Patrick T. Taylor

Geologic implications of aeromagnetic data for the eastern continental margin of the United States

An aeromagnetic survey extending from the Gulf of Maine to the tip of Florida was conducted by the U. S. Naval Oceanographic Office between 1964 and 1966. Flight traverses were flown in a northwesterly direction at right angles to the geologic grain. The flight lines were approximately 800 km long and had an 8‐km separation. The survey traversed part of the New England, Piedmont, and Coastal Plain provinces and extended some 320 km beyond the continental shelf into the Atlantic Ocean. Despite the wide flight‐line spacing, numerous geological and structural features became apparent from this survey. Interpretation of these features was aided by using the available gravity and seismic data in addition to the State and Provincial geologic maps. The residual aeromagnetic map shows a continuous magnetic high on or near the continental slope as far south as the 31st parallel. At about the 36th parallel, this east‐coast magnetic anomaly splits into two branches, and both of them parallel the 850‐fathom contour. ...

The Canada Basin: Aeromagnetic constraints on structure and evolution

Abstract Aeromagnetic data over the Canada Basin and Alpha Ridge-Ellesmere Island junction are re-examined together with available bathymetry, free-air gravity and magnetic source depths calculated from the aeromagnetics. The plate tectonic evolution of this area remains largely conjectural; for example, the data from the basin and the adjacent continental margins have not settled the Alaska rotation controversy. Did the Canada Basin open by a rotation of the Alaska and Chukchi blocks away from the Canadian Arctic margin, or did the southern Canada Basin open by motion of the Chukchi block parallel to the Alaska North Slope? What we do conclude is: 1. (1) The southern Canada Basin consists of oceanic crust produced by spreading from a now-extinct axis marked by a gravity high and magnetic low. The opening took place largely or entirely during the time of the M-Series (155-115 m.y. B.P.) although the lineations are too few and too irregular to allow certain correlation to the reversal time scale. 2. (2) The central Canada Basin, north of Prince Patrick Island, probably opened by spreading normal to the margin. The segmented character of the magnetics and perhaps topography, suggests small transform faults (ca. 100 km spacing) extending at least several hundred kilometers into the basin. If these trends are real, major subsequent motion parallel to the Canadian Arctic margin can be ruled out. 3. (3) We speculate that the southern Canada Basin extinct axis extended to a triple junction near 79°N, 140°W, with one rift trending toward Ellesmere Is and the other into the Mendeleev Abyssal Plain. 4. (4) The transition between the low magnetic amplitudes of the southern Canada Basin to the high amplitudes of the northern Canada Basin-Alpha Mendeleev Ridge province is gradual and linear anomalies extend through the transition. 5. (5) The area north of Ellesmere Island can be divided into five discrete magnetic anomaly provinces, some of them sharply demarcated by major lineaments (faults?) which may provide plate kinematic constraints. The Alpha Ridge basement is depressed by 3–4 km along a 100 km wide magnetic basement trough along the Alpha Ridge-Ellesmere Is. margin.

Shoreline changes along the Rosetta-Nile Promontory: Monitoring with satellite observations

Abstract A study to test the effectiveness of satellite data for monitoring shoreline change is reported, the rapidly changing Rosetta Promontory of the Nile Delta, Egypt, being selected as the study area. Landsat multispectral scanner (MSS) image data in the near infrared (0.8–1.1 μm, 80 m resolution) covering a 15 year period (1972–1987) at roughly 3 year intervals were compiled. These were digitally overlain, registered and differenced relative to the initial 1972 shoreline, and composite changes (erosion and accretion) are mapped. The area studied is one undergoing both aggradation and degradation, depending on location, sediment supply and man-made barriers. It was found that the general trend of the regional processes can be monitored with Landsat imagery due to its repetitive coverage and good spectral contrast of land and sea. Improved satellite imagery with higher resolution should be a valuable tool for complementing traditional shoreline monitoring surveys in easily eroded, lowlying areas such as the Nile Delta.

