Henry C. Halls

The use of converging remagnetization circles in palaeomagnetism

Abstract A method based on converging circles of remagnetization is used to find the direction of a secondary component of magnetization that has overprinted two areas of Precambrian rocks in the Lake Superior region of North America. It is shown that the method can successfully recover a secondary direction in cases where the coercivity spectra relations between two superimposed remanences prevent the use of other conventional palaeomagnetic techniques.

The Matachewan dyke swarm, Canada : an early Proterozoic magnetic field reversal

Abstract Consistent age relationships between oppositely magnetized dykes of the 2.45 Ga Matachewan dyke swarm suggest that only a single magnetic field reversal occurred during the period of igneous activity. The magnetic field throughout most of this time was characterized by a SSW declination and shallow negative inclination but reversed toward the waning stages of magmatism. The new paleomagnetic data provide the oldest known magnetic reversal for which the relative reversal sense is known.

Multiple episodes of dike emplacement along the northwestern margin of the Superior Province, Manitoba

Three stable magnetization components A, B, and C have been isolated from a set of dikes that have been collectively referred to as the Molson swarm. Positive baked contact tests and square-shouldered thermal decay curves indicate that these components were acquired during three periods of dike emplacement. Dikes carrying A with a pole at 15.4°N and 96.5°W (A95 = 4.1°, N = 16 sites) have been dated by the U-Pb method at 1883 Ma and are closely related to the Paleoproterozoic Trans-Hudson Orogen. Ages of B with a pole at 27.1°N and 140.8°W (A95 = 3.8°, A95 = 3.0°, N = 30) and C with a pole at 40.7°N and 156.3°W (A95 = 9.7°, N = 19) are estimated at 2170–2120 Ma, on the basis of paleomagnetic characteristics that are distinct from the A component, the analysis of metamorphic history of the Pikwitonei Domain, and comparison with well-dated paleomagnetic poles from mafic igneous rocks in the Superior Province. The associated B and C dikes were emplaced in normal and reversed geomagnetic fields when the northwestern margin of the Superior Province experienced initial rifting prior to the Trans-Hudson Orogen. These earlier dikes may correlate with known 2094 Ma mafic magmatism documented in the lower Hurwitz Group of the Hearne Province. A stable magnetization component D with a pole at 20.9°N and 133.6°W (A95 = 7.6°, N = 30 samples) was also identified from unbaked granulites throughout the Pikwitonei Domain. This component predates all dike intrusion in the region, has an age range of 2200–2170 Ma, and may have been caused by earlier uplift of the northwestern Superior Province.

Uplift structure of the southern Kapuskasing zone from 2.45 Ga dike swarm displacement

The ca. 2 Ga Kapuskasing zone, a 500-km-long belt of uplifted crust, cuts across the regional structural trend of the Archean Superior Province in Canada and disrupts the continuity of 2.45 Ga Matachewan dikes that form a major swarm in its vicinity. As expressed by positive gravity and magnetic anomalies, it appears to end about 100 km east of Lake Superior. Until now, the nature of this termination has remained obscure. Regional variations in paleomagnetic polarity and in the intensity of feldspar clouding caused by magnetite exsolution are two physical properties of the dike swarm that depend upon postemplacement erosion level. They show that the Kapuskasing zone, as a narrow, fault-bounded crustal uplift, continues, after sinistral offset, for a further 60 km to the southwest before terminating in a major cross fault. Dual magnetic polarity and variable levels of feldspar clouding are features found in many other Proterozoic dike swarms worldwide, and therefore, in principle, may be used to document other examples of craton response to tectonism.

Magnetic susceptibility anisotropy of rocks saturated with ferrofluid: a new method to study pore fabric?

Abstract The shape effect of ferrofluid-filled cavities on the anisotropy of magnetic susceptibility (AMS) is examined for different geometries. Principal directions of magnetic susceptibility correspond to principal axes of the shape of the cavity. Elongation directions and axial ratios are reflected in the AMS response which for simple geometries is explained by the anisotropy of the demagnetizing factor. The experiments can be carried further for the analysis of pore structure in natural rock samples. Techniques developed for the injection of fractionated ferrofluid into a porous sandstone successfully lead to a homogeneous saturation of the sample. The method potentially provides a new means to determine three-dimensional pore fabric anisotropy and to relate it to the anisotropy of various petrophysical parameters.

A geophysical investigation concerning the continuation of the Clarendon-Linden fault across Lake Ontario

More than 400 km of high-resolution seismic and magnetic data were collected over central Lake Ontario to investigate a possible lakeward continuation of the Clarendon-Linden fault structure of western New York State. The seismic data show that a bathymetric lineament that crosses the lake along the projected strike of the fault is a west-facing partly buried bedrock ridge, here called the Scotch Bonnet Rise. Its 30-m relief is similar in magnitude and orientation to observed offsets on the north-striking Clarendon-Linden fault. Magnetic data from the lake and earlier magnetic and gravity data collected in western New York show that the Scotch Bonnet Rise and the Clarendon-Linden structure coincide with the east flank of a linear magnetic trough and the west flank of a series of circular Bouguer gravity highs. The source of these geophysical anomalies is clearly in the Precambrian basement, suggesting that the observed deformation in the Paleozoic section is superimposed on a pre-existing Precambrian structure. Although the unconsolidated lake sediments show no evidence for Holocene movement of the fault, the geophysical evidence indicates that the Scotch Bonnet Rise is a continuation of the Clarendon-Linden structure and that together they form a linear feature more than 150 km long.

