Aeromagnetic Signatures Research Articles

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.

Insights into geological evolution of Princess Elizabeth Land, East Antarctica-clues for continental suturing and breakup since Rodinian time

Svenner Islands-Brattstrand Bluffs-Larsemann Hills constitutes ~70 km long coastal outcrops of Princess Elizabeth Land (PEL), comprising complexly deformed metapelites and orthogneisses. Pelitic granulites from these outcrops are investigated in this work. Conventional geothermobarometric estimations and Pseudosection modelling consistently indicate that the peak metamorphic grade throughout the area is high to ultra-high temperature (800–950 °C) at low to medium pressure (2–5 kbar). A high pressure (~10 kbar) relict metamorphic event and a substantial decompression component of ~5 kbar, corresponding to >15 km uplift, are inferred through petrographic as well as pseudosection analysis. Two set of ages are estimated (~800 Ma and ~500 Ma), corresponding to Tonian and Pan-African metamorphic events, respectively. Field data, petrographic studies and ages estimated from orthogneisses from the Brattstrand Bluffs and the Grovnes Peninsula suggest that this unit is a product of in-situ melting of the pelitic granulites.Pelitic granulites of PEL possess similarities with those exposed in the Eastern Ghat Mobile Belt of India. We propose that these sectors represent a contiguous terrane with two major orogenic imprints, reflecting Rodinia and Gondwana amalgamations. An attempt is made to mark out paleo-orogenic belt axes, supported by both field as well as recent aero-magnetic signatures in interior PEL. We support that the parent sediments of the pelitic granulites were deposited during Stenian period, which underwent compressional UHT-HP(?) metamorphism at ~800 Ma. Another extensive basin is proposed at ~600 Ma prior to the Pan-African orogenic event. We propose that the Pan-African orogeny marked the collision of Indo-Australo-Africo-Antarctic cratons and stitched the East Gondwana. We also propose a thinned lithosphere along the system of subglacial lakes-canyons confirmed by ICECAP data. Analog modelling is used to demonstrate the influence of pervasive mechanical anisotropy of the basement in defining the orientation of this rift system and its connection to the Lambert Graben.

A reassessment of the Archean-Mesoproterozoic tectonic development of the southeastern Chhattisgarh Basin, Central India through detailed aeromagnetic analysis

We constrained the geological framework over polydeformed Paleoproterozoic Sonakhan Greenstone Belt and addressed the tectonic evolution of Singhora basin in the fringes of Bastar Craton, central India by utilizing aeromagnetic data interpretation, 2.5D forward modelling and 3D magnetic susceptibility inversions. The Sonakhan Greenstone Belt exposes volcano-sedimentary sequences of the Sonakhan Group within NNW-SSE to NW-SE trending linear belts surrounded by granite gneisses, which are unconformably overlain by sedimentary rocks of Chhattisgarh Basin. The orientations of aeromagnetic anomalies are coincident with geological trends and appear to correlate with lithology and geologic structure. Regional magnetic anomalies and lineaments reveal both NNW-SSE and NE-SW trends. Prominent E-W trending linear, high amplitude magnetic anomalies are interpreted as the Trans-Chhattisgarh Aeromagnetic Lineament (TCAL).NW-SE trending aeromagnetic signatures related to Sonakhan Greenstone Belt extends below the Singhora sedimentary rocks and forms the basement in the west. The analysis suggests that TCAL is a block fault with northern block down-thrown and affected the basement rocks comprising the Sonakhan Greenstone Belt and Samblapur Granitoids. The episode of faulting represented by the TCAL is pre-Singhora sedimentation and played a vital role in basin evolution. The basement configuration image generated by estimates of depth to magnetic basement suggests a complex pattern of NNE-SSW to NE-SW trending depressions separated by a linear N-S trending basement ridge. It is inferred from the 3D magnetic susceptibility inversion that the thickness of sediments is more towards the eastern basin margin and the N-S ridge is a manifestation of post sedimentary faulting. Results of 2.5D modelling of a WNW-ESE profile across the Singhora Basin combined with results from 3D inversion suggest suggests the basin subsidence was controlled by NE-SW trending regional faults in an active system. The basin geometry evolved by E-W block faulting overprinted by NE-SW trending pre- to syn-depositional normal faults generating NE-SW depression, which are affected by N-S trend post-sedimentary faulting. Though the present work relates the basin evolution with the initiation of rift basin, it warrants further work to establish the deformation within the basin pertaining to the proximal thrust and uplift along the craton fringe.

