Patrick James

Late Archaean granites of the Napier Complex, Enderby Land, Antarctica: A comparison of Rb-Sr, Sm-Nd and U-Pb isotopic systematics in a complex terrain

Abstract Late Archaean granites have been identified within the northern part of the Napier Complex of Enderby Land, Antarctica, although intrusives of this age are not common elsewhere in the complex. The oldest intrusive (3070 ± 34 Ma), synorogenic granite at Proclamation Island, was probably derived from melting of felsic crustal rocks of igneous origin. Two younger granites were emplaced at about 2840 Ma and 2481 ± 3 Ma, and the latter has geochemical similarities with other late and post-orogenic intrusives elsewhere in the East Antarctic Shield. Because of their fractionated character, it is difficult to discern whether the granites were derived by melting of sedimentary or igneous protoliths, but a granulite-facies source is probable. TNdCHUR model ages of the three analysed granites suggest that the Napier Complex is the product of at least two temporally discrete episodes of continental crust formation. The granite precursors probably formed at about 3100 Ma, whereas most of the exposed Napier Complex is much older (back to almost 4000 Ma). Two of the granites appear to record a significant hiatus (in one case of about 600 Ma) between the formation of this crustal precursor and final emplacement. The 3100 Ma old episode of crustal formation was roughly synchronous with a widespread, intense, and high-grade tectonothermal event (D1-M1), which produced metamorphic fabrics and mineral assemblages that are widely preserved within the Napier Complex. It is therefore likely that this new crust formation, deformation and metamorphism are all attributable to a single tectonic episode, possibly related to emplacement of magma into the lower crust. The two younger granites were emplaced at about the time of two subsequent and less intense tectonothermal events (D2-M2 and D3-M3). Concordant Rb-Sr total-rock and U-Pb zircon ages, which are interpreted as emplacement ages, have been obtained for two of the intrusions. However, the third exhibits the unusual relationship of a Rb-Sr isochron age older than the U-Pb zircon age. In this case the Rb-Sr age is believed to date magmatic emplacement. The complex interpretation required for the zircon data is forewarned by a non-perfect alignment of analytical points, in contrast to the perfect analytical alignments produced by the isotopically coherent granites. Two of the three granites yield similar and relatively precise lower intercepts with the U-Pb concordia but these do not appear to have direct geological significance. The new isotopic data are combined with earlier results to derive an integrated tectonothermal evolution for the Napier Complex.

The geochronology, structure and metamorphism of early Archaean rocks at Fyfe Hills, Enderby Land, Antarctica

Abstract The Fyfe Hills of Enderby Land, Antarctica, are composed of Archaean cratonic rocks of the Napier Complex. Initial crustal formation (possibly of plutonic, volcanic and sedimentary origin) was in the early Archaean, but probably significantly later than the 4000 Ma ages previously reported. At ∼ 3100 Ma, the rocks underwent granulite facies metamorphism and an intense regional strain (D1) which rotated into parallelism pre-existing discordances and produced a recumbent gneissic pile. Approximate Sr isotopic homogenisation over at least a metre and new zircon growth and/or major resetting apparently occurred at this time. Subsequent ductile deformation (D2), also characterised by tight to isoclinal folding, produced characteristic microstructure only within F2 folds, but no axial-plane foliation was developed. Peak granulite-facies metamorphic conditions close to 900°C and 9–11 kbars were reached during D1-D2. Considerable Pb loss occurred in zircon during asymmetric open to tight folding (D3) and upper amphibolite to granulite-facies metamorphism (∼ 650°C; 7 kbars) at ∼ 2500 Ma. Rb-Sr equilibration on the scale of centimetres occurred at that time, after which the Napier Complex was essentially cratonised. A subsequent semiductile deformation did not affect the whole Napier Complex, but produced upright shear zones in the Fyfe Hills area which lies close to the boundary between the Napier Complex and the younger Rayner Complex to the south.

