Aeromagnetic Interpretation Research Articles

Estimating the Depth to the Basement of Anah and Its Surroundings Area, Western Iraq by Using the Source Parameter Imaging and Power Spectrum Techniques

The varying characteristics of the Precambrian basement rocks and the depth thickness variation of sedimentary cover are significant for geological structure. The Aeromagnetic data interpreted in order to delineate the depth of the basement of Anah and the surrounding areas by using Source Parameter Imaging Technique. Aeromagnetic data Total Magnetic Intensity were processed by using the Reduction to the pole, and Source Parameter Imaging tool which was applied to RTP data. The results of Source Parameter Imaging showed that the minimum depth of basement rocks in the study area was about 5500 m, this is located at the center and northeast side of the area, while the maximum basement depth was about 11000 m , this is located at the southeastern part. A comparison made between the current study results and the results of previous studies has been done with basement depth inferred from the aeromagnetic interpretation and particularly for three selected cities (Rawa, Anah, and Hadithah). The power spectrum method was used by applying a high pass filter with a cutoff value of 90 km wavelength In order to obtain the residual data, the depth to anomalies basement rock bodies were obtained by using the spectrum analysis technique with the help of GET grid software which used to transform data into spectra of two functions (amplitude and frequency). Each part of the spectrum curve can be selected, and then drawing a straight line, each straight line indicates the average depth of the anomaly.

A geophysically constrained crustal element map of East Antarctica between Enderby Land and Princess Elizabeth Land

East Antarctica along with Greater India played a vital role in the accretion and breakup of the Indo-Antarctic landmasses during the supercontinents Nuna, Rodinia and Gondwana. Without geophysical potential field methods, interpreting the architecture of the ice-covered geological provinces of Antarctica is impossible. We present here a crustal element map of East Antarctica between Enderby Land and Princess Elizabeth Land (Indo-Antarctica tectonic element) using aerogeophysical data interpretation. The data reveal distinct anastomosing geophysical provinces that correlate with sparse geological data. Our crustal element map shows the Oygarden Province and the Northern and Southern Rayner provinces are arcuate belts that wrap around the Archean Napier Province. These provinces represent the remnants of an accretionary tectonic margin, which evolved between ca 1300 Ma and 900 Ma. The arcuate geometry of these Meso- to Neoproterozoic provinces formed during the collision with the Napier Province, which represents a microcontinent. This collision triggered widespread extension and ultra-high temperature metamorphism in the Northern and Southern Rayner provinces. The southernmost provinces include the Fisher Province, Lambert Province and a transition zone. The provinces are truncated by a suture zone with the Archean Ruker Province, following north-dipping subduction during the Meso- to Neoproterozoic. Our interpretation provides a template upon which to correlate geological provinces with the terranes on the conjugate eastern Indian margin. KEY POINTS An aeromagnetic interpretation is given for Enderby Land and Princess Elizabeth Land of East Antarctica. Napier Province is a microcontinent that collided with the Rayner Province during a ca 1000 Ma orogenic event. A new interpretation of potential field data suggests anastomosing provinces accreted as part of a collisional event. New structures/piercing points are identified at the Mawson Coast and in Kemp Land.

Aeromagnetic Interpretation of Basement Structures and Geometry in Parts of the Middle Benue Trough, North Central, Nigeria

The research of an analysis of aeromagnetic data collected in the middle Benue Trough in north-central Nigeria is presented. A detailed analysis of basement structures is conducted in order to identify regions with high hydrocarbon potential that is different from those discovered by earlier researchers. Aeromagnetic data were filtered by using the Butterworth and Gaussian filters, transformed by engaging the reduction to the equator technique, and subsequently enhanced. To estimate magnetic basement depths at various places throughout the basin, the Euler deconvolution depth weighting approach was used. Eleven (11) sub-basins with depths ranging from –2000 m to –8000 m were also identified by Euler's findings. The sub-basins trend in the NE-SW direction while the average sediment thickness is found to be more than 3 km. The extracted structural features indicate areas like Kadi Blam and Kado areas in the southeastern part and Ogoja and Obudu in the southern part of the study area as regions with high structural densities. These areas coincide with the areas delineated as the sub-basins. The cross-sections generated reveal depressions caused by the action of some tectonic activities in the area. This study identified undulating basement topography believed to be due to tectonic activities as well as five areas that are possible targets for hydrocarbon exploration.

