Modelling Of Potential Field Data Research Articles

Joint interpretation and modeling of potential field data for mapping groundwater potential zones around Debrecen area, Eastern Hungary

The Debrecen area, as part of the Great Hungarian Plain (GHP), is associated with a multi-aquifer system that is overly exploited to fulfill the development plans. This research aims to jointly interpret and model gravity and magnetic data to map the subsurface geology and structures that govern groundwater occurrence. Various potential field techniques, including spectral analysis, anomaly derivatives, analytical signal, and Euler deconvolution were employed to map the distribution and depth of the buried geological structures. The combination of the potential field techniques enabled the construction of a detailed lineament map, providing valuable insights into the distribution of the subsurface structural features. It was indicated that the main structural trend is NW–SE and NE–SW, that coincides with the main structural trends in Hungary. Subsequently, a lineament density map is derived, indicating that the eastern, central, and northwestern parts of the area form the most promising zones for groundwater prospection. The joint inversion of gravity and magnetic data has further enhanced the understanding of subsurface geology. The depth to the basement rock varied between 1.18 and 2.2 km. The highest depth to the basement meets with thick sedimentary sequences bounded by normal faults forming graben and horst structures. Moreover, the distribution of these sediments is investigated using lithological logs indicating the thickness of the main hydrostratigraphic units in the Debrecen area. These units include Nagyalföld Aquifer, Algyő Aquitard, Endrőd Aquitard, and Miocene Badenian Aquifer units, which mainly consist of sand, silt, marl, and gravel. The recent study demonstrated the effectiveness of the joint interpretation in enhancing the knowledge of lithology and geological structures. However, a detailed geophysical survey is recommended to characterize the hydrostratigraphic units in the Debrecen area.

2D fast Fourier transform analytical solutions in all space for all gravity and magnetic components

ABSTRACTForward modelling of potential field data is an important part of optimization algorithms used to invert large datasets such as those involving rugged terrain or borehole data. Two‐dimensional fast Fourier transform modelling with a prism or a dipole is one of the most efficient methods compared to the forward modelling in the space domain. However, the exact solution of a prismatic source is limited to the case of a half‐space with the computation of data on a horizontal datum above the topography. Starting from the three‐dimensional Fourier forward modelling analytical formulation for a prism, an integration according to the wavenumber w is accomplished which allowed to find a two‐dimensional Fourier exact analytical formulation outside, at the interfaces of, and inside a prism for all potential field components. This new formulation requires the calculation of only four integrals. The gravity and magnetic fields are computed with this two‐dimensional fast Fourier transform formulation in the entire domain and compared with the analytical space domain and the three‐dimensional fast Fourier transform formulations. From the three‐dimensional calculated field, each component can be interpolated with the tri‐linear interpolation method along a borehole or on a drape surface simulating an airborne survey. Based on experiments demonstrated in this work, the two‐dimensional formulation in the Fourier domain gave accurate results with greater speed of execution in comparison to modelling in the space domain. The forward modelling method is tested on real gravity data from the north of Alberta (Canada).

Large silicic magma chambers at the Moho depth characterize the multi-level plumbing system of back-arc spreading ridges

The plumbing system of volcanoes in crustal convergent settings consists of vertically superimposed, sill-like reservoirs. The shallower reservoirs are generally filled by less dense and SiO2-richer magmas. The plumbing systems of spreading ridges are supposed to be simpler and characterized by prevailing sub-parallel vertical dykes. Here, we present the results of thermo-barometric determinations from minerals and modelling of potential field data at the Marsili spreading ridge in the Southern Tyrrhenian Sea back-arc oceanic basin. The Marsili plumbing system consists of sill-like, large SiO2-rich magma reservoirs located at the Moho depth and sub-vertical basaltic dykes crossing the whole oceanic crust. The formation of deep-seated silicic reservoirs is associated with a decrease in the back-arc spreading rate. The geometry of the plumbing system of spreading ridges may be not dissimilar from that of volcanoes at convergent margins.

