Gravity Lows Research Articles

The Hunan-Jiangxi strike-slip fault system in southern China: southern termination of the Tan-Lu fault

The NNE-trending Hunan-Jiangxi strike-slip fault system (HJSFS) of southern China comprises three NNE-trending primary faults (the Anhua-Chengbu fault, the Chalin-Chenxian fault, and the Ganjiang fault) with associated NE- and NW-trending secondary faults. A series of linear Bouguer gravity lows coincide with these NNE-trending faults and their associated secondary ones. En echelon basins and ranges occurring at fairly regular intervals of 20–40 km are strictly situated along the NE- and NNE-trending faults and within their releasing and restraining bends or stepovers, respectively. Several large streams, in general, follow the strike-slip faults. These features demonstrate a close relationship between the modern landforms and the HJSFS. Sub-linear seismically active zones and hot springs are also distributed along or adjacent to the faults, indicating this strike slip fault system is still active. The spatial and temporal relationships between the strike-slip faults and Mesozoic granitic rocks suggest that these faults acted as the loci for magmatic activity at that time. The deformed fold axes, together with the structural framework of the HJSFS itself, indicate a clockwise sub-vertical rotation of large-scale blocks restricted by the primary strike-slip faults. A variety of structural evidence suggests that the HJSFS experienced three deformation stages, i.e. transpression, transtension and compression. On the basis of offsets of pre-Late Triassic lithofacies, Mesozoic granites, and the Late Silurian suture zone between the Yangtze and Cathaysia Blocks, the total sinistral displacement of the HJSFS is estimated to be 50–60 km. The HJSFS is comparable to the Tan-Lu fault with respect to the fault geometry and evolution history. However, the HJSFS has a different structural style, displacement and earthquake intensities from those of the central segment of the Tan-Lu fault, resulting from the differences in crustal structures of the North China Block and South China Block.

Latent outflow activity for western Tharsis, Mars: Significant flood record exposed

Observations permitted by the newly acquired Mars Observer Laser Altimeter data have revealed a system of gigantic valleys northwest of the huge Martian shield volcano, Arsia Mons, in the western hemisphere of Mars (northwestern slope valleys (NSVs)). These features, which generally correspond spatially to gravity lows, are obscured by veneers of materials including volcanic lava flows, air fall deposits, and eolian materials. Geologic investigations of the Tharsis region suggest that the system of gigantic valleys predates the construction of Arsia Mons and its extensive associated lava flows of mainly late Hesperian and Amazonian age and coincides stratigraphically with the early development of the outflow channels that debouch into Chryse Planitia. Similar to the previously identified outflow channels, which issued tremendous volumes of water into topographic lows such as Chryse Planitia, the NSVs potentially represent flooding of immense magnitude and, as such, a source of water for a northern plains ocean.

Segmentation in gravity and magnetic anomalies along the U.S. East Coast passive margin: Implications for incipient structure of the oceanic lithosphere

Segmentation is a characteristic feature of seafloor spreading along the global mid‐ocean ridge system. While segmentation of active spreading centers has been the focus of much recent research, the process by which a rifted continental margin develops into a segmented mid‐ocean ridge is still poorly understood. In this study we investigate the segmentation character of the U.S. East Coast margin through a modeling study of the margin basement structure, magnetics, and gravity anomalies. The East Coast margin is of particular interest because it is one of several rifted continental margins that display thick sequences of high seismic velocity igneous crust, presumably formed during rifting. The East Coast Magnetic Anomaly (ECMA), a distinct total field magnetic high running offshore along the margin, is commonly located seaward of the thickest sections of the high‐velocity crust and displays segmentation on length scales (100–120 km) similar to the segmentation observed at the Mid‐Atlantic Ridge (MAR). Isostatic gravity anomalies were calculated by removing from free‐air gravity the predicted effects of seafloor, sediments, and a crust‐mantle model assuming local isostatic compensation. The resultant residuals show a corridor of high anomaly running along the margin, situated close to the maximum thickness of the high seismic velocity crust as determined from the two available seismic refraction lines. Reduction to the pole (R‐T‐P) of the total field magnetic anomaly shows that after the removal of skewness from the ECMA, the location of the isostatic gravity high is closely correlated to the ECMA. The isostatic gravity high is also segmented but in two distinct wave bands: 100–150 km and 300–500 km. The short‐wavelength (100–150 km) segmentation in the R‐T‐P magnetic and isostatic gravity anomalies is similar in wavelength to segmentation in magnetization and mantle Bouguer anomaly observed along the present‐day MAR. The 300–500 km segmentation in the along‐margin isostatic gravity anomaly is similar in length scale to both intermediate‐wavelength tectonic segmentation observed in the South Atlantic and variations in lithospheric strength observed along the African margin. Furthermore, two of the intermediate‐wavelength (300–500 km) isostatic gravity lows correspond to the early traces of the Kane and Atlantis fracture zones, suggesting that these two fracture zones may define boundaries of a single tectonic corridor in the North Atlantic. We hypothesize that the direct cause of the intermediate‐wavelength segmentation may be along‐margin variations in both the amount of underplated igneous crust and the strength of the lithosphere, although the relative importance of these two effects remains unresolved. Our results imply that segmentation is an important feature of margin development and that segmentation at mature oceanic spreading centers may be directly linked to segmentation during continental rifting.