Field Observations and Geophysical Research Applied to the Detection of Manganese (Mn) Deposits in the Eastern Part of Oban Massif, South-Eastern Nigeria: An Integrated Approach

The growing need for an industrialized world, especially in Africa, cannot be feasible without adequate mineral resources. Thus, the search for more mineral deposits will continue to be necessary. An integrated approach involving geological mapping and a high-resolution geophysical (aeromagnetic) investigation was conducted to assess the manganese mineralization in parts of the Oban Massif, southeast Nigeria. The aeromagnetic data were processed using regional-residual anomaly separation techniques, first vertical derivative (1VD), analytical signals, source parameters imaging (SPI), and Euler deconvolution to better understand magnetic source distributions and their depths of occurrence. The geological investigation revealed a dominant variety of metamorphic rock types, including migmatitic (banded) gneisses hornblende granite gneisses, amphibolites, charnockites, and some quartzite ridges. Also present are some indications of pockets of dolerites. The study area also observed epithermal Mn+Fe+Qtz vein type mineralization associated with hydrothermal alteration zones whose orientation coincides with dominant structural orientation from aeromagnetic interpretation. Analysis of aeromagnetic data shows that the study area is dominated by ENE, NNE, and E-W structural directions (near-surface basement structures), with the ENE trends related to mineralization in the area. The manganese mineralization within Oban Massif is structurally controlled. The depths of the magnetic anomalies in the study area were estimated using SPI and Euler decomposition algorithms. SPI delineated the shallow, intermediate, and deep magnetic anomalies at 84–142 m, 152–200 m, and 215–656 m, respectively. Euler decomposition, however, revealed that shallow, intermediate, and deep depths occurrence of the magnetic anomalies are at 200–377 m, 393–472 m, and 499–793 m, respectively.

The Patos-Pernambuco shear system of NE Brazil: Partitioned intracontinental transcurrent deformation revealed by enhanced aeromagnetic data

The Neoproterozoic-Cambrian (Brasiliano) intracontinental Patos-Pernambuco shear system is developed in Archean to Neoproterozoic crust, reactivating rift margins N of the São Francisco Craton. We here use enhanced aeromagnetic images to reveal internal details of the broad E-W dextral Patos and Pernambuco shear zones, the intervening Central (Transversal) Domain and the adjacent Northern Domain. We estimate shear strains up to 50–60 and a total of ca. 350 and 200 km dextral displacement for the Patos and Pernambuco shear zones, respectively. Deflection of foliations, fold axial surfaces and plutons as well as internal S-C fabrics and asymmetric folds within these large shear zones confirm dextral displacement. However, the dominant population of NE-trending shear zones in the Central Domain, by many considered dextral, are here confirmed to be sinistral and thereby elongating this domain. The Northern Domain behaved differently, hence large-scale Ediacaran strain partitioning in the region is suggested, where different blocks experienced different deformation between major simple-shear dominated shear zones. We show how much more structural information can now be obtained from enhanced aeromagnetic data than from field mapping alone; aeromagnetic interpretations, nevertheless, still draw on analysis techniques epitomized by John Ramsay.