Crustal architecture of North Singhbhum Mobile Belt, Eastern Indian Shield: Constraints from two‐dimensional and three‐dimensional modelling of Bouguer gravity and aeromagnetic data

The North Singhbhum Mobile Belt (NSMB) that lies in the Eastern Indian Shield between the Chotanagpur Granite‐Gneiss Complex (CGGC) and the Singhbhum Craton (SC) provides a unique opportunity to understand the Proterozoic crustal evolution of the Indian shield. In the present study, an integrated interpretation of petrophysical, Bouguer gravity, and aeromagnetic data was carried out to examine the crustal architecture and structural framework of the region. Three‐dimensional (3‐D) inversion of the aeromagnetic anomalies reveals that the magnetic basement depths range from 0.2 to 9.0 km in the region. The crustal models obtained from two‐dimensional (2‐D) and 3‐D forward modelling of potential field data reveal the Moho at 37–38 km depth beneath the NSMB and at a relatively deeper depth of 39–40 km and 40–44 km beneath SC and CGGC, respectively. Further, the distinct gravity high and the coinciding bipolar magnetic anomalies associated with the Dalma Formation can be attributed to the mafic/ultramafic intrusions. Gravity models across the Singhbhum Shear Zone (SSZ) and South Purulia Shear Zone (SPSZ) exhibit the crustal density inhomogeneity and Moho offset, typical of ancient suture zones of Precambrian terrains. In comparison, the absence of such crustal‐scale signatures across the North Purulia Shear Zone represents its intracratonic nature. Further, the Bouguer gravity and aeromagnetic signatures across the SPSZ, together with the crustal architecture and metamorphic ages, suggest that SPSZ marks the eastward continuity of the Gavilgarh‐Tan Shear Zone along which the amalgamation of northern and southern Indian cratonic blocks possibly took place at ca. 1.0–0.9 Ga.

Crustal structure and Curie point depth in central Mexico inferred from the spectral analysis and forward modeling of potential field data

Different interpretations of crustal thickness variations have been proposed for central Mexico. The studied region encompass the transition between the Mesa Central, an elevated plateau with low relief, and the Sierra Madre Oriental, a late Mesozoic–early Cenozoic fold-thrust belt at lower mean elevation. Crustal models discussed here are based on three different published hypotheses: 1. Homogenous crustal thickness and a flat Moho beneath both provinces; 2. Thicker crust beneath the Mesa Central compared to thinner crust under the Sierra Madre Oriental; and 3. Thinner crust below the plateau and thicker crust beneath the fold-thrust belt. This last configuration implies the presence of lower-density mantle under the plateau to produce the higher mean elevation. We tested these conflicting configurations using gravity and magnetic data to produce forward models of the previously published interpretations and to obtain new crustal thickness and Curie point depth estimates. Our results show significantly higher error for model 1 and similar errors for models 2 and 3, ruling out model 1 but preventing discrimination between models 2 and 3 through forward modeling. However, the geothermal state results along with our depth-to-Moho estimates indicate the presence of a wide region beneath the eastern portion of the Mesa Central with thicker (40–52 km) and colder (−35 to −45 mWm−2) crust as compared to thinner crust (34 to <40 km) with elevated heat flow values (−50 to −85 mWm−2) beneath the portion of the Sierra Madre Oriental in the study area. Broad negative isostatic anomalies in the Mesa Central are evidence for excess mass/density under the plateau. Our findings are more consistent with model 2 and incompatible with model 3, in which the presence of shallow asthenosphere and partial melt under the plateau implies lower density rocks beneath the plateau and a corresponding steeper geothermal gradient.

Depth estimates of anomalous subsurface sources using 2D/3D modeling of potential field data: implications for groundwater dynamics in the Ziway-Shala Lakes Basin, Central Main Ethiopian Rift

