A recent paradigm shift has occurred in our understanding of the importance of Neogene intra-plate tectonics for surface hydrology and groundwater systems in Australia. While active deformation zones are readily identified by seismicity monitoring and high-resolution satellite and airborne terrain mapping, advances in airborne electromagnetic (AEM) technology and data optimization have enabled rapid mapping of more extensive 'blind' Neogene intra-plate fault systems in the near-surface within seismically quiescent depositional landscapes. However, to date, the geometries, displacements, and properties of these Neogene faults have remained ambiguous due to a paucity of outcrop, AEM footprint resolution, and the non-uniqueness of the conductivity models and derived hydrostratigraphy and fault geometry solutions produced by AEM equivalent inversion models. In this study, a novel, inter-disciplinary approach has been developed to characterise the hydrogeology of intra-plate fault systems within unconsolidated, near-surface depositional landscapes and underlying sedimentary basins. The approach integrates morphotectonic analysis with sub-surface mapping of hydrostratigraphy and 'blind' intra-plate fault systems using airborne geophysics (AEM, airborne magnetics) and satellite remote sensing (e.g. Landsat). Fault zone geometry and displacement is examined using both 1D and 2.5D inversions of AEM data, validated at local scales by ground geophysics (e.g. seismic reflection, resistivity and ground nuclear magnetic resonance (GMR)) and drilling. Fault zone hydrogeology has been assessed through the integration of hydrogeophysics with hydrochemistry, hydrodynamics, and studies of vegetation response to water availability (using Landsat time-series analysis). The approach has also adapted deterministic and stochastic structural analysis approaches developed for use with seismic reflection data in the oil industry, to analyse AEM datasets. This inter-disciplinary approach has revealed complex fault zone conduit-barrier system behaviour that determines lateral and vertical groundwater flow, inter-aquifer leakage and localised recharge and discharge. This approach has revealed that Neogene deformation is a 'keystone element' of the Lower Darling floodplain and other surface hydrological and groundwater systems in Australia.
In Australia, groundwater investigations in remote frontier areas face challenges including the cost and difficulty in obtaining drilling approvals due to often lengthy heritage and environmental approvals processes. Non-invasive geophysical techniques, including airborne electromagnetics (AEM) and new Ground Magnetic Resonance (GMR) techniques are particularly attractive in these circumstances, as key hydrogeological parameters including depth to watertable, bound and free water contents, and transmissivity can be obtained cost-effectively in short timeframes with limited clearance approvals required.
This study reports the results of a comparative evaluation of 1D, 2.5D and 3D AEM inversions for resolving hydrostratigraphy and structural elements in two contrasting settings: unconsolidated Quaternary floodplain sediments affected by Neogene deformation; and a tectonically inverted Palaeozoic sedimentary basin.Previous studies have demonstrated the importance of airborne electromagnetic (AEM) data optimization to ensure that key elements of the hydrogeological system, including geological faults, are appropriately represented in inversion models. In the inverted sedimentary basin study, 1D inversions of AEM data indicated greater structural complexity than previously known. Initially, a suite of equivalent 1D inversion models produced very similar inversion model results. However, 2.5D inversions produced a disparity in solutions in key locations. To resolve these differences, 3D AEM inversion methods have been trialled. In the second study (floodplain setting), 3D inversions have helped resolve the geometry of hydrostratigraphic units and tectonic elements (folds and faults). In both study areas, independent validation of inversion results has involved an inter-disciplinary approach incorporating a range of borehole and ground geophysics techniques (e.g. passive seismic and Ground Magnetic Resonance (GMR)), tectonic mapping and analysis, hydrochemistry and drilling.In summary, comparative evaluation of 1D, 2.5D, and 3D AEM inversions in two contrasting settings demonstrates the importance of optimizing inversion procedures, taking into consideration all available geological, hydrogeological and tectonic data. The benefits of using 2.5D and/or 3D inversion procedures are particularly evident in areas of structural complexity. Confidence in 3D inversions is maximised when all elements of the system response are modelled appropriately.
