Positive Bouguer Anomalies Research Articles

Geophysical studies of the West Antarctic Rift System

Geophysical studies of the West Antarctic Rift System

The regional meaning of the Bouguer gravity anomalies in the Mediterranean

In the qualitative interpretation of gravity anomalies, the first rule is to try to separate the long-wave anomalies from the short-wave ones: in a relative sense, the former are due to deeper mass variations, the latter to shallower ones. Main emphasis will be given here to long-wave phenomena: that is, to the regional anomalies. In the Mediterranean Sea, the regional Bouguer anomalies are almost everywhere positive, so the relative differences will be in the “intensity” of the anomalies. It can be observed immediately that the main basins are all characterized by very strong positive anomalies: Provencal: +210 mGal; Tyrrhenian: +250 mGal; Levantine: +200 mGal; Jonian: +310 mGal; Aegean: +170 mGal. The left column indicates the areas of oceanic crust, with a thickness of a few kms and denser mantle very near to the surface. In the Western Mediterranean, this crust is relatively young, since it was formed during different stages of the late Tertiary and Quaternary (35-20 Ma in the Provencal basin, 8–2 Ma in the Tyrrhenian). Reduced thickness of the crust, due to extensional phenomena, can be deduced from the pronounced positive Bouguer anomalies in the Valencia Basin, in the Ligurian Sea, in the Sicily Channel, and, on land, in the Tuscany and Latium-Campania provinces in Italy. The strong gradients in gravity all around the Western Mediterranean coasts are connected with the very strong and extended fault systems which characterized the (intense) subsidence and formation of the present basins from the Miocene to the present time (as indicated by seismicity). The extended peri-Apenninic external negative anomalies, mainly on land but continuing also into the sea, from Piemonte to Sicily, are caused by thick sedimentary basins; smaller negative anomalies indicate sedimentary basins all around the Tyrrhenian Sea, between Corsica and Elba, etc. The strongest positive anomaly of all in the Mediterranean Sea region is the Ionian area (+310 mGal), probably indicative of a paleo-Tethyan oceanic domain, at least in its northern part. The rest of the Eastern Mediterranean—at least as it results from the deep seismic reflection and refraction profiles—is composed of a thin continental (African) crust, covered by a very thick sedimentary series (that could be as old as Paleozoic). The Aegean Sea positive anomaly is due to thinning and rifting (in the South) of the continental (European) crust: a situation similar to the Tyrrhenian evolution, but younger.

Gravity and vertical magnetic gradient investigations of a localised area of the Benue Trough, Nigeria

Gravity and vertical magnetic gradient studies were carried out in Lokpanta-Lekwesi area in the Lower Benue Trough to investigate the local structures which might be associated with the Lead-Zinc mineralisation as evident in the adjacent area (Ishiagu Pb-Zn mineralisation). For this reason 134 gravity and vertical magnetic gradient readings were taken at intervals of 250 m along major roads and track routes that connect widely separated parts of the 25 km 2 area. The instruments used are the Canadian Scintrex CG-2 gravimeter (No. 318), the American Paulin altimeter and the Proton Precession Magnetometer. The geology of the areacomprises the Ezeaku Formation, and the Awgu Formation, their ages ranging from Middle to Upper Cretaceous. The study was used to delineate two igneous intrusions in the Ezeaku Formation. The intrusive bodies show high positive Bouguer anomalies which ranged from 0.5 mgal to 3.3 mgal in contrast to the negative Bouguer anomaly of the country rock that ranged from 0 to − 3.0 mgal. The vertical magnetic gradient method identified the two intrusive bodies as negative anomalies having vertical magnetic gradient values ranging from −5.0 to −25.0 gammas per meter in contrast to 5.0 to 10.0 gammas per meter values of the country rock. The Profiles were interpreted using computer assisted evaluation procedure of non-linear optimization and interactive techniques. Based on two dimensional model, the minimum depth computations to the upper surfaces of the anomalies A and B by gravity method are 60mm and 160 m respectively while the vertical magnetic gradient method gave a minimum depth estimates of the anomaly A as 75 m and that of B as 180 m.

