Development Of Ductile Shear Zones Research Articles

Reactivation mechanisms and related-porosity enhancement of shear zones in the context of basement-hosted uranium mineralization: case of the Spitfire discovery in the Patterson Lake corridor, Canada

Uranium mineralization in the Patterson Lake corridor (southwestern Churchill province, Canada) is hosted in the metamorphosed Paleoproterozoic basement covered to the north by the flat-lying sandstone formations of the Athabasca Basin. The mineralization is exclusively contained within inherited ductile structures that were reactivated under a brittle regime. Petrographic and micro-structural studies of drill core samples from the Spitfire discovery (Hook Lake project) reveal the linkages between structural evolution of the basement, alteration and mineralization. During basement exhumation, localization of non-coaxial deformation led to the formation of a large anastomosing shear zone system made of mylonitic rocks. Strain localization associated with fluid circulation induced strong mineralogical and rheological changes, forming discontinuities in mechanical anisotropy. During and post-deposition of the Athabasca Basin after 1.80 Ga, these zones of anisotropy localized brittle reactivation, expressed by a network of micro-fractures later amplified by dissolution processes which enhanced porosity later filled with phyllosilicates and uranium oxides. Cross-cutting relationships between alteration minerals and structures indicate that fluid circulation was active after the basement exhumation. Uranium-bearing fluids moved through the network of micro-fractures. As shown for the Spitfire prospect, fertile structures in the basement below the Athabasca Basin have a combined poly-phase structural and alteration history during which development of ductile shear zones followed by brittle reactivation and dissolution processes led to the formation of superimposed shear and damaged zones in which uranium orebodies are located.Thematic collection: This article is part of the Uranium Fluid Pathways collection available at: https://www.lyellcollection.org/cc/uranium-fluid-pathways

Tectonic Mode Switches Recorded at the Northern Edge of the Australian Plate During the Pliocene and Pleistocene

AbstractWe report new data from medium‐high grade metamorphic rocks found at the northern margin of the Lengguru Fold Belt in West Papua. The study involved a systematic analysis of cross‐cutting structures to establish the relative timing of deformation, together with isotopic dating to define when these tectono‐thermal events occurred. These data show that the region underwent multiple episodes of deformation within the last six million years. Metamorphic mineral growth was associated with the development of ductile shear zones. This episode occurred during a phase of crustal stretching associated with the formation of a metamorphic core complex. Metamorphic zircon growth at 4.9 to 5.3 Ma was documented in two of the dated samples. These data are interpreted to postdate the peak pressure and temperature conditions of the phase of regional crustal stretching. The shear fabrics associated with the metamorphic core complex were later overprinted by at least two generations of folds. The change in mode from crustal extension to shortening reflects a tectonic mode switch. A subsequent mode switch is documented by numerous brittle extensional faults that cross‐cut the earlier formed ductile fabrics. We interpret ca. 0.75–1.51 Ma (U–Th)/He age data to reflect cooling associated with the later stages of crustal shortening (marked by folds) or the later extensional unroofing of the peninsula. This work demonstrates that an orogen can record multiple tectonic mode switches within several million years. These outcomes should be considered in studies of ancient orogens where analytical uncertainties associated with isotopic dating may mask short‐lived mode switches.

Development of Ductile Shear Zones during Diapiric Magmatism of Nepheline Syenite and Exhumation of Granulites-Examples from Central Rajasthan, India

The present communication discusses two separate instances where features commonly observed in DSZ are noted, one along the margin of the Kisengarh nepheline syenite and the other in the granulite bodies the Sandmata Complex in Rajasthan, India. The foliations in the nepheline syenite pluton show features similar to the mylonite gneisses that characterize DSZs in orogenic belts. Apart from simulating LS tectonite- type fabric the continuity of similar structures in adjacent cover rocks provides evidence of heterogeneous deformation during upward ascent of nepheline syenite. Based on tectono-metamorphic studies on granulites suggestion is made about the uplift of deepseated granulites accompanied by ductile shearing on along the margins. The development of DSZ along margins helped in reducing the frictional resistance during upward ascent and emplacement into Archaean gneissic terrane. The process is comparable to buoyancy- induced diapiric uplift of hot plutonic bodies through cooler upper crust.