Unique thermoremanent magnetization of multidomain sized hematite: Implications for magnetic anomalies

Intense magnetic remanence (100^1000 A/m) associated with MD hematite and/or titanohematite and associated with high Koenigsberger ratios (40^1000) indicate that magnetic remanence may dominate the total magnetization if these minerals are volumetrically significant. Titanohematite behaves similarly to hematite and, thus, the grain size dependence of TRM acquisition in hematite is considered as a generalization. The transition between truly MD behavior and tendency towards SD behavior in hematite has been established to be between grain sizes of 0.1 and 0.05 mm. In contrast to magnetite and titanomagnetite, hematite exhibits inverse grain size dependence, with MD hematite acquiring a relatively intense TRM in the geomagnetic field, comparable to sub-micrometer sized magnetite and only an order of magnitude less than SD magnetite. Consequently MD hematite (and by analogy titanohematite) remanence may be of significance as a source of magnetic anomalies at all scales. MD hematite exhibits TRM weak field acquisition behavior that is different from all other magnetic minerals, being the only magnetic mineral having an REM (TRM/SIRM) value E0.1 for TRM acquisition in the geomagnetic field. The very different TRM behavior of MD hematite in contrast to magnetite is due to two factors. The first is the lesser influence of demagnetizing energy with respect to wall pinning energy, at temperatures almost up to the Curie temperature for hematite. The second is the greater importance of the magnetostatic energy in the applied field, which for hematite dominates the total energy at high temperatures. fl 2000 Published by Elsevier Science B.V. All rights reserved.

Microstructural observations of polycrystalline diamond: a contribution to the carbonado conundrum

Abstract The formation mechanism of carbonado, a polycrystalline variety of diamond, remains unresolved. Here we report a microstructural analysis of systematic defects and inclusions in carbonados from Brazil and the Central African Republic (CAR). Transmission electron microscopy (TEM) has revealed the presence of pervasive defect lamellae in both the CAR and Brazilian carbonados that are distinct from planar defects observed in synthetic polycrystalline diamond (PCD). Analyses of included minerals by energy dispersive spectroscopy (EDS) yielded strong evidence for metallic Fe, Ti, and Si, as well as SiC and Fe–Cr alloys, within the diamond matrix. Our results offer strong evidence for a close genetic relationship between Brazilian and Central African carbonados, suggesting formation within a united landmass during the late Archaean. The data do not preclude carbonado nucleation by meteoritic impact, though this issue remains contentious. The role of water in the growth of carbonado merits further exploration.

INVESTIGATION OF MAGNETIZATION AND DENSITY OF A NORTH ATLANTIC SEAMOUNT USING POISSON’S THEOREM

The relationship between the gravitational and magnetic potentials caused by a uniform distribution of mass and magnetization may be used to obtain independent information about these physical properties. The general relationship in the frequency domain between the Fourier transforms of the gravity and magnetic anomaly fields is established through the Poisson theorem. The discrete Fourier transforms of the sampled continuous functions are used in an analysis which leads to a system of linear equations involving terms in density, magnetization, and calculated finite Fourier‐series coefficients. A least squares solution of the system yields the three components of the total magnetization vector divided by the density. From these results, the direction of total magnetization and the minimum of the Koenigsberger ratio Q can be determined uniquely. The remanent magnetization direction and certain other information can be derived for special cases in which the value of one or more of the physical property term...

Magsat anomaly data over the Kursk region, U.S.S.R.

Abstract A new method of reducing Magsat orbits was developed to produce a map of the Kursk magnetic anomaly at satellite altitude. Two different techniques to estimate the direction of magnetization were applied to the components of the anomaly to determine the presence of remanent magnetization. We propose that there is a significant component of remanent magnetization, however, we are unable to determine the importance of remanence relative to induced magnetization.