The geothermal gradient map of Central Tunisia: Comparison with structural, gravimetric and petroleum data

Five hundred and fifty temperature values, initially measured as either bottom-hole temperatures (BHT) or drill-stem tests (DST), from 98 selected petroleum exploration wells form the basis of a geothermal gradient map of central Tunisia. A “global-statistical” method was employed to correct the BHT measurements, using the DST as references. The geothermal gradient ranges from 23° to 49°C/km. Comparison of the geothermal gradient with structural, gravimetric and petroleum data indicates that: 1. (1) the general trend of the geothermal gradient curves reflects the main structural directions of the region, 2. (2) zones of low and high geothermal gradient are correlated with zones of negative and positive Bouguer anomalies and 3. (3) the five most important oil fields of central Tunisia are located near the geothermal gradient curve of 40° C/km. Such associations could have practical importance in petroleum exploration, but their significance must first be established through further investigation and additional data.

Allochthonous terranes in the Andes of Ecuador and northwestern Peru

In Ecuador and northwestern Peru the Andes and adjacent country, particularly on the Pacific side, are composed of at least five distinctive geologic terranes. The terranes are distinguished from one another and from cratonic South America to the east by dissimilar basements, cover rocks, intrusive rocks, and Bouguer gravity anomaly fields.The Piñón terrane, occupying most of coastal Ecuador, has a basaltic basement characterized by the largest known on-land positive Bouguer anomalies in the western hemisphere. The Tahuín terrane occupies most of northwestern Peru and the southwestern corner of Ecuador. The terrane has an especially complex basement and is the site of generally positive Bouguer anomalies. The small Birón terrane has an unusual basement composed in part of cordierite gneiss and amphibolite that give consistent Late Cretaceous K – Ar mineral ages. The wedge-shaped Chaucha terrane lies in part on the western Andean slope, between the oceanic Piñón terrane on the north and the continental Birón terrane on the south. The vast Santiago terrane composes the high Andes of southern Ecuador and northwestern Peru. It is the site of the unique Santiago Formation, a thick succession of Lower Jurassic limestones found nowhere else in the region.Geologic and geophysical evidence supports the view that the five terranes are parautochthonous or allochthonous fragments emplaced against cratonic South America from Middle Jurassic to Late Eocene time. Continental-border subduction alone (at the so-called "Andean margin") may have been an inadequate engine for orogeny. Additional allochthonous terranes perhaps await identification at other places along the Andes. Whether the emplacement of allochthonous terranes has been an important process elsewhere in the tectonic development of the Andes remains to be established. Geologic mapping on the oceanward western border of the Andean orogen, studies of basement petrology and chronology, and paleomagnetic studies are particularly needed.The distribution of mineral deposits (including petroleum) in Ecuador and northwestern Peru is not uniform but is instead related spatially to the five terranes and cratonic South America. This relationship can be useful to prospectors.

Correlation of gravity with the Maquereau Group, southern Gaspe Peninsula, Quebec

Gravity observations at 150 locations in the Port-Daniel and Chandler area, southeastern Gaspé, Quebec, delineate an E-W to ENE-WSW trending elliptical positive Bouguer gravity anomaly with an amplitude of 19 mGal and a residual gravity anomaly of circa 22 mGal. This anomaly correlates closely with the Lower Cambrian and/or Hadrynian Maquereau Group, the steep gradients defining it coinciding with the contact between the group and the surrounding Ordovician and Siluro-Devonian sedimentary rocks. On the basis of its similar geological setting, this gravity anomaly may be compared with the positive Bouguer anomalies observed over the Fleur de, Lys zone to the east of the Baie Verte lineament in Newfoundland. Tridimensional modelling of the gravity data is inconsistent with a serpentinite dome as originally proposed and it suggests that the Maquereau Group represents an upthrusted slice of deep-seated volcanosedimentary sequences. These were originally located on the lower segment of the continental slope; they constituted a topographic high during the final process of the mountain building at the Taconic phase (Middle to Upper Ordovician time). The most probable way of emplacement of the Maquereau block is through a tear-fault running E-W to ESE-WNW in the southern Gulf of St. Lawrence west of Cape Breton and resulting from Early Paleozoic continental collision between a Eurafrican continent and a North American continent which had a salient located beneath the site of the modern Gulf. The Maquereau may then have been reactivated with a left-lateral motion accompanied of a rotation about a sub-horizontal axis. The presumed Baie Verte-Brompton line ought to be displaced to the north and the Humber zone is narrowed.

