D2 Shear Research Articles

Buckle folding and deep-crustal shearing of high-grade gneisses at the junction of two major high-strain zones, Central Gneiss Belt, Grenville Province, Ontario

Low-plunging, transport-parallel F3 folds are common at all scales in the Central Gneiss Belt of the Grenville Province, but few of these folds are sheath folds. Where the D1–D2 Parry Sound shear zone intersects the D3 Shawanaga shear zone (SSZ) at a high angle, F3 folds formed at several scales (centimetre to greater than outcrop scale) in layered D1–D2 "straight" gneisses. At the start of their evolution, the F3 folds formed just beyond the SSZ with hinges near orthogonal to the D3 shear direction and with typical buckle features, e.g., wavelengths vary with layer thickness, and hinges are discontinuous and bifurcate. The buckle folds evolved within the SSZ by rotation of hinges towards the shear direction. Even though hinges initiated at a high angle to the shear direction, sheath folds were not produced. In addition to tightening the buckles, the ductile reorientation produced thin–thick (extended–shortened) limb pairs and very straight, ridge-like fold hinges and removed small folds from the extended limbs of larger folds. Such features may serve as criteria to distinguish transport-parallel folds that initiated in layering at high angles to the shear direction from those formed in layers containing the shear direction. A general shear parallel to the SSZ can reproduce several features inferred to mark stages in the progressive reorientation of the folds; the pure shear component of the general shear is inferred to have had a positive stretch direction down the dip of the shear zone, at a high angle to the transport (simple shear) direction. The interplay of buckling and shearing in the study area is, plausibly, the expression of deformation at the upper boundary of a channel-like flow that succeeded initial crustal thickening.

Relative timing of deformation and two-stage gold mineralization at the Hutti Mine, Dharwar Craton, India

Gold mineralization at Hutti is confined to a series of nine parallel, N–S to NNW–SSE trending, steeply dipping shear zones. The host rocks are amphibolites and meta-rhyolites metamorphosed at peak conditions of 660±40°C and 4±1 kbar. They are weakly foliated (S1) and contain barren quartz extension veins. The auriferous shear zones (reefs) are typically characterized by four alteration assemblages and laminated quartz veins, which, in places, occupy the entire reef width of 2–10 m, and contain the bulk of gold mineralization. A <1.5 m wide distal chlorite-sericite (+biotite, calcite, plagioclase) alteration zone can be distinguished from a 3–5 m wide proximal biotite-plagioclase (+quartz, muscovite, calcite) alteration zone. Gold is both spatially and temporally associated with disseminated arsenopyrite and pyrite mineralization. An inner chlorite-K-feldspar (+quartz, calcite, scheelite, tourmaline, sphene, epidote, sericite) alteration halo, which rims the laminated quartz veins, is characterized by a pyrrhotite, chalcopyrite, sphalerite, ilmenite, rutile, and gold paragenesis. The distal chlorite-sericite and proximal biotite-plagioclase alteration assemblages are developed in microlithons of the S2–S3 crenulation cleavage and are replaced along S3 by the inner chlorite-K-feldspar alteration, indicating a two-stage evolution for gold mineralization. Ductile D2 shearing, alteration, and gold mineralization formed the reefs during retrograde evolution and fluid infiltration under upper greenschist to lower amphibolite facies conditions (560±60°C, 2±1 kbar). The reefs were reactivated in the D3 dextral strike-slip to oblique-slip environment by fault-valve behavior at lower greenschist facies conditions (ca. 300–350°C), which formed the auriferous laminated quartz veins. Later D4 crosscutting veins and D5 faults overprint the gold mineralization. The alteration mineralogy and the structural control of the deposit clearly points to an orogenic style of gold mineralization, which took place either during isobaric cooling or at different levels of the Archean crust. From overlaps in the tectono-metamorphic history, it is concluded that gold mineralization occurred during two tectonic events, affecting the eastern Dharwar craton in south India between ca. 2550 – 2530 Ma: (1) The assemblage of various terranes of the eastern block, and (2) a tectono-magmatic event, which caused late- to posttectonic plutonism and a thermal perturbation. It differs, however, from the pre-peak metamorphic gold mineralization at Kolar and the single-stage mineralization at Ramagiri. Notably, greenschist facies gold mineralization occurred at Hutti 35–90 million years later than in the western Dharwar craton.

