even though some minerals tend to break in a brittle manner.

No description available.

The White Mountain shear zone (WMSZ) is a zone of intense penetrative deformation that lies along the western front of the northern White-Inyo Range in eastern-most California and western-most Nevada. The northern half of the WMSZ is characterized by a NNE to NNW-striking steeply dipping foliation and associated shallowly plunging NNE to NW-trending stretching lineations. S-C fabrics observed in outcrop, microstructural shear sense indicators and kilometer-scale foliation geometry all indicate dextral movement. Localized discrete zones of coeval steeply plunging stretching lineations are present in the northern half of the WMSZ. Microstructural data from these domains indicate a high component of pure shear within a separate coeval kinematic framework and hence a transpressional history. The WMSZ appears to be tectonically related to both the Sierra Crest shear system to the west and the Santa Rita shear system to the south. Correlation between the WMSZ and the Santa Rita shear system indicates that Late Cretaceous dextral transpression may extend up to ~120 km along the western front of the White-Inyo Range. Cross-cutting relationships with Late Cretaceous plutons bracket the age of the WMSZ at between 72-92 Ma. A lack of annealing recrystallization in deformed quartz and the presence of high temperature crystallographic fabrics near the margins of the ca. 72 Ma Boundary Peak pluton indicate significant strain accumulation within the WMSZ subsequent to emplacement of the Boundary Peak pluton. These observations extend the duration of Late Cretaceous dextral transpression in eastern California to at least as recent as 72 Ma. / Master of Science

Thesis advisor: Seth C. Kruckenberg / The Potters Pond migmatite domain (PPMD) is a heterogeneous zone of migmatites located ~10 km southwest of Cascade, Idaho within the western Idaho shear zone (WISZ). The PPMD is the only known exposure of migmatites within the WISZ over its ~300 km length, occurring where the shear zone orientation changes from 020° south to 000° north of the migmatite domain. Structural mapping within the PPMD has identified multiple generations of migmatite with varied structural fabrics. Leucosome layers were sampled from distinct migmatite localities and morphologies (e.g., metatexite, diatexite) to determine the timing and duration of partial melting in the PPMD. U-Pb age determinations of zircon by means of LA-ICP-MS document two periods of protracted migmatite crystallization during the Early and Late Cretaceous. Early Cretaceous (ca. 145 to 128 Ma) migmatite crystallization ages are coeval with the collision and suturing of oceanic terranes of the Blue Mountains province with North America, and the formation of the Salmon River suture zone (SRSZ). Migmatite crystallization ages from ca. 104 to 90 Ma are associated with Late Cretaceous dextral transpression in the WISZ. Field observations and geochronology of cross cutting leucosome relationships are interpreted to record deep crustal deformation and anatexis associated with formation of the SRSZ, subsequently overprinted by solid-state deformation and renewed anatexis during the evolution of the WISZ. These data are the first direct evidence of the synmetamorphic fabric related to the SRSZ east of the initial Sr 0.706 isopleth, and that the WISZ is a temporally distinct overprinting structure. / Thesis (MS) — Boston College, 2016. / Submitted to: Boston College. Graduate School of Arts and Sciences. / Discipline: Earth and Environmental Sciences.

Mineralised shear zones in Otago are often truncated by regional low-angle faults, which juxtapose schist of different metamorphic grade. The Footwall Fault and the Hyde-Macraes Shear Zone are one example for this kind of tectonic setting, and are the subject to this study. Although, the mechanisms for the development of the mineralised thrust-origin shear zones are well studied, the relationship to the truncating faults is still poorly understood. Currently, the truncating low-angle faults are assumed to be related to crustal extension, starting in the early Cretaceous after the schist passed the ductile-brittle transition. This study presents new kinematic data for the Footwall Fault, suggesting development of normal sense movement under ductile conditions due to an abundant shear band cleavage in the footwall, which dynamically recrystallises quartz grains. However, brittle high-angle normal faults truncating these shear bands indicate either reactivation of normal sense movement after passing the ductile-brittle transition or continuous normal sense movement during the transition. Furthermore, this study presents a model, which suggests a regional scale rolling hinge development, consisting of an array of individual low-angle normal faults along the boundary of the textural zone change from TZ IV to TZIII, and strike-parallel high-angle faults at the NE margin of the Otago schist.