Oxygen isotope, aeromagnetic, and gravity anomalies associated with hydrothermally altered zones in the Yankee Fork mining district, Custer County, Idaho

Epithermal Ag-Au vein and disseminated deposits in the Yankee Fork district are hosted in altered volcanic rocks having low delta 18 O values, low magnetic susceptibilities, low remanent magnetizations, and relatively high densities. These isotopic and physical quantities provide sensitive indices of rock alteration that can be contoured over areas that are much larger than zones of strong visible alteration. Such contour maps reveal a marked spatial association between the ore deposits, the steepest delta 18 O gradients, and the perimeter of zones where the rocks have extremely low total magnetizations. The oxygen isotope results indicate that the ore zones coincide with steep gradients in temperature and water/rock ratio in two fossil hydrothermal convective systems that involved fluids derived from Tertiary meteoric waters. Our magnetic measurements suggest that these same high-gradient zones also coincide with oxidation-reduction boundaries. Low aeromagnetic intensities and positive Bouguer anomalies are also associated with the altered zones in the Yankee Fork district. The intensities of these geophysical features are consistent with the size of the altered zones and with the contrast in physical properties between altered and unaltered rocks. Contour maps of the magnetic and isotopic properties of host rocks should find application in exploration for a variety of types of hydrothermal ore deposits, as well as in the study of ore controls and conditions of mineralization.

The Calabrian Arc and the Ionian Sea in the dynamic evolution of the Central Mediterranean

In the Central Mediterranean area it is possible to recognize three main realms: the Tyrrhenian Sea, the Pelagian block and the Ionian block. These domains are limited by important fracture zones that are easily recognizable on land and offshore. These fracture zones can be grouped into four main trends: (1) the NW-SE trend which is characterized by dextral horizontal movements; (2) the NE-SW trend which is generally characterized by sinistral lateral horizontal movements; (3) the E-W trend which also shows dextral shear movements; and (4) the N-S trend which is characterized by normal faults only. The Tyrrhenian Sea realm is characterized by a thinned crust of oceanic type and is connected to the Apenninic chain by means of listric faults. From a geophysical point of view, the area shows a high heat flow which follows the main structural trends, and positive gravimetric anomalies which are compatible with seismic refraction data and can be explained by the proposed model. The Pelagian block shows a continental crust, about 20 km thick, which progressively increases beneath the Sicilian chain. Here an abrupt interruption with the Tyrrhenian block takes place. In the Pelagian realm, rifting processes developed according to the NW-SE and N-S trend. The N-S trend characterizes the westernmost zone of the realm where volcanic processes and a relatively high heat flow can also be observed. The Ionian block is characterized by the following: a thinned crust (17–20 km) which increases (40 km) towards the Southern Calabrian chain; a thick sedimentary sequence; high positive Bouguer anomalies (+300 mGal); and very low heat flow (50 mW m −2). Geophysical and structural analyses allowed us to evaluate the post-Tortonian evolution of the whole area under consideration. This evolution resulted in continental crust formation according to a rigid—plastic deformation model. According to our model, the Ionian block can be considered as a domain, characterized by a thinned continental crust and affected by intrusive material derived from the upper mantle. Furthermore, this domain, of probably Jurassic age, can be considered as a pelagic basin bordered by the Pelagian block to the west and by the Apulian block to the east-northeast. This basin, according to its position with respect to the Apenninic and Sicilian chains, has not been affected by tectogenesis and still represents a site of continuous sedimentation. From a kinematic point of view, the Ionian realm can be linked with the Pelagian block shifting towards the east-northeast, whereas it is limited towards the north by an E-W fracture zone which regulates the opening of the Tyrrhenian Sea and the counterclockwise rotation of the Apenninic chain. According to the proposed model, the opening of the Tyrrhenian Sea can be considered as a megaextension-fracture which developed parallel to the maximum principal stress direction and evolved as a triangular-shaped structural feature. In preparing the model the authors have taken into account a large set of data collected both on land and offshore.