Evolution of a crustal‐scale transpressive shear zone in the Albany–Fraser Orogen, SW Australia: 2. Tectonic history of the Coramup Gneiss and a kinematic framework for Mesoproterozoic collision of the West Australian and Mawson cratons

AbstractWithin the Albany–Fraser Orogen of southwestern Australia, the Coramup Gneiss is a NE–SW trending zone of high‐strain rocks that preserves a detailed record of orogenesis related to Mesoproterozoic convergence of the West Australian and Mawson cratons. New structural, metamorphic and U–Pb SHRIMP zircon age data establish that the Coramup Gneiss underwent high‐grade tectonism during both Stage I (c. 1290 Ma) and Stage II (c. 1170 Ma) of the Albany–Fraser Orogeny. Stage I commenced with c. 1300 Ma high‐T, low‐P M1a metamorphism during extension, and the formation of small‐scale ptygmatic folds within a subhorizontal S1a gneissosity. High‐P M1b metamorphism at c. 1290 Ma was accompanied by the transposition and shearing of S1a into a composite, shallow SE‐dipping S1b foliation, and the development of tight recumbent F1b folds with S1‐parallel axial surfaces and asymmetries indicating NW‐directed thrusting. The preservation of a similar P–T–time record in the Fraser Complex (NE of the Coramup Gneiss) is consistent with large‐scale, NW‐directed Stage I thrusting of the Mawson Craton margin over the south‐eastern edge of the West Australian Craton. Stage II tectonism in the western Coramup Gneiss involved high‐T, low‐P M2a metamorphism and the formation of subvertical SE‐dipping D2 shear zones, shallow SW‐plunging L2 mineral stretching lineations, and NW‐verging F2 folds with S2‐parallel axial surfaces. A synkinematic pegmatite dyke emplaced into a D2 shear zone yielded a U–Pb SHRIMP zircon age of 1168 ± 12 Ma. Kinematic indicators suggest a combination of pure shear flattening perpendicular to S2, and dextral simple shear. However, contemporaneous structures elsewhere in the Albany–Fraser Orogen are consistent with continued NW–SE convergence at craton‐scale during Stage II, and oblique compression in the Coramup Gneiss is attributed to the arcuate geometry of the orogen‐scale deformation front.

Evolution of a crustal‐scale transpressive shear zone in the Albany–Fraser Orogen, SW Australia: 1. P–T conditions of Mesoproterozoic metamorphism in the Coramup Gneiss

AbstractThe Albany–Fraser Orogen in southwestern Australia preserves an important thermo‐tectonic record of Australo‐Antarctic cratonic assembly during the Mesoproterozoic. New petrologic and thermobarometric data from the Coramup Gneiss (a 10 km wide zone of high strain rocks within the NE‐trending eastern Albany–Fraser Orogen) indicate at least two high‐grade metamorphic events during 1345–1140 Ma convergence and amalgamation of the West Australian and Mawson cratons. The first event (M1) involved c. 1300 Ma granulite facies metamorphism of the Coramup Gneiss (M1a: 800–850 °C, 5–7 kbar), followed by burial and recrystallization under high‐P conditions (M1b: 800–850 °C, c. 10 kbar) prior to high‐T decompression (M1c: 700–800 °C, 7–8 kbar) and the 1290–1280 Ma emplacement of Recherche Granite sills. The second event (M2) entailed high‐T, low‐P metamorphism within dextral D2 shear zones (M2a: 750–800 °C, 5–6 kbar), followed by fluid‐present amphibolite facies M2b retrogression. Subsequent sinistral D3 mylonites and pseudotachylites are considered contemporaneous with similar structures in the adjacent Nornalup Complex that postdate the c. 1140 Ma Esperance Granite. Our petrological and thermobarometric data permit two end‐member P–T‐time relationships between M1 and M2: (1) a single post‐M1b event involving continuous M1b–M1c–M2a–M2b cooling and decompression, and (2) a two‐stage post‐M1b evolution involving M1c metamorphism during the waning stages of an event unrelated causally or temporally to subsequent M2a metamorphism and D2 deformation. In a companion paper, new structural and U–Pb SHRIMP zircon data are presented to support a two‐stage P–T evolution for the Coramup Gneiss, with M1 and M2, respectively, reflecting thermo‐tectonic activity during Stage I (1345–1260 Ma) and Stage II (1215–1140 Ma) of the Albany–Fraser Orogeny.

Birimian lithological succession and structural evolution in the Goren segment of the Boromo–Goren Greenstone Belt, Burkina Faso

