Two-dimensional Gravity Models Research Articles

Constraints upon models of greenstone belt evolution by gravity modelling, Birch-Uchi greenstone belt, northern Ontario

Abstract Eight two-dimensional gravity models, incorporating most of the major hypotheses proposed for the development of Precambrian greenstone belts, are computed. Maximum use is made of geological and geophysical constraints, in an attempt to explain the disparity between the 11 000 m measured stratigraphic thickness and the maximum 5000 m interpreted vertical extent of a portion of the Birch-Uchi metavolcanic-metasedimentary belt. The two-dimensional modelling concentrates on an east-trending profile across a dominantly volcanic portion of the belt comprising three superposed basalt to rhyolite cycles. It is supplemented by a computed second vertical derivative map of the gravity field. Metamorphic and geophysical considerations show simple, isoclinal, parallel folding of the mapped stratigraphic thickness to be unacceptable. Proposed mechanisms which could explain the shallow interpreted vertical extent are a density gradient, a granite root, more complex folding of the belt and restriction of the metavolcanic basin. Taken individually, however, none of these mechanisms is entirely adequate. The model which best fits the gravity and geological data suggests that the shallow vertical extent is the result of both magmatic stoping by a subjacent granitic magma chamber during the caldera stage of cycle III volcanism and partial melting of the basaltic rocks during earlier cycles of volcanism.

The crustal structure of the Madagascar Ridge

Summary. The Madagascar Ridge is an elongated aseismic plateau in the southwest Indian Ocean, which separates two ocean basins of mid-to late Cretaceous age. From interpretations of a reversed refraction line on the ridge and a continuous gravity profile across it we conclude that it formed during the Cretaceous as thickened oceanic crust, at a mid-ocean ridge which possibly overlay a mantle hot-spot. The seismic data consist of record sections from five recording sonobuoys. Forty-one shots were fired along a 210 km long NNE—SSW line across the central part of the ridge. Simple horizontal-layer analysis of first arrivals reveals a lower crustal refractor with Vp= 7 kms−1 at a depth of 8–8.5 km and the Moho at an apparent depth of 18 km, but fails to characterize the crustal structure as either oceanic or continental. We obtain a definitively oceanic structure by using synthetic seismogram techniques together with Tau(p) analysis and second arrival correlations. The model which best fits the data consists of 2.2 km of water and 1 km of sediments overlying a velocity gradient from 4 to 7 kms−1. Beneath this, at a depth of 7.5 km, is a 9.5 km thick layer with Vp= 6.9–7.1 kms−1. A 7.5 kms−1 layer, not observed in the first arrival analysis, is found at the base of the crust. The Moho is a transition zone 1 km thick and occurs at ∼ 25 km depth. The gravity data consist of a continuous NE—SW profile across the ridge and on to the flanking basins. Two-dimensional gravity modelling allows a structure consistent with the synthetic seismogram results, and supports the existence of a high-density basal layer in the crust. We use in addition a time-series analysis approach to estimate the depth of compensation assuming an Airy model, and lithospheric thickness assuming an elastic plate model. Convolution of bathymetry along the profile with a range of model filters gives minimum rms gravity residuals for a crustal compensation depth of 23 ± 5 km, and a plate thickness of less than 2–5 km.