The role of hematite-ilmenite solid solution in the production of magnetic anomalies in ground- and satellite-based data

Abstract Remanent magnetization (RM) of rocks with hematite–ilmenite solid solution (HISS) minerals, at all crustal levels, may be an important contribution to magnetic anomalies measured by ground and satellite altitude surveys. The possibility that lower thermal gradient relatively deep in the crust can result in exsolution of HISS compositions with strong remanent magnetizations (RM) was studied for two bulk compositions within the HISS system. Samples from granulite-terrane around Wilson Lake, Labrador, Canada contains titanohematite with exsolved ferrian ilmenite lamellae. Other samples from the anorthosite-terrane of Allard Lake, Quebec, Canada contain ferrian ilmenite with exsolved titanohematite lamellae. In both cases, the final exsolved titanohematite has similar Ti content and carries dominant magnetic remanence with REM (=NRM/SIRM, where NRM is the natural remanent magnetization and SIRM is the saturation isothermal remanent magnetization) that is comparable to the Ti-free end member. The RM was acquired prior to exsolution and the ilmeno-hematite-rich rock possesses thermal remanent magnetization (TRM), whereas rocks with hemo-ilmenite possess chemical remanent magnetization (CRM). In both cases, we found fairly large homogeneous grains with low demagnetizing energy that acquired intense RM. The magnetism of the ilmeno-hematite solid solution phases is not significantly perturbed by the continuous reaction: ilmeno-hematite≧titanohematite solid solution. Hence, the occurrence of HISS in rocks that cooled slowly in a low intensity magnetic field will have an intense magnetic signature characterized by a large REM.

A possible impact origin for the Bangui magnetic anomaly (Central Africa)

Abstract Bangui is one of the most impressive magnetic anomalies on Earth and by far the largest over Africa. At Magsat altitudes, it has an amplitude of 28 nT and covers an area of 700,000 km 2 . Evidence presented suggests that the anomaly might be related to a large impact structure early in the history of the Earth: north-south, west-east, and vertical derivatives of the 5-min topographic data base all reveal a double ring structure with outer ring diameter ~ 810 km and inner ring diameter ~ 490 km. The Precambrian geology shows several small ancient basins within a large basin with a basement high in the middle such as found in large impact structures. The basement complex is early Precambrian and hence the impact must be of this age. The magnetic anomaly is interpreted as being due to strong remanent magnetism in the ancient crater floor and surrounds. The anomaly can be satisfactorily modelled by a 4.5-km-thick disc, 800 km in diameter beneath the ancient basins. To model the amplitude and shape of the anomaly requires an intensity of 10 A m −1 with direction D = N 18° W , I = + 25°, i.e., significantly different from the present dipole field. The magnetisation is likely to include strong shock remanence (SRM) acquired at the time of impact, thermal remanence (TRM), partial thermal remanence (PTRM), thermochemical remanence (TCRM) and chemical remanence (CRM) acquired soon after the impact. All depend on the size of the impact and all contribute to the high intensity required to explain the anomaly.

An interpretation of the Magsat anomalies of central Europe

Abstract Magsat studies of central Europe have revealed correspondence between tectonic elements /geological structures and long-wavelength satellite-altitude anomalies. The most striking correlation is the two Magsat anomalies (one positive and the other negative) associated with either side of the Tornquist-Teisseyre Zone (TTZ). The TTZ, which extends from the Black Sea to the Baltic Sea, represents the structural boundary (suture) between the younger, thinner, and hotter crust of the Hercynide or Paleozoic block (negative anomaly) and the older, thicker, and colder crust of the East European Precambrian Platform (positive anomaly). This produces a gradient of approximately 18 nT (peak-to-trough) and a half-wavelength of 720 km along a Magsat orbit at 325 km altitude. This anomaly pair is the result of the juxtaposition of these two significantly different crustal blocks. To model this field, two irregular three-dimensional bodies were used to represent the Precambrian and Paleozoic crust. The former was modeled with a normal magnetization vector while the latter was modeled with a reverse vector. Aeromagnetic anomalies in western Germany and rock magnetic measurements from Germany and Scandinavia support a model in which reversely magnetic geologic units, extending from upper-crustal metasediments to mid-crustal sources produce this negative anomaly.