Interpretation of the experimental data provided by Gómez-Rivas and Griera (2012) in terms of the MEM-criterion

The systematical experiments carried out by Gomez-Rivas and Griera (2012) demonstrate that the ductile shear zones initiate at ∼55° to σ1 just as predicted by the MEM-criterion. However, the data are explained in terms of dilatancy, which requires many prerequisites and implies that the ±55° angle is only valid for the used material. In contrast, the ∼55° predicted by the MEM-criterion is material independent, which makes it widely applicable to explaining the development of ductile shear zones in nature.

Reply to comment by P. Olivier on “Preorogenic exhumation of the North Pyrenean Agly Massif (Eastern Pyrenees, France)”

We make this reply to Philippe Oliver's comments on the deformation of the Mesozoic cover of the northern Agly massif.

Deformation features of garnet-bearing granites from Huwan, western Dabie Mountains

Foliated garnet-bearing granite, usually associated with high pressure and ultrahigh -pressure (UHP) metamophic rocks, is a particular rock-type extensively exposed in the Mesozoic Dabie-Sulu orogenic belt of China. This study focuses on deformation features and SHRIMP zircon dating of foliated garnet granite in a high-pressure metamorphic unit from Huwan, western Dabie Mountains in order to resolve discrepancies in current versions of its petrogenesis and structural evolution. SHRIMP dating reveals a zircon age of 762 ± 15 Ma (MSWD=1.7) for Huwan granites, representing the Middle to Late Neoproterozoic age of intrusion and crystallization. Field and microstructural studies show that the Huwan granite body underwent multiple-stage deformation. The deformation was manifested by an early stage of rootless folding and imposition of relict foliation (S1); an Indosinian main stage marked by imposition of north-dipping penetrative gneissosity (S2) and development of ductile shear zones under NNE-SSW directed compression; and a final Indosinian stage of southward thrusting of the Huwan high-pressure unit. Shallow level extension prevailed after the Late Triassic, giving rise to south-dipping thrust faults and north-dipping normal faults.

CARACTERIZAÇÃO LITOLÓGICA E EVOLUÇÃO METAMÓRFICA DA PORÇÃO LESTE DO COMPLEXO METAMÓRFICO BRUSQUE, SANTA CATARINA

The eastern portion of the Brusque Metamorphic Complex comprises dominantly metasedimentary rocks, with subordinate metavolcanics. Sheets of peraluminous leucogranites are concordantly intrusive in this sequence, as well as larger granitic bodies, as the Compra Tudo, Valsungana and Serra dos Macacos granitoids. Regional deformation and orogenic metamorphism are here interpreted as polyphase events of Neoproterozoic age associated to continental collision. A younger metamorphic event is related to heat transfer from granitic magmas and to the development of ductile shear zones. Protoliths are dominantly marine sediments probably deposited in rift systems evolving towards continental margin enviroments. No evidence of aceient oceanic crust is found. Turbidites are characterized by systematically alternate pelites, feldspathic sandstones and quartz sanstones, with minor amounts of limestone and marble. Massive metavolcanic rocks of tholeiitic affinity are interpreted as former subaqueous basalt flows. Minor occurrences of chemical exhalative interflows are found as banded iron formations. Metamorphic evolution is complex and intimately related to deformation events. M 1 e M 2 metamorphic events are related to low-pressure, regional orogenic metamorphism synchronous to the development of foliations S 1 and S 2 . Both were formed during continental collision, which caused important tectonic interleaving of the volcano-sedimentary sequence. Temperature conditions range from those of the greenschist facies to lower amphibolite, marked by chlorite, biotite. Garnet, andaluzite and cordierite zones. Metamorphic zoning pattern is complex, with repetition and incompatibility of zones due to tectonic interleaving. M 3 metamorphic event is related to the intrusion of syn-transcurrence (D 3 ), Neoproterozoic Valsungana and Serra dos Macacos granites, which develop contact metamorphic aureoles containing pelitic and calc-silicate rocks formed under albite-epidote hornfels to pyroxene hornfels facies conditions. Mylonitic rocks formed under greenshist facies conditions are found along the Major Gercino and Itajaí-Perimbó shear zones.