Tracing Archaean terranes under Greenland's Icecap: U–Th–Pb–Hf isotopic study of zircons from melt-water rivers in the Isua area

The Archaean gneisses of the Nuuk area (southern West Greenland) are partitioned into tectonostratigraphic terranes – blocks of arc-like crust that evolved independently until they coalesced by collisional orogeny. To ‘map’ terranes to the east under the Inland Ice, sand samples were taken from rivers issuing from the edge of the Icecap; three from the Isua area and one from ∼20km to the south. Bedrock along this part of the ice front consists of ∼3820–3600Ma amphibolite facies rocks. 40km south of Isua (Kapisilik terrane in the Ivisaartoq area) and also from ∼10km to the north there are Mesoarchaean amphibolite facies gneisses (3070–2950Ma) exposed at the ice front. In the moraine fields in the Isua area there are erratic blocks of granulite facies gneisses. These were sourced from a hidden terrane to the east under the ice because no such rocks are exposed in the Isua area.The majority of the zircons from the sand samples yielded close to concordant U–Pb ages. Apart from one 2414Ma grain, all are Archaean. The Isua sands show ∼2695, 2710 and 2730Ma; then 2790, 2805 and 2840Ma clusters with a few grains back to 2950Ma and then a complex 3440–3960Ma spectrum. Less than 1% of the grains have ages between 3000 and 3400Ma. In the Isua sands, Neoarchaean and late Mesoarchaean zircons form >50% of the population. The sand 20km to the south shows a similar span of zircon ages, but there is a 2960Ma peak not seen in the Isua sands.As the Eoarchaean and late Mesoarchaean–Neoarchaean cycles progress, the spread of Th/U and ɛHf values seen in each zircon age population increases. This is interpreted as repeated addition of juvenile material to the crust, but also with increasing amounts of high temperature tectonothermal recycling of older materials within a package. However, there is no isotopic evidence for any contribution of Eoarchaean crust to the later ≥2750Ma Archaean cycle(s), showing evolution in disparate terranes. Furthermore, no evidence for Hadean material beneath the ice is seen in either the U–Pb ages or Hf isotopic compositions of the detrital zircons. On the other hand, 2750–2500Ma metamorphic zircons show a spread of initial ɛHf values of zero to −25, with the most negative indicating zircon growth in Eoarchaean rocks. Overall these results closely mirror those obtained from direct sampling of rocks of the Archaean terranes exposed beyond the Icecap.The zircon data indicate that Mesoarchaean rocks equivalent to the Kapisilik terrane are absent under the ice. Instead, zircons were sourced from a late Mesoarchaean to Neoarchaean terrane affected by high grade metamorphism, whose age spectra perfectly matches the Tasiusarsuaq terrane to the southeast of Nuuk. Guided by the zircon results and aeromagnetic signatures, a terrane map is presented that extrapolates the Tasiusarsuaq terrane northwards under the Inland Ice towards the Tuno terrane in the north of the craton. Implications for different syntheses on the evolution of Archaean crust in southern West Greenland are discussed.

A preliminary regional geothermal assessment of the Gulf of Suez, Egypt

The tectonic setting of Egypt, in the northeastern corner of the African continent, suggests that it may possess significant geothermal resources, especially along its eastern margin. The most promising areas for geothermal development in the NW Red Sea-Gulf of Suez rift system are locations along the eastern shore of the Gulf of Suez that are characterized by surface thermal manifestations, including a cluster of hot springs with varied temperatures. The Gulf of Suez region is one of the most interesting geothermal areas in Egypt because of the high temperatures of its springs. Geothermal studies were performed based on bottom-hole temperature logs of 103 offshore oil wells in the Gulf of Suez. The eastern margin of the Gulf of Suez, especially in the vicinity of hot springs, is characterized by high temperature gradients and heat flow values of more than 100 mW/m 2. Geophysical studies, which characterized the gravity and aeromagnetic signatures of the Gulf of Suez, were used to investigate the origin of the high heat flow and to constrain the depths of the heat sources in the hottest areas. Based on these data, conceptual and numerical models of hydrothermal systems were developed of the Hammam Faraun and Hammam Musa hot springs, which are the hottest springs in Egypt. The modeling results show that the origin of the high heat flow is the uplifted basement rocks.