The Gawler Craton is an equidimensional crustal province located on the southern margin of Australia. It is comprised of a sheared, irregularly shaped, dominantly metasedimentary Archaean nucleus surrounded by Proterozoic-aged plutonic and orogenic rocks (Daly et al., 1998; Hand et al., 2007). The Gawler Craton has undergone three major orogenies; the Sleafordian Orogeny (2480-2420Ma), the Kimban Orogeny (1730-1690Ma) and the Kararan Orogeny (1570-1540Ma) (Betts and Giles, 2006; Daly et al., 1998; Hand et al., 2007). Voluminous magmatism dominated the period between the Kimban and Kararan Orogenies. This included the I-type Tunkillia Suite (1690-1670 Ma) and St Peter Suite (1631-1608 Ma), and the A-type Hiltaba Suite and its volcanic equivalent the Gawler Range Volcanics (1595-1575 Ma) (Hand et al., 2007). This period in the evolution of the Gawler Craton marks the transition from St Peter Suite magmatism at an inferred former plate margin to intracontinental magmatism associated with the Hiltaba tectonothermal event (Betts and Giles, 2006; Betts et al., 2009; Betts et al., 2007; Hand et al., 2007; Swain et al., 2008). Tectonic models reconstructing the history of the Gawler Craton and Proterozoic Australia have been created (Betts and Giles, 2006; Betts et al., 2009; Betts et al., 2002; Giles et al., 2004; Myers et al., 1996; Wade et al., 2006) although lack of basement outcrop and paucity of data especially at former plate margins means that these models are often poorly resolved (Stewart and Betts, 2010). Additional structural and geochronological data from former plate margins during this significant period is thus required to better constrain tectonic reconstructions. The evolution of arc magmas above subduction zones is a complex multistage process that is still poorly understood despite considerable research and numerous publications (Winter, 2010). It is thought that dehydration of the downgoing slab releases LILE-rich fluid into the overriding mantle causing partial melting forming tholeiitic basaltic magmas (Hawkesworth et al., 1993; Stern, 2002). The production of intermediate to silicic calc-alkaline magmas, which are so voluminously preserved in the upper crust in volcanic arcs, from primary tholeiitic magmas is complex with numerous processes at work (Winter, 2010). Fractional crystallisation (Bachmann and Bergantz, 2004; Gill, 1981; Grove et al., 2003) and assimilation of crustal material and crust-derived magmas during ascent (Hildreth and Moorbath, 1988) are thought to be the dominant processes. The role of fractional melting of earlier arc rocks is less well understood and has been mostly inferred from geochemistry (Tamura and Tatsumi, 2002; Vogel et al., 2004), petrological features within lavas (Bachmann et al., 2002; Murphy et al., 2000) and zircon recycling within arc magmas (Weinberg and Dunlap, 2000; White et al., 2012). This thesis investigates the Palaeoproterozoic arc represented by the calc-alkaline St Peter Suite at two localities: Rocky Point and Westall. These two outcrops have undergone more strain than other St Peter Suite rocks investigated elsewhere (Chalmers, 2009; Wolfram, 2011). Previous investigations on the St Peter Suite have focused on its geochemistry and geochronology (Chalmers, 2009; Flint et al., 1990; Knight, 1997; Rahilly, 2011; Swain et al., 2008; Wolfram, 2011). This research aims to use a multidisciplinary approach, including structural, geochemical and geochronological analysis, to decipher the igneous and structural evolution of the St Peter Suite at these localities thus contributing to constraining the timing and evolution of this plate margin at a time preceding the intracontinental A-type Hiltaba magmatism, and contributing to better understanding magmatic arc processes such as remelting, magma mingling and mixing and continental crust formation.