The gravity field of the U.S. Atlantic continental margin

Approximately 39,000 km of marine gravity data collected during 1975 and 1976 have been integrated with U.S. Navy and other available data over the U.S. Atlantic continental margin between Florida and Maine to obtain a 10 mgal contour free-air gravity anomaly map. A maximum typically ranging from 0 to +70 mgal occurs along the edge of the shelf and Blake Plateau, while a minimum typically ranging from −20 to −80 mgal occurs along the base of the continental slope, except for a −140 mgal minimum at the base of the Blake Escarpment. Although the maximum and minimum free-air gravity values are strongly influenced by continental slope topography and by the abrupt change in crustal thickness across the margin, the peaks and troughs in the anomalies terminate abruptly at discrete transverse zones along the margin. These zones appear to mark major NW—SE fractures in the subsided continental margin and adjacent deep ocean basin, which separate the margin into a series of segmented basins and platforms. Rapid differential subsidence of crustal blocks on either side of these fractures during the early stages after separation of North America and Africa (Jurassic and Early Cretaceous) is inferred to be the cause of most of the gravity transitions along the length of margin. The major transverse zones are southeast of Charleston, east of Cape Hatteras, near Norfolk Canyon, off Delaware Bay, just south of Hudson Canyon and south of Cape Cod. Local Airy isostatic anomaly profiles (two-dimensional, without sediment corrections) were computed along eight multichannel seismic profiles. The isostatic anomaly values over major basins beneath the shelf and rise are generally between −10 and −30 mgal while those over the platform areas are typically 0 to +20 mgal. While a few isostatic anomaly profiles show local 10–20 mgal increases seaward of the East Coast Magnetic Anomaly (ECMA: inferred to mark the ocean-continent boundary), the lack of a consistent correlation indicates that the relationship of isostatic gravity anomalies to the magnetic anomalies and the ocean—continent transition is variable. Two-dimensional gravity models have been computed for two profiles off Cape Cod, Massachusetts and Cape May, New Jersey, where excellent reflection, refraction and magnetic control appear to define 10 and 12 km deep sedimentary basins beneath the shelf, respectively and 10 km deep basins beneath the rise. The basins are separated by a 6–8 km deep basement ridge which underlies the ECMA and appears to mark the landward edge of oceanic crust. The gravity models suggest that the oceanic crust is between 11 and 18 km thick beneath the ECMA, but decreases to a thickness of less than 8 km within the first 20–90 km to the southeast. In both profiles, the derived crustal thickness variations support the interpretation that the ECMA occurs over the ocean-continent boundary. The crust underlying the sedimentary cover appears to be 12 to 15 km thick on the landward side of the ECMA and gradually thickens to normal continental values of greater than 25 km within the first 60 to 110 km to the northwest. Multichannel seismic profiles across platform areas, such as Cape Hatteras and Cape Cod, indicate the ocean-continent transition zones there are much narrower than profiles across major sedimentary basins, such as the one off New Jersey.

The Owen Basin off the south-east margin of Arabia and the evolution of the Owen Fracture Zone

Summary. The history of the Owen Basin, which lies between Arabia and the Owen Fracture Zone (OFZ) in the north-west Indian Ocean, is relevant to the break-up of Gondwanaland and the subsequent sea-floor spreading history of the region. Bathymetric, magnetic and gravity studies of the basin were made in 1975 in conjunction with seismic reflection and refraction profiles. A slightly thickened oceanic crust exists under the basin and thickens towards the continental margin. There is limited evidence of an ENE grain in both the magnetic anomalies and the acoustic basement but, on the other hand, there are major ridges which parallel the shelf edge and the OFZ. Two-dimensional gravity modelling indicates areas of abnormally thinned crust under some sediment troughs and areas of abnormally thickened crust under ridges not associated with the OFZ. The Owen Basin does not appear to be a region of simple oceanic crust but possesses features whch possibly relate to the formation of the adjacent continental margin. Free-air gravity anomalies suggest that the OFZ is a continuous feature from 15’ 30” to as far north as, and inclusive of, thg Dalrymple Trough. The arcuate shape of the northern OFZ is probably a relict of an earlier phase of sea-floor spreading before the Gulf of Aden opened although there has been some vertical and horizontal readjustment at the OFZ as a consequence of this opening. Thus the northern OFZ is probably no longer an active transform fault. The Owen and Chain Ridges exhibit rotational symmetry about a point midway between them and the latter ridge, being the inactive extension of a transform fault, traces a history of changing spreading direction during their formation. It is not possible at present to relate the history of the Owen Basin to Mesozoic tectonic events in the Somali. Basin because the age and origin of two intervening seamount chains are unknown.

Gravity models of the Wairarapa region, New Zealand

Abstract Two-dimensional and three-dimensional gravity models of the Mesozoic basement topography in the Wairarapa show that the major feature is the Wairarapa Trough, aligned SSW—NNE through the area. The Trough has two arms, one north of Eketahuna and the other south of Carterton. All three features are fault-angle depressions, bounded to the west by the West Wairarapa, Wellington, and Dry River Faults, respectively, and reaching their greatest depths at these faults. A structural contour map of the Mesozoic surface in the Wairarapa shows that the basement is covered by more than 3000 m of Cenozoic material. The West Wairarapa Fault is shown to be reverse in the south, with a dip as low as 15°. The other faults are apparently near-vertical.

An Interpretation of the Golden Grove Gravity High in Southern New Brunswick

The Golden Grove gravity high in southern New Brunswick is associated with mafic intrusive rocks ranging from diorite to gabbro. The anomaly was interpreted with the aid of an iterative two-dimensional gravity modelling program used in conjunction with a non-linear optimization computer package MINUIT. For minimization, the simplex method of Nelder and Mead and the variable metric method of Fletcher were used with constraint. The optimum causative body parameters indicate that the mafic intrusive mass is a sill-like (tabular) body, having a depth extent of about 1 km, with outward dipping contacts.

