Lyal B. Harris

Fault-valve behaviour in optimally oriented shear zones: an example at the Revenge gold mine, Kambalda, Western Australia

Abstract Quartz vein systems developed in and adjacent to shear zones host major gold deposits in the Kambalda region of the Norseman–Wiluna greenstone belt. At the Revenge Mine, two groups of mineralised reverse shear zones formed as conjugate, near-optimally oriented sets during ESE subhorizontal shortening adjacent to a major transpressional shear system. The shear zones developed at temperatures of about 400°C in a transitional brittle–ductile regime. Deformation was associated with high fluid fluxes and involved fault-valve behaviour at transiently near-lithostatic fluid pressures. During progressive evolution of the shear system, early brittle and ductile deformation was overprinted by predominantly brittle deformation. Brittle shear failure was associated with fault dilation and the formation of fault-fill veins, particularly at fault bends and jogs. A transition from predominantly brittle shear failure to combined shear along faults and extension failure adjacent to faults occurred late during shear zone evolution and is interpreted as a response to a progressive decrease in maximum shear stress and a decrease in effective stresses. The formation of subhorizontal stylolites, locally subvertical extension veins and minor normal faults in association with thrust faulting, indicates episodic or transient reorientation of the near-field maximum principal stress from a subhorizontal to a near-vertical attitude during some fault-valve cycles. Local stress re-orientation is interpreted as resulting from near-total shear stress release and overshoot during some rupture events. Previously described fault-valve systems have formed predominantly in severely misoriented faults. The shear systems at Revenge Mine indicate that fault-valve action, and associated fluctuations in shear stress and fluid pressure, can influence the mechanical behaviour of optimally-oriented faults.

Reworking of Archaean and Early Proterozoic components during a progressive, Middle Proterozoic tectonothermal event in the Albany Mobile Belt, Western Australia

Abstract Isotopic data, derived mainly from UPb zircon ion-microprobe analyses, are presented for each of the three tectonic domains of the Albany Mobile Belt, Western Australia. They show that not only the Northern Domain, but also parts of the Central domain originally crystallised about 3 100 Ma ago; they represent reworked Archaean rocks similar to those of the adjoining Yilgarn Block. On the basis of SmNd model ages, a somewhat younger, less precisely defined crustal formation event of probable Early Proterozoic (∼ 2100 Ma) age can be inferred for the southern Domain and the remainder of the Central Domain. The younger component of the Albany Belt is older than some previous estimates, being of similar initial age to most of the Early Proterozoic orogenic provinces of northern and central Australia. Interpretation of the SmNd data is not strictly based on conventional model ages, for it appears that significant fractionation of the rare-earth elements occurred during Middle Proterozoic crustal melting. Pegmatites and granites dated by UPb on zircon provide a temporal framework for the Middle Proterozoic evolution of the Albany Mobile Belt. Progressive deformation, incorporating foliation development, up to four superimposed fold generations, late-tectonic granite intrusion and conjugate shear zone arrays took place under a consistent orientation of maximum compressive stress over a geologically brief time span, about 1190 Ma ago. Dextral transpression and thrusting at 1190 Ma postdates granulite facies metamorphism and major deformation events in the Fraser Mobile Belt. The widespread 1190 Ma rocks are of comparable age to intrusives rocks emplaced during a similar high-grade metamorphic event in the Bunger Hills of Antarctica (a region that is commonly juxtaposed with the Albany Mobile Belt in Gondwana reconstructions).

Mechanisms for folding of high-grade rocks in extensional tectonic settings

Abstract This review of structures developed in extensional high-grade terrains, combined with results of centrifuge analogue modelling, illustrates the range of fold styles and mechanisms for folding of amphibolite to granulite facies rocks during rifting or the collapse of a thrust-thickened orogen. Several extensional fold mechanisms (such as folding within detachment shear zones) are similar to those in contractional settings. The metamorphic P–T–t path, and not fold style or mode of formation, is therefore required to determine the tectonic setting in which some folds developed. Other mechanisms such as rollover above and folding between listric normal shear zones, and folding due to isostatic adjustments during crustal thinning, are unique to extensional tectonic settings. Several mechanisms for folding during crustal extension produce structures that could easily be misinterpreted as implying regional contraction and hence lead to errors in their tectonic interpretation. It is shown that isoclinal recumbent folds refolded by open, upright folds may develop during regional extension in the deep crust. Folds with a thrust sense of asymmetry can develop due to high shear strains within an extensional detachment, or from enhanced back-rotation of layers between normal shear zones. During back-rotation folding, layers rotated into the shortening field undergo further buckle folding, and all may rotate towards orthogonality to the maximum shortening direction. This mechanism explains the presence of many transposed folds, folds with axial planar pegmatites and folds with opposite vergence in extensional terrains. Examples of folds in high-grade rocks interpreted as forming during regional extension included in this paper are from the Grenville Province of Canada, Norwegian Caledonides, Albany Mobile Belt and Leeuwin Complex of Western Australia, Ruby Mountains in the Basin and Range Province of Nevada, the Atâ Sund area of Greenland, the Napier Complex of Enderby Land in East Antarctica and the Kigluaik Mountains in western Alaska.