CARACTERIZAÇÃO LITOLÓGICA E EVOLUÇÃO METAMÓRFICA DA PORÇÃO LESTE DO COMPLEXO METAMÓRFICO BRUSQUE, SANTA CATARINA

The eastern portion of the Brusque Metamorphic Complex comprises dominantly metassedimentary rocks, with subordinate metavolcanics. Sheets of peraluminous leucogranites are concordantly intrusive in this sequence, as well as larger granitic bodies, as the Compra Tudo, Valsungana and Serra dos Macacos granitoids. Regional deformation and orogenic metamorphism are here interpreted as polyphase events of Neoproterozoic age associated to continental collision. A younger metamorphic event is related to heat transfer from granitic magmas and to the development of ductile shear zones. Protoliths are dominantly marine sediments probably deposited in rift systems evolving towards continental margin enviroments. No evidence of ancient oceanic crust is found. Turbidites are characterized by systematically alternate pelites, feldspathic sandstones and quartz sanstones, with minor amounts of limestone and marble. Massive metavolcanic rocks of tholeiitic affinity are interpreted as former subaqueous basalt flows. Minor occurrences of chemical exhalative interflows are found as banded iron formations. Metamorphic evolution is complex and intimately related to deformation events. M 1 e M 2 metamorphic events are related to low-pressure, regional orogenic metamorphism synchronous to the development of foliations S 1 and S 2 . Both were formed during continental collision, which caused important tectonic interleaving of the volcano-sedimentary sequence. Temperature conditions range from those of the greenschist facies to lower amphibolite, marked by chlorite, biotite, garnet, andaluzite and cordierite zones. Metamorphic zoning pattern is complex, with repetition and incompatibility of zones due to tectonic interleaving. M 3 metamorphic event is related to the intrusion of syn-transcurrence (D 3 ), Neoproterozoic Valsungana and Serra dos Macacos granites, which develop contact metamorphic aureoles containing pelitic and calc-silicate rocks formed under albite-epidote hornfels to pyroxene hornfels facies conditions. Mylonitic rocks formed under greenshist facies conditions are found along the Major Gercino and Itajai- Perimbo shear zones.

Mineralogy, geochemistry and the development of ductile shear zones in the Grampian Slide zone of the Scottish Central Highlands

The Grampian Slide zone comprises a system of shear zones characterized by strain-induced, syn-tectonic muscovitization, growth of porphyroblasts of plagioclase, garnet and muscovite, concordant quartz veinlets, and by later-tectonic sillimanite and late- to post-tectonic kyanite. Traverses across shear zones in the Central Highland division show progressive modification of original gneissic fabrics, demonstrating that metamorphism and gneissification preceded the shearing event. Mylonitization involved bulk mobilization of alkalies (Na, Ca, K) and trace elements (Ba, Rb, Sr, Ni, Cr, Zr), accompanied by decreasing Fe oxidation ratios; and increasing shearing progressively changed the mineral chemistry of reversely zoned plagioclase, zoned garnet and micas. Advanced syn-shearing blastesis produced a suite of lenticular pegmatitic segregation veinlets in the mylonites. A coeval suite of larger, intrusive pegmatites represents more mobile material that has migrated through the shear zones. The pegmatites and the mylonite matrices yield U-Pb monazite ages of c. 806 Ma, corresponding to the Precambrian Knoydartian event in the Moine rocks of the Northern Highlands. The shear zones record compressional(?) transport directed towards the NNE, and in the northern sector of the Central Highlands were reactivated by later (Caledonian?) extensional SE-directed movement. The presence of the c. 806 Ma event in the Grampian Group challenges recent proposals that early deformation across the entire Dalradian Supergroup can be correlated with the c. 590 Ma Tay Nappe.