Quantitative analysis of potential field data are made in the Ziway-Shala lakes basin over an area bounded by 38°00′ E - 39°30′ E and 7°00′ N - 8°30’ N. Most previous geophysical studies in the region under consideration focus on mapping the deep crustal structures and undulation of the Moho depth. Only few studies are targeted at mapping the shallow subsurface structures. The main focus of this paper is mapping geometries of the major lithological and structural units of the shallow subsurface using gravity and magnetic data. The ultimate objective of the research is to understand the hydrogeological dynamics of the region through mapping interfaces geometries. Automatic inversions, 2D joint forward modeling and 3D inversion are the major techniques employed. The 2D Werner de-convolution based on both gravity and magnetic data along the rift axis showed source depths tending to deepen northwards. Source depths estimates determined by Source Parameter Imaging also showed similar tendency. This is further strengthened by the joint 2D forward modeling of gravity and magnetic data which showed the top of the basement is sloping northwards. The result of the 3D gravity interface inversion agrees with results of the above mentioned depth estimation techniques. Finally, the gravity power spectral analysis resulted in two depth estimates, 1.53 km and 2.87 km which approximate the positions of two density interfaces. The shallow depth interface is thought to presumably delineate the low density Fluvio-lacustrine sediments including the rift floor volcanic units and crystalline basement. Our investigation results agree with the results of previous seismic studies which identified low velocity (“sediment-volcanic”) horizon in the rift floor with low resolution. The information obtained with regard to water balance of the basin, salinity level of the lakes and the conceptual hydrological flow model appears to reveal that the groundwater flow in the study region is controlled by subsurface structures, particularly, the mapped interface topographies.

A comprehensive lithospheric study of Black Sea using thermal modeling and simultaneous joint 3D inversion of potential field data

We present a comprehensive lithospheric model of the Black Sea comprised of thermal structure, sediment thickness, and depths to Moho and lithosphere-asthenosphere boundary (LAB). The crustal structure (depths to Moho boundary and basement) is first estimated via a simultaneous joint 3D nonlinear inversion of satellite gravity data and the Gzz component of gravity gradiometry tensors. The results are then used for detailed 2D iterative forward modeling of potential field data based on the thermal structure of the lithosphere. In addition to potential field data, heat flow measurements are used for the iterative forward modeling. Eight 2D profiles across the Black Sea were selected to achieve a full coverage of the main geological structures. The average depth to basement is estimated as 10–11 km in the East Black Sea (EBS) basin and 11–12 km in the West Black Sea (WBS) basin. The average depth to basement beneath the ridges is estimated between 2 and 5 km. The modeling results show a mean thickness of 20 km and 21 km for the crust and 150 and 100 km for the lithosphere in the central parts of EBS and WBS, respectively. While the lithosphere is thicker in the EBS than in the WBS, a LAB uplift in the central basins is obvious in the reconstructed model. Also, the calculated surface heat flow is highest at the ridges while the lowest values are found over thick sediments in the central basins. After correcting the calculated heat flow data for the sedimentation effect, the resulting values match the published measurements indicating the reliability of the obtained thermal structure for the Black Sea.

Imaging the 2–D crust and upper mantle structure of the Iranian plateau resolved by potential field and seismic data

In this study, the crust and upper mantle density distribution across the Iranian plateau along a ~1500 km profile are investigated using an integrated modeling of potential field data (gravity, geoid and topography) and by assuming local isostatic equilibrium. Our profile is initiated in Arabian platform and running through the main geological structures of Iran including Zagros orogen, Sanandaj – Sirjan Zone (SSZ), Urumieh – Dokhtar Magmatic Arc (UDMA), Central Iran (CI) block, Tabas/Lut blocks, and East Iranian Range (EIR). In the second step, in order to verify the integrated model, the 1–D S–wave absolute velocities for the crust and upper mantle beneath 21 seismic stations which partially covered the density profile are obtained using the joint inversion of P–receiver function (PRF) and surface–wave dispersion data. Our results indicate a thick crust (~60 km) beneath the SSZ and UDMA which thinning toward the east and experienced its minimum value (~40 km) beneath the central Iran, as well as Tabas/Lut blocks. Moreover, a thick lithosphere (>150 km) is observed under the Zagros, SSZ and parts of the UDMA, whereas a thinner lithosphere (~120 km on average in velocity model) is presented under the most of our profile. Both density and velocity models indicate a lithospheric thinning toward the CI block which imply the slab breakoff or lithospheric delamination followed by the upwelling of the asthenosphere in the Neogene. The presence of a relative fast structure in the crust beneath the Tabas block which is coinciding with a shallow high velocity body in the uppermost mantle elucidates a signature of former oceanic crust, which disappeared after the collision of Lut and Afghan continental blocks and E-directed subduction of Neotethys under the Afghan microcontinent either in post-Albian or middle Eocene times.