Redefining the boundary between crystalline and sedimentary rock of Eastern Dahomey Basin

This study defines a new boundary between the crystalline and sedimentary rocks of Eastern Dahomey Basin at the southwestern part of Nigeria using a geophysical approach that combines regional aeromagnetic and ground resistivity data. Aeromagnetic data covering the entire Eastern Dahomey Basin were acquired at 500 m line spacing, 80 m mean terrain, and processed into grids of Residual Magnetic Intensity (RMI) map. Filters and corrections such as upward continuation, and reduction to equator were applied to enhance deep magnetic sources and correct for magnetic inclination and declination. Tilt Derivative Angles (TDR) was applied for edge detection. To support the aeromagnetic analysis and interpretation, 104 Vertical Electrical Sounding (VES) surveys and 8 Electrical Resistivity Tomography (ERT) data were also acquired, processed and interpreted along the basement-sedimentary rock boundary. The TDR revealed a significant trend that corresponds to the edge between the basement complex and the sediments of the Eastern Dahomey Basin. A strong match was also noticed between the VES positions and the TDR map. Areas interpreted as basement rocks from the VES stations align with positive values on the TDR maps while the sedimentary terrains have negative TDR values. Our work demonstrates that areas that were previously fixed as sedimentary terrains on geological maps belong to the crystalline basement or transition zone. A new and reliable geological boundary is hereby drawn between the basement and sedimentary rocks. Thus, providing a revised map of the Eastern Dahomey Basin.

Aeromagnetic interpretation of basement structure and architecture of the Dahomey Basin, Southwestern Nigeria

ABSTRACT Significant yet untapped resources still abound in the Nigerian sector of the Dahomey Basin. Although the presence of an extensive Cretaceous Petroleum System has been confirmed following recent discoveries in Aje and Ogo Fields offshore Lagos, exploration outputs in the Dahomey Basin has so far not been encouraging. Proper understanding of the basement architectural framework and controls on tectonic development remains key to unlock the unrealised potentials in the basin. Hence, a geophysical interpretation of the basement structure and architecture of the Dahomey Basin southwestern Nigeria has been carried out in this study. Various edge enhancement techniques were applied to the high-resolution residual magnetic intensity (HRRMI) grid of the area. This includes first vertical derivative (FVD), total horizontal derivative (THDR), tilt derivative (TDR) and total horizontal derivative of upward continuation (10 km). Determination of the depth to magnetic sources and sedimentary thicknesses in the study area were achieved using standard Euler deconvolution and source parameter imaging (SPI) techniques, with depth range of 4.5–6.3 km attained in the two identified sub-basins located offshore of the study area. Lineament analysis gave insights on the tectonic trends and stress-field orientation in the basin with major trends in the NNE-SSW, NE-SW, NW-SE, and WNW-ESE directions. 2D forward modelling of some selected profiles was employed to characterise the basement pattern and architecture, which depicted a horst-graben architecture. The basement structure and architecture have a major control on the distribution of sub-basins, petroleum systems elements and trap styles in the basin. The study demonstrates the robust application of high-resolution aeromagnetic data in basin-wide mapping of regional subsurface geological features, basement architecture and determination of sedimentary thickness in a frontier basin.

A geophysical investigation of the 2018 Lake Muir earthquake sequence: reactivated Precambrian structures controlling modern seismicity

Seismicity in the intraplate southwest of Western Australia is poorly understood, despite evidence for potentially damaging earthquakes of magnitude > M6. Identifying stress-focusing geological structures near significant earthquake sequences assists in understanding why these earthquakes occur in seemingly random locations across a region of more than 250 000 km2. On 16 September 2018, an ML5.7 earthquake occurred near Lake Muir in the southwest of Western Australia and was followed by an ML5.4 aftershock. The main earthquake formed a mainly north-trending fault scarp ∼5 km in length and with a maximum vertical displacement of ∼40 cm. The main event was followed by a series of aftershocks, one of which had a magnitude of ML5.4. Using high-resolution aeromagnetic data, we analyse bedrock geology in a wide area surrounding the new scarp and map a series of major ∼ east–west-trending faults segmenting eight distinct geological domains, as well as a network of less prominent northwest-trending faults, one of which aligns with the southern segment of the scarp. Surface faulting, surface deformation and earthquake focal mechanism studies suggest movements on north- and northeast-trending structures. The main shock, the aftershocks, surface faulting and changes in InSAR-derived surface elevation all occur in a region bounded to the south by a prominent northwest-trending fault and to the north by a west-northwest-trending domain-bounding structure. Thus, we interpret the north-trending thrust fault associated with the main Lake Muir event as due to local stress concentration of the regional east–west stress field at the intersection of these structures. Further, we propose that a particularly large west-northwest-trending structure may be broadly focusing stress in the Lake Muir area. These findings encourage similar studies to be undertaken in other areas of Australia’s southwest to further the current understanding of seismic release in the region. KEY POINTS A large ML5.7 earthquake occurred in southwest Western Australia in September 2018, damaging infrastructure and forming a 40 cm-high thrust fault scarp. An aeromagnetic interpretation of the Lake Muir region revealed that the new scarp aligned with pre-existing faults that were reactivated. Local and regional structures likely acted as regional stress focusers in the Lake Muir area. Identifying regional stress-focusing structures is important for the purpose of understanding seismic risk in intraplate areas such as Western Australia.