Pan-African Magmatism, and Sedimentation in the NW Himalaya

Correlation of early Palaeozoic, Pan-African (500 ± 50 Ma) granites that intruded the Chail, Salkhala, Haimanta Formations in the Lesser Himalaya, Zanskar crystallines, and Lower Taglang La of Tso-Morari crystallines in the northwestern Himalaya, is based on the field relationship, tectonic setting, mineralogical, and geochemical characteristics, and isotope dating of the granites. These granite plutons exhibit identical petrographical, and geochemical character. The mineralogical composition of the granites is quite similar, consisting of quartz, K-feldspar, plagioclase feldspar, biotite, muscovite, garnet, tourmaline, ± cordierite, andalusite, and sillimanite fibrolite. The granite which are massive, and inequigranular in the core of the plutons, show strongly foliated character indicating development of ductile shear zone at the margins. These are peraluminous S-type granites having high A/CNK value (> 1). Presence of normative corundum, rounded shape of zircon, and high initial Sr ratio suggest crustal source of the granites. Mantle normalized spider-diagram exhibits similar characters for all these granitoids. The intrusion of the Pan-African granites mark an abrupt end of the sedimentation that continued virtually uninterrupted from Palaeoproterozoic. The sudden break in sedimentation towards the terminal phases of the Lower Cambrian has been observed in almost all parts in Lesser as well as the Tethys Himalaya. Occurrences of large number of plutons along different tectonic belts of northwestern Himalaya are indicative of widespread tectono-thermal event during early Palaeozoic (500 ± 50 Ma). The bracketing of the two features like, the break in sedimentation during post-Late Cambrian, and the intrusion of granites around 500 ± 50 Ma, is considered to be the result of a strong diastrophic orogenic event correlatable to the late phases of the Pan-African Orogeny in Africa.

Trace element redistribution in high‐temperature deformed gabbros from East Ligurian ophiolites (Northern Apennines, Italy): constraints on the origin of syndeformation fluids

Abstract Mg‐gabbros from East Ligurian ophiolites (Northern Apennines, Italy) display a high‐temperature/low‐pressure recrystallization localized along ductile shear zones. In deformed gabbros, the igneous diopside is recrystallized into granoblastic aggregates of neoblastic diopside and minor red‐brown amphibole. The latter displays a pargasitic composition, with high amounts of AlIV Na(A) and Ti (± 1.8, 0.7 and 0.4 atoms per formula unit, respectively). Major element composition of neoblastic minerals highlight equilibration temperature conditions in the range 800–950° C. Red‐brown Ti‐pargasite also occurs as a minor interstitial constituent, presumably growing from a residual trapped liquid, in the differentiated lithologies (Fe‐Ti‐diorites) of the plutonic ophiolitic complex.By means of ion microprobe (SIMS technique), rare earth (La, Ce, Nd, Sm, Eu, Gd, Dy, Er, Yb) and selected trace elements (Sr, Y, Cr, V, Sc, Zr, Ti) have been analysed in igneous and neoblastic diopside, as well as in Ti‐pargasites. Ti‐pargasites have also been analysed for F and Cl, and compared with the halogen composition of the amphiboles, mainly hornblendes to actinolites, which are related to the subsequent low‐temperature brittle evolution.Neoblastic Ti‐pargasite from deformed Mg‐gabbros bears close compositional similarities with igneous Ti‐pargasite from undeformed Fe‐Ti‐diorites, whereas it is geochemically distinct from the amphiboles post‐dating the ductile event. In particular, Ti‐pargasites have relatively high contents of F, REE, Y, Zr and Sr, which are not consistent with crystallization in the presence of seawater‐derived hydrothermal fluids.High‐grade recrystallization probably developed in the presence of volatile‐rich igneous fluids, either trapped between the cumulus minerals or injected into the ductile shear zones from outside the local system. An alternative hypothesis comprises the absence of fluid phase and the development of ductile shear zones in Ti‐pargasite‐rich layers. The petrological features of deformed gabbros recovered from present‐day slow‐spreading ridges and fracture zones bear close similarities with the investigated ophiolitic metagabbros. In East Ligurian ophiolites, high‐grade ductile shear zones have been related to the initial stages of the uplift of the gabbro‐peridotite complex to the sea‐floor.

P wave velocity across a noncoaxial ductile shear zone and its associated strain gradient: Consequences for upper crustal reflectivity