System of Gigantic Valleys northwest of Tharsis, Mars: Latent catastrophic flooding, northwest watershed, and implications for Northern Plains Ocean

Mars Orbiter Laser Altimeter (MOLA) reveals a system of gigantic valleys to the northwest of the huge martian shield volcano, Arsia Mons, in the western hemisphere of Mars. These newly identified northwestern slope valleys (NSVs) potentially signify previously undocumented martian catastrophic floods and may corroborate the northern ocean hypotheses. These features, which generally correspond spatially to gravity lows, were previously obscured in Mariner and Viking Orbiter imagery by veneers of materials, including volcanic lava flows and air fall deposits. Geologic investigations of the Tharsis region suggest that the NSVs were mainly carved prior to the construction of Arsia Mons and its associated Late Hesperian and Amazonian age lava flows, concurrent with the early development of the outflow channels that debouch into Chryse Planitia.

Application of gravity and magnetic methods to assess geological hazards and natural resource potential in the Mosida Hills, Utah County, Utah

Gravity and magnetic data were collected in the Mosida Hills, Utah County, Utah, at over 1100 stations covering an area of approximately 58 km2(150 mi2) in order to help define the subsurface geology and assess potential geological hazards for urban planning in an area where the population is rapidly increasing. In addition, potential hydrocarbon traps and mineral ore bodies may be associated with some of the interpreted subsurface structures. Standard processing techniques were applied to the data to remove known variations unrelated to the geology of the area. The residual data were used to generate gravity and magnetic contour maps, isometric projections, profiles, and subsurface models. Ambiguities in the geological models were reduced by (1) incorporating data from previous geophysical surveys, surface mapping, and aeromagnetic data, (2) integrating the gravity and magnetic data from our survey, and (3) correlating the modeled cross sections. Gravity highs and coincident magnetic highs delineate mafic lava flows, gravity lows and magnetic highs reflect tuffs, and gravity highs and magnetic lows spatially correlate with carbonates. These correlations help identify the subsurface geology and lead to new insights about the formation of the associated valleys. At least eight new faults (or fault segments) were identified from the gravity data, whereas the magnetic data indicate the existence of at least three concealed and/or poorly exposed igneous bodies, as well as a large ash‐flow tuff. The presence of low‐angle faults suggests that folding or downwarping, in addition to faulting, played a role in the formation of the valleys in the Mosida Hills area. The interpreted location and nature of concealed faults and volcanic flows in the Mosida Hills area are being used by policy makers to help develop mitigation procedures to protect life and property.