ADVANCES ON THE GEOLOGY AND EVALUATION OF POTENTIAL PETROLEUM SYSTEMS OF THE SOKOTO BASIN IN NW NIGERIA

The stratigraphy of the Sokoto Basin has the Illo/Gundumi Formation at the bottom, followed successively upward by the Taloka, Dukamaje, Wurno, Dange, Kalambaina, Gamba and Gwandu Formations. Re-mapping of the basin carried out in this study shows that the geological framework remains largely as previously outlined except that some hitherto unreported tectonically controlled structures have been documented. The basin is generally shallower at the margin and deepens towards the centre such that the areas around the border with Niger Republic are deepest and hence most prospective on the Nigerian side. Geophysical aeromagnetic interpretation has assisted to analyze the depth to basement configurations. Organic geochemical studies show that the dark shales and limestones of the Dukamaje Formation constitute the source rocks in the potential petroleum system. With averages for source rock thickness of 50m, area of basin of 60,000km2, TOC of 7.5wt%, and HI of 212mgHC/gTOC, charge modeling indicates 808.10 million barrels of oil equivalent extractable hydrocarbons in the Sokoto Basin, at current knowledge of the geology and if the appropriate maturity has been attained at deeper sections. The sandstones of the Illo/Gundumi Formation as well as in the Taloka and Wurno Formations and carbonates of the Kalambaina Formation provide potential reservoir packages. The paper shale of the Gamba Formation and the clays of the Gwandu Formation provide regional seals. If the presently mapped tectonic structures are ubiquitous in the whole basin, structural and stratigraphic traps may upgrade the petroleum system. Other petroleum systems may exist in the basin with either or both the Illo/Gundumi and Taloka Formation(s) providing the source and reservoir rocks. Keywords: Sokoto Basin, Dukamaje Formation, Hydrocarbons, Petroleum System, Reservoirs

Interpretation of Aeromagnetic Data to Investigate Crustal Structures of the Contact Congo Craton - Pan-African Belt at the Eastern Cameroon

The collision between the Congo Craton and the Pan African fold belt of Central Africa had great impacts on the geological and tectonic points of view, notably the installation of several tectonic accidents such as faults, fractures, dikes, folds, domes. This aeromagnetic study is based on Paterson's aeromagnetic data interpretations through the use of multiple operators. These data were processed by Oasis Montaj software. The total magnetic intensity map reduced to the equator (RTE-TMI) shows important anomalies features the major important regional anomalies. Maps of the vertical gradient, analytical signal and tilt angle maps have meanwhile highlighted several short wavelength anomalies assimilated to folding, dykes, fractures or faults. The map of maxima upward to 2 km allowed to establish the structural map of the study area. It turns out that the different types of geological accidents follow ENE-WSW, ESE-WNW, NE-SW, NW-SE and even E-W and N-S directions. All these directions are very similar to the geological history of the area. Anything that seems to confirm that the study area was the scene of intense tectonic movements resulting from the collision between the Congo Craton and the Central Africa Fold Belt.