In order to simulate a normal incidence reflection profile across a noncoaxial ductile shear zone, we determined P wave velocities of samples cut parallel and normal to mylonite foliation along a closely spaced profile (≈ 27 cm long) through a transition zone and its associated strain gradient. The ductile shear zone, developed within an aplitic leucogranite, was sampled from a kilometer‐wide ductile transcurrent fault in the northern French Massif Central. Strain analysis indicates that the sample experienced heterogeneous and progressive simple shear deformation; shear strain (γ) systematically increases from zero in the undeformed protolith to ≈ 30 in the mylonite. The transition zone thickness (T) is about 30 cm, and the mylonite thickness (M) is about 10 cm. The amount of quartz and mica increases relative to feldspar toward the mylonite, indicating that a mineralogical composition change accompanied mylonitization. Mica and quartz developed a strong crystallographic preferred orientation (CPO). In the least strained domain, seismic anisotropy is low and mean Vp is 6 km/s at 600 MPa. Anisotropy increases up to 10% and Vp decreases up to 5.35 km/s for propagation normal to the mylonite foliation through the transition zone. This systematic velocity change correlates with the increasing γ through the transition zone and can be directly related to the CPO of mica and the increase in volume percent mica within the mylonite zone. These results indicate that velocity and anisotropy gradients may, in some cases, be associated with ductile shear zones and that mylonite boundaries may not represent first‐order discontinuities. The reflectivity of a ductile shear zone depends on the thickness of the transition zone relative to the seismic wavelength (λ) and on the T/M ratio. Synthetic seismograms show that for a given seismic wavelength the reflectivity decreases when the transition zone thickness increases and when the ratio T/M increases. We show that layers with second‐order boundaries (velocity gradients in transition zones) are only seismically detectable within a narrow thickness range. Extrapolation to thicker shear zones is based on the assumption that the strain gradient thickness relative to shear zone thickness is, to a first approximation, scale independent. In granitic domains, ductile shear zones with similar geometrical and petrophysical features to the example studied here will be detected on deep seismic profiles only if their width is between 20 and 400 m. Development of ductile shear zones with strain gradients of the appropriate thickness to enhance reflectivity is favored under low‐temperature conditions in the granitic upper crust. Indeed, low‐temperature strain gradient may explain the high seismic reflectivity of the upper crust in the Scandinavian Caledonides, whereas high‐temperature strain gradient may explain, in part, the relative transparency of the European Variscan upper crust.

Basement-cover relationships and regional structure in the Grandjean Fjord - Bessel Fjord region (75°–76°N), North-East Greenland

Geological mapping and isotopic investigations demonstrate that the Grandjean Fjord – Bessel Fjord region can be divided into three rock groups: (1) a Lower Proterozoic basement gneiss complex; (2) a Middle Proterozoic supracrustal cover (Smallefjord sequence); and (3) Upper Proterozoic metasediments (Eleonore Bay Supergroup). The basement gneiss complex largely comprises c. 2.0–1.7 Ga calc-alkaline granitoid orthogneisses with intercalated migmatitic supracrustal rocks. The complex is deformed by at least two sets of approximately coaxial folds which may be either Proterozoic or Caledonian in age. The Smallefjord sequence is comprised mostly of migmatitic schists and gneisses which underwent high-grade metamorphism during the late Middle Proterozoic. The dominant deformation structures within the Smallefjord sequence are associated with the development of ductile shear zones along the boundaries of all the major tectonostratigraphic units and are thought to be Caledonian in age.

Relations between thrust belt evolution, grain-scale deformation, and metamorphic processes: Cape Smith Belt, northern Canada

An important step towards understanding how continental thrust belts evolve is to study the relationship between the map-scale structures and the associated grain-scale deformation mechanisms and metamorphic processes. This relationship is examined in the early Proterozoic Cape Smith Thrust Belt, in the context of the belt's P- T- t history and fluid-rock interactions. Overprinting deformation-metamorphism relationships allow the thrust belt's evolution to be described in terms of four events: 1. (1) piggyback thrusting above a basal décollement localized at the basement-cover contact; 2. (2) greenschist to amphibolite grade metamorphism and development of a ductile basal shear zone; 3. (3) re-imbrication (out-of-sequence faulting) of the internal zone at pre- to syn-thermal peak conditions; 4. (4) post-thermal peak out-of-sequence thrusting and retrograde metamorphism. Out-of-sequence thrusting was accommodated by the development of ductile shear zones. In general, relatively broad ductile shear zones developed adjacent to thrusts during prograde metamorphism, while relatively narrow shear zones developed adjacent to thrusts during retrograde metamorphism. The width of the shear zones was controlled by factors such as the competition between softening and hardening processes (prograde shear zones), and the extent of fluid influx (retrograde shear zones). Four processes were primarily responsible for accomplishing penetrative deformation and accommodating thrust faulting during the belt's history: 1. (1) partitioning of strain into pre-existing weak zones or into relatively weak layers in a compositionally-stratified sequence; 2. (2) growth and/or rotation of metamorphic products into orientations of maximum resolved shear stress; 3. (3) transitions in dominant deformation mechanism from brittle or semi-brittle to crystal-plastic; 4. (4)retrograde metamorphism, involving fluid influx and hydration reactions producing weaker products than reactants.