Tectonic Development of the Bismarck Sea Based on Gravity and Magnetic Modelling

The Bismarck Sea is a small marginal sea situated north of the island of New Britain and its associated trench. The land areas which surround it on three sides are island arc structures which have been active episodically since the Middle Cretaceous but which were most active during the Tertiary. Magnetic and sediment distribution maps of the Sea have been compiled and the results of two-dimensional magnetic and gravity modelling along five north-south traverses are presented. The magnetic map shows that the general trend of the anomalies is east-west. Individual anomalies can be traced for more than 150 km, mainly in the eastern half of the Sea, but in the western half persistent anomalies are difficult to discern except along the coast of mainland New Guinea. The water depth in the Bismarck Sea is about 2000 m and Bouguer gravity values are of the order of + 150 mGal. There is a substantial sediment-free area across the centre of the Sea, and sediment thickness increases towards land, reaching 2 km along the north New Guinea coast. Magnetic models and sediment distribution indicate that in the eastern half of the Sea north-south extension is taking place, thus implying northwards movement of the North Bismarck Plate relative to the South Bismarck Plate. The extension is occurring at about 8 cm yr−1 and is single limbed, accretion taking place mainly to the south of the Bismarck Sea Seismic Zone. Earthquake focal-mechanism solutions from the eastern part of the seismic zone, which extends E-W across the Sea, indicate that the N-S movement is being accommodated along two north-west-trending segments of the zone, about a pole to the south-west of the Sea. This interpretation supports the concept of an extensional origin for marginal seas. In addition the mechanics of the process are shown to be (a) different from those at mid-ocean ridges, (b) probably episodic, and (c) possibly reactivated across established trends, thus effectively erasing magnetic lineaments. The western half of the Sea is more complex but some north-south extension has occurred at the seismic zone.

Evidence for high-density crust and mantle beneath the Chile Trench due to the descending lithosphere

Gravity models of oceanic trenches computed prior to the advent of plate tectonic concepts fell into two classes of solutions: (1) if a homogeneous mantle density was assumed, then the gravity models required an abnormally thin oceanic crust in the first 100 km seaward to the trench axis; or (2) if the oceanic crust was not thinned, then high-density mantle was included beneath the Moho near the trench axis. In contrast to the gravity models, however, seismic refraction studies near trench axes and on the seaward trench slope have generally observed normal oceanic crustal thicknesses and normal mantle velocities. This apparent conflict between the refraction data and the gravity interpretations can be resolved by taking into account density anomalies in the descending lithosphere. A theoretical density model for the downgoing slab was computed from thermal and petrologic data and was then compared with the observed gravity data of Hayes (1966) and the refraction data of Fisher and Raitt (1962) over the Chile trench at 23°S. Considerations of pressure-temperature data suggest that the oceanic crust and the lithosphere may transform to eclogite (3.55 g/cm3) and garnet peridotite (3.38 g/cm3), respectively, at depths as shallow as 30 km in the descending slab. This model predicts density anomalies of +0.08 to +0.28 g/cm3 at depths between 30 and 80 km and +0.04 to +0.024 g/cm3 between 80 and 150 km and predicts a mean density anomaly of +0.05 g/cm3 at depths between 150 and 300 km. Incorporation of these high density mantle zones into a two-dimensional gravity model allows a gravity solution that is in much better agreement with the refraction data seaward of the Chile trench than earlier gravity models, which assumed a homogeneous mantle.

Crustal and Upper Mantle Structure of the Central Aleutian Arc

Seven reflection profiles over the central Aleutian trench and terrace are presented. Undeformed trench turbidites terminate abruptly against the inner wall which is acoustically opaque for the next 30 km. The southern half of the Aleutian terrace is an uplifted anticlinal arch, Hawley ridge. Acoustically visible sediments thicken to at least 2 km in the northern part of the terrace. A sonobuoy refraction profile near the terrace axis indicates 5 to 8 km of sediment over an oceaniclike crust (6.4 km per sec). Incorporation of this data with previous near-bottom geophysical and dredge work suggests that thrusted and complexly deformed sediments outcrop between the trench and terrace. Two alternative two-dimensional gravity models extending 200 km south and 350 km north of the trench are presented which take into account both the crustal and upper mantle structure. The fundamental ambiguity between the two models is the dip of the Benioff zone beneath the Aleutian terrace (between 5° and 20°). Low-density (2.2 to 2.4 g per cc) deformed sediments appear to make up the first 30 to 70 km north of the trench axis. The total thickness of sediments near the terrace axis appears closer to 7 to 8 km according to the gravity data. The density contrast between the descending lithosphere and surrounding aesthenosphere appears to be near +0.05 g per cc for an 80-km-thick plate, but a low-density zone in the crust and mantle beneath the volcanic chain is also necessary to fit the observed gravity. Assuming that the Pacific crust inverts into ecologite at a depth of 30 km produced reasonable results, but this estimate is difficult to confirm with present data. The effects of modifying some of these boundaries and density contrasts are calculated and discussed.