A gravity survey of the central Labrador Trough, northern Quebec

1977 ◽  
Vol 14 (1) ◽  
pp. 45-55 ◽  
Author(s):  
P. Kearey
Keyword(s):  
Gravity Anomalies ◽  
Igneous Rocks ◽  
Gravity Survey ◽  
Iron Formation ◽  
The North ◽  
Density Control ◽  
Residual Gravity Anomaly ◽  
Fold Belts ◽  
Persistent Depression ◽  
North And South

The Labrador Trough is the best preserved and exposed of several Aphebian (lower Proterozoic) fold belts which surround the Archaean Ungava Craton of northern Quebec and is characterised by three longitudinal facies zones: predominantly meta-sedimentary rocks in the west and east and predominantly basic meta-igneous rocks in the centre. The results of a detailed gravity survey of the central part of the Labrador Trough between latitudes 55° 45′ and 57° 30′ and longitudes 66° 30′ and 70° are presented. Over 1500 rock samples provide density control for the interpretation of four residual gravity anomaly profiles in terms of the surface geology.In the eastern part of the Labrador Trough positive gravity anomalies correlate with outcrops of basic meta-igneous rocks. Their causative bodies extend subsurface to the east and reach depths of up to 9 km in the central part of the area, but are considerably thinner to the north and south. This interpreted depth is considerably less than the 15–20 km that has been inferred by other workers from surface geological investigations. Small positive gravity anomalies are associated with iron formation. A persistent depression in the observed gravity field over the centre of the trough in the south coincides with deposits of the basal sedimentary unit. Uncertainty in the location of the regional level prohibits accurate thickness determinations of the causative bodies of negative anomalies in this area, but the approximate values of 2–3 km obtained for the basal unit are of the same order as estimates based on geological investigations. In the northern part of the area the causative bodies of the negative anomalies are probably elevated areas of granitic basement. These elevated basement features may be related to a ridge that controlled sedimentation during much of the trough's history.

The Virginia earthquake of April 9, 1918

1918 ◽  
Vol 8 (4) ◽  
pp. 105-116
Author(s):  
Thomas L. Watson
Keyword(s):  
Horizontal Displacement ◽  
Igneous Rocks ◽  
Blue Ridge ◽  
Valley Bottom ◽  
The North ◽  
Lower Ordovician ◽  
State Boundary ◽  
Virginia Earthquake ◽  
The Town ◽  
North And South

Summary of geology and conclusions Since, from the reports received, the shock seems to have been most severe in the northern part of the Valley of Virginia, a very brief summary of the geology of the northern valley region is of some interest in seeking the probable cause of the earthquake. The Valley of Virginia is bounded on the southeast by the Blue Ridge, the central portion of which is composed of pre-Cambrian igneous rocks and on the northwest by the Valley Ridges subprovince of folded sedimentary rocks ranging up to Devonian and Mississippian in age. The valley maintains an approximate width of twenty miles from the state boundary southwestward to nearly the latitude of Greenville, Augusta County (Map, Plate I). From near the latitude of Strasburg and Riverton to that a short distance south of Harrisonburg, the valley is divided lengthwise by Massanutten Mountain, which is synclinal in structure and composed of sedimentary rock ranging up to and including Devonian in age. The mountain extends southwestward for a distance of about forty-five miles, and divides the valley lengthwise into two narrow valleys which average from five to ten miles in width. The Massanutten syncline, however, which involves the Martinsburg shale (Ordovician) at the surface, continues for a considerable distance both to the northeast and to the southwest of the north and south ends of the mountain proper. The valley bottom is developed on folded limestone and shales of Cambro-Ordovician age, underlain by quartzites, sandstones, and shales of Lower Cambrian age which, because of their structure and greater resistance, are exposed along the northwest flank of the Blue Ridge. No igneous rocks are known to occur in the valley proper north of the latitude of northern Rockingham County. The valley rocks are faulted, but in some localities at least the faulting appears to be slight, since the displacement is frequently not great enough to cut one or more formations. Bassler has recognized faulting at Winchester, one of the localities of highest intensity (VI R.-F. scale), during the earthquake of April 9, 1918. He says:5 “Although the full geologic structure in the vicinity of Winchester could not be determined because of lack of continuous exposures, the quarries and other outcrops just west and east of the town indicate that by faulting a band of Lower Ordovician dolomitic limestones has been interpolated between a band of Stones River limestones on the west and argillaceous limestones and shales of Chambersburg and Martinsburg age on the east.” Faulting occurs at the base of Little North Mountain along the northwest side of the valley, and along the northwest front of the Blue Ridge on the southeast side of the valley a great overthrust fault, which apparently follows the Blue Ridge, has a horizontal displacement in places of at least four miles. It seems probable, therefore, that the seismic disturbance of April 9, 1918, had its origin in one or more of the faults which characterize the region.