40Ar/39Ar geochronolgy and Neoproterozoic tectonics along the northern margin of the Eastern Ghats Belt in north Orissa, India

Abstract The post-1100–950 Ma cooling history of the northern margin of the high-grade Eastern Ghats Belt is determined from 40Ar/39Ar data collected from schist and gneiss samples. The data define two groups of dates with distinct cooling histories between ca. 700 and 420 Ma. The oldest group of ages define a slow apparent cooling path, whereas a younger group of 40Ar/39Ar dates from the west and north margin of the Eastern Ghats Belt and along the southern margin of the Rengali Province define a more rapid cooling path. The younger 40Ar/39Ar dates are interpreted to reflect a variable and locally intense thermal event at ca. 550–500 Ma associated with reactivation of major shear zones. Final exhumation to current exposed levels occurred by ca. 300 Ma as constrained by unconformable Permo-Triassic coal-bearing sediments overlying the northern Eastern Ghats Belt and northwestern Rengali Province. Dextral-reverse reactivation after ca. 950 Ma and possibly prior to ca. 700 Ma along the Kerajang and Barakot Fault Zones, was associated with regional shortening within the Eastern Ghats Belt and Rengali Province. Further reactivation of major shear zones within the same shortening regime at ca. 550–500 Ma is indicated by the 40Ar/39Ar data. Resetting of the K/Ar system was locally more intense along the Eastern Ghats Boundary Fault and Kerajang Fault Zones and was associated with ingress of high-temperature fluids. The resulting post-Grenvillian tectonic framework for northern Orissa provides insight into the character of intra-cratonic Pan-African tectonism within East Gondwanaland.

The effect of cover composition on extensional faulting above re-activated basement faults: results from analogue modelling

Abstract Analogue models were used to investigate differences in deformation between sedimentary cover sequences of contrasting composition above reactivated basement faults during extension. Two types of model arrangements were used. The first represented clastic sediments directly overlying faulted basement, and the second represented a basal ductile layer in an overburden overlying the same faulted basement. The basement configuration consisted of faults both oblique and orthogonal to the bulk extension direction. Particular attention was paid to the resulting map-view fault patterns. In experiments without a basal ductile layer in the overburden, grabens only developed in areas directly above basement faults. In section, cover faults above oblique basement faults curved in profile and, in places, exhibited reverse movement at shallow depth. In experiments involving a ductile layer at the base of the overburden, faults developed oblique to both the bulk extension direction and the basement faults. The resulting fault pattern was influenced by both bulk extension and basement fault reactivation. The results indicate that cover sequences that include thick basal ductile layers above reactivated basement faults are more likely to develop faults with strikes perpendicular to the regional extension direction, compared to those without. Faulting in an overburden lacking a basal ductile sequence is more likely to be directly influenced by the underlying reactivated basement faults.

DELINEATION OF LITHOSTRUCTURAL DOMAINS IN NORTHERN ORISSA (INDIA) FROM LANDSAT THEMATIC MAPPER IMAGERY

Abstract Image-based reconnaissance geological mapping at 1:100,000 scale using Landsat TM data has delineated a college of Precambrian lithostructural domains within a 50,000-km 2 region which encompasses the northern portion of the Archaean to Proterozoic granulite-grade Eastern Ghats Tectonic Province and the adjacent Archaean-Mesoproterozoic Singhbhum Craton. The domains identified in the present study display distinctive internal structures on satellite imagery. Most are bounded by clearly recognisable major shear zones and faults on imagery; displacement directions may frequently be ascertained through local reorientation of planar structures adjacent to large strike-slip shear zones and through juxtaposition of domains with different structuring. The macroscopic/megascopic structural overview provided by the Landsat interpretation, supported by preliminary field investigation, suggests that the N-S shortening (E-W fold/thrust packages with associated NE and NW strike-slip faulting), is the dominant structural style in the northernmost part of the Eastern Ghats Tectonic Province, followed by regional dextral transpression, expressed in the form of major strike-slip faults. The largest of these structures (Kerajang Fault), which may be traced for over 250 km, has demonstrable Paleozoic-Mesozoic dextral motion associated with coal basin formation. A precursor Kerajang shear zone with dextral movement in excess of 100 km of indeterminate age may also have been instrumental in juxtaposing the Singhbhum Nucleus into its present position to the north of the Eastern Ghats Tectonic Province.