Rate of Arunta Inlier evolution at the eastern margin of the Entia Dome, central Australia

Abstract Zircon dating in the Entia Dome of the Arunta Inlier helps to temporally define the multiphase, successive Strangways and Harts Range Orogenies. It appears that the entire activity of both orogenies took place between 1767 ± 2 Ma and 1731 ± 1 Ma, with a major shearing and thrusting event associated with plutonic activity at 1747−2+3 Ma. These comparatively short time spans indicated are quite sufficient for the large horizontal movements required for the intra-continental basin formation and deformation that has been postulated by James and Ding. The earlier of these events are later than those currently proposed for equivalent stages of other Middle Proterozoic inliers of the North Australian Orogenic Province which raises difficulties with detailed correlations in grouping the inliers together. Monazite (UPb), muscovite (RbSr) and biotite (RbSr) isotopic systems had not closed until 360 Ma, 325 Ma and 308 Ma, respectively, indicating uplift and cover removal from > 20 km to

Structural evolution of the Harts Range Area and its implication for the development of the Arunta Block, central Australia

Abstract A division of the high grade metamorphics of the Arunta Block in the Harts Range Area into two main groups on the basis of lithology, structure, metamorphism and igneous activity is suggested as a result of detailed mapping. The lower group is regarded as a basement terrain and comprises major layered felsic or granitic gneiss, minor amphibolite and mafic granulite, while the structurally overlying group is regarded as a cover sequence (Irindina supracrustal assemblage) and comprises predominantly metasediments (pelitic and semi-pelitic gneiss, calc-silicate and quartzite) and amphibolite, anorthosite and ultramafic rock of the Harts Range meta-igneous complex. The cover sequence might have been deposited in an aulacogen or adjacent to a continental margin which developed by rifting of a pre-existing basement. The two groups both show early major ductile tectonothermal stages with four major fold generations and a later discrete shear event in the basement and two major fold generations and a later series of stacked nappes developing in the cover. The basements early orogenic evolution with associated granulite facies metamorphism is typical of intensely reworked early Precambrian terrains and developed spatially and or temporally separated from the cover. Cover deformation closely resembles continental margin Cordillera type overthrusting and thus thin-skinned tectonics associated with large scale crustal shortening. During the later Harts Range remobilization, the basement and cover were juxtaposed by displacement on a fundamental crustal dislocation (recumbent contraction fault). Mylonites repeatedly developed along the dislocation which was also isoclinally folded, intruded by a major megacrystic granitoid (Bruna gneiss) sheet and repeatedly rejuvenated. The history of development of the dislocation is documented and described as a process of underthrusting of basement beneath cover which led to the final termination of the orogenic activity.

‘Caterpillar tectonics’ in the Harts Range area: a kinship between two sequential Proterozoic extension-collision orogenic belts within the eastern Arunta inlier of central Australia

Abstract We propose that in the Harts Range area of the Arunta Inlier of central Australia, a basement and cover relationship previously described by us, represents a thick complexly deformed contact zone between an Early-Middle Proterozoic fold belt, the Strangways Orogenic Belt (SOB) and a younger Middle Proterozoic orogenic belt, the Harts Range Orogenic Belt, both of which contain their own supracrustal sequences. Rocks of the SOB were deposited and possibly metamorphosed to a maximum grade of granulite facies at ∼ 1800 Ma during an early phase of crustal extension. They were subsequently intruded by a significant proportion of acidic magmas at 1767 Ma and compressed to form a distinctively NNE-SSW trending orogenic belt. A WNW-ESE trending shallowly north-dipping zone of intense and repeated deformation, termed the Harts Range Detachment Zone (HRDZ), formed initially during a period of NNE-SSW crustal extension as a major low-angle normal shear/detachment zone within, and crosses at a high angle to the trend of, the basement rocks of the SOB. The HRDZ has been intermittently active as a zone of crustal weakness since its initiation. The first extensional movement on this zone began the formation of the Harts Range Mobile Belt (HRMB) which displays many of the characteristics of modern tectonic zones activated by plate movement. A supracrustal cover sequence, the Harts Range Cover (HRC), formed in the major geosynclinal basin which developed on the subsiding upper plate of the HRDZ. Continued crustal extension juxtaposed the cover sequence, which was then metamorphosed to a maximum grade of amphibolite facies, and granulite facies basement of the HRDZ, before a fundamental change in movement sense led to crustal contraction and a collision-related, thrusting phase. The HRMB which includes the HRDZ and the HRC, was then internally deformed over a relatively very short time span (15–20 Ma), before voluminous granitic magma intruded mainly along the HRDZ, during the Harts Range Orogeny (HRO). The intrusion of this major granitoid at 1747 Ma precisely fixes the time-scale of evolution for the earlier phases of the HRO. The HRO is documented in detail as a peculiar orogenic style of crustal contraction which was largely taken up by reverse-sense, discrete thrusting and shearing on faults and shear zones associated with the HRDZ. Displacement on the HRDZ was a stick-slip process with repeated punctuation of thrust-slip movement by pinning at points located progressively to the south. Each period of cessation of thrust-slip movement was followed by folding of discrete thrusts, of captured basement lobes within the HRDZ and of the overlying supracrustal sequence. The individual discrete movement phases and folding episodes are each named and described, as is a model of this unusual type of Proterozoic tectonic activity.