Structural Features of the Grey Granites of Aberdeenshire

1945 ◽  
Vol 82 (5) ◽  
pp. 189-204 ◽  
Author(s):  
James Cameron
Keyword(s):  
Structural Features ◽  
Igneous Rocks ◽  
The North ◽  
The City ◽  
North And South

INTRODUCTIONThe area containing the quarries discussed in this paper extends inland from the City of Aberdeen for a distance of about twenty miles and is bounded on the north and south by the rivers Don and Dee. This is the area of the Newer Granites of Aberdeenshire, which C. B. Bisset has described in “A Contribution to the Study of Some Granites near Aberdeen” and has divided the acid igneous rocks into:—1. The Skene Complex: consisting of diorite, adamellite, grey granites, transition types and minor intrusions.2. Later Group: consisting of coarse red granites.

A New Fluid-Flow Model for the Genesis of Banded Iron Formation-Hosted Martite-Goethite Mineralization, with Special Reference to the North and South Flank Deposits of the Hamersley Province, Western Australia

2020 ◽  
Vol 115 (3) ◽  
pp. 627-659
Author(s):  
Caroline Perring ◽  
Matt Crowe ◽  
Jon Hronsky
Keyword(s):  
Physical Process ◽  
Vadose Zone ◽  
Process Model ◽  
Fluid Phase ◽  
Mining Area ◽  
Iron Formation ◽  
Banded Iron Formation ◽  
Ore Formation ◽  
The North ◽  
North And South

Abstract The North and South Flank deposits are located on the flanks of the Weeli Wolli anticline at Mining Area C in the central Hamersley Province. Supergene martite-goethite mineralization is hosted within the Marra Mamba Iron Formation and is developed over a strike length of more than 60 km. This multibillion metric ton resource has been drilled out on a 150- × 50- to 50- × 50-m grid, thus providing us with an unprecedented data set for analysis. This study synthesizes the drill hole data and presents a physical process model that can account for the observed distribution of mineralization. A fluid and mass flux model is proposed which envisages a three-stage process: (1) leaching of Fe from banded iron formation (BIF) in the vadose zone by reduced, acidic, meteoric-derived fluids; (2) penetration of an Fe-rich supergene-fluid plume, driven by gravity and focused by bedding-parallel permeability into the body of ambient alkaline groundwater, effecting nonredox, mimetic replacement of magnetite by hematite and of the gangue minerals (carbonate, silicate, and chert) by goethite coupled with the release of silica into the fluid phase; and (3) a change from silica leaching to silica deposition on the downdip margins of the system before the ore-fluid plume is eventually diluted and becomes indistinguishable from the surrounding body of groundwater. Despite the undoubted secondary role played by structurally enhanced permeability, the primary control on ore-fluid hydrology is gravity-driven flow along bedding planes. This central observation explains every observed feature of the three-dimensional distribution of martite-goethite mineralization, and the inherited structural architecture simply provides the context for this process to play out. This type of control is by no means obvious–the ingress of meteoric fluids during later lateritic weathering of the mineralization does not show this control and produces broadly subhorizontal, bedding-discordant zones of overprinting. The fundamental control exerted on the distribution of martite-goethite mineralization by bedding-plane permeability within BIF horizons suggests that the supergene ore-fluid plume created its own porosity via the relevant ore-forming reactions, and that these were in turn controlled by bedding. A corollary of the pseudomorphic replacement process, both the generation of hematite after magnetite and goethite after gangue phases, is that it typically introduces porosity. The mineralizing process thus creates porosity (and potentially permeability) and is likely to be self-propagating as long as there is continuous supply of ore fluid. This putative active porosity-generation process may be an important clue as to the unique conditions of martite-goethite ore formation. Indeed, it may be that the distribution of magnetite is the critical controlling feature of these ore systems, as the nonredox transformation to hematite not only releases Fe2+ to the fluid phase but concurrently introduces porosity. Further research is required to formulate a comprehensive chemical (as opposed to physical) process model for supergene martite-goethite ore formation. Based on the physical process model presented here, the development of a large-scale martite-goethite mineralizing system requires continued delivery of unleached BIF (and, perhaps ultimately, previously mineralized martite-goethite ore) into the vadose zone. The Hamersley Province has been undergoing significant uplift since at least 60 Ma. Preliminary dating of martite-goethite ores from Mining Area C indicates that they formed at about 45 Ma, at a time when the local climate was temperate and wetter than today. The combination of ongoing uplift and a wet, temperate climate is likely to be the key to the widespread formation of martite-goethite deposits in the Hamersley Province.