A structural and metamorphic traverse across the Albany Mobile Belt, Western Australia

The Albany Mobile Belt is divided into three domains (the Southern, Central and Northern Domains) based on their aeromagnetic response and their distinctive structural and metamorphic characteristics. Amphibolite-facies rocks of the Northern Domain are reworked Archaean Yilgarn Block gneisses and dolerite dykes with a progressive, southwards increase in the intensity of deformation and metamorphism. The Northern Domain-Central Domain contact is marked by an oblique thrust zone with a component of dextral transcurrent movement developed during the second deformation event (D2). The Central Domain consists of two-pyroxene granulites, together with quartz-magnetite gneiss, which has undergone penetrative, ductile, non-coaxial deformation with a dominant dextral transcurrent component during the first deformation event (D1), and has subsequently been folded around kilometric-and metric-scale folds overturned to the NNW during D2. The Central Domain grades transitionally southwards into the Southern Domain, where retrograde amphibolite facies rocks dominate. The Albany-Fraser Province is a Proterozoic mobile belt, formed during two prolonged tectonothermal events, both of which consist of a combination of dextral transcurrence and northwards thrusting.

Neoarchean disaggregation and reassembly of the Superior craton

The southern and western Superior craton of Canada (SWSC) is widely considered to be a tectonic collage accreted from north to south by multiple coeval subduction zones. We propose an alternative non–plate tectonic scenario where SWSC continental fragments are not exotic but derived by partial disaggregation of a heterogeneous older (Superior I) craton in response to a mantle overturn event that started at ca. 2780 Ma. During overturn, radial mantle outflow stretched and disaggregated the lithosphere to create the concentric Neoarchean fabric of the Superior craton, the southern part of which (SWSC) broke up into ribbon continents separated by oceanic tracts. Neoarchean calc-alkaline magmas record interaction between plume-related magma and older crust. A change in the mantle flow field at ca. 2720 Ma caused southward drift of the Northern Superior cratonic block as a result of mantle traction on its lithospheric keel, and SWSC terranes accreted to its leading edge.

Folding in high-grade rocks due to back-rotation between shear zones

Abstract Folds with a sense of asymmetry opposite to the bulk shear sense may form in high-grade rocks due to back-rotation between ductile shear zones. Layers back-rotated into the shortening field can undergo additional buckle folding. With progressive deformation, shear zones and domains of back-rotated foliation between shears rotate towards the X – Y (flattening) plane of the finite strain ellipsoid. In high-grade gneiss, the migration of melt into shear zones may facilitate displacement along them and accentuate back-rotation of material between shear zones. Folding during back-rotation occurs when the spacing between shear zones does not increase sufficiently to accommodate the length of back-rotated layers or between non-parallel shear zones. Folds formed by back-rotation between shear zones commonly have pegmatites sub-parallel to their axial planes, thickened overturned limbs and occur in localised packages. Associated shear zones have the same sense of shear on both attenuated limbs of folds formed by back-rotation. Examples are given for folds between transcurrent and normal ductile shear zones in the Albany Mobile Belt and Leeuwin Complex (Western Australia) and the Dharwar Craton (India).

Palaeomagnetic dating and tectonic significance of dolerite intrusions in the Albany Mobile Belt, Western Australia

Abstract Dolerite dykes and sills in the Mesoproterozoic Albany Mobile Belt and Neoproterozoic Stirling Range Formation metasediments in southwestern Australia provide evidence for extensional events between Australia, East Antarctica and Greater India. Paleomagnetic analysis shows that primary remanence was carried by single domain magnetite for samples from seven localities. Palaeomagnetic poles for the majority of sites fall close to the earliest Cambrian part of the Australian apparent polar wander path (APWP). The interpreted earliest Cambrian dykes intrude both extensional (dominant) and shear fractures, indicating NNE-SSW extension between the Western Australian and East Antarctic Shields and a horizontal, ESE-WNW maximum compressive stress. Dyke emplacement appears to be contemporaneous with mafic volcanism in central and northern Australia and igneous intrusion in once contiguous parts of the East Antarctic Shield. These features are interpreted as being related to Gondwanaland assembly, and may represent a response to far-field stresses resulting from the collision between East and West Gondwanaland. A second suite of NW-striking, mid-Carboniferous dykes is related to NE-SW extension and rifting between Greater India and the Western Australian Shield during early stages in the formation of the Perth Basin in a sinistral transtensional regime along the southern Western Australian margin. A palaeopole from the primary remanence in a NNW-striking dolerite dyke falls on the poorly defined Triassic to Early Cretaceous part of the Australian APWP, indicating a possible third phase of dyke emplacement. Regional tectonic constraints for rifting in the Perth Basin suggest that this youngest dyke most likely intruded during the Triassic.