Lithological and structural investigations indicate that the Goren segment of the Boromo–Goren Greenstone Belt in the Sanmatenga District is dominated by conformable successions of meta-sedimentary rocks that are interbedded with volcanic deposits. These successions are intercalated by a package of mafic-intermediate volcanic rocks, volcanic breccias and tuffs. The intercalated clastic–volcanoclastic–volcanic assemblage indicates interplay of marine-deltaic sedimentary processes and volcanic activity. Deposition is interpreted to have occurred within a marginal marine setting adjacent to an emergent volcanic centre or island arc setting. An angular unconformity between a volcanic-sedimentary package (the Lower Birimian) and a volcanic–volcanoclastic–clastic package (the Upper Birimian) does not occur within the Sanmatenga District. The supracrustal rocks were subjected to several phases of deformation and are metamorphosed to greenschist facies. Gabbro–diorite–pyroxenite bodies, and several granodiorite–tonalite and granite plutons have intruded them. Structural studies have indicated that the Goren segment underwent at least four phases of deformation, D1–D4. D1 resulted in the formation of a gently north-westerly plunging macroscopic anticline (F1), which is crosscut by D2 shear zones and D3 thrusts. D2 involved a period of north–south crustal shortening that is correlated with the Eburnean Orogeny at ∼2.1 Ga. It is characterised by the development of NE-trending shear zones (sinistral reverse) and folds (F2), and NW-trending (dextral) strike-slip faults. Deformation in D2 was concomitant with emplacement of the Baskoudré Granite, and the granodiorite–tonalite plutons (Banta, Yargo, Kob, Tantogo, Tompelse, Zitenga, Goudren and Loudo granodiorites, and Koukoundi and Soubsi tonalites). It was succeeded by emplacement of the Silmidougou Megacrystic Granite and aplite dykes. D3 involved a period of E–W crustal shortening that resulted in the formation of a series of NW to NNW-trending thrusts and northerly-plunging mesoscopic folds (F3). Deformation culminated in the repetition of stratigraphy through stacking of crustal blocks. The final deformation, D4, resulted in the formation of a series of WNW-trending thrusts (dextral reverse) collectively termed the Wabi-Tampelse Shear Zone. The zone is significant in that it marks a period of north–south crustal shortening that postdates the Eburnean Orogeny.

A structural reappraisal of the Beardmore–Geraldton Belt at the southern boundary of the Wabigoon subprovince, Ontario, and implications for gold mineralization

The Beardmore–Geraldton Belt occurs along the southern margin of the Archean Wabigoon subprovince, Superior Province, Ontario. The belt consists of shear-bounded interleaved metasedimentary and metavolcanic units. The units were imbricated from 2696 to 2691 Ma during D1 thrusting and accretion of the Wabigoon, Quetico, and Wawa subprovinces. Post-accretion D2 deformation produced regional F2 folds that transposed lithological units parallel to the axial plane S2 cleavage of the folds. During D3 deformation, the folds were overprinted by a regional S3 cleavage oriented anticlockwise of F2 axial planes, and lithological contacts and S2 cleavage were reactivated as planes of shear within dextral regional shear zones that generally conform to the trend of the belt. D3 is a regional dextral transpression event that also affected the Quetico and Wawa subprovinces, south of the Beardmore–Geraldton Belt. Gold mineralization at the Leitch and MacLeod-Cockshutt mines, the two richest past-producing gold mines in the Beardmore–Geraldton Belt, is associated with D3 shear zones and folds, overprinting regional F2 folds. The plunge of the ore zones is parallel to F3 fold axes and to the intersection of D3 shear zones with F2 and F3 folds.

Structure of the eastern margin of the East African Orogen in central Madagascar

The island of Madagascar straddles the poorly understood eastern margin of the East African Orogen, which is characterised by a high-grade gneissic basement that was structurally and thermally reworked during the Neoproterozoic collision of the Dharwar craton of India with the Congo/Tanzania/Bangweulu craton of Africa. An east-west traverse across the eastern margin of the Antananarivo block of central Madagascar and the overlying Beforona belt of the Tsaratanana sheet has identified four north-south trending structural domains, which represent the easternmost units of the East African Orogen. Dominant structures in these four domains are ascribed to two main deformation events, D2 and D3, whereas a poorly preserved early foliation and late folding event are classified as D1 and D4, respectively. D2 structures developed during east-west shortening comprise relatively low-strain, upright, north-south trending folds and steeply dipping, planar, north-south trending high-strain zones, both of which are associated with a stretching lineation that plunges gently north-south. D3 structures comprise a >20 km wide mylonitic high-strain zone and smaller scale discrete shear zones that dip gently west and exhibit top-to-the-east movement, again interpreted as a result of east-west shortening. The Antananarivo block and Beforona belt were juxtaposed sometime before D2 folding, which deforms the contact between the two units, but some time after an early granulite-facies metamorphic event that is preserved only in low strain zones of the Antananarivo block. D2 coaxial strain and D3 non-coaxial strain both occurred under amphibolite-facies metamorphic conditions and, although D3 shear zones locally cut D2 structures, these two events need not be temporally distinct. Published geochronological data from rocks exposed along this traverse constrain the likely timing of D2 and D3 to 630–515 Ma. The age, style and magnitude of strain along the eastern Antananarivo block indicates that eastern Madagascar was a focus for deformation during the final stages of collision between the Dharwar and Congo/Tanzania/Bangweulu cratons.