Crustal uplift in the southern Superior Province, Canada, revealed by paleomagnetism

Abstract A change in the polarity of magnetization with depth in the 2.45 Ga Matachewan dyke swarm is used to document vertical crustal movements that occurred at ∼1.9–2.3 Ga along the Kapuskasing Structural Zone, a 500-km-long fault zone that transects the Archean Superior Province of Canada. At shallow crustal levels dykes have a primary magnetization dominantly of one polarity, but at greater depths (∼20 km down) a polarity change occurs associated with the growth of exsolved magnetite in feldspar due to slow crustal cooling after cessation of Matachewan igneous activity. Regions of the dyke swarm with one dominant polarity are separated from those with opposite polarity by major faults. Using this polarity distribution and associated variations in the intensity of feldspar clouding and hydrous alteration, maps of the southern Superior Province are produced that display regional crustal tilting on which are superimposed more local fault-bounded blocks associated with the Kapuskasing zone. Some of these blocks have been recognized for the first time as a result of this study. The paleomagnetic work has also shown that the Matachewan swarm is regionally distorted both within and north of the Kapuskasing zone, and originally had a more radial disposition. This widespread distortion suggests that the lower crust was still relatively ductile at the time of deformation, perhaps due to high heat flow associated with the waning stages of the Matachewan mantle plume beneath.

Separation of multicomponent NRM: combined use of difference and resultant magnetization vectors

For rock specimens carrying three superimposed remanence components with partially overlapping coercivity spectra, a modified form of the vector difference method proposed by Hoffman and Day (1978) can lead to greater precision in the estimate for the direction of the intermediate coercivity component. The modification is that the convergence point of great circles defined by differenceand resultant vectors is used to determine the direction of the intermediate component, rather than by using great circles defined only by difference vectors as in the Hoffman-Day technique.

Magnetic pore fabric analysis: Verification through image autocorrelation

The pore fabric of sandstones is studied by impregnating porous samples with a magnetic suspension (ferrofluid) and subsequently measuring their anisotropy of magnetic susceptibility (AMS). Similar to the use of natural AMS for estimation of grain fabric, we determine average orientation and shape anisotropy of pore bodies in three dimensions. This new approach overcomes restrictions of previous methods of pore structure characterization. The interpretation of AMS in terms of pore fabric anisotropy is tested by comparison with pore shape observed in thin section. An image analytical technique is used to derive statistically the elongation direction of digitized pore cross sections. By computing the two-dimensional autocorrelation function we obtain section ellipses reflecting average pore shape and orientation. Combination of ellipses of three mutually perpendicular images leads to fabric ellipsoids comparable to AMS. Comparison of the two anisotropies shows a close correspondence between the principal axis directions of AMS and fabric ellipsoids. Owing to the bedded nature of the samples, the ellipsoids are of oblate shape with minimum axes perpendicular to bedding. However, even small anisotropies within the bedding plane can be detected successfully. Axial ratios of AMS and fabric ellipsoids correlate closely if rock sample AMS is corrected for pore directional scattering. The method represents a powerful tool to analyze pore fabric of sedimentary rocks and to study its effects on petrophysical properties.

Significance of anisotropy of magnetic susceptibility fabrics in Proterozoic mafic dykes, Hopedale Block, Labrador

Abstract Anisotropy of magnetic susceptibility fabrics have been studied in a suite of mafic early Proterozoic (Kikkertavak) dykes and two mid-Proterozoic (Kokkorvik) sills in the Hopedale Block of Labrador. Two sets of en-echelon dyke arrays from the Kikkertavak Swarm are considered to represent two linear dyke intrusions at depth. Three main fabrics were revealed in the Kikkertavak dykes. In the first, the maximum susceptibility axis is sub-horizontal or gently inclined and parallel to the dyke walls and is interpreted as the primary magma flow direction. A second fabric affects only dykes along strike from a much younger dyke and is interpreted as the result of local stress anisotropy. The third fabric with significantly lower susceptibility is only locally developed in the southeast, near the mid-Proterozoic Kanairiktok Shear Zone and is also interpreted as the result of local superimposed tectonic stress. The Kokkorvik sheets preserve a single flow-induced fabric, sub-parallel to the measured crack propagation lineation, indicating that the magma flow was parallel to the crack propagation direction. The gently inclined magma flow direction inferred for the Kikkertavak dykes is believed to reflect later-stage magmatic emplacement when consolidation had already commenced. At this stage, the buoyancy contrast between magma and host rock had been reduced to the point when the initially vertical magma flow was no longer sustainable and was replaced by lateral flow.