Comparison of ground TEM and VTEM responses over kimberlites in the Kalahari of Botswana

Kalahari cover greater than 60 m thick precludes indicator sampling from identifying individual kimberlite intrusions in the Kokong kimberlite field of Botswana. Employing concurrent multidisciplinary geophysical surveying methods is the exploration technique for the discovery of kimberlites. Time Domain Electromagnetic (TEM) surveying is an effective technique for the prioritization of aeromagnetic and gravity signatures. Diagnostic discrete conductive TEM signatures are usually returned from kimberlites overlain by up to 120 m of Kalahari cover. Following the success of kimberlite identification by ground TEM, a VTEM helicopter time domain survey was flown over the Kokong field to increase the TEM coverage in this prospective kimberlite terrain. A comparison of responses over covered kimberlites traversed by both TEM survey methods and their comparative effectiveness for kimberlite identification is presented.

Aeromagnetic exploration over the East Antarctic Ice Sheet: A new view of the Wilkes Subglacial Basin

The Wilkes Subglacial Basin represents an approximately 1400 km-long and up to 600 km wide subglacial depression, buried beneath the over 3 km-thick East Antarctic Ice Sheet. Contrasting models, including rift models and flexural models, have been previously put forward to explain the tectonic origin of this enigmatic basin, which is located in the largely unexplored hinterland of the Transantarctic Mountains. A major aerogeophysical survey was flown during the 2005–06 austral summer to explore the Wilkes Subglacial Basin. Our new airborne radar dataset reveals that the Wilkes Subglacial Basin contains several subglacial basins, which are considerably deeper than previously mapped. Major aeromagnetic lineaments are detected from total field, pseudo-gravity, tilt derivative and Euler Deconvolution maps. These aeromagnetic lineaments reveal that the Wilkes Subglacial Basin and its sub-basins are structurally controlled. Comparison between aeromagnetic signatures over the Wilkes Subglacial Basin region and the Cordillera in North America, suggests that the basin contains a former broad backarc basin and fold-and-thrust belts, forming the transition between a Precambrian craton and the Ross Orogen. The eastern margin of the Wilkes Subglacial Basin is imposed upon the Prince Albert Fault System and the Priestley Fault. These faults may have been reactivated in the Cenozoic, as major strike–slip faults. The western margin of the Wilkes Subglacial Basin is located along the southern extension of the Precambrian-age Mertz Shear Zone and marks the edge of the Terre Adélie Craton. High-frequency aeromagnetic anomalies in the Wilkes Subglacial Basin image large volumes of Jurassic tholeiites, which were intruded into and extruded over Beacon sediments in a possible rift setting. Depth-estimates of magnetic anomaly sources and forward modelling indicate that major Cretaceous and Cenozoic rift basins with thick sedimentary infill, comparable to the deep Ross Sea Rift basins, are however unlikely beneath this part of the Wilkes Subglacial Basin. More localised graben-like features, with up to 1.5 km of sedimentary infill are identified in the Central Basins. These grabens may be transtensional features related to regional Cenozoic intraplate strike–slip deformation, which has been extensively mapped over the adjacent Transantarctic Mountains, and/or older Cretaceous grabens. Over 3 km deep sedimentary basins are also imaged beneath the Western Basins and are inferred to contain older sedimentary infill of Ross-age, based on recent dating of glacial erratics between Mertz and Ninnis Glacier.

A regional 1.92 Ga tectonothermal episode in Ostrobothnia, Finland: Implications for models of Svecofennian accretion

