Laurent Ailleres

Synthesis of the proterozoic evolution of the Mt Isa Inlier

By virtue of its large area of exposure of different crustal levels, and preservation of a protracted (∼400 million years) Palaeoproterozoic to Mesoproterozoic tectonic evolution, the Mt Isa Inlier is an excellent natural laboratory to study Proterozoic tectonic processes. The inlier preserves evidence of intracontinental basin development, plutonism, low-pressure metamorphism, orogenesis at different crustal levels, and crustal-scale metasomatism. In addition, the Mt Isa Inlier is endowed with a variety of ore deposits, including the Mt Isa Pb – Zn – Ag and Cu deposits, Century Zn – Pb – Ag deposit, Cannington Ag – Pb – Zn deposit, and the Osborne and Ernest Henry iron oxide Cu – Au deposits. Basement rocks were deformed and metamorphosed during the ca 1900 – 1870 Ma Barramundi Orogeny and intruded by the granitic rocks of the ca 1850 Ma Kalkadoon and Ewen Batholiths and their coeval Leichhardt Volcanics. Three stacked and superimposed superbasins evolved between ca 1800 and ca 1595 Ma. These basins evolved in an environment characterised by elevated heat flow and transient episodes of magmatism and basin inversion in an inferred continental backarc setting. The ca 1600 – 1500 Ma Isan Orogeny probably records two phases of orogenesis. The first phase (ca 1600 – 1570 Ma) involved approximately north – south to northwest – southeast shortening in which a northwest-vergent fold-thrust belt evolved in the Eastern Fold Belt and localised basin inversion occurred in the Western Fold Belt. The second phase (ca 1550 – 1500) involved thick-skinned deformation in the Eastern and Western Fold Belts, characterised by upright folding, reverse faulting, and dextral wrenching. Voluminous granites were emplaced throughout the Eastern Fold Belt between ca 1550 and 1500 Ma. Exhumation and cooling of the crustal pile following the Isan Orogeny were related to crustal extension and widespread erosion in eastern and southern Australia. Subtle reactivation of faults within the inlier following the Isan Orogeny records the distal effects of Mesoproterozoic to Neoproterozoic breakup events and orogenesis in central Australia.

Evolution of the Isan Orogeny at the southeastern margin of the Mt Isa Inlier

We present the results of field mapping, structural and metamorphic analysis in an area at the southeastern margin of the Mt Isa Inlier. The results provide insight into the evolution of the Isan Orogeny and a comparison with structural studies conducted along the line of the Mt Isa Deep Seismic Transect. Our observations are consistent with a two-stage orogenic scheme in which there was a transition from early thin-skinned to later thick-skinned deformation as the orogen evolved. Metasedimentary rocks of the Maronan Supergroup (ca 1700 – 1650 Ma) were deposited in a basin marginal to the presently exposed Mt Isa Inlier. They were then thrust toward the north and west during the early stages of shortening, ca 1600 – 1580 Ma. This produced an arcuate fold and thrust belt with east – west-trending folds in the east of the study area and north – south-trending folds in the west. Amphibolite to upper amphibolite facies, high-temperature – low-pressure metamorphic conditions, evidenced by garnet, andalusite and sillimanite porphyroblasts, were reached during the early phase of deformation. Subsequent deformation, ca 1550 – 1500 Ma, resulted in upright to steeply inclined folding of the earlier fabrics and steeply east-dipping reverse faults. This orogenic phase was characterised by the growth of staurolite in aluminous schists, and its subsequent replacement by biotite, consistent with a distinct cycle of prograde metamorphism at higher pressures than the first. Based on the differing orientation and style of structures and the association with separate metamorphic events, we argue that the two phases of deformation, ca 1600 – 1580 Ma and ca 1550 – 1500 Ma, represent discrete tectonic events that may have had different driving forces and boundary conditions.