Application of data integration for deformation potential mapping using remotely acquired data sets within the Lynn Lake Greenstone Belt, northwestern Manitoba, Canada

The purpose of this paper is to demonstrate the ability of combining remotely acquired data using data integration techniques to define lineament trends that may represent high-strain zones within the Lynn Lake Greenstone Belt. A knowledge-driven data integration approach has been adopted for this study that makes use of theoretical principles and good working knowledge of the regional geology to translate components of a shear-hosted gold conceptual model into a spatial model. Lineaments extracted from airborne vertical gradient magnetic data (VG), very low frequency electromagnetic data (VLF-EM), and RADARSAT synthetic aperture radar (SAR) data are used as evidence for the occurrence of shear zones. Proximity maps created from the extracted lineaments are weighted based on deformational characteristics of shear zones. Combination of these proximity maps, using fuzzy logic and Dempster-Shafer theory, produces a potential for deformation map. Field mapping was undertaken to verify the accuracy of the deformation potential maps generated. Detailed strike-perpendicular structural mapping in areas of above-average outcrop exposure identified a series of subparallel D2 shear zones, which correlated with areas of high deformation potential. Other potential areas of high deformation were field checked and found to represent D2 shear zones not identified during structural mapping. Deformation potential mapping proved invaluable in the interpretation of the western strike extension of the Johnson Shear Zone (JSZ) and demonstrates the broadening of the JSZ west of the Gemmel Lake area. The results from this paper have major implications for future gold exploration in the belt and have demonstrated that VG, VLF, and SAR data can be useful tools to assist in the delineation of major structural trends and high deformation zones within greenstone belts.

Marymia: an Archean, amphibolite facies-hosted, orogenic lode-gold deposit overprinted by Palaeoproterozoic orogenesis and base metal mineralisation, Western Australia

The Marymia gold deposit, comprising two orebodies, Keillor 1 and Keillor 2, is at the northern end of the Plutonic Well greenstone belt in the Marymia Inlier, in the southern Capricorn Orogen, just north of the Yilgarn craton. The Marymia Inlier is a discrete fault-bounded Archean gneiss–granitoid–greenstone domain surrounded by sedimentary basins that were formed and variably metamorphosed and deformed during several Palaeoproterozoic orogenic cycles. The greenstone sequence at Marymia is stratigraphically and geochemically similar to greenstone sequences in the Yilgarn craton, but was subjected to further deformation and metamorphism in the Palaeoproterozoic. Late Archean deformation (D1–D2) was ductile to brittle–ductile in style, whereas Palaeoproterozoic deformation was predominantly brittle. Equilibrium mineral assemblages indicate that peak amphibolite-facies metamorphism (540–575 °C, <3 kb) was overprinted by greenschist-facies metamorphism (300–360 °C). Petrographic textures indicate that prograde metamorphism was coeval with D1–D2, with peak metamorphism early to syn D2. Gold mineralisation at Marymia is hosted in metamorphosed tholeiitic basalts and banded iron formation. On a gross scale, the distribution of gold is controlled by D2 folds and shear zones. Lithological contacts with strong rheological or chemical contrasts provide local controls. Gold-related alteration comprises subtle millimetre- to centimetre-wide zones of silicification with variable amounts of quartz, hornblende, biotite, K-feldspar, plagioclase, calcite/siderite, scheelite, titanite, epidote, sulfide and telluride minerals. Quartz veins are generally narrow and discontinuous with low total volume of quartz. Gold is sited in the wall rock, at vein salvedges or within stringers of wall rock within veins. There are two distinct opaque-mineral assemblages: pyrite–pyrrhotite–chalcopyrite–galena and hessite–petzite–altaite–Bi-telluride–galena. Ore samples are variably enriched in Ag, Te, Pb, W, Cu, S and Fe reflecting heterogeneity of the ore mineralogy. Structural timing and temperature of formation of alteration and ore minerals support deposition of gold during late peak amphibolite-facies metamorphism from neutral to alkaline (pH=5–6), moderately oxidising (log ƒO2≈–21–22) and CO2-bearing (XCO2 ≈0.2) fluids. The total sulfur content of the fluid is estimated at 1mΣS. Lead isotope compositions support derivation of lead from within the local greenstone sequence. Gold lodes were deformed by faults and shear zones in the Palaeoproterozoic, with only limited remobilisation. Subeconomic, carbonate vein- and breccia-hosted base metal mineralisation is locally hosted within Palaeoproterozoic fault zones, which clearly cut gold lodes. Base-metal-related alteration is characterised by intense carbonatisation, chloritisation, and albitisation of the mafic host rocks. Mineral assemblages are consistent with formation at greenschist facies conditions. Lead isotope compositions support crystallisation at ca. 1.7 Ga from lead that is similar in composition to earlier gold-related galena.