A Short Note on the Petroleum Potential of the Sokoto Basin in North-western Nigeria

The stratigraphy of the Sokoto Basin has the Illo/Gundumi Formation at the bottom, followed successively upward by the Taloka, Dukamaje, Wurno, Dange, Kalambaina, Gamba and GwanduFormations. Re-mapping of the basin carried out in this studyshows that the geological framework remainslargely as previously outlined except that some hitherto unreported tectonically controlled structures have been documented. The basin is generally shallower at the margin and deepens towards the centre such that the areas around the border with Niger Republic are deepest and hence most prospective on the Nigerian side. Geophysical aeromagnetic interpretation has assistedto analyze the depth to basement configurations. Organic geochemical studies show that the dark shales and limestones of the Dukamaje Formation constitute the source rocks in the potential petroleum system. With averages for source rock thickness of 50m, area of basin of 60,000km², TOC of 7.5wt%, and HI of 212mgHC/gTOC, charge modeling indicates 808.10 million barrels of oil equivalent extractable hydrocarbons in the Sokoto Basin, at current knowledge of the geology and if the appropriate maturity has been attained at deeper sections. The sandstones of the Illo/Gundumi Formation as well as in the Taloka and Wurno Formations and carbonates of the Kalambaina Formation provide potential reservoir packages. The paper shale of the Gamba Formation and the clays of the Gwandu Formation provide regional seals. If the presently mapped tectonic structures are ubiquitous in the whole basin, structural and stratigraphic traps may upgrade the petroleum system. Other petroleum systems may exist in the basin with either or both the Illo/Gundumi and Taloka Formation (s) providing the source and reservoir rocks.

Updates on the Geology and Potential Petroleum System of the Bida Basin in Central Nigeria

The Bida Basin is one of Nigeria’s inland frontier basins. The conventional stratigraphy of the Bida Basin has the Bida Formation at the base in the northern sector, followed successively upward by the Sakpe, Enagi, and Batati Formations. In the southern sector, the Lokoja Formation is at the base, followed by the Patti Formation and ending with the Agbaja Formation at the top. Detailed field mapping carried out in this study has not identified the Batati Formation at the previously assigned Type Locality at Batati village. A new formational name of “Pattishabakolo Formation” has been proposed. The Patti Formation is also not mappable at the previously assigned Type Locality of the Mount Patti in Lokoja but mappable at Gegu, Ahoko and midway on the Agbaja Plateau. A new Type Locality at Ahoko has been proposed. Sediment thicknesses in the Bida Basin are shallower at the margin and generally deepen towards the centresuch that the central portions constitute the most prospective areas. Geophysical aeromagnetic interpretation has assisted in the interpretation of the geology of the basin. Organic geochemical studies show that the Kudu Shale in the Northern Bida Basin equivalent to the Ahoko Shale in the Southern Bida Basin constitutes the source rocks in the potential petroleum system. With averages for source rock thickness of 40m, area of basin of 45,000km2, TOC of 9.0wt%, and HI of 220mgHC/gTOC, charge modeling indicates 623 million barrels of oil equivalent extractable hydrocarbons in the Bida Basin, at current knowledge and if the appropriate maturity has been attained at deeper sections. The Bida/Lokoja Formation sandstones as well as the well sorted sandstones in the Enagi Formation constitute potential reservoirs in the basin. Regional seals are provided by the clayey members of the Enagi and Batati Formations. Potential traps are both structural and stratigraphical.

Qualitative aeromagnetic interpretation over Ikogosi Warm Spring, southwestern Nigeria

In this paper, lineaments of Ikogosi warm spring and its environs have been delineated using qualitative methods. Boundary structures or lineaments between magnetic units were delineated using the first horizontal derivative map and skeletonization of derivative-based data. Also, the residual anomaly contour map was digitized into ten profiles, profiles’ plots were made and used for interpretation. Dominant lineaments suspected were elongated features trending North East-South West direction and are housed by the shallow basements beneath the study area and surroundings.