Regional crustal and lithospheric thickness model for Alaska, the Chukchi shelf, and the inner and outer bering shelves

SUMMARYThis study presents for the first time an integrated image of the crust and lithospheric mantle of Alaska and its adjacent western shelves of the Chukchi and Bering seas based on joint modelling of potential field data constrained by thermal analysis and seismic data. We also perform 3-D forward modelling and inversion of Bouguer anomalies to analyse density heterogeneities at the crustal level. The obtained crustal model shows northwest-directed long wavelength thickening (32–36 km), with additional localized trends of thicker crust in the Brooks Range (40 km) and in the Alaska and St Elias ranges (50 km). Offshore, 28–30-km-thick crust is predicted near the Bearing slope break and 36–38 km in the northern Chukchi Shelf. In interior Alaska, the crustal thickness changes abruptly across the Denali fault, from 34–36 to the north to above 30 km to the south. This sharp crustal thickness gradient agrees with the presence of a crustal tectonic buttress guiding block motion west and south towards the subduction zone. The average crustal density is 2810 kg m−3. The denser crust, up to 2910 kg m−3, is found south of the Denali Fault likely related to the oceanic nature of the Wrangellia Composite Terrane rocks. Offshore, less dense crust (&lt;2800 kg m−3) is found along the sedimentary basins of the Chukchi and Beaufort shelves. At LAB levels, there is a regional SE–NW trend that coincides with the current Pacific Plate motion, with a lithospheric root underneath the Brooks Range, Northern Slope, and Chuckchi Sea, that may correspond to a relic of the Chukotka-Artic Alaska microplate. The obtained lithospheric root (above 180 km) agrees with the presence of a boundary of cold, strong lithosphere that deflects the strain towards the South. South of the Denali Fault the LAB topography is quite complex. East of 150°W, below Wrangellia and the eastern side of Chugach terranes, the LAB is much shallower than it is west of this meridian. The NW trending limit separating thinner lithosphere in the east and thicker in the west agrees with the two-tiered slab shape of the subducting Pacific Plate.

The Brunswick magnetic anomaly: Geophysical signature and geologic source

Research Article| February 28, 2019 The Brunswick magnetic anomaly: Geophysical signature and geologic source Patrick D. Duff; Patrick D. Duff 1School of Earth, Ocean, and Environment, University of South Carolina, Columbia, South Carolina 29208, USA Search for other works by this author on: GSW Google Scholar James N. Kellogg James N. Kellogg 1School of Earth, Ocean, and Environment, University of South Carolina, Columbia, South Carolina 29208, USA Search for other works by this author on: GSW Google Scholar Author and Article Information Patrick D. Duff 1School of Earth, Ocean, and Environment, University of South Carolina, Columbia, South Carolina 29208, USA James N. Kellogg 1School of Earth, Ocean, and Environment, University of South Carolina, Columbia, South Carolina 29208, USA Publisher: Geological Society of America Received: 08 Aug 2018 Revision Received: 27 Jan 2019 Accepted: 29 Jan 2019 First Online: 28 Feb 2019 Online Issn: 1943-2682 Print Issn: 0091-7613 © 2019 Geological Society of America Geology (2019) 47 (4): 355–358. https://doi.org/10.1130/G45462.1 Article history Received: 08 Aug 2018 Revision Received: 27 Jan 2019 Accepted: 29 Jan 2019 First Online: 28 Feb 2019 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Patrick D. Duff, James N. Kellogg; The Brunswick magnetic anomaly: Geophysical signature and geologic source. Geology 2019;; 47 (4): 355–358. doi: https://doi.org/10.1130/G45462.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract We used integrated analyses and forward and inverse modeling of potential field data to interpret the enigmatic Brunswick magnetic anomaly (BMA), a prominent negative magnetic lineation that is developed within the ocean-continent transition of the Southeast Georgia Embayment. From magnetic profiles extracted from gridded data, residual magnetic maps, and simultaneous 2.5-dimensional forward and 3-dimensional Euler inverse modeling of the gravity and magnetic fields, we suggest that the source of the BMA is a series of semicontinuous to discrete, late-stage rift-related mafic intrusions of Mesozoic age. This interpretation is supported by the observation that the BMA is independent of the East Coast magnetic anomaly, implying a predrift source, and that the amplitude and frequency of the anomaly change nearshore across a fracture zone that divides the offshore BMA, where continental breakup occurred, from the onshore BMA. Modeling demonstrates for the first time that a Mesozoic rift-related mafic body can explain the anomaly onshore. The lack of a BMA equivalent on the West African margin may indicate that Atlantic rifting began with a single lithospheric dislocation. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Reinterpretation of adcoh and cocorp seismic reflection data with constraints from detailed forward modeling of potential field data — Implications for Laurentia-Peri-Gondwana suture