This study combines structural field data with microstructural observations in an analysis of a mid-crustal shear zone related to the emplacement of the Misty pluton during a high-flux magmatic event in Northern Fiordland, New Zealand. These high-flux magmatic events transport massive amounts of heat and material as they develop along accretionary continental margins, and represent a primary source of continental crust. Fiordland, New Zealand possesses, perhaps, the most extensive middle and lower crustal exposure of these systems on earth. Therefore, this study area provides a significant opportunity to understand processes of continental crust formation in the mid-crust and how these events relate to the broader construction of continents. Herein, I document the four-stage geologic history of the Cozette Burn field area. Pre-existing structures along the Gondwana accretionary margin hosted a regional flare-up magmatic event that produced the Misty pluton and several other large plutons of the West Fiordland Orthogneiss (WFO). This study primarily focuses on the mid-crustal emplacement of the Misty pluton during oblique convergence along the accretionary margin, forming the upper-amphibolite facies Misty Shear Zone (MSZ). The exposures of the MSZ within the Cozette Burn preserve rare structural relationships between host rock and the intrusive Misty pluton. Together, these structures developed during end-stage contractional tectonics that constructed a long-lived (~270+ Ma) composite batholith. Heterogeneous ductile shearing defines the MSZ, with microstructural evidence indicating an interplay of high-temperature crystal plastic deformation along with partial melting of host rock and melt channeling. This resulted in focused, melt-assisted shearing under regional transpressive deformation. These accommodative processes provided an efficient mechanism for moving heat, fluids and magma sourced from the lower crust/mantle boundary into the mid-crust during 15-25 km of crustal thickening related to arc flare-up magmatism. This flare up magmatism and MSZ formation occurred during the final stages of crustal thickening along Gondwana continental margin. High-strain, mylonitic- ultramylonitic shear zones developed in a later phase of deformation, cutting MSZ fabrics near contacts between the Misty pluton and host rock. These more localized shear zones can be attributed to either accommodation of localized melt-pressure buildup or the shift to extensional tectonics. Brittle faulting cut these structures with oblique-thrust in the Tertiary. These mid-crustal structures carry economic relevance: thickened-crust events along accretionary continental margins produce deep-crustal sourced, metal-bearing magmas that are transferred into mid-crust prior to their hydrothermal emplacement as ore deposits in the upper crust. The lasting influence of these processes warrants consideration when assessing continental crust architecture at all scales.

The George Sound Shear Zone (GSSZ) exposed in Bligh Sound within Fiordland, New Zealand allowed us to reconstruct the kinematics of transpressive flow in >100 km2 of exhumed Cretaceous lower crust. We compare the three-dimensional characteristics of the deformation to theoretical models of transpression that assume steady-state flow in a homogeneous medium. This assumption is rarely the case for shear zones that experience metamorphism during deformation. We determined the three-dimensional kinematics of the GSSZ and evaluated the effects of metamorphism on strain accommodation and structural fabric evolution in the GSSZ to determine if metamorphism is an important parameter that transpressional models should account for. We found that metamorphism aided strain localization within the GSSZ and resulted in a style of structural fabric development that deviates from predictions made by theoretical models. We used foliation and lineation orientation data and field observations to determine GSSZ kinematics. Asymmetric pyroxene σ-porphyroclasts and hornblende fish show top-down-to-the-SW apparent normal shear sense with a sinistral component. The Z-axes of oblate SPO ellipsoids define the vorticity normal section and the moderately WNW-plunging vorticity vector. Foliation deflections relative to the shear zone boundaries yielded a vorticity magnitude (Wk) of ≥0.8. Our kinematic results suggest that the GSSZ records inclined, triclinic transpression with sinistral, top-down-to-the-SW simple shear-dominated flow. We used finite strain analysis and petrographic analysis to determine that metamorphism influences strain accommodation. Finite strain analyses were performed in 3D on 16 samples using the Rf/ɸ, Fry, and Intercept methods to determine the SPO fabric ellipsoids at different stages of deformation. Petrographic analysis was performed to identify metamorphic reactions using syn-kinematic minerals and constrain deformational temperatures using deformation mechanisms of plagioclase. Early deformation formed a ~13 km wide prolate fabric at granulite facies. Deformation later localized into a ~2-4.6 km wide oblate, mylonitic fabric at upper amphibolite facies. This fabric cross-cuts the prolate fabric and is characterized by metamorphic hornblende and biotite produced from retrogressive hydration reactions. Samples with syn-kinematic biotite contain more shear bands and display more grain size reduction of plagioclase than samples without this phase, suggesting these samples may have accommodated more strain. Changes in syn-kinematic metamorphic minerals were accompanied by steepening of stretching lineations and by changes in foliation orientation. Our analyses show that retrogressive hydration metamorphism aided strain localization within a cross-cutting oblate fabric, and the uneven distribution of biotite within this domain potentially influenced along strike variation in strain magnitude and fabric ellipsoid symmetry. Our results highlight the influence of fluid-induced metamorphism on shear zone evolution and call for new transpressional models to incorporate changes in rheology due to syn-kinematic metamorphism.

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Mineralised shear zones in Otago are often truncated by regional low-angle faults, which juxtapose schist of different metamorphic grade. The Footwall Fault and the Hyde-Macraes Shear Zone are one example for this kind of tectonic setting, and are the subject to this study. Although, the mechanisms for the development of the mineralised thrust-origin shear zones are well studied, the relationship to the truncating faults is still poorly understood. Currently, the truncating low-angle faults are assumed to be related to crustal extension, starting in the early Cretaceous after the schist passed the ductile-brittle transition. This study presents new kinematic data for the Footwall Fault, suggesting development of normal sense movement under ductile conditions due to an abundant shear band cleavage in the footwall, which dynamically recrystallises quartz grains. However, brittle high-angle normal faults truncating these shear bands indicate either reactivation of normal sense movement after passing the ductile-brittle transition or continuous normal sense movement during the transition. Furthermore, this study presents a model, which suggests a regional scale rolling hinge development, consisting of an array of individual low-angle normal faults along the boundary of the textural zone change from TZ IV to TZIII, and strike-parallel high-angle faults at the NE margin of the Otago schist.