An anomalous high-velocity layer at shallow crustal depths in the Narmada zone, India

The Narmada zone in central India is a zone of weakness that separates the region of Vindhyan (Meso-Neoproterozoic) deposition to the north from Gondwana (Permo-Carboniferous–lower Cretaceous) deposits to the south. The reinterpretation of analogue seismic refraction data, acquired during the early 1980s, using 2-D ray-tracing techniques reveals a basement (velocity 5.8–6.0 km s−1 ) topography suggesting that the Narmada zone, bounded by the Narmada North and Narmada South faults is a region of basement uplift. A layer of anomalously high velocity (6.5–6.7 km s−1 ) at depths between 1.5 and 9.0 km appears to be present in the entire region. Within the Narmada zone this layer occurs at shallower depths than outside the Narmada zone. At two places within the Narmada zone this layer is at a depth of about 1.5 km. This layer cannot be considered as the top of the lower crust because in this case it should have produced large positive gravity anomalies at the shallowest parts. Instead, these parts correspond to Bouguer gravity lows. Furthermore, lower crust at such shallow depths has not been reported from any other part of the Indian shield. Therefore, this layer is likely to represent the top of a high-velocity mafic body that has different thicknesses in different places.

Crustal thickness and structure along three contrasting spreading segments of the Mid‐Atlantic Ridge, 33.5°–35°N

The crustal thickness and crustal and upper mantle structure along the rift valleys of three segments of the northern Mid‐Atlantic Ridge with contrasting morphologies and gravity signatures are determined from a seismic refraction study. These segments lie between the Oceanographer and Hayes transforms and from north to south have progressively deeper axial valleys with less along‐axis relief and smaller mantle Bouguer gravity lows. Major variations in seismic crustal thickness and crustal velocity and density structure are observed along these segments. The thickest crust is found near the segment centers, with maximum crustal thicknesses of 8.1, 6.9, and 6.6±0.5 km, decreasing from north to south. However, the mean crustal thickness is similar for each segment (5.6±0.4, 5.7±0.4 and 5.1±0.3 km). Near the segment ends, crustal thickness is 2.5 to 5±0.5 km with no systematic variation from north to south. At segment ends, both crustal velocities and vertical velocity gradients are anomalous and may indicate fracturing and alteration of thin igneous crust and underlying mantle. Away from segment ends, the thickness of the upper crust is relatively uniform along axis (∼3 km), although its internal structure is laterally heterogeneous (velocity anomalies of ±0.6 km s−1 over distances of 5 km), possibly related to the presence of discrete volcanic centers. The along‐axis crustal thickness variations are primarily accommodated in the lower crust. The center of the northern segment (OH‐1) has an unusually thick crustal root (excess thickness of 2–4 km and along‐axis extent of 12 km). Our results are consistent with an enhanced supply of melt from the mantle to the segment centers and redistribution of magma along axis at shallow crustal levels by lateral dike injection. Along this portion of the Mid‐Atlantic Ridge, our results suggest that differences in axial morphology, seismic crustal thickness, and gravity anomalies are correlated and the result of variations in melt flux from the mantle. A surprising result is that the melt flux per segment length is similar for all three segments despite their different morphologies and gravity signatures. This argues against excess melting of the mantle beneath segment OH‐1. Instead, we suggest that the thickened crust at the segment center is a result of focusing of melt, possibly due to the influence of the thermal structure of the Oceanographer fracture zone on melt migration in the mantle.

The nature of the basement to the Cooper Basin region, South Australia

Analyses of aeromagnetic and gravity data in the Cooper Basin area of South Australia, reveals varied basement lithologies and structure. Most of the observed magnetic responses arise from sources beneath the Cooper Basin sequence. A prominent northeast-trending structural basement grain is evident. Emplacement of a large igneous mass into the upper crust is interpreted beneath the Patchawarra Trough. Interpreted faulting along the southeastern edge of the igneous body resulted in up to 3.5 km of block uplift, coincident with the Gidgealpa and Merrimelia Ridges. A broad and prominent gravity low in the Nappamerri Trough region is attributed to Late Carboniferous Big Lake Suite granodiorite masses, which have intruded Warburton Basin strata. Intense gravity lows within the broader gravity low are attributed to granodiorite cupolas, which became non-tectonic palaeo-topographic highs during Cooper Basin deposition. Another broad gravity low in the Tenappera Trough region is due to a belt of Early Devonian granitoids that have intruded Proterozoic basement. A number of small magnetic anomalies, in the south of the survey, are interpreted as sourced from skarns above and adjacent to the granitoid intrusions. The only magnetic units above Proterozoic basement in the region are the basalts in the Warburton Basin sequence, which coincide with small magnetic anomalies. Depth to magnetic source modelling correlates well with the known depths of the basalts.