The architecture, kinematics, and lithospheric processes of a compressional intraplate orogen occurring under Gondwana assembly: The Petermann orogeny, central Australia

We ally aeromagnetic interpretation with constrained three-dimensional (3D) gravity inversion over the Musgrave Province in central Australia to produce a 3D architectural and kinematic model of the ca. 550 Ma compressional intraplate Petermann orogeny. Our model is consistent with structural, metamorphic, and geochronological constraints and crustal-scale seismic models. Aeromagnetic interpretation indicates that divergent thrusts at the margins of the province are cut by transpressional shear zones that run along the axis of the orogen. Gravity inversion indicates that the marginal thrusts are crustal-scale and shallow-dipping, but that the transpressional shear zones of the axial zone are more steeply dipping, and penetrate the crust-mantle boundary, accommodating offsets of 10–25 km. This thick wedge of mantle within the lower crust has been in isostatic disequilibrium for more than 500 Ma, and we suggest that this load may be supported by local lithospheric strengthening resulting from the emplacement of relatively strong lithospheric mantle within the relatively weak lower crust. Other orogenic processes inferred from the model include: probable inversion of relict extensional architecture; crustal-scale strain partitioning leading to strain accommodation by the vertical and lateral extrusion of relatively undeformed crustal blocks; and escape tectonics directed toward the relatively free eastern margin of the orogen. These processes are consistent with the concept that mechanical and thermal heterogeneities in the lithosphere, and the resulting feedbacks with deformation, are the dominant controls on intraplate orogenesis. This model also demonstrates that the architecture and kinematics of the Petermann orogeny require modifi cation of leading models of Gondwana assembly.

Deformation history of the Hampden Synform in the Eastern Fold Belt of the Mt Isa terrane

The moderately metamorphosed and deformed rocks exposed in the Hampden Synform, Eastern Fold Belt, in the Mt Isa terrane, underwent complex multiple deformations during the early Mesoproterozoic Isan Orogeny (ca 1590–1500 Ma). The earliest deformation elements preserved in the Hampden Synform are first‐generation tight to isoclinal folds and an associated axial‐planar slaty cleavage. Preservation of recumbent first‐generation folds in the hinge zones of second‐generation folds, and the approximately northeast‐southwest orientation of restored L1 0 intersection lineation suggest recumbent folding occurred during east‐west to northwest‐southeast shortening. First‐generation folds are refolded by north‐south‐oriented upright non‐cylindrical tight to isoclinal second‐generation folds. A differentiated axial‐planar cleavage to the second‐generation fold is the dominant fabric in the study area. This fabric crenulates an earlier fabric in the hinge zones of second‐generation folds, but forms a composite cleavage on the fold limbs. Two weakly developed steeply dipping crenulation cleavages overprint the dominant composite cleavage at a relatively high angle (>45°). These deformations appear to have had little regional effect. The composite cleavage is also overprinted by a subhorizontal crenulation cleavage inferred to have developed during vertical shortening associated with late‐orogenic pluton emplacement. We interpret the sequence of deformation events in the Hampden Synform to reflect the progression from thin‐skinned crustal shortening during the development of first‐generation structures to thick‐skinned crustal shortening during subsequent events. The Hampden Synform is interpreted to occur within a progressively deformed thrust slice located in the hangingwall of the Overhang Shear.

Sequential development of a mid-crustal fold-thrust complex: evidence from the Mitakoodi Culminationin the eastern Mt Isa Inlier, Australia