U-Pb data on granulite facies rocks from fold island, Kemp Coast, East Antarctica

Abstract The northern tip of Fold Island (67°16′S; 59°20′E) on the Kemp Coast is composed largely of quartzofeldspathic gneisses (charnockitic and enderbitic) and pyroxene granulite. Garnet-biotite-sillimanite gneiss, garnet-clinopyroxene-plagioclase-calc-silicate rock, magnetite-bearing rocks, and ultramafic rocks are very subordinate. The gneisses were deformed by the two phases of isoclinal folding and were subsequently intruded by mafic dikes. A second stage of deformation involving tight, ductile folding post-dates the dikes. This deformation culminated with the emplacement of hypersthene-bearing pegmatites in discordant planar veins and irregular masses, and with the hornblende-granulite facies metamorphism that resulted in the present-day mineral assemblages. U-Pb isotope data on zircon and monazite from two pegmatites define a chord intersecting concordia at 0.94 ± 0.08 Ga and 0.21 ± 0.21 Ga. The 0.94 Ga age dates deformation, granulite facies metamorphism, and the pegmatite emplacement of Stage II. U-Pb data on a total of five fractions of highly rounded zircons from two quartzofeldspathic gneisses define a chord with intersections at 3.08 ± 0.17 Ga and 0.82 ± 0.48 Ga. Our results suggest that the gneiss zircons are xenocrysts and do not provide any direct indication of the age of the host gneisses. The Fold Island gneisses are interpreted to be older (Napier Complex?) rocks that were reworked during the Late Proterozoic (∼ 1000 Ma) Rayner event. A similar sequence of events has been reported for a coastal portion of the Eastern Ghats Province of South India, suggesting that these two areas were part of the same metamorphic complex in Gondwanaland.

Influence of basin architecture on the style of inversion and fold-thrust belt tectonics—the southern Adelaide Fold-Thrust Belt, South Australia

Abstract The southern Adelaide Fold-Thrust Belt involves Neoproterozoic (Adelaidean) strata deposited during protracted rift and sag phases, and an overlying Cambrian (Kanmantoo) sequence represented by a thick wedge of turbidites, rapidly deposited in an eastwards-deepening basin, deeply incised into the underlying rocks. The belt was deformed during the Delamerian orogeny, developed at the southeastern margin of the Australian craton during early Palaeozoic NW-directed displacement. During this shortening event the margin of the Kanmantoo Basin was reactivated and formed a significant strain guide that controlled key features of the fold-thrust belt. Strain-integrated and restored sections reveal contrasting geometric and kinematic styles along the strike of the belt. A transpressional zone is developed at the steep basin margin in the south, oriented oblique to the structural grain. The central parts of the belt are dominated by strong buttressing, growth fault reactivation and basement-involved footwall shortcut thrusting. In the northern part of the belt, shortening is accommodated by homogeneous folding above a master de´collement. These variations in regional structural style and internal strain partitioning reflect significant variations in the original Kanmantoo Basin margin geometry. Rheological parameters control the partitioning of strain associated with pervasive fabrics, for example cleavage and transposed bedding. Folding is accompanied by negative line-length changes (shortening) in the northern part of the belt. In its thrusted central part, pelites commonly show positive line-length changes. Psammites were least affected by line-length changes and thus provide key beds for section balancing. Pervasive strain makes a full restoration of cross-sections impossible. However, incorporating regional strain data assists establishment of balanced and restorable sections, which provide a powerful tool in understanding both pre-deformational basin geometry and its shortening.