The mostly metasedimentary Svecofennian Western Pohjanmaa belt in Ostrobothnia, Finland, can be divided into two stratigraphic groups separated by a major unconformity that reflects deformation following regional metamorphism. The western Lappfors group, interpreted as a Svionian basement complex, has strong W-trending folding and aeromagnetic signatures that contrast with the overlying eastern Evijärvi group, interpreted as lower Bothnian, which has more open N-trending folding and magnetic patterns. Several lines of evidence date the unconformity at ∼1.92 Ga. Detrital zircons from two samples of Lappfors group metasediment, and a sample of the basal Nivala gneisses in the Eastern Pohjanmaa belt, have 1.92–1.91 Ga post-depositional low-Th/U metamorphic overgrowths. The maximum deposition age of the Lappfors sedimentary protoliths, based on detrital zircon ages, is between ∼1.99 and ∼1.95 Ga. Three samples of Bothnian sediments lack pervasive ∼1.91 Ga overgrowths, instead having a variety of detrital zircons as young as ∼1.95–1.91 Ga, reflecting recycling of the underlying basement complex. The maximum deposition age of the Bothnian sedimentary protoliths is inferred to be ∼1.91 Ga. The Niska granitoid, which intrudes the Evijärvi group and is deformed only by the younger tectonic episode affecting that sequence, has a zircon age of 1896 ± 6 Ma. That episode, which established the present relationships between basement and cover, is dated by ∼1.88 Ga metamorphic zircon overgrowths in both the Svionian and Bothnian samples, and by 1878 ± 4 Ma metamorphic monazite from a metasediment from the Savo belt, east of the Nivala district. The post-1.91 Ga volcanic sequences of the Svecofennian Province are unlikely to represent arc accretion. The Svionian metamorphic sequences are probably the remnants of a widespread marginal basin that formed between ∼1.97 and 1.92 Ga, then was accreted to the craton during an Early Svecofennian (∼1.92–1.91 Ga) orogenic phase, forming the basement on which the Bothnian volcano-sedimentary sequences were subsequently deposited.

Tectonic and magmatic patterns in the Jutulstraumen rift (?) region, East Antarctica, as imaged by high-resolution aeromagnetic data

Abstract The Jutulstraumen ice stream in western Dronning Maud Land may conceal a Jurassic continental rift. Delineating the geometry and the magmatic patterns of this inferred glaciated rift in East Antarctica is important to improve our understanding of the regional tectonic and magmatic processes associated with Gondwana break-up. A high-resolution aeromagnetic survey provides new insights over the largely buried tectonic and magmatic patterns of the Jutulstraumen area. Prominent NE-SW oriented aeromagnetic trends are detected over the Jutulstraumen. These trends delineate major inherited structural boundaries, active in Grenvillian (about 1.1 Ga) and Pan-African times (about 500 Ma), which appear to strongly control the location of the later Jurassic rift. The postulated eastern flank of the rift is marked by a broad positive anomaly over H. U. Sverdrupfjella. Buried Grenvillian age rocks may be the source of the long-wavelength anomaly. However, the higher frequency components correlate with granitoids of late Pan-African age. The inferred western flank of the rift features short-wavelength anomalies over the Borgmassivet and Ahlmannryggen areas, indicating a considerably greater extent of mid-Proterozoic tholeiitic sills than apparent in outcrop. In contrast, aeromagnetic signatures suggest that alkaline plutons, which relate to Jurassic rifting, are restricted to outcrop areas along the eastern rift flank. The prominent magnetic low over the Jutulstraumen indicates either a largely amagmatic rift, or perhaps subglacial sediments within the rift basin.

Aeromagnetic And Aeroradiometric Response To Hydrothermal Alteration

In Precambrian terrains all regional and most local intensive magnetic anomalies are produced by magnetite. Monoclinic pyrrhotite is responsible for some local, but often intensive, magnetic anomaly patterns. Both magnetite and pyrrhotite are affected by hydrothermal alteration processes in various ways, resulting in changes either in abundance or in grain fabric. These changes are recorded in the magnetic properties of the altered rock units and reflected in their aeromagnetic signatures. Hydrothermal alteration in deformed bedrock zones is commonly controlled by structural or tectonic features. Regional high-resolution aerogeophysical surveys can be utilized, in both re- gional and detailed investigations, to map the overall geological and tectonic setting or to estimate local changes in magnetic mineralogy and the relative abundance of radionuclides. Magnetite is most commonly destroyed in alteration processes, such as biotitization, carbonation, sulfidization and silicification. The progressive destruction of magnetite begins at grain margins and results first in broken and cracked grain texture and smaller grain size, then progresses to total disap- pearance of magnetite. Alteration in magnetite- bearing rock units may be recognized by decreased magnetic intensity and by the broken, disrupted magnetic pattern. The abundance of monoclinic pyrrhotite is enhanced by reducing hydrothermal fluids, and typical crystal anisotropy is developed due to tectonic stress. The relative contents of radioelements are changed in the same hydrothermal processes and partly for the same reasons as the ferrimagnetic minerals. Potassic alteration often results in elevated K radiation particularly for mafic rocks, and then anomalous K/Th ratios along local shear or fracture zones may be indicative of gold-bearing mineralization. On the other hand, high U/Th ratios within metasedimentary units may point out prospects for sulphidization. Although variation of U/Th ra- tios largely reflects the environmental conditions during primary diagenesis or a later deformational phase, mainly the decrease in Th radiation close to sulphide mineralization seems to be responsible for the elevated U/Th ratios.