To better constrain the structure of the Laurentian – Peri-Gondwana suture zone, maps and a 2-dimensional regional cross-section model constrained by seismic data and surface geology have been developed by forward and inverse modeling the aeromagnetic and gravity fields. The Central Piedmont Suture (CPS), the boundary between the Laurentian Inner Piedmont and the Peri-Gondwanan Carolina terrane is a low-angle thrust fault (~30°) ramping up from an Alleghanian mid-crustal detachment at depths of about 12km. ADCOH and COCORP seismic data image anticlinal structures in the footwalls of the Hayesville thrust and the CPS, above the Alleghanian decollement. The footwall rocks have previously been interpreted as Paleozoic shelf strata on the basis of sub-horizontal seismic reflectors; however, the high densities required to fit the observed gravity anomaly suggest that the folded footwall reflectors may need to be reinterpreted as horse blocks or duplex structures of Grenvillian basement. The Appalachian paired gravity anomaly can be explained by an increase in crustal thickness and a decrease in upper crustal density moving northwestward from the Carolina Terrane toward the Appalachian core. A change in lower crustal density is not required, so that Grenville basement rocks may extend farther to the southeast than previously thought. The 5 to 10km of Alleghanian uplift and exhumation predicted by P-T crystallization data compiled in this paper can be easily accommodated by thrusting on four major low-angle thrust systems: Great Smoky Mountain Thrust (GSMT), Hayesville, Brevard, and CPS. Unroofing of metamorphic core complexes by normal faulting may therefore not be required to explain the observed exhumation. Alleghanian collision along the southeastern Appalachian margin was predominately orthogonal to strike consistent with the previous reconstructions that call for the counter-clockwise rotation of Gondwanan West Africa, creating head-on collision in the southern Appalachians and at least 370km of shortening.

Quo vadis inversion?

Modelling in 3D on a 2D screen is a difficult endeavour. The outcome depends mainly on how software implements modeling-tools. Tools used in the movie industry require a lot of time for user training. Usually geoscientists are specialized in their fields and they are not trained in computer-based interactive 3D modelling. They require simple-to-use modelling strategies, which can be used by geo-experts not familiar with sophisticated 3D manipulators and – most importantly – allow the creation and, most often, handling existing models (e.g., salt structures coming from seismic interpretation). Since models get more and more complex in geometry (e.g., complex multi-z geometries), modelling tools must be designed in a way that model-topology is conserved while constraints are respected, when such tools are applied. Thereby it is important that the tools must guarantee topology conservation, no matter what operations are performed to geo-models. Therefore tools for interactive 3D modelling of potential field data have been rethought and implemented in the IGMAS+ platform (Alvers et al., 2013; 2014, Lahmeyer et al., 2010). Responding to the question ‘Quo Vadis Inversion?’, this paper presents a new method (based on Alvers et al., 2013; 2014) for an automated interactive 3D-inversion. Thereby the methodology is, to let the user select a certain part of the model, thought to be wrong, by simply opening a box around it and start an automated inversion, which can be watched in (ideally) real time. The new approach is that now users can stop the automated inversion at any time, rewind if required, reset parameters (such as constraints) and rerun the process from there. The aim is to put the geo-expert in the driver’s seat to easily test geological and geophysical hypotheses. Based on a hybrid model (Schmidt et al., 2011) structure consisting of grids, voxel cubes and triangulated surfaces, complex models can be created and modified either interactively or by automated interactive inversion as described below.