Use of immobile trace elements in gold exploration in the Neoarchean Sandstone Greenstone Belt, Yilgarn Block, Western Australia

Exploration in the Sandstone Greenstone Belt (SSGB), with its widespread complex regolith cover, is extremely difficult as targeted orogenic gold deposits have no consistent parameters that allow direct geophysical detection, and gold and ore-metal surface geochemistry reflects variations from in situ to highly transported responses. Based on aeromagnetic interpretations, the gold deposits of the SSGB can be viewed within broad litho-structural frameworks, and NNW-NW and NNE-NE–trending domains, with more favourable orientation with respect to the inferred principal maximum stress during the time of gold mineralisation, can be defined. Fault jogs and high-angle corridors of cross faults further constrain more-prospective zones. However, the aeromagnetic data cannot identify relatively small volumes of critical host rocks which can localize ore-fluid flux and fluid-rock reactions that control the deposition of gold mineralisation. In the SSGB, these critical host rocks include thin granodiorite intrusions within ultramafic schists or specific mafic bodies in mafic-ultramafic sequences. A reconnaissance study using the immobile trace elements, Ti and Zr, demonstrates that these critical rock types can be readily recognized at surface and in drill chips. In particular, Ti–Zr geochemistry identifies previously unknown 20-km-long differentiated dolerite units with granophyric zones that localize gold mineralisation. A combination of aeromagnetic interpretation of structures and major Domains, plus the location of critical host units using Ti–Zr geochemistry, allows production of an exploration targeting map for this district with definition of priority targets based on a mineral system approach. Thus, in the SSGB, for gold exploration under regolith cover that represents in situ weathering of those host rocks, rather than later cover sequences, immobile trace element geochemistry provides an effective, low-cost methodology, as an adjunct to conventional ore-metal geochemistry, and should be used in other orogenic gold provinces worldwide.

Aeromagnetic data interpretation of east Qattara Depression, Northwest Desert, Egypt

The study area is located at the east of Qattara Depression at the north of the Western Desert of Egypt. The study area contains Abu Gharadig basin, which is the most petroliferous basin in the Egyptian Western Desert. Only three exploratory wells are presented in the study area, showing a thick sediment section overlying basement rocks. Magnetic data have been frequently used in geophysical exploration. Aeromagnetic data are mainly utilized to estimate the depth to the magnetic basement as well as to delineate the possible structures of the study area. The depth to magnetic basement has been estimated using the analytical solution of exponential equations obtained from the Fourier transformation of magnetic data, assuming multi-prisms. The depths obtained from this technique vary from 0.70 to 2.91 km with an average depth of 2.08 km. Local phase filters have been mainly used as edges detector where the possible occurrences structures can be delineated. Hyperbolic tilt angle, second-order tilt angle, and normalized total horizontal derivative (TDX) provide the best results for delineating the possible structures, showing the possible contacts within the basement of the study area. The edge enhancement filters show that the study area has been affected by different structural trends taking E-W, NE-SW, NNE-SSW, N-S, and ENE-WSW directions.

Integration of Aeromagnetic Interpretation and Induced Polarization Methods in Delineating Mineral Deposits and Basement Configuration within Southern Bida Basin, North-West Nigeria

Aeromagnetic data interpretation and Induce Polarization (IP) method has been integrated and interpreted with a view of delineating magnetic minerals within some part of the Southern Bida Basin, Nigeria. Spectral analysis method was used in delineating the depth to magnetic basement and model prominent magnetic anomaly. Analysis of Induce Polarization data were used in delineating the potential fractures zones of mineralization. Visual inspection of the magnetic anomalies and first vertical derivative maps reveals that the area is highly faulted with major faults trending East-West (E-W) and minor ones Northeast-Southwest (NE-SW) directions. The qualitative interpretation results of both the resistivity and chargeability pseudosections reveal potential fractures zones trending East-West (E-W) and the depth to the anomalous body ranges from (0.3-2.5 km) with average overburden thickness of 2.1 km. Two depth source models were interpreted using Discrete Fourier Transform method (spectral analysis) namely; the shallower sources which range from 0.45 to 1.49 km and the deeper ones which range from 1.81 to 3.24 km. The quantitative interpretation of the aeromagmatic data, depict that the average sedimentary thickness ranges from (2.3-3.2 km) and the average depth to the Curie isotherm in the area is 24.76 km. The result also shows that the Curie temperature isotherm within the basin is not a horizontal level surface, but is undulating. The regional average results for both geothermal gradient and heat flow across the study area are 23.07°C/km and 57.66 W/m2 respectively. Based on the computed sedimentary thicknesses (2.3-3.2 km), the geothermal gradient (22.27 and 37.00°C/km.) and the prevalent fractures, the possibility of hydrocarbon accumulation in the northern and southeastern part of the study area is feasible whereas other parts of the study area with low sedimentary thicknesses will favour magnetic mineral deposits such as the prevalent oolitic iron ore deposits at Agbaja and Kotonkarifi axes.