Horizontal tectonic deformation geometries in a late Archaean sedimentary sequence, Belingwe greenstone belt, Zimbabwe

In the Belingwe greenstone belt of Zimbabwe, structural evidence from the circa 2.65 Ga old sedimentary Cheshire Formation which overlies and is imbricated with a mafic volcanic unit is consistent with thrusting of the greenstone sequence. The Cheshire Formation consists of a karstified carbonate ramp sequence overlain by siliciclastic turbidite deposits that formed in a southeast deepening basin. The earliest deformational structures formed during a syndepositional to postdepositional, thin‐skinned thrusting event (D1) that affected poorly consolidated sediments and is recorded in bedding‐parallel ductile shear zones, boudins, folds, and block‐in‐matrix structures. D1 shear zones separate the volcanic sequence and the overlying sediments and occur between the carbonate and siliciclastic units. Syntectonic sulphide mineralization and silicification of mainly fine‐grained sediments along the main thrust faults gave rise to the formation of rocks similar in appearance to banded iron formations. Stratigraphic units were locally duplicated along the D1 thrust faults, including a tectonic slice of mafic volcanics that was emplaced onto carbonates along a chaotic unit similar to a tectonic melange. The elongation of inclusions in the melange zone and lineations in ironstones together with kinematic indicators suggest that stratigraphic duplication resulted from northwestward directed tectonic transport. Deposition of the Cheshire Formation took place in an asymmetric, foreland‐type basin contemporaneously with thrusting. Soon after deposition the formation was incorporated into the thrust stack. Subsequent deformation events include tight upright folding, gentle cross‐folding, and dextral strike‐slip faulting of the greenstone succession. Evidence for early thrusting suggests that horizontal tectonic processes played an important role in the evolution of the Belingwe greenstone belt.

Structural-metamorphic evolution of the Southern Yenisey Range of Eastern Siberia: implications for the emplacement of the Kanskiy granulite Complex

¶The Southern Yenisey Range of Eastern Siberia consists of the granulite facies Kanskiy Complex bordered in the west by the lower-grade Yeniseyskiy and Yukseevskiy Complexes. Three deformational events were recognized in each of the three complexes along the Yenisey River cross-section: a D1 fabric forming event, a D2 shear and folding event, and a D3 shear event. Thrust kinematics across the Southern Yenisey Range suggest that during the D2 event the Kanskiy Complex was thrusted along a regional ductile shear zone onto the lower-grade complexes. This resulted in shearing and folding as well as the development of a dynamic metamorphic zonation. In the low-grade greenstone belt part of the cross section (Yukseevskiy complex) D2 shearing is associated with peak prograde (T ∼ 660 °C and P ∼ 5.8 kbar) metamorphism. The retrograde P-T path of the Yukseevskiy Complex coincides with minimum T of the near-isobaric cooling P-T paths for the adjacent granulites of the Kanskiy Complex (Perchuk et al., 1989). The metamorphism can therefore be attributed to deformation and heat transfer caused by exhumation of the Kanskiy Complex in the time period 2000–1800 Ma which also defines the most significant tectono-thermal event in the Southern Yenisey Range. The tectono-metamorphic pattern and evolution of the low- to high-grade metamorphic complexes of the Southern Yenisey Range is very similar to that described for the ∼ 2600 Ma Limpopo Complex of Southern Africa and the ∼ 1900 Ma Lapland Complex of the Kola Peninsula. Similar geodynamic processes were therefore possibly responsible for the formation of these high-grade terrains suggesting that their formation is linked to a general geodynamic model.

Change from thrusting to syncollisional extension at a mid-crustal level: an example from the Palaeoproterozoic Nagssugtoqidian Orogen (West Greenland)

The Palaeoproterozoic Nagssugtoqidian Orogen in West Greenland represents a mid- to deep-crustal section through a collisional orogen with a complex intrusive, tectonic, and metamorphic history. In the northeastern central part of the orogen, in the Ussuit area, Palaeoproterozoic intrusive and supracrustal rocks are sandwiched between slices of Archaean rocks forming a stack of lithotectonic units. Juxtaposition of these units occurred during west- to northwest-vergent thrusting along ductile shear zones (= D1) associated with a foliation formed at upper amphibolite facies conditions. D1 structures were folded (= FA) and then reactivated and locally omitted by localized east- to east-northeast-vergent extensional ductile shear zones (= D2) at near-peak metamorphic conditions. Shallowly east-plunging, transport-parallel upright folds (= FB) affect but are also truncated by D2 extensional shear zones, suggesting an interplay between FB folding and D2 shearing, compatible with a scenario of overall shortening perpendicular to fold axial surface during simultaneous extension parallel to fold axes. Consequently, the structures preserved in the Ussuit area document a deformation history which results from a change from thrusting to syncollisional extension occurring in mid- to lower crustal levels at peak metamorphic conditions.