Aeromagnetic signatures over western Marie Byrd Land provide insight into magmatic arc basement, mafic magmatism and structure of the Eastern Ross Sea Rift flank

Aeromagnetic signatures over the Edward VII Peninsula (E7) provide new insight into the largely ice-covered and unexplored eastern flank of the Ross Sea Rift (RSR). Positive anomalies, 10–40 km in wavelength and with amplitudes ranging from 50 to 500 nT could reveal buried Late Devonian(?)–Early Carboniferous Ford Granodiorite plutons. This is suggested by similar magnetic signature over exposed, coeval Admiralty Intrusives of the Transantarctic Mountains (TAM). Geochemical data from mid-Cretaceous Byrd Coast Granite, contact metamorphic effects on Swanson Formation and hornblende-bearing granitoid dredge samples strengthen this magnetic interpretation, making alternative explanations less probable. These magnetic anomalies over formerly adjacent TAM and western Marie Byrd Land (wMBL) terranes resemble signatures typically observed over magnetite-rich magmatic arc plutons. Shorter wavelength (5 km) 150 nT anomalies could speculatively mark mid-Cretaceous mafic dikes of the E7, similar to those exposed over the adjacent Ford Ranges. Anomalies with amplitudes of 100–360 nT over the Sulzberger Bay and at the margin of the Sulzberger Ice Shelf likely reveal mafic Late Cenozoic(?) volcanic rocks emplaced along linear rift fabric trends. Buried volcanic rock at the margin of the interpreted half-graben-like “Sulzberger Ice Shelf Block” is modelled in the Kizer Island area. The volcanic rock is marked by a coincident positive Bouguer gravity anomaly. Late Cenozoic volcanic rocks over the TAM, in the RSR, and beneath the West Antarctic Ice Sheet exhibit comparable magnetic anomaly signature reflecting regional West Antarctic Rift fabric. Interpreted mafic magmatism of the E7 is likely related to mid-Cretaceous and Late Cenozoic regional crustal extension and possible mantle plume activity over wMBL. Magnetic lineaments of the E7 are enhanced in maximum horizontal gradient of pseudo-gravity, vertical derivative and 3D Euler Deconvolution maps. Apparent vertical offsets in magnetic basement at the location of the lineaments and spatially associated mafic dikes and volcanic rocks result from 2.5D magnetic modelling. A rift-related fault origin for the magnetic lineaments, segmenting the E7 region into horst and graben blocks, is proposed by comparison with offshore seismic reflection, marine gravity, on-land gravity, radio-echo sounding, apatite fission track data and structural geology. The NNW magnetic lineament, which we interpret to mark the eastern RSR shoulder, forms the western margin of the “Alexandra Mountains horst”. This fundamental aeromagnetic feature lies on strike with the Colbeck Trough, a prominent NNW half-graben linked to Late Cretaceous(?) and Cenozoic(?) faulting in the eastern RSR. East–west and north–north–east to NE magnetic trends are also imaged. Magnetic trends, if interpreted as reflecting the signature of rift-related normal faults, would imply N–S to NE crustal extension followed by later northwest–southeast directed extension. NW–SE extension would be compatible with Cenozoic(?) oblique RSR rifting. Previous structural data from the Ford Ranges have, however, been interpreted to indicate that both Cretaceous and Cenozoic extensions were N–S to NE–SW directed.