Gravity profiles across the Samail Ophiolite, Oman

Four hundred and sixty gravity stations have been established over the Samail ophiolite and adjacent units exposed in the Oman Mountains. The Bouguer anomaly pattern follows more or less parallel to ophiolite exposures. Along the coastal areas the Bouguer anomaly values range from +20 to +140 mGal and increase northward. Four gravity profiles across the ophiolite are correlated with known geology and structure. The main gravity profile extending 325 km northward from Ibra to Muscat and into the Gulf of Oman shows two positive Bouguer anomalies (+85 over the southern folded ophiolite nappe and +110 mGal near the shoreline at Muscat) that are associated with two nappes of tabular form. The nappes are nearly continuous, and there is no evidence that they are rooted in the mantle. The gravity high across the southern nappe is centered over the axis of the synform (maximum nappe thickness); its position over the gabbro‐peridotite contact rather than over the peridotite suggests extensive serpentinization of the peridotite. At the southern end of the profile, in the Ibra Dome area, the negative Bouguer anomaly (−10 to −15 mGal) is interpreted as an Infracambrian salt piercement similar to those in the Zagros fold belt. Geologic modeling of the main gravity profile provides a good correlation in terms of known thicknesses of the mappable ophiolite units. The nature of the gravity anomaly across the continental margin indicates a transition from ∼42‐km‐thick continental crust to a thinner (16–18 km) oceanic crust. The two gravity profiles in the north Oman region show a doublet of positive residual Bouguer anomalies (+50 to +90 mGal). The anomalies correlate well with imbricated ophiolite exposures, a result of low‐angle thrusting caused by strong compressive forces. In the absence of imbricate thrusting, the gravity profile in the central region has only one large positive residual Bouguer anomaly (+130 mGal). The anomalies of all the four profiles are further analyzed to elucidate the nappe‐transport distances.

The structure of the shallow crust beneath Olkaria geothermal field, Kenya, deduced from gravity studies

The Olkaria region of the Kenya rift valley is a potentially important geothermal area due to the occurrence of young volcanic activity and surface geothermal manifestations which include hot springs, fumaroles and thermally altered ground. In order to determine subsurface structure nearly 255 new gravity measurements, together with existing regional gravity data, were used to propose a subsurface model. A few surface density measurements have been made within Olkaria and in adjoining areas, and these combined with density values inferred from the measured P-wave seismic velocities within Olkaria are used to define the density distribution within the shallow crust. A two-dimensional model to explain the local gravity anomalies indicates that an approximately horizontal three-layer volcanic sequence overlying the basement system is downfaulted on the western part of Olkaria, and is intruded by denser dyke-like bodies of rhyolitic composition along the north-south-trending fractures that occur mainly in the central part of the Olkaria. An increase in the thickness of volcanics in the western part of Olkaria is responsible for the observed negative Bouguer anomaly in that area, while vertical density contrasts between the dyke-like intrusions and the host rock generate model anomalies in good agreement with the observed positive Bouguer anomalies in the central part of Olkaria. The largest and most recent intrusion, which is probably still in a magmatic state, occurs along the Ololbutot fracture zone and is apparently the main heat source for the geothermal phenomena in Olkaria.

Three-dimensional seismic structure of subducting lithospheric plates under the Japan islands

The three-dimensional seismic structure of subducting lithospheric plates under the Japan Islands has been investigated in detail by using an inversion method developed by Aki and co-workers. The present analysis clearly reveals the subduction of the Pacific plate, with a gradual narrowing in breadth down to about 600 km beneath the Sea of Japan, and also gives some indication of the Philippine Sea plate in the uppermost mantle, less than 50 km under the southernmost part of Japan. The upper boundary of the descending Pacific plate shows sharp velocity contrasts with respect to the overlying low-velocity zone, while the lower boundary appears to have a transitional nature. Large positive Bouguer anomalies in northeastern Japan may be explained, in part, by the effects of the subducting Pacific plate, but those over the Sea of Japan cannot be accounted for by lateral heterogeneities in the upper mantle.

The geological interpretation of gravity and magnetic surveys over the exposed southern margin of the Galway Granite, Ireland

AbstractGravity measurements across the southern margin of the Galway granite, on the Islands of Gorumna and Lettermullen, reveal small positive Bouguer anomalies of amplitudes less than 40 g.u. associated with the denser country rock. These anomalies suggest that the ‘country rock’ is no more than a thin wedge, thickening southwards. Such a structure may form part of a large roof pendant or roof section with the true granite margin lying further south. Magnetic anomaly profiles across the granite contact reveal that the rock groups, making up the country rock, have distinct magnetic character which may provide a means of mapping their southern extent beneath Galway Bay.