Ultramafic exposures and the gravity signature of the lithosphere near the Fifteen-Twenty Fracture Zone (Mid-Atlantic Ridge, 14°–16.5°N)

We analyze geological and geophysical data near the Fifteen-Twenty Fracture Zone to constrain the processes of magmatic accretion and tectonic extension in the area, characterized by extensive outcrops of ultramafic rocks at the seafloor. Close to the fracture zone (<60 km) thin crust is associated with irregular, oblique faults and extensive outcrops of serpentinized peridotite, suggesting a highly heterogeneous lithosphere both compositionally and mechanically. Immediately under the transform valley the crust is thick, probably as a result of enhanced fracturing and serpentinization of the lithosphere along the transform fault. Away from the fracture zone (>60 km) the crust is thicker, associated with circular or elongated gravity lows (bull's eyes), axis-parallel faults and abyssal hills and no peridotite outcrops, consistent with a more homogeneous and magmatic crust. Simple melting models predict magmatic crustal thicknesses that differ substantially from the gravity estimates, and the outcrop of peridotites seems inconsistent with the high degrees of partial melting in the area inferred from basalt and peridotite geochemistry. We explain these discrepancies primarily (a) by the entrapment of melt in the lithosphere at deep levels, particularly near the transform, (b) by along-axis melt migration away from the transform, and (c) by variations in the degree and depth of serpentinization of a heterogeneous crust composed of gabbro and peridotite. The along-axis variations in melt supply, lithospheric thickness, and alteration of the lithosphere can result in a change of rheological structure, and therefore in fault pattern as observed along the ridge axis. Identified detachment faults in the area provide a mechanism for the uplift of ultramafic rocks to the seafloor. These detachments are not systematically associated with or formed at the inside corner of discontinuities as described elsewhere. The origin and evolution of these structures is yet unconstrained, but we suggest that they can be initiated at mechanical heterogeneities in the deep lithosphere, and that once formed they localize strain due to the presence of weak serpentinites along the fault planes.

A distinction technique between volcanic and tectonic depression structures based on the restoration modeling of gravity anomaly: a case study of the Hohi volcanic zone, central Kyushu, Japan

In this study, we propose a numerical modeling technique which restores the gravity anomaly of tectonic origin and identifies the gravity low of caldera origin. The identification is performed just by comparing the restored gravity anomalies with the observed gravity anomalies, thus we do not need detailed geophysical and geological information around the buried caldera. The technique has been successfully applied to distinguish the gravity low originated in the buried Shishimuta caldera from other gravity lows in the Hohi volcanic zone, central Kyushu in Japan.

An Integrated Gravity and Remote Sensing Assessment of Ground-Water Resources in Central Chihuahua, Mexico

The City of Chihuahua is located in the central portion of the state of Chihuahua in northwestern Mexico. The water resources in this semi-arid region are scarce and the city is experiencing difficulty meeting its water needs. Most of its water comes from nearby basins whose structure is poorly known, other than the recognition that the area is located in the Basin and Range Province. For this study 1,000 gravity stations distributed over an area of 1,200 km 2 provided a Bouguer gravity anomaly map of portions of three basins. Satellite imagery confirmed and defined the local geology and provided a base map for the interpretation of the Bouguer anomalies. Gravity data and a compilation of drill hole and geologic data were used to construct two gravity models of the basins. These integrated structural models reveal grabens filled with Cenozoic sediments with maximum depths in the range of 1,500 to 3,000 m. Some gravity lows indicate possible deep basins obscured by igneous cover. These results suggest the basins may contain aquifers capable of providing additional potable water for the growing population.