Hydrothermal brecciation due to fluid pressure fluctuations: examples from the Olary Domain, South Australia

Abstract Two breccias, the Doughboy and Cathedral Rock breccias, hosted by the Willyama Supergroup in the Olary Domain, northeastern South Australia, were mapped and described in this study. Mapping of the breccias has demonstrated that they are controlled by the regional deformational history of the area and they are intimately related to episodes of hydrothermal fluid flow and alteration. The clast morphology, particle size distribution, dilation ratio, structural position and contact relationships with the host rocks allow the degree of maturity and the nature of the fluid rock interactions for each breccia to be determined. Breccia clast size distribution and morphology were quantified using fractal analysis techniques. Textural analysis revealed significant differences in particle size distribution and clast morphology for the different breccias. The results of the fractal analysis indicate that the Doughboy breccia is the result of multiphase tectonic and fluid-assisted failure on a fault plane, whereas the Cathedral Rock breccia is the result of fluid-assisted failure during fold-related faulting. Due to the high fluid pressure constraints on reverse fault initiation, large fluid pressure differentials are generated during the opening of dilatant sites. It is the opening of these dilatant spaces which provides the room to accommodate and the mechanism for brecciation. Reverse faulting created dilatant spaces that, due to the large pressure differential between the wall rock and the dilational space, initiated failure of the wall rock. Whether this was a single event such as at Cathedral Rock or occurred during multiple events such as at Doughboy can be deduced via the fractal characteristics. Collectively, these observations shed light on the processes that occur during regional deformational events associated with multiple generations of hydrothermal activity. The relationship between fluid pressure and brecciation may explain why they are the sites where the most intense effects of hydrothermal activity in the Olary Domain occur.

Using Concept Maps to Plan an Introductory Structural Geology Course

A concept map is a visual representation of concepts and their relationship to each other in a body of knowledge. They show the hierarchy of these concepts and emphasize the links between them. Concept maps are valuable pedagogical tools used to design the syllabus for an undergraduate structural geology course. Their value as an aid to student learning has been widely documented (Novak, 1990), and we have found them particularly suitable in the initial planning of courses such as structural geology where many new concepts are introduced. Concept maps used in the design stage of our structural geology course has resulted in a significant re-ordering of the topics. A more logical sequence begins with descriptive topics (joints and faults) and progresses to more abstract topics (stress and strain and continuum mechanics). The resultant sequence of topics is not that used in most traditional structural geology textbooks. Although it is not necessary for a course to be taught in the same sequence as material is presented in a textbook it is more convenient for students if it does.

Geometric and kinematic evolution of asymmetric ductile shear zones in thrust sheets, southern Adelaide Fold–Thrust Belt, South Australia

Abstract The Southern Adelaide Fold–Thrust Belt, South Australia contains unusually asymmetric shallow to moderately inclined low-grade phyllonitic shear zones, uniformly floored by basal imbricate thrusts. The fabrics and minor structures that reveal the geometric and kinematic evolution of these shear zones are described, analysed and presented in detail. Incipient fabrics and structures are developed in upper transitional zones and then develop progressively more intensely downwards into the shear zones. They ultimately intensify along the lower boundary thrusts that make up the base of all of the shear zones. Microstructural analysis of multiple cleavages and grain-scale geometries from within the shear zones demonstrate that deformation is spatially and temporally concentrated along the lower boundary thrusts. The shear zones initiated by strain-softening at the base of a number of stacked thrust sheets during Mid- to Late-Cambrian orogenic shortening of the area when the fold–thrust belt was generated. Kinematic indicators confirm consistent NW-directed transport. The shear zones progressively developed upwards by further superposition of shear and flattening strain during which time deformation was localised within the base of the thrust sheets to develop distinctively asymmetric shear zones.