Discussion on the generation of the Tay Nappe, Scotland, by large-scale SE-directed shearing

J. R Mendum &amp; C. W. Thomas write: Krabbendam et al. 1997 present a revised model for the development of the Tay Nappe, in part based on their recent mapping around Glenshee. Although they consider this a new model, it follows those of numerous other authors, notably Harris et al. 1976 and Bradbury et al. 1979, but also Roberts &amp; Treagus (1977), Treagus (1987) , Lambert &amp; McKerrow (1976) and, ultimately, Sturt (1961) and Rast (1963) . Briefly, the Tay Nappe is considered to have developed by southeasterly directed D2 shearing of upright D1 structures; this shearing produced a grossly inverted stratigraphy in the ‘Flat Belt’. As limited space precludes a full discussion and presentation of alternative models, we comment on those aspects with which we have most difficulty. We find the evidence cited in support of their model is wanting in critical areas and we propose alternative interpretations. Our discussion is concentrated on the nature and geometry of D1 and D2 structures in the Dalradian of the Southern Highlands. D2 structures are generally well-developed in much of the Southern Highlands, but to understand the nature of, and relationships between, D1 and D2, it is necessary to examine localities where D1 is virtually unaffected by D2 and where D2 begins to modify D1. Such outcrops are confined mainly to the Highland Border ‘Steep Belt’, southeast of the Highland Border Downbend, and are not well-exposed in the Glenshee district. Critical sections that demonstrate these relationships occur north of Stonehaven, in Little Glenshee [NN9835],

Evolution of the Archaean granite-greenstone terranes of the Eastern Goldfields, Western Australia: SHRIMP UPb zircon constraints

Thirty-five high-precision SHRIMP UPb zircon dates (with an average 95% confidence error of ±6 Ma) have been obtained from the granite-greenstone terranes in the southern part of the Eastern Goldfields of Western Australia. These dates establish the timing of major deformation, metamorphism and granite intrusion events, and test inferred stratigraphic relationships within the greenstones. These new data reveal that felsic volcanic rocks were erupted throughout the southern part of the Eastern Goldfields between c. 2713 and c. 2672 Ma. Analytically indistinguishable dates of c. 2705 Ma were obtained for felsic volcanic rocks interbedded with komatiitic lavas at three sites. Dates on felsic volcanic rocks within tectonostratigraphic packages that include komatiites are also consistent with an inferred eruption age for these komatiites of c. 2705 Ma. Komatiitic lavas in the southern part of the Eastern Goldfields were probably emplaced during a single voluminous eruption that resulted in the flooding of a vast area of the late Archaean landscape, filling the topographic lows around older felsic volcanic vents. The stratigraphic complexity within tectonostratigraphic domains is attributed to the existence of an undulating topography during the eruption of the mafic and ultramafic lavas, and to the rapid and localized deposition of felsic volcanic rocks close to a number of isolated volcanic centres prior to, and after, the komatiite lava eruption event. Between c. 2685 and c. 2675 Ma, early (pre-D2) intrusive equivalents of the felsic volcanic rocks were emplaced as thick granite sheets into the base of the greenstone sequences. Regional east-west compression between c. 2675 and c. 2657 Ma reactivated early extensional structures and resulted in the stacking of the greenstones. This was accompanied by further intrusion of granite within developing antiforms. Burial of greenstone slices by stacking, and the effect of hydrothermal fluids emanating from both the granites and buried greenstones, resulted in regional metamorphism to greenschist facies prior to c. 2640 Ma. Monzogranite and granodiorite emplaced near the western, southern and eastern margins of the greenstones were deformed and metamorphosed to upper amphibolite facies, and converted into gneisses during D2 and D3 compression events. Movement along large-scale transcurrent D3 shear zones had ceased by c. 2635 Ma. Remnants of a c. 2958 to c. 2930 Ma greenstone sequence preserved near Norseman and Ravensthorpe in the southeastern Yilgarn Craton, and xenocryst zircons ranging in age from c. 2910-2730 Ma in the c. 2713-2672 Ma felsic volcanic rocks, indicate that the exposed greenstone sequences were deposited on continental basement. The Eastern Goldfields granite-greenstone terranes were formed by the closure of a series of narrow back-arc rift basins that developed along a continental margin and above an active subduction zone located to the east of the rift basins.