Gravity signatures of terrane accretion

In modern collisional environments, accreted terranes are bracketed by forearc gravity lows, a gravitational feature which results from the abandonment of the original trench and the initiation of a new trench seaward of the accreted terrane. The size and shape of the gravity low depends on the type of accreted feature and the strength of the formerly subducting plate. Along the Central American trench, the accretion of Gorgona Island caused a seaward trench jump of 48 to 66 km. The relict trench axes show up as gravity lows behind the trench with minimum values of −78 mgal (N of Gorgona) and −49 mgal (S of Gorgona) respectively. These forearc gravity lows have little or no topographic expression. The active trench immediately seaward of these forearc gravity lows has minimum gravity values of −59 mgal (N of Gorgona) and −58 mgal (S of Gorgona), respectively. In the north, the active trench has a less pronounced gravity low than the sediment covered forearc. In the Mariana arc, two Cretaceous seamounts have been accreted to the Eocene arc. The northern seamount is most likely a large block, the southern seamount may be a thrust slice. These more recent accretion events have produced modest forearc topographic and gravity lows in comparison with the topographic and gravity lows within the active trench. However, the minimum values of the Mariana forearc gravity lows are modest only by comparison to the Mariana Trench (−216 mgal); their absolute values are more negative than at Gorgona Island (−145 to −146 mgal). We speculate that the forearc gravity lows and seaward trench jumps near Gorgona Island were produced by the accretion of a hotspot island from a strong plate. The Mariana gravity lows and seaward trench jumps (or thrust slices) were the result of breaking a relatively weak plate close to the seamount edifice. These gravity lows resulting from accretion events should be preserved in older accreted terranes.

The nature of the basement to the Kimberley Block, Northwestern Australia

Magnetic effects of the extensive hot-spot related Hart Dolerite, Carson Volcanics, and related feeders that occur within the sedimentary section complicate interpretations of aeromagnetic data in defining the nature of the basement beneath the sediments of the Kimberley Basin, of Northwestern Australia. The magnetic effects of these sources can however be suppressed by upward continuation and stabilised downward continuation to reveal what appears to be a largely granitic basement to the area. Computer modelling has defined the geometry, density, and magnetic characteristics of what may be a remnant of an ophiolite slice emplaced under the southeastern margin of the Kimberley Block during subduction related to the convergence of a Kimberley “microcontinent” with the main Australian continent. Linear gravity lows over the Kimberley Block may define granites formed as a result of the subduction process. Five northeast trending zones of differing lithologies can be identified in the basement on the basis of differing characteristic in their associated magnetic and gravity fields. A conjugate dyke-filled fracture system, apparently related to Devonian-Carboniferous rifting processes, has been superimposed on the block. Suites of these fractures are located over areas interpreted as junctions between zones of different lithological character in the basement.

Diapiro de Salinas de Añana: ¿un diapiro con un Overhang? Modelo 3D grav¡mético

A new gravity map of the Cubeta Alavesa has been compliled and interpreted. In this map the gravity lows produced by the outcroping evaporitic diapirs and other deeper structures are very well depicted. The gravity low of the Salinas de Anana diapir has been modelled using two different 3D methods. The density model obtained has a great extention at surface but not in depth, given a lateral extention or overghang dipping to the SW.

Gravity signature of the western Paraná basin, Brazil

Volcanism in the Paraná basin, one of the largest continental flood basalt provinces on Earth, was contemporaneous with the Early Cretaceous opening of the South Atlantic Ocean. Problematic is the lack of crustal extension associated with this flood basalt event, although three Palaeozoic–Jurassic extensional episodes have been reported in parts of the basin. Gravity measurements at 185 new stations were established to constrain the geometry and magnitude of Bouguer anomalies that characterise the poorly understood western portion of the Paraná basin, and to interpret new and existing data in light of its poly-phase extensional history. The Bouguer map of the Paraná basin is characterised by two gravity lows (up to −100 mGal) in the eastern and western portions with a relative high in its centre (around −50 mGal). The spectral characteristics of the N–S-oriented western gravity lows and flanking regions suggest density contrasts at 7.4–4 km, which we interpret as variable depth to basement beneath the Palaeozoic–Cretaceous sedimentary and Cretaceous volcanic rocks. Forward modelling (2.5 D) of two profiles across the N–S-trending western gravity low shows that the low can be explained by shallow, steeply deepening structures, bounding what we believe to be a rift basin buried beneath the Mesozoic flood basalts and sedimentary strata. Comparison with other gravity data, heat flow, contour maps of crustal extension, basement and structural maps all support our interpretation of a pre-Cretaceous rift basin. We also compare our results to Palaeozoic–Mesozoic rift basins in Brazil, the North Sea and Central Africa with similar gravity signatures and better subsurface control. These comparisons also support a 3–5 km-deep, thick rift sequence beneath the western Paraná basin sequences.