The Australian Government has recently provided Aus$100.5M to Geoscience Australia over 4 years (2016-2020) to manage the Exploring for the Future (EFTF) programme designed to increase investment in minerals, energy and groundwater resources, primarily in Northern Australia. The programme includes Aus$30.8M for groundwater-specific investigations, recognizing that there are major gaps in our knowledge of Northern Australia’s groundwater systems and resources. The groundwater component of the EFTF programme is focused on addressing these knowledge gaps, with the aim of underpinning future opportunities for irrigated agriculture, mineral and energy development, and community water supply. The groundwater programme will include identification and assessment of potential groundwater resources and water banking options in priority regional areas, while also analyzing the salinity risk (including seawater intrusion).To rapidly map, characterise and assess regional groundwater systems and resources in the data-poor ‘frontier’ areas of Northern Australia, a multi-physics, inter-disciplinary approach has been developed. The programme involves the initial use of temporal remote sensing ‘data cube’ technologies for surface hydrology and landscape mapping, and acquisition of airborne electromagnetic (AEM) and Ground Magnetic Resonance (GMR) datasets. This provides a framework for targeted investigations including passive seismic, microgravity and GPR; borehole geophysics (Induction, gamma and Nuclear Magnetic Resonance (NMR)); drilling and pump testing; hydrochemistry and geochronology (water, landscapes and geology); as well as soils, regolith and basin/bedrock geological, hydrogeological and structural mapping and modelling.This methodology has enabled rapid identification and assessment of potential groundwater resources, salinity and seawater intrusion hazards, and groundwater dependent ecosystems in several priority regions.
Summary Airborne electromagnetics (AEM) has successfully mapped and characterised groundwater systems in a range of landscapes and geological settings in the East Kimberley Region of north-western Australia. The AEM data enabled rapid imaging of key elements of hydrogeological systems in near-surface Cenozoic paleovalley, alluvial fan and colluvial sediments, and in underlying tectonically-inverted sedimentary basins. Rapid mapping and assessment of groundwater systems, MAR targets and salinity hazards involved the integration of AEM data with Ground Magnetic Resonance (GMR), seismic reflection, drilling and pump tests, borehole geophysics, soils, regolith, geological and structural mapping, and hydrogeological and hydrochemical investigations. AEM survey design was aided by the use of spatio-temporal analysis of Landsat data to identify areas of potential surface-groundwater interaction. A suite of equivalent 1D AEM inversion models produced comparable images of the sub-surface hydrostratigraphy and faults. However, 2.5D inversions produced different solutions in key locations. 3D inversions were subsequently performed, and drilling and tectonic analysis was used to assess all AEM inversion models. Recognising zones of structural complexity was important in the successful development of appropriate AEM inversion strategies and models. Overall, the success of groundwater system mapping has been due to the use of AEM within a broader, inter-disciplinary, multi-physics project framework.
'/%3e%3c/defs%3e%3cpath fill='%23012169' d='M0 0h10080v5040H0z'/%3e%3cpath stroke='%23fff' stroke-width='.6' d='m0 0 6 3m0-3L0 3' clip-path='url(%23b)' transform='scale(840)'/%3e%3cpath stroke='%23e4002b' stroke-width='.4' d='m0 0 6 3m0-3L0 3' clip-path='url(%23c)' transform='scale(840)'/%3e%3cpath stroke='%23fff' stroke-width='840' d='M2520 0v2520M0 1260h5040'/%3e%3cpath stroke='%23e4002b' stroke-width='504' d='M2520 0v2520M0 1260h5040'/%3e%3cg fill='%23fff'%3e%3cuse xlink:href='%23d' x='2520' y='3780'/%3e%3cuse xlink:href='%23a' x='7560' y='4200'/%3e%3cuse xlink:href='%23a' x='6300' y='2205'/%3e%3cuse xlink:href='%23a' x='7560' y='840'/%3e%3cuse xlink:href='%23a' x='8680' y='1869'/%3e%3cuse xlink:href='%23e' x='8064' y='2730'/%3e%3c/g%3e%3c/svg%3e)