Generation of the Tay Nappe, Scotland, by large-scale SE-directed shearing

Recent mapping in the Glen Shee Area, Central Scotland, has revealed an array of several large-scale F1 folds within the 'Flat Belt' of the Tay Nappe. Strata within the 'Flat Belt', forming part of the lower limb of the Tay Nappe, are thus not entirely inverted, and the traditional view that the Nappe in this area comprises a single SE-facing, recumbent anticline is rejected. It is demonstrated that the sense of shear of D2 was top-to-the-SE, resulting in NW-verging F2 folds by strong D2 strain partitioning. Strain estimates indicate adisplacement of 10–50 km during D2 shearing. In the proposed model, originally upright F1 folds were modified by a major D2 shearing event that produced a shallow NW dipping S2 foliation in the 'Flat Belt'. The upper limit of D2 deformation forms the boundary between the strongly deformed and inverted 'lower limb' and the right-way-up 'upper limb' which is undeformed by D2, and is now mostly removed by erosion. The D2 shear event was responsible for the gross stratigraphic inversion in the Flat Belt. The Tay Nappe is considered here as formed by a crustal-scale shear zone; in this model there is no necessity to invoke a root zone.

Geochronologic, petrologic and kinematic constraints on the evolution of the Err-Platta boundary, part of a fossil continent-ocean suture in the Alps (eastern Switzerland)

The boundary between the Austroalpine Err nappe and the Penninic Platta unit represents part of a Late Cretaceous continent-ocean suture preserved in the highest stockwerk of the Tertiary Alpine orogen. Subduction and nappe stacking (D1) and subsequent extensional uplift (D2) along part of this suture are dated with the K-Ar method on two generations of syn-kinematic white mica in the Err nappe and with Ar-Ar step-heating of syn- to post-kinematic riebeckitic amphiboles in the Platta nappe. The first generation of white micas associated with isoclinal folds and W-directed mylonitic thrusting along nappe contacts yields a 76 to 89 Ma age range, whereas the second generation of white micas that grew during tight folding and top-to-the-E extensional D2 shearing yields 67 to 80 Ma ages. The K-Ar white mica ages are interpreted as ages of formation at the approximately 300°C temperature of lower green-schist facies regional metamorphism in the southern part of the Err nappe. The two amphibole samples from the Platta nappe yield 69 and 73 Ma Ar-Ar ages, interpreted as syn-D2 formational ages. The Si-content of the white micas in chlorite-K-feldspar-quartz-bearing meta-radiolarites in the southern part of the Err nappe indicate a pressure decrease from D1 to D2 deformation, probably accompanied by an increase in temperature. This can be related to D2 extensional uplift accommodated along reactivated D1 nappe contacts in the footwall of the Julier-Bernina nappe. When combined with stratigraphic evidence, the radiometric ages and deformation-crystallization relations indicate clockwise P-T paths for the Err, Platta, and Margna nappes: Suturing of the Austroalpine continental and Liguro-Piemontese oceanic crust began with near surface thrusting and flysch sedimentation between 88 and 100 Ma and culminated with D1 folding and nappe stacking under high P/T greenschist facies conditions sometime during the period 80 to 88 Ma. The unroofing of this suture was a multi-stage process involving Late Cretaceous D2 extension under greenschist facies conditions, Early Tertiary D3 thrusting under lower anchizonal conditions, and mid-Tertiary D4 normal faulting above the uplifting Lepontine dome.

Giant metasomatic system formed during exhumation of mid‐crustal Proterozoic rocks in the vicinity of the Cloncurry Fault, northwest Queensland

A giant alteration system including approximately 200 km2 of rocks dominated by the secondary mineral assemblages lies near the prominent lineament formed by the Cloncurry Fault in the southeast of the Proterozoic Mt Isa Inlier. Alteration affected rocks that had previously undergone sillimanite zone metamorphism under pressures of at least 300 MPa. The main lineament was produced by postorogenic brittle transcurrent motion with limited displacement. It exploits older D1 faults and occupies different structural positions within F2 fold limbs that were reactivated as late/post‐D2 shear zones. Metasomatism and vein emplacement occurred under a range of retrograde conditions during the transition from a compressional regime in a ductile regional metamorphic environment, through late‐ to post‐orogenic granitoid emplacement, and ultimately high‐level brittle faulting. The dominant sodic‐calcic alteration occurred in multiple overprinting fluid buffered systems at 400–500°C with pressures initially above 200 MP...