Volcano-tectonic variability along segments of the Mid-Atlantic Ridge between Azores platform and Hayes fracture zone: evidence from submersible and high-resolution sidescan sonar data

Abstract Three contrasting segments of the Mid-Atlantic Ridge (MAR) between the Azores platform and Hayes fracture zones have been studied to determine variations in the magmato-tectonic processes between segment centre and segment ends. Two of the segments (OH1 and OH3) were surveyed with the submersible Nautile and the third (Lucky Strike or PO1), with high resolution deep-tow side-scan sonar (TOBI). Segment OH1 is a long and robust segment with a wide and shallow axial valley floor. Submersible observations support the inference that this segment is characterized by intense volcanic activity concentrated principally at the segment centre. Segment OH3, with a narrow and deep rift valley, is dominated by tectonic features at the segment centre, whereas the most recent volcanic constructions are found to outcrop in its southern part. Lucky Strike contains a shallow centrally located volcanic platform but tectonic features predominate all along the segment. The only evidence of fresh volcanism are a lava lake and fresh sheet-flows located at the middle and northern part of the platform, respectively. Each of the three segments show ‘bull’s eye’ gravity lows suggesting a focused mantle diapiric source.

Gravity inversion of basement relief using approximate equality constraints on depths

We present a gravity interpretation method for estimating the relief of an arbitrary interface separating two homogeneous media. The upper medium is discretized into rectangular, juxtaposed prisms whose thicknesses represent the depths to the interface and are the parameters to be estimated from the gravity anomaly. The density contrast of each prism is assumed to be constant and known. To stabilize the inversion, we introduce two kinds of constraints on the depths. The first one requires proximity between the observed and computed depths at isolated points such as those obtained from boreholes (absolute equality constraint). The second one requires that groups of depths approximately follow an established linear relationship among the depths (relative equality constraint). Both kinds of constraints are imposed in the least‐squares sense. We illustrate the method performance by applying it to a synthetic anomaly produced by a simulated basement relief consisting of four narrow and adjacent structural lows. Only two structural lows produced isolated gravity lows. Nonetheless, the whole basement topography was successfully reconstructed with an average error of 4% of the maximum relief amplitude. In this example, the relative constraints established that the thicknesses of adjacent prisms should be as close as possible to each other (overall relief smoothness). As absolute constraints we used point information about the basement depth at five points. In addition to the overall relief smoothness, other relevant geologic information, such as localized relief smoothness, occurring at structural terraces, for example, can be incorporated by assigning different weights to the relative equality constraints. The method was applied to the gravity anomaly of Recôncavo Basin, Brazil, leading to a sharper definition of the basement features relative to previous gravity interpretations of this area.

An integrated analysis of lithospheric structure across the East African plateau based on gravity anomalies and recent seismic studies

Data from recent gravity surveys carried out by the University of Texas at El Paso (USA) and other institutions and existing data were combined and integrated with seismic results from the KRISP project and geologic data to analyze the regional lithospheric structure beneath the East African plateau. A series of gravity maps were constructed, and they show that the East African plateau is associated with a broad negative gravity anomaly which is about 1200 ± 100 km wide and which has an amplitude of −150 ± 20 mGal. Superimposed on this anomaly are narrower, steeper-sided negative anomalies about 200 km wide with amplitudes of −30 to −60 mGal. These anomalies are associated with the eastern and western arms of the East African rift. These gravity maps also show low gravity gradients in the Tanzanian craton, Kasai craton and Congo basin areas while there are high gravity gradients within the rifts and at the mobile belt-craton boundaries. With increased filter wavelength cutoff, the rift-related gravity lows broaden and eventually merge into one broad low whose apex is centered on Lake Victoria. Seismic refraction, teleseismic, mantle xenolith, petrochemical and geologic data were used as constraints during the construction of a series of gravity models which represent integrated models of lithospheric structure across the East African plateau. Three W-E profiles across the plateau and one N-S profile along the axis of the Eastern rift in Kenya and Tanzania were modeled. These models together with the filtered gravity maps demonstrate lithospheric thinning beneath the rifts and suggest the existence of a deep mantle anomaly centered beneath the East African plateau and the Tanzanian craton. The mantle anomaly is widely believed to be a plume and our models indicate that the diameter of its head is about 600 km. Two arms with diameters less than 250 km extend from the plume head and penetrate the lithosphere to shallow levels under the Eastern and Western rifts. In the Western rift and the southern part of the Eastern rift (Tanzania), these features are deeper than under Kenya. This study provides additional evidence that the long wavelength Bouguer gravity anomaly over the East African plateau can be explained by lithospheric heating and thinning. The model along the axis of the Eastern rift suggests that it is propagating southwards into central Tanzania which is also suggested by the age of initiation of volcanism.