Bigger, better, smarter: High performance computing applied to government geophysics

Government Geological Surveys study the Earth at the regional, province or national scale, and acquire vast volumes of technically complex data. These data must be high quality, fit for purpose, durable, and readily accessible and usable by industry. Increasingly, users require the geological information contained within the data as well as the data itself. High performance computers facilitate a step-change in advanced processing and modelling of large, complex data, and will help Government deliver more sophisticated products to industry. Data enhancement and manipulation are no-longer limited by the computational effort required, and there are no artificial limits to the size of the data or model, or the data resolution that can be processed. Geoscience Australia is collaborating with the National Computational Infrastructure facility (NCI) at the Australian National University to develop advanced methods for extracting the maximum geological information from large data volumes. The new methods include: Modelling of potential-field data in spherical coordinates to create continental-scale reference models of density and magnetic susceptibility; Inversion of magnetotelluric tensor data to a full 3D mesh of resistivities, and; Monte Carlo inversions of AEM responses to assess the reliability and sensitivity of conductivity-depth images. These algorithms are being implemented in a new Virtual Geophysical Laboratory where Government data and advanced processing methods are brought together in a single high performance computer environment.

Equivalent-Source from 3D Inversion Modeling for Magnetic Data Transformation

The well-known non-uniqueness in modeling of potential-field data results in an infinite number of models that fit the data almost equally. This non-uniqueness concept is exploited to devise a method to transform the magnetic data based on their equivalent-source. The unconstrained 3D magnetic inversion modeling is used to obtain the anomalous sources, i.e. 3D magnetization distribution in the subsurface. Although the 3D model fitting the data is not geologically feasible, it can serve as an equivalent-source. The transformations, which are commonly applied to magnetic data (reduction to the pole, reduction to the equator, upward and downward continuation), are the response of the equivalent-source with appropriate kernel functions. The application of the method to both synthetic and field data showed that the transformation of magnetic data using the 3D equivalent-source gave satisfactory results. The method is relatively more stable than the filtering technique, with respect to the noise present in the data.

Regional-scale geometry of the central Skellefte district, northern Sweden—results from 2.5D potential field modeling along three previously acquired seismic profiles

The Skellefte district in northern Sweden is one of the most important mining districts in Europe hosting approximately 80 volcanic massive sulfide (VMS) deposits. Due to its economical importance, geological and geophysical studies were carried out in order to create an image of the geometry of the upper crustal structure and integral geological elements and to evaluate their relationship to mineral deposits . Consequently, seismic reflection data along three sub-parallel profiles were acquired during 2009–2010 to map the spatial relationships between the geological structures down to a depth of ~ 4.5 km. Although these seismic studies helped researchers understand the regional relationship between geologic units in the central Skellefte district (CSD), the seismic reflection data did not succeed entirely in mapping the lithological contacts in the area. In this study, the model derived from the seismic reflection data was examined by using 2.5D modeling of potential field data (down to a 5 km depth) constrained by physical properties of the rocks and surface geology. Moreover, we modeled gravity and magnetic data along the non-reflective or poorly reflective parts of the seismic profiles to identify major lithological contacts and shear zones in the CSD, which could not be modeled on the basis of the seismic reflection data. Gravity and magnetic data helped reveal the spatial relationship between the Skellefte volcanic rocks, Vargfors group meta-sedimentary rocks and two meta­intrusive complexes. Results suggest a maximum depth extent of 2.1 km for the tectonic contact at the southern border of the Jörn granitoid . Furthermore, this north-dipping Skellefte–Jörn contact coincides closely with magnetic lows and gravity highs, which implies that the Jörn intrusive rocks have a greater thickness than the underlying basalt . Further to the NW, gravity and magnetic data suggest a depth extent of 2 km for the Gallejaur complex, which coincides with a set of gently dipping reflectors. In addition, this study supports previous concepts of fault geometries and fault patterns as a result of upper-crustal extension and subsequent inversion during crustal shortening . In the final model interpretations of the IP data were included, thus relating indications of mineralization to the geological structures. ► Potential field modeling along seismic profiles has successfully been applied. ► Model based on potential field data coincide with several of the seismic reflectors. ► IP results from the upper 0.43 km of the crust, highlights possible VMS deposits. ► Major granite intrusions, Jörn and Gallejaur, were modeled down to 1–2 km depth. ► Forward and inverse modeling of joint geophysical/geological data proved successful.