The Eastern Fold Belt, Mt Isa Inlier preserves possible backarc inversion in the Mesoproterozoic, which occurred in a ca 1600 – 1500 Ma Isan Orogen that was distal to the plate margin. Like many of the Palaeoproterozoic to Mesoproterozoic orogens of the Australian continent, this orogen displays several unusual characteristics compared with modern convergent orogens. The thermal history is dominated by high-temperature, low-pressure metamorphism, which pre-dates maximum crustal thickening. Crustal thickening has been achieved by stacking younger and relatively hotter basin phases over older and colder basin phases during thin-skinned lateral crustal translations. The structural geometries produced during the Isan Orogeny suggest that wholesale inversion was accomplished via the development of a fold and thrust belt at mid-crustal levels. The early evolution involved thin-skinned (D1) deformation characterised by northwest-directed transport of nappes and thrusting of young supracrustal successions over older supercrustal successions along mylonitic overthrusts. As the fold and thrust belt evolved, crustal translations (D2) were facilitated along a subhorizontal structure (Argylla Detachment) located at the interface between crystalline basement and the overlying basin successions. Upright to inclined folds (F2) with north-northeast-trending axial traces formed above underlying ramps and as fault propagation folds in the hangingwall of the Argylla Detachment. D1 nappes and shallow dipping mylonites were refolded by F2 folds and translated towards the west. Continued deformation involved thick-skinned crustal shortening producing upright local F3 folds, broad flexures in the basement – cover interface and steeply east-dipping basement-rooted reverse faults. Whilst the architecture of the fold and thrust belt preserved in the Eastern Fold Belt resembles that preserved in the foreland of many orogenic belts, there are several notable differences. Younger and hotter sedimentary successions have been thrust over older sedimentary successions, rather than older over younger as we might expect. Stratigraphy has been excised, rather than repeated, across major mylonite overthrusts. These observations are consistent with an episode of lithospheric extension immediately before a switch to crustal thickening and orogenesis. In this interpretation, high-temperature metamorphism during the earliest stages of orogenesis may have been inherited from the earlier extensional regime, and extensional detachments may have been reactivated as thrusts during regional shortening. Such a model has important implications for understanding basin inversion processes at mid-crustal levels and provides a framework to interpret the timing and context of high-temperature, low-pressure metamorphism, which is a common characteristic in many Proterozoicz orogens of Australia.

Australia and Nuna

Abstract The Australian continent records c. 1860–1800 Ma orogenesis associated with rapid accretion of several ribbon micro-continents along the southern and eastern margins of the proto-North Australian Craton during Nuna assembly. The boundaries of these accreted micro-continents are imaged in crustal-scale seismic reflection data, and regional gravity and aeromagnetic datasets. Continental growth (c. 1860–1850 Ma) along the southern margin of the proto-North Australian Craton is recorded by the accretion of a micro-continent that included the Aileron Terrane (northern Arunta Inlier) and the Gawler Craton. Eastward growth of the North Australian Craton occurred during the accretion of the Numil Terrane and the Abingdon Seismic Province, which forms part of a broader zone of collision between the northwestern margins of Laurentia and the proto-North Australian Craton. The Tickalara Arc initially accreted with the Kimberley Craton at c. 1850 Ma and together these collided with the proto-North Australian Craton at c. 1820 Ma. Collision between the West Australian Craton and the proto-North Australian Craton at c. 1790–1760 Ma terminated the rapid growth of the Australian continent.

Interpreting subsurface volcanic structures using geologically constrained 3‐D gravity inversions: Examples of maar‐diatremes, Newer Volcanics Province, southeastern Australia

We present results and a method to geophysically image the subsurface structures of maar volcanoes to better understand eruption mechanisms and risks associated with maar-forming eruptions. High-resolution ground gravity and magnetic data were acquired across several maar volcanoes within the Newer Volcanics Province of southeastern Australia, including the Ecklin maar, Red Rock Volcanic Complex, and Mount Leura Volcanic Complex. The depth and geometry of subsurface volcanic structures were determined by interpretation of gridded geophysical data and constrained 2.5-D forward and 3-D inverse modeling techniques. Bouguer gravity lows identified across the volcanic craters reflect lower density lake sediments and pyroclastic debris infilling the underlying maar-diatremes. These anomalies were reproduced during modeling by shallow coalesced diatremes. Short-wavelength positive gravity and magnetic anomalies identified within the center of the craters suggest complex internal structures. Modeling identified feeder vents, consisting of higher proportions of volcanic debris, intrusive dikes, and ponded magma. Because potential field models are nonunique, sensitivity analyses were undertaken to understand where uncertainty lies in the interpretations, and how the models may vary between the bounds of the constraints. Rather than producing a single “ideal” model, multiple models consistent with available geologic information are created using different inversion techniques. The modeling technique we present focuses on maar volcanoes, but there are wider implications for imaging the subsurface of other volcanic systems such as kimberlite pipes, scoria cones, tuff rings, and calderas.