Regional mapping of Precambrian basement beneath Phanerozoic cover in southeastern Trans-Hudson Orogen, Manitoba and Saskatchewan

The northern edge of Phanerozoic platformal rocks of the Western Canada Sedimentary Basin overlies the Flin Flon Belt (Trans-Hudson Orogen) in Manitoba and Saskatchewan. A program of regional mapping of the Phanerozoic-covered basement has been undertaken, involving the integration of high-resolution aeromagnetic and gravity data with extensive drill core information. Several major domains are recognized in the buried basement, each with a distinct lithotectonic character and potential field anomaly pattern. Three lithotectonic domains in the buried basement (Clearwater, Athapapuskow, and Amisk Lake domains) are characterized by northerly-trending positive gravity and aeromagnetic anomalies and correlate with the 1.92–1.83 Ga volcanic and plutonic rocks of the exposed Flin Flon Belt (Amisk collage and Snow Lake assemblage). An upper amphibolite grade orthogneiss complex (Namew Gneiss Complex), containing calc-alkaline intrusive rocks ranging in age from 1.88 to 1.83 Ga and screens derived from the older volcano-sedimentary rocks, is interpreted as the middle crust of a 1.88–1.84 Ga arc exposed in the Flin Flon Belt. Discordant intrusive complexes, such as the 1.830 Ga Cormorant Batholith, are centred on magnetic–gravity lows and truncate the structural trend of adjacent lithotectonic domains. Correlation of Flin Flon Belt geology with that beneath the Phanerozoic cover shows that its constituent lithotectonic elements have north–south strikes of up to 150 km, and form a predominantly east-dipping crustal section, consistent with Lithoprobe seismic reflection profiles.

Magnetic investigations of the West Hawk, Deep Bay and Clearwater impact structures, Canada

Abstract— Due to the effects of erosion, tectonism and burial, impact structures are often obscured or destroyed. Geophysical methods are increasingly being used in detecting the signatures of impact structures. While gravity lows associated with impact structures are well understood, associated magnetic anomaly lows are not. In this study, drill cores from three Canadian impact structures were analyzed for rock magnetic properties and mineralogy, in order to explain the magnetic anomaly lows associated with these structures. Samples from the drill cores were cut and measured for anisotropy of magnetic susceptibility (AMS) and natural remanent magnetization (NRM) parameters. Drill cores from the twin impact craters of the Clearwater structure exhibited different NRM characteristics, and samples from their respective drill cores were subject to demagnetization by alternating field and thermal techniques. The difference noted in their NRM characteristics was attributed to the acquisition of a viscous remanent magnetization (VRM) at depth in Clearwater East.At all three structures, both magnetic susceptibilities and remanent magnetizations are well below regional values in impact generated breccias, melt rocks, shocked crystalline rocks, and in postimpact sedimentary infill. The processes of brecciation, alteration, shock, and infill by nonmagnetic sediments contribute to the development of the magnetic lows. However, a significant contribution to the observed magnetic anomalies was found, by first‐order forward modelling, to arise from basement rocks beneath the impact structures. This zone of reduced magnetization may be caused by the partial demagnetization of magnetite by the impact‐induced transient stress wave traveling away from the point of impact.