Low temperature, low pressure deformation and metamorphism of the Vangorda massive sulphide orebody, Yukon, Canada

The Vangorda orebody is a small stratiform massive sulphide orebody located in Anvil District, Yukon, Canada. The orebody consists of fineto medium-grained semi-massive and massive sulphides with a common sulphide mineralogy of pyrite, pyrrhotite, sphalerite, galena, and minor chalcopyrite. The host rocks and the sulphide lithofacies have been complexly deformed during two phases of deformation (D1 and D2) and associated metamorphism (M1 and M2). The effects of d1 and M1 are penetratively overprinted by D2 and M2. D2 and M2 resulted in tight to isoclinal F2 folding of the orebody, remobilisation of the sulphides, recrystallisation and development of shear zones along the limbs of the F2 folds. Chlorite thermometry and sulphide thermobarometry have been carried out on the host phyllites and on the sulphides. Chlorite was analysed from the S1 and S2 foliations in the phyllites to determine M1 and M2 temperatures, respectively. However, no difference was found between chlorite compositions in these foliations and a mean temperature of 363 °C was calculated from the tetrahedral A1IV occupancy. Arsenopyrite thermometry yielded a comparable mean temperature of 336 °C. Sphalerite inclusions in M2 pyrite porphyroblasts from D2 shear zones were analysed for pressure using the sphalerite + hexagonal pyrrhotite + pyrite barometer. Inclusions were analysed in an attempt to determine if relic m1 sphalerite, and hence pressure signature, was preserved. Inclusion compositions appear to reflect only M2 conditions and yielded a mean pressure of 4.0 kb. Sphalerite + hexagonal pyrrhotite assemblages were analysed from D2 shear zones to determine the M2 pressure using the sphalerite + hexagonal pyrrhotite barometer. These calculations yielded a mean pressure of 6.1 kb. The M2 temperatures and pressures calculated using these calibrations are in good agreement with those estimated from petrogenetic relationships.

Major southeast-directed Caledonian thrusting and folding in the Dalradian rocks of mid-Ulster: implications for Caledonian tectonics and mid-crustal shear zones

AbstractThe dominant structure controlling the disposition of Dalradian stratigraphy in mid-Ulster has hitherto been regarded as a southeast-facing gently inclined F1 anticline, a gross geometry modelled on, and thought to be a possible correlative of, the Tay Nappe in Scotland. Remapping of the supposedly inverted southern limb of this major fold reveals that much of it is in fact the correct way up. However, a stratigraphie repetition coupled with a reversal in younging does occur in the Sperrin Mountains, much further south than previously realized. This hitherto unrecognized upward southeast-facing isoclinal Sperrin Nappe is, however, a D2 structure, traceable for at least 40 km along strike and responsible for a regional stratigraphie inversion over an area of 300 km2. Following D2, a major 10 km thick D3 ductile shear zone resulted in translation towards the east-southeast. In the south, this deformation carried the Dalradian over Ordovician volcanics of the Tyrone Igneous Complex along the Omagh Thrust. Penecontemporaneity of magmatism with deformation clearly demonstrates that D3 is Caledonian (Arenig-Llanvirn). This deformation correlates with similar southeast-directed Caledonian thrusting in southern Donegal and Connemara. The apparent absence of Dalradian deformation of this age in southwestern Scotland may imply that Caledonian collision of outboard terranes with the miogeoclinal margin was initiated in Ireland and/or subsequent strike-slip has removed the evidence for deformation of this age from southwestern Scotland. The D3 shear zone in the Sperrin Mountains affects a very large volume of psammitic rocks. Within this shear zone the strain is not markedly higher than surrounding areas; however, its existence is demonstrated by the reorientation of mineral lineations over a large area. Such broad zones of only moderate strain may, we believe, be typical of translatory tectonics in areas of the mid-crust where there is little lithological diversity.

Dating transcurrent terrane accretion: An example from the Meguma and Avalon Composite Terranes in the northern Appalachians

The polygenetic Minas Geofracture separates the Meguma and Avalon Composite Terranes of the Maritime Appalachians. Dynamically recrystallized muscovite and biotite occur within D3 shear zones which developed during dextral movement along the Geofracture. These record 40Ar/39Ar plateau ages between circa 360 and 370 Ma. Comparable plateau ages are recorded by micas within undeformed portions of granitic plutons and their low‐pressure, contact metamorphic aureoles. These are similar to U‐Pb ages previously reported for igneous monazite, therefore indicating relatively rapid postmagmatic cooling at shallow crustal levels. The D3 fabrics were superimposed on earlier (S1) structures which formed during initial amalgamation of the Meguma and Avalon Composite Terranes. Whole‐rock slate/phyllite samples collected outside high‐temperature portions of the plutonic contact aureoles yield 40Ar/39Ar plateau ages which correspond to groupings of circa 400–395 Ma, 390–385 Ma, and 380–375 Ma. These are interpreted to date diachronous formation of the S1 fabrics and concomitant low‐grade regional metamorphism. In the circa 25 Ma between initial amalgamation and granitic plutonism, it is inferred that there was a slow rise of isotherms through the crust as a result of overthrust tectonic thickening. This led to crustal melting and the production of widespread granitic magmas which were emplaced immediately prior to D3 dextral shearing.