Refinements to the Fry method (1979) using image processing

Abstract The Fry method is a very powerful way to determine the finite strain ellipse in a deformed rock, but the problems of reproducibility and objectivity of the measurements still remain. Using image processing, the program presented here extracts the central void from Normalized Fry diagrams and computes the characteristics of the best fitted ellipse. It runs automatically on Personal Computers, but remains interactive with the operator, as does the videographic image analyzer.

Inversion and Geodiversity: Searching Model Space for the Answers

Geophysical inversion employs various methods to minimize the misfit between geophysical datasets and three-dimensional petrophysical distributions. Inversion techniques rely on many subjective inputs to provide a solution to a non-unique underdetermined problem, including the use of a priori model elements (i.e. a contiguous volume of the same litho-stratigraphic package), the a priori input model itself or inversion constraints. In some cases, inversion may produce a result that perfectly matches the observed geophysical data, but can still misrepresent the geological system. A workflow is presented here that offers objective methods to provide inputs to inversion: (1) simulations are performed to create a model suite that contains a range of geologically possible models; (2) stratigraphic variability is determined via uncertainty analysis to identify low certainty model regions and elements; (3) geodiversity analysis is then conducted to determine geometrical and geophysical extremes and commonalities within the model space; (4) geodiversity metrics are simultaneously analysed using principal component analysis to identify the contribution of different model elements toward overall model suite uncertainty; (5) principal component analysis also determines which models exhibit diverse or common geological and geophysical characteristics which (6) facilitate the selection of models as inputs to geophysical inversion. This workflow is applied to a three-dimensional model of the Ashanti Greenstone Belt, southwestern Ghana in West Africa in order to reduce the subjectivity incurred during decision making, explore the range of geologically possible models and provide geological constraints to the inversion process to produce geologically and geophysically robust suites of models. Results further suggest that three-dimensional uncertainty grids can optimize inversion processes and assist in finding geologically reasonable solutions.

Mineral system analysis of the Mt Isa–McArthur River region, Northern Australia

The Mt Isa–McArthur region is renowned for a range of commodities and deposit types of world-class proportions. The region is described here in the context of a ‘mineral system,’ through consideration of processes that operate across a range of scales, from geodynamics and crustal architecture, to fluid sources, pathways, drivers and depositional processes. The objective is to improve targeting of Pb–Zn, Cu and Cu–Au deposits. Repeated extension and high heat flow characterise much of the history prior to 1640 Ma. The pre-Barramundi Orogeny (pre-1.87 Ga) metamorphic basement was the substrate on which a volcanic arc developed, focussed along the Kalkadoon-Leichhardt Belt. This is related to an inferred east-directed subduction between 1870 and 1850 Ma. From 1755 to 1640 Ma, three successive volcano-sedimentary basins developed, the Leichhardt, Calvert and Isa Superbasins, in an interpreted distal back-arc environment. The Isan Orogeny, from 1640 to 1490 Ma, overlapped with Isa Superbasin sedimentation, suggesting a transition from back-arc to a foreland basin setting. Most crustal thickening occurred in the Eastern Fold Belt, an area earlier characterised by thinned crust and deep marine environments. This region was deformed into nappe-like structures with high-temperature–low-pressure regional metamorphism and associated granites; the latter are absent from the Western Fold Belt. Metal deposition mainly occurred late in the history, with all known (and preserved) major base metal occurrences either hosted by Isa Superbasin rocks or formed during the Isan Orogeny. Earlier superbasins were potential fluid source regions. Sedimentary formation waters, metamorphic and magmatic fluids were present at prospect scale, while meteoric and possibly mantle sources are also implicated. The spatial distribution of metallogenic associations (i.e. iron oxide–copper–gold, Pb–Zn–Ag, U, Au) across the inlier may result from differences in the geodynamic make-up and evolution of the pre-1.87 Ga tectonic elements. Penetrative faults are interpreted as predominantly steeply dipping and to have acted as pathways for fluids, both in extension and compression. Fluid mixing was a potentially significant ore deposit control. Examples are drawn from the Ernest Henry iron oxide–copper–gold-related hydrothermal breccias in the east and from the Mt Isa Copper deposit in the west. Stress switching during late-stage deformation appears to have triggered a fluid mixing event that led to formation of the major copper deposits.