Locating a major Proterozoic crustal boundary beneath the Eastern Officer Basin, Australia

The Archaean to Mesoproterozoic basement of northern South Australia is almost completely overlain by thick Neoproterozoic and younger basins (≪1% outcrop), yet is likely to preserve an important record of the interactions between the Archean-Proterozoic Gawler Craton and the Proterozoic Musgrave Province during the amalgamation of Australia in the Proterozoic. However, constraints on the location and geometry of the boundary between these provinces are poor. We use potential field data to determine the 3D basement architecture and so constrain where this Palaeo-Mesoproterozoic boundary may be located beneath the Eastern Officer Basin. We establish the geometry and properties of the overlying basins and explicitly include them during forward and inverse modelling of potential field data to highlight the structure of the underlying basement. Our analysis identifies three crustal domains. (1) Southeast of the steep northeast–southwest Middle Bore Fault, positive gravity and magnetic anomalies are co-located and sourced from bodies in the upper crust, these bodies overlie middle to lower crust that is apparently uniform. (2) Between the Middle Bore Fault and the southern edge of the Munyarai Trough, the highest amplitude gravity and magnetic anomalies are not co-located and are sourced from large northwest dipping bodies. (3) To the northwest, the crust underlying the Munyarai Trough has similar properties to the Musgrave Province, suggesting that the Musgrave Province extends at least 50 km beneath the Eastern Officer Basin. Although details of the geology in the second (central) domain are poorly constrained, the domain preserves large crustal-scale Precambrian structures and is interpreted to mark the boundary between the Gawler Craton and the Musgrave Province. In particular, the multiply reactivated Middle Bore Fault forms a major crustal boundary and is interpreted to mark the northern limit of the Gawler Craton. The Middle Bore Fault may have formed as early as the Kimban Orogeny (∼1.7 Ga), although substantial reactivation and modification of the crustal architecture could have occurred between the Kimban Orogeny and intrusion of the cross-cutting Gairdner Dolerite Dykes (827 Ma).

Basin geometry and salt diapirs in the Flinders Ranges, South Australia: Insights gained from geologically-constrained modelling of potential field data

The Adelaide Basin in Australia is a complex of late Neoproterozoic to Early Cambrian rift and sag basins which was inverted during the Cambro–Ordovician Delamerian Orogeny. The deposition of evaporitic sediments during the earliest stage of basin development in the late Neoproterozoic (Willouran age) played a major role in the subsequent tectonic evolution of the basin. Previous studies have shown that early mobilization, vertical transport and withdrawal of the evaporites influenced the sedimentation during the late Neoproterozoic and Early Cambrian. The evaporites also influenced deformation during the inversion of the basin and the development of the Delamerian fold and thrust belt. However, the control exerted by basement structures in the deposition of the evaporitic beds and the role of these tectonic structures in the later inversion of the basin have been poorly constrained. In this work, we use a combination of published and original geological observations along with the interpretation of potential field datasets (total magnetic intensity and Bouguer anomaly data) to better constrain the geology of the basin at depth. We construct two and a half dimensional forward models of the potential field data along selected profiles across the Adelaide Basin. These models are constrained by the geology at the surface, drill hole data and measured petrophysical properties (specific gravity and magnetic susceptibility). We achieved the best fit between observed and modelled potential fields with a model favouring thick-skinned deformation, where the diapirs are not randomly distributed, but located directly above basement-penetrating normal faults. Furthermore, these normal faults were probably active during the sedimentation of the late Neoproterozoic and Early Cambrian sediments, and underwent partial or total inversion during the Cambro–Ordovician Delamerian Orogeny. Mobilisation and withdrawal of the evaporites was therefore initiated and facilitated by the extension coeval with the opening of the basin, and continued during the subsequent Delamerian Orogeny.