Relationship between Shear Zones and Sr Isotope Compositions of Granitic Rocks in the Hida Belt, Japan

The dextral shear zones in the Hida belt, central Japan, show close temporal and spatial relationships with some of the Late Triassic to Early Jurassic Funatsu granitic intrusions. The Sr isotopic compositions of intrusions associated with the shear zones suggest large crustal contributions to the magma chemistries. In contrast, granitic intrusions not accompanied by shear zones show less crustal signatures. Various models could possibly explain the genetic relationship between shear zones and granitic magmas with crustal isotope characteristics, though it is not clear which model is most applicable. It is suggested that contrasting characteristics in the two types of intrusions reflect different stress conditions. The granitic rocks without large crustal contributions are likely to be the products of extensional stress conditions. Those with a larger amount of crustal contamination seem to be generated in compressional stress situations that accompanied the development of ductile shear zones along the co...

Crustal deformation and regional metamorphism across a terrane boundary, Coast Plutonic Complex, British Columbia

The Coast Plutonic Complex between Prince Rupert and Terrace, British Columbia, developed in two stages between mid‐Cretaceous and mid‐Eocene time. The early stage (>100–70 Ma) involved crustal thickening as the amalgamated Alexander and Wrangellia terranes were emplaced against the Stikine terrane to the east. This thickening resulted from tectonic stacking of crustal slabs, lubricated by intrusion of melt generated at the base of the thickened crust. Emplacement of westward directed thrust slabs along the western margin of the orogen was accompanied by intrusion of a high pressure epidote‐bearing tonalite pluton and associated sills. Thrusting resulted in inversion of the metamorphic sequence as supracrustal rocks buried early in the collision were tectonically exhumed. Pluton emplacement also accompanied eastward directed thrusting of high temperature gneisses over low‐grade rocks on the eastern margin of the orogen as the tectonic welt was backthrust over Stikinia. The earliest recognized events in the core of the orogen involve pervasive ductile deformation accompanied by emplacement of tonalite sills between about 85 and 50 Ma. Uplift of the orogen core between 60 and 48 Ma coincided with and was facilitated by emplacement of large volumes of tonalitic magma, accompanied by anatexis and the development of ductile shear zones. One of these shear zones, the Work Channel lineament, presently separates the core of the orogen from the schists to the west. The main orogenic cycle ended by 48 Ma when the rocks cooled rapidly through the biotite and hornblende Ar blocking temperatures. The time and space associations of deformation, metamorphism, and plutonic intrusion imply that substantial differential movement occurred across zones which contained fluid during deformation of the Coast Plutonic Complex. In the region of westward directed thrusting and inverted metamorphism, melts generated in the tectonically thickened lower crust and fluids released during metamorphic recrystallization of supracrustal rocks, which were rapidly buried during tectonic thickening, acted to reduce the strength of the crust. In the core zone of the complex, injections of tonalite magma into the lower crust induced pervasive anatexis. The weakened core zone yielded along both low angle and steep shear zones that were further lubricated by melt as the relatively buoyant hot crust rose rapidly in response to continued compression and underthrusting.

On folding and shear zone-development: a mesoscale structural study on the transition between two different tectonic styles

Abstract An example of coeval development of ductile shear zones and folds in schistose terrains is documented by detailed mapping and structural analysis. Folds and shear zones were formed under retrogressive conditions after the peak of the regional Hercynian metamorphism, and both affect a previous regional schistosity. A gradual change in structural style, on a regional scale, is evidenced by the disappearance in higher grade domains of late folds and the preponderance of shear zones. Geometrical analysis reveals that the bulk structural pattern is compatible with a non-coaxial strain history. Structural variation reflects a gradual change in mechanical behaviour of schists, which show a marked variation in crystallinity, at the time of deformation.