Preliminary interpretations from the 2015 Coompana aeromagnetic survey

The Coompana airborne survey is a large-scale pre-competitive aeromagnetic and radiometric survey in western South Australia flown in 2015. The survey was undertaken in order to provide greater geological controls on the basement geology in this area, which lies beneath the cover sequences of the Eucla, Bight, Denman and Officer Basins. This survey covers much of the South Australian part of the Coompana Province, which includes the Coompana Magnetic Anomaly. In this preliminary interpretation of the aeromagnetic survey, we have distinguished five basement domains based on the magnetic data and attempted to differentiate where domain boundaries are structural or intrusive. Integrating the aeromagnetic interpretation with existing drillhole and outcrop constraints suggests that the western part of South Australia had a long-lived, dynamic geological history with at least four major rock-forming events identified in the Coompana Province: c. 1610 Ma, c. 1500 Ma, c. 1180 Ma and c. 860 Ma. Detailed interpretation of the 200 m line-spaced infill region has enabled contact and relative age relationships of the intrusive bodies causing the Coompana Magnetic Anomaly. We suggest a multi-phase intrusive complex, with an early deep disc-like intrusion, followed by pipe-like satellite bodies intruding higher into the crust. The relative age of this intrusive complex is interpreted to be between c. 1120 Ma and c. 860 Ma.

Aeromagnetic interpretation in the south-central Zimbabwe Craton: (reappraisal of) crustal structure and tectonic implications

Regional aeromagnetic data from the south-central Zimbabwe Craton have been digitally processed and enhanced for geological and structural mapping and tectonic interpretation integrated with gravity data, to constrain previous interpretations based on tentative geologic maps and provide new information to link these structural features to known tectonic events. The derived maps show excellent correlation between magnetic anomalies and the known geology, and extend lithological and structural mapping to the shallow/near subsurface. In particular, they reveal the presence of discrete crustal domains and several previously unrecognised dykes, faults, and ultramafic intrusions, as well as extensions to others. Five regional structural directions (ENE, NNE, NNW, NW, and WNW) are identified and associated with trends of geological units and cross-cutting structures. The magnetic lineament patterns cut across the >2.7 Ga greenstone belts, which are shown by gravity data to be restricted to the uppermost 10 km of the crust. Therefore, the greenstone belts were an integral part of the lithosphere before much of the upper crustal (brittle) deformation occurred. Significantly, the observed magnetic trends have representatives craton-wide, implying that our interpretation and inferences can be applied to the rest of the craton with confidence. Geological–tectonic correlation suggests that the interpreted regional trends are mainly 2.5 Ga (Great Dyke age) and younger, and relate to tectonic events including the reactivation of the Limpopo Belt at 2.0 Ga and the major regional igneous/dyking events at 1.8–2.0 Ga (Mashonaland), 1.1 Ga (Umkondo), and 180 Ma (Karoo). Thus, their origin is here inferred to be inter- and intra-cratonic collisions and block movements involving the Zimbabwe and Kaapvaal Cratons and the Limpopo Belt, and later lithospheric heating and extension associated with the break-up of Gondwana. The movements produced structures, or reactivated older fractures, that were exploited by Late Archaean and Proterozoic mafic intrusions. There was interplay between vertical and horizontal tectonics as seen in similar terrains worldwide.