Aeromagnetic legacy of early Paleozoic subduction along the Pacific margin of Gondwana

Comparison of the aeromagnetic signatures and geology of southeastern Australia and northern Victoria Land, Antarctica, with similar data from ancient subduction zones in California and Japan, provides a framework for reinterpretation of the plate tectonic setting of the Pacific margin of early Paleozoic Gondwana. In our model, the plutons in the Glenelg (southeastern Australia) and Wilson (northern Victoria Land) zones formed the roots of continental-margin magmatic arcs. Eastward shifting of arc magmatism resulted in the Stavely (southeastern Australia) and Bowers (northern Victoria Land) volcanic eruptions onto oceanic forearc crust. The turbidites in the Stawell (southeastern Australia) and Robertson Bay (northern Victoria Land zones) shed from the Glenelg and Wilson zones, respectively, were deposited along the trench and onto the subducting oceanic plate. The margin was subsequently truncated by thrust faults and uplifted during the Delamerian and Ross orogenies, leading to the present-day aeromagnetic signatures.

Magnetic Properties and Magnetic Signatures of BIFS of the Hamersley Basin and Yilgarn Block, Western Australia

Magnetite-rich banded iron-formations (BIFs) exhibit characteristic magnetic properties, including strong anisotropy. Interpretation of the geological structure of BIF units from the associated magnetic anomalies is complicated by anisotropy of susceptibility and, frequently, by remanent magnetisation. The timing of remanence acquisition relative to folding exerts a crucial influence on the form of the anomalies. Neglect of susceptibility anisotropy and remanence can lead to large errors in interpreted dips and thicknesses of BIF units. The effective susceptibility of BIFs parallel to bedding exceeds the susceptibility normal to bedding, typically by a factor of 2 to 4. Bedding-parallel susceptibilities of magnetite-rich BIFs are typically 0.5 to 2.0 SI (0.05–0.16 G/Oe). Remanence directions in BIFs usually lie close to the bedding plane. Koenigsberger ratios (Q) of BIFs vary widely, but characteristic values can often be determined for individual units. Q values in the range 1 to 2 are common. The magnetisations of the haematite-rich supergene-enrichment iron ores are much lower than those of their BIF precursors.Magnetic properties of outcropping BIFs are usually greatly modified by weathering, which substantially decreases the bulk susceptibility, the degree of anisotropy and the remanence intensity. Deeper and more intense weathering of BIFs is encouraged by faulting and can be associated with reduced magnetic response over intensely faulted zones.The remanence of BIFs from the Hamersley Basin is carried by late diagenetic to low-grade metamorphic magnetite after primary haematite. At Wittenoom and Paraburdoo, the remanent magnetisation of Brockman Formation BIFs is pre-folding, whereas in an area of higher metamorphic grade, the Turner Syncline, the remanence of the BIFs is probably post-folding. Aeromagnetic signatures over the Turner Syncline clearly reflect anisotropy and remanence.Anisotropy and remanence effects are also evident in observed magnetic signatures over Archaean BIFs in the Yilgarn Block. Magnetic property measurements on samples from the Mount Magnet area and elsewhere confirm the high anisotropy and strong remanent magnetisations of these rocks.

Geophysics as an Aid to Geological Mapping – A Case History from the Hoggar Region of Algeria

The Hoggar Region of Algeria, situated in the central part of the Sahara, is geologically complex and logistically difficult. Programmes of geological mapping and mineral exploration are being conducted by Algerian Government Agencies in which geophysical studies are playing useful roles. The region is covered by aerogeophysical surveys at regional scale. The interpretation of this data, integrated with the results of ground magnetic, radiometric and gravity traverses and of measurements of geophysical properties of outcrop samples is the first stage in the mapping programmes. Examples of results from the Timissao-Timgaouine region are presented. The several episodes of granite were identified and mapped by their radiometric expressions. The sequence of granites is interpreted, from older (early Pan African) to younger (Eocambrian), as Timgaouine granite, migmatitic granite, Abankor and Timissao magmatic granites and finally the intrusive granites of Taourirt and Bahouinet. The various magnetic bodies (conformable metavoicanics, diorite and gabbro intrusives, effusive complexes and mafic dikes) show characteristic aeromagnetic signatures by which they can be mapped. Source body parameters are calculated from magnetic and gravity data. The evoiution of the Pharusian Chain west of the great 4°50' submeridional fault has been re-interpreted in the light of modern theories of global tectonics. In the Timgaouine region, the computed structures, the distribution of magnetic units and the systems of faults all support the hypothesis that the Pharusian Chain evolved as a back-arc basin during a period of separation of the West African Craton from the Touareg Shield as a result of the Pan African Orogeny.