Modelling magnetic anomalies of solid and fractal bodies with defined boundaries using the finite cube elements method

SUMMARY The finite cube elements method (FCEM) is a numerical tool designed for modelling gravity anomalies and estimating structural index (SI) of solid and fractal bodies with defined boundaries, tilted or in normal position and with variable density contrast. In this work, we apply FCEM to modelling magnetic anomalies and estimating SI of bodies with non-uniform magnetization having variable magnitude and direction. In magnetics as in gravity, FCEM allows us to study the spatial distribution of SI of the modelled bodies on contour maps and profiles. We believe that this will impact the forward and inverse modelling of potential field data, especially Euler deconvolution. As far as the author knows, this is the first time that gravity and magnetic anomalies, as well as SI, of self similar fractal bodies such as Menger sponges and Sierpinsky triangles are calculated using FCEM. The SI patterns derived from different order sponges and triangles are perfectly overlapped. This is true for bodies having variable property distributions (susceptibility or density contrast) under different field conditions (in case of magnetics) regardless of their orientation and depth of burial. We therefore propose SI as a new universal fractal-order-invariant measure which can be used in addition to the fractal dimensions for formulating potential field theory of fractal objects.

Finite cube elements method for calculating gravity anomaly and structural index of solid and fractal bodies with defined boundaries

SUMMARY In this paper, we present the finite cube elements method (FCEM); a novel numerical tool for calculating the gravity anomaly g and structural index SI of solid models with defined boundaries and variable density distributions, tilted or in normal position (e.g. blocks, faulted blocks, cylinders, spheres, hemispheres, triaxial ellipsoids). Extending the calculation to fractal objects, such as Menger sponges of different orders and bodies defined by polyhedrons, demonstrates the robustness of FCEM. In addition, approximating the cube element by a sphere of equal volume makes the calculation of gravitation and related derivatives much simpler. In gravity modelling of a sphere, cubes with edges of 100 m and 200 m achieve a good compromise between running time and overall error. Displaying the distribution of SI of the studied models on contour maps and profiles will have a strong impact on the forward and inverse modelling of potential field data, especially for Euler deconvolution. For Menger sponges, plots of gravity elements g and its derivatives show similar patterns independent of fractal order. Moreover, both the pattern and magnitude of SI are independent of fractal order, allowing the use of SI as a new invariant measure for fractal objects. However, SI pattern and magnitude strongly depend on the depth to the buried bodies as do other elements In this study, we also present a new type of plot; the structural index against distance variation diagrams from which we extract the three critical SI (CSI) values, one per axis. The inversion of gravity anomaly data at CSI values gives the optimal mean location of the buried body.

Integrated geophysical appraisal of crustal architecture in the eastern Lachlan Orogen

Forward modelling of potential field data, combined with new geological mapping and deep seismic reflection transects acquired by the Australian Geodynamics Cooperative Research Centre (AGCRC), has led to testing of robust crustal architectural models for the eastern Lachlan Orogen of New South Wales. This integrated analysis has led to new subsurface inferences unlikely to be deduced solely from any of the individual datasets used.The overall Palaeozoic history of the eastern Lachlan Orogen implied by the integrated analysis of geophysical and geological datasets, involves: formation of an Ordovician oceanic island arc ;early Silurian thin-skinned east-directed overthrusting of a continental backarc basin over the arc; generation of Silurian granites due to crustal thickening; early Silurian to early Devonian rifting, resulting in the dismemberment of the deformed arc-basin complex, and formation of a series of arc fragments with intervening thin basins; tight, asymmetric folding of these basins during the Early to Mid Devonian, with some granite magmatism; extensive Carboniferous thin-skinned thrust stacking involving both east-, and west-directed transport, and slicing of the upper structural level of earlier antiformal stacks, together with granite roof zones; and final granitic arc magmatism during amalgamation with the New England Orogen.