THE STRUCTURE OF THE CONCEALED PRE-CARBONIFEROUS BASEMENT OF THE DERBYSHIRE DOME FROM GRAVITY DATA

SUMMARY Bouguer anomalies over the Derbyshire Dome indicate a positive structural relief for the pre-Carboniferous basement, and their qualitative and quantitative interpretations show that the basement reaches its maximum elevation towards the southern edge of the dome. The Bouguer map contours in the central area suggest that the surface of the basement there may be at a greater depth than in both the northern and southern parts. It is also inferred from the gravity models that the basement is at a shallower depth to the west of the limestone outcrop than to the east and that its eastern margin has a comparatively steeper slope. Lateral extensions of the basement beyond the area of the dome are postulated, based on the existence of features with positive Bouguer anomalies.

Gravity and Seismic Reflection Investigations into the Crustal Structure of the Aves Ridge, Eastern Caribbean

Summary The results of bathymetric, gravimetric and seismic reflection surveys over the Aves Ridge and Grenada Trough (eastern Caribbean) are presented. The Aves Ridge is a submerged linear prominence typically formed of two flanking ridges enclosing a sediment filled trough in which occur seismic reflectors similar to those found in the Venezuela Basin and Grenada Trough. The short wavelength gravity anomalies of the Aves Ridge may be adequately explained in terms of structures in the sediment/basement interface. The Aves Ridge is underlain by a mass deficiency which may be interpreted in terms of a thickened crust with consequent depression of the Moho and lowest crustal layer. The positive Bouguer anomalies of the Grenada Trough are probably due to a relatively shallow Moho. The Aves Ridge and Grenada Trough typically exhibit negative isostatic anomalies and the Lesser Antilles define a linear belt of positive isostatic anomalies. These new results support the suggestion that the Aves Ridge is an ancient island arc.

Regional Gravity Anomalies and Crustal Structure in Northern Colombia

The central range of the Colombian Andes gives way northward to a series of Cenozoic fault-bordered basins and uplifts near the Caribbean Sea. Pre-Cenozoic structures exposed in the uplifts curve increasingly toward the east to become parallel to the continental margins along the south side of the Caribbean. Major Cenozoic faults, with large vertical and horizontal displacements, cut across older structures which include Permian-Triassic(?) and Late Cretaceous to early Tertiary metamorphic zones, Precambrian gneiss, and Jurassic batholiths. Gravity anomalies have large amplitudes in the Santa Marta area. Bouguer anomalies rise to +130 mgals over the crystalline rocks of the high Santa Marta massif. Over adjacent Cenozoic basins, they range down to −80 mgals over the Lower Magdalena basin and to −65 mgals over the Baja Guajira basin. Steep gravity gradients characterize the Santa Marta and Oca faults on the west and north sides of the massif, respectively. In the Guajira Peninsula region, Bouguer anomalies increase to +105 mgals over a serpentinite zone at Cabo de la Vela and to +55 mgals over Cretaceous volcanic rocks in the southern peninsula. Two smaller basins on the peninsula are characterized by negative Bouguer anomalies. Steep gradients characterize many of the major Cenozoic faults, and two concealed faults are postulated on this basis. Though useful for evaluating the relative vertical displacements, which may exceed 10 km along faults bounding the Santa Marta massif, the gravity data yield no definitive information on the large horizontal displacements postulated along some of the major faults of the area. The Bouguer anomalies do indicate, however, the extension of some of the Cenozoic basins into offshore areas. Strong positive Bouguer anomalies of the Santa Marta massif and its great relief, which exceeds 9 km relative to the floor of the adjacent Caribbean, indicate thin continental crust, lack of isostatic balance, and relatively recent uplift for the massif. After corrections are made for the gravitational effects of Tertiary sedimentary basins in the Guajira Peninsula, most of the peninsular region also has positive anomalies, suggesting a relatively thin continental crust and a lack of isostatic balance. A mechanism of overthrusting, in relatively recent time, of the continental margin over the adjacent Caribbean upper mantle and crust to the northwest can account for the observations.

Positive Bouguer anomalies in some orogenic belts

Positive Bouguer anomalies in some orogenic belts

Lower Quaternary of Kyushu Island, with special reference to its tectonic Framework