The palaeomagnetism of some igneous rocks from Antarctica

Polar Record ◽  
1961 ◽  
Vol 10 (67) ◽  
pp. 349-352 ◽  
Author(s):  
D. J. Blundell
Keyword(s):  
Continental Drift ◽  
Unknown Origin ◽  
Igneous Rocks ◽  
Climatic Zones ◽  
The North ◽  
North And South

A century ago geologists first began to consider the possibility of large lateral shifts of the continents. Reconstructions of the ancient positions of the continents have been proposed at various times since to try to explain past climatic zones, faunal distributions, similar orogenic sequences and structural trends on adjacent continents, and many other detailed geological events. Prominent amongst the hypotheses is that generally known as the Continental Drift Hypothesis, developed independently by F. B. Taylor and A. Wegener fifty years ago and later modified by A. L. du Toit andothers. In this hypothesis two primeval continents, Laurasia and Gondwanaland, are supposed to have formed at the north and south poles, to have broken up and possibly to have grown, and the pieces to have drifted to the positions of the present continents. The continents are moved around by forces of unknown origin and their interaction with each other and with the substratum gives rise to orogenesis. Due in large part to the lack of any known forces capable of producing these movements the hypothesis is nowadays less favoured than others requiring no drift. Recently, however, the study of palaeomagnetism has provided an independent line of evidence. From it the ancient latitudes and orientations of the continents can be worked out and relative displacements may possibly be revealed.

Multiple Deformation in Archean Rocks of the Vermilion District, Northeastern Minnesota

1971 ◽  
Vol 8 (4) ◽  
pp. 423-434 ◽  
Author(s):  
P. R. Hooper ◽  
R. W. Ojakangas
Keyword(s):  
Iron Formation ◽  
Rock Outcrop ◽  
Kink Bands ◽  
Precambrian Rocks ◽  
The North ◽  
Multiple Deformation ◽  
North And South ◽  
East West

The structure of the Precambrian rocks of the Vermilion district is critically examined. It is demonstrated that two significant deformations (F1 and F2) have affected the area in addition to a later set of faults, joints, and kink-bands (F3). The F1 folds are tight to isoclinal with gently plunging axes and vertical axial planes trending west–northwest and containing iron formation and greenstones in anticlinal cores. The second deformation forms a series of open to close asymmetric folds (F2) with steep axes and vertical axial planes trending east–west. Of these, F1 has the more significant effect on the pattern of rock outcrop and it is suggested that it is directly related to the diapiric rise of the batholiths lying immediately to the north and south. The F2 folds may be accounted for by further compression between the two granite bodies.

Gravity modelling of the lherzolite body at Lers (French Pyrenees); some regional implications

1985 ◽  
Vol 122 (1) ◽  
pp. 51-56 ◽  
Author(s):  
Helen J. Anderson
Keyword(s):  
High Temperature ◽  
Seismic Data ◽  
Gravity Anomalies ◽  
Sole Source ◽  
Gravity Survey ◽  
Gravity Modelling ◽  
Maximum Volume ◽  
The North ◽  
High Temperature Metamorphism ◽  
Central Pyrenees

AbstractLherzolites outcrop throughout the North Pyrenean Zone of the Pyrenees and are everywhere associated with metamorphosed carbonates. It has been suggested that heat from the cooling of the lherzolites was responsible for the high temperature metamorphism of the carbonates. A gravity survey reported here shows that the volume of the lherzolite body at Lers is approximately 0.8 km3. The maximum volume of carbonates that such a body could metamorphose is 3.2 km3. This latter value is so much less than the volume of carbonates inferred from field mapping that the lherzolite body cannot have been the sole source of heat for metamorphism of the carbonates.It has been suggested from seismic data that there is a step in the Moho beneath the North Pyrenean Fault in the central Pyrenees. Gravity anomalies reported here show that either the step is less than 10 km high or that the density contrast is very low at the base of the crust in the Pyrenees.

Sign in / Sign up

Export Citation Format

Share Document