Interpretation of tectonic setting in the Phetchabun Volcanic Terrane, Northern Thailand: Evidence from enhanced airborne geophysical data

Re-processed aeromagnetic data with enhancement approaches of reduction to the pole, high pass filtering and shaded relief have been used to interpret complex subsurface structures of the Carboniferous to Triassic Phetchabun Volcanic Terrane (PVT) which is largely covered by thick Cenozoic sediment deposits. Interpretation of the enhanced aeromagnetic data reveals four distinct structural domains in the PVT, viz. Northern, Eastern, Central, and Western domains. Within these domains, high magnetic units are recognized, namely elongate, ring, circular, and dipolar spot units. The elongate unit in the Central domain is characterized by a deformation zone with northwest–southeast trending, sinistral shearing. East–west trending and the northeast–southwest trending faults cross-cut several magnetic units in the Central domain, with sinistral and dextral movements, respectively. Three major fault directions have been identified, including the northeast–southwest trending sinistral faults, north–south trending dextral faults, and northwest–southeast trending dextral faults. The younger spot units are small intrusive bodies largely situated along these latest fault segments.The aeromagnetic interpretation results, together with relevant current field verification, as well as previous geochronological and petrochemical investigations, have lead to the clarification of structural development in the PVT. The elongate units are interpreted to represent Late Carboniferous intrusive bodies. They occurred as a result of an eastward subduction of the Nakhonthai oceanic plate beneath the Indochina continental plate, along the Loei suture. The elongate units are also reflected in a north–south trending deformation zone formed by the east–west compressional tectonics. The ring units are considered to have formed in a Permo-Triassic volcanic arc, whereas the circular units represent equigranular intrusive bodies which formed in a response to the second phase of eastward subduction. East–west extensional tectonics formed during the Late Triassic period is inferred to overlapp with subsidence along the Permo-Triassic volcanic arc. The magnetic spot units represent shallow, porphyritic intrusive bodies occurring along downthrown fault zones during this stage.

Extracting ore-deposit-controlling structures from aeromagnetic, gravimetric, topographic, and regional geologic data in western Yukon and eastern Alaska

Aeromagnetic lineaments interpreted from reduced-to-pole (RTP) magnetic grids were compared with gravity, topography, and field-based geologic maps to infer regional structural controls on hydrothermal mineral occurrences in a poorly exposed portion of the North American Cordillera in western Yukon and eastern Alaska. High-frequency and variable-intensity aeromagnetic lineaments corresponding to discontinuities with an aeromagnetic domain change were interpreted as steep-dipping and either magnetite-destructive or magnetite-additive faults. These structures were interpreted to be predominantly Cretaceous in age and to have formed after the collision of the Intermontane terranes with the ancient Pacific margin of North America. To demonstrate the reliability of the aeromagnetic interpretation, we developed a multidata set stacking methodology that assigns numeric values to individual lineaments depending on whether they can be traced in residuals and first vertical derivative of RTP aeromagnetic grids, isostatic residual gravity grids, digital topography, and regional geologic maps. The sum of all numeric values was used to estimate the likelihood of the aeromagnetic lineament as a true geologic fault. Fault systems were interpreted from zones of lineaments with high spatial density. Using this procedure, 10 major northwest-trending fault systems were recognized. These were oriented subparallel to the regional Cordilleran deformation fabric, the mid-Cretaceous Dawson Range magmatic arc, and well-established crustal-scale dextral strike-slip fault systems in the area. These orogen-parallel fault systems were interpreted to play a structural role in the emplacement of known porphyry Cu-Au and epithermal Au systems of mid-Cretaceous (115–98 Ma) and Late Cretaceous (79–72 Ma) age. The procedure also identified seven northeast-trending, orogen-perpendicular fault-fracture systems that are prominent in eastern Alaska and exhibit sinistral-to-oblique extensional kinematics. These structures were interpreted to govern the emplacement of Late Cretaceous (72–67 Ma) porphyry Mo- and Ag-rich polymetallic vein and carbonate replacement systems in the region.