The Oita Group and its equivalent strata, extensively developed in the middle Kyushu, are documented with the tremendous volcanic products and Villafranchian to early classical Quaternary fauna and flora. Regional geologic evidence shows that the strata occupy a part of the Second Setouchi basin, which represents the final collapse of the axial zone of the Honshu geanticline at the last phase of the geosynclinal cycle. However the strata in the middle Kyushu markedly differ from those of other parts of the Setouchi by the extensive volcanism and great thickness.The basin of the middle Kyushu is divided into two parts by the Kunizaki-Chikugo upheaval zone of NEE-SWW trend. The southern part is a half graben bordered by the Oita-Kumamoto fault line at the south. The half graben and its north and south upheaval areas are corresponded respectively with the area of negative and positive Bouguer anomaly. This fact suggests that the early Pleistocene events have the relation to the geophysical character of present time in the middle Kyushu. Hence the early Pleistocene basin is examined in regard to the present geophysical features. There is a distinctive zone of the negative isostatic anomaly along the east coast of Kyushu, where the crust is still sinking with deeper MOHOROVICIC plane. The focal plane of the medial- and deep-focused earthquakes intersects with the earth surface in this zone and the intersection coincides with the zero line of the Bouguer anomaly. It should be noted about the earthquake that the more north the position, the shallower the foci. These features seem to be the expression of the Ryukyu arc at its northern extremity. Thus the overlap of the activities of the younger Ryukyu arc to the final collapse of the Honshu geosyncline results the unique geologic events in the middle Kyushu.

Gravity Anomalies and Inferred Crustal Structure in the Eastern Mediterranean Sea

Gravity data in the eastern Mediterranean area have been compiled from Woods Hole Oceanographic cruises of the R/V Chain in 1964 and 1966, and supplemented by other shipboard and land measurements. Over most of the area, free-air anomalies are negative and Bouguer anomalies are consistently lower, by 50 to 100 mgal, than in the western Mediterranean. Broad negative free-air anomalies (−80 to −240 mgal) and low positive Bouguer anomalies (0 to 80 mgal) follow an arcuate zone, concave northward, south of Crete from western Greece to the Rhodes abyssal plain. Relatively high anomalies (free-air anomaly as high as −60 mgal and Bouguer anomaly of +100 to +140 mgal) centered over the Anaximander seamounts separate the arcuate low south of Crete from a similar trend south of Cyprus that gradually becomes less distinct toward northern Syria. Cyprus is characterized by free-air anomalies greater than 100 mgal, Bouguer anomalies of from +100 to +250 mgal, and some of the world9s largest positive isostatic anomalies (as great as +173 mgal). A crustal-structure model was constructed for a profile south from Turkey, approximately along long. 31°E., toward Egypt. The simulated structure showed a range in depth to the top of the mantle from 23 km beneath the outer margin of the Nile Cone to 34 km beneath the axis of the Mediterranean Ridge. A free-air anomaly gradient of about −1.2 mgal/km to the north over the southern margin of the Mediterranean Ridge corresponds to a downward slope on the crust-mantle interface of about 7° to the north. The thickening of the crust by almost 50 percent north of 34°N. is interpreted as largely due to underthrusting of Mediterranean crust beneath Cyprus, the Anaximander Mountains, and Turkey. A concomitant thick accumulation of sediment may have further downwarped the crust beneath the Mediterranean Ridge.

A gravity survey of coastal areas of West Finnmark, northern Norway

A gravity survey carried out in coastal regions of West Finnmark is described. The survey reveals a positive Bouguer anomaly with an amplitude of approximately 100 mgal centred over the island of Söröya and forming a culmination on a persistent positive anomaly axis extending along the entire coastal zone. The Söröya anomaly is similar in amplitude and width to a large anomaly overlying Lofoten-Vesterålen 300 km to the south-west. These anomalies are interpreted as being caused by an underlying upper crustal zone of high density, formed either by a massive enrichment of the upper crust in basic and ultrabasic material or by a culmination of granulitic facies metamorphic rocks, perhaps containing abundant basic intrusions. The proposed crustal structure is similar in important respects to that of the Western Alps as indicated by recent geophysical surveys, and is consistent with modern Russian views on the role of asthenoliths in geosynclinal development.

Regional Gravity Anomalies Attributable to Basic Intrusions in Orogenic Belts

Summary Large positive Bouguer anomalies overlie the coastal region of northern Norway. In terms of amplitude, extent and their location within an orogenic belt they resemble the Ivrea anomaly of the southern border region of the Alps (Closs 1965). This latter anomaly is interpreted as being due to a large basic intrusion which might have some direct causal relationship with the development of Alpine nappes (Closs 1965, p. 98). The Norwegian anomaly is similarly interpreted in terms of massive basic or ultra-basic bodies. These intrusions appear to predate considerably the late Caledonian development of overthrust tectonics but may well be related to nappe tectonics earlier than the main Caledonian metamorphism.