Earthquakes are common in Atlantic Canada, but instrumental^ determined magnitudes have rarely exceeded 5 on the Richter scale. One exception is the Grand Banks earthquake of 1929 which had a magnitude of 7.2. However, it is probable that the Richter scale overestimates earthquake magnitude in Atlantic Canada. A recalculation of magnitudes for many ot the non-inslrumentally recorded earthquakes, using the various empirical relationships between felt area and magnitude, rather than maximum ntensity and magnitude, shows a reduction in magnitudes for historically reported earthquakes.
Because of the paucity of available instrumentation - there has been only one three-component seismograph in the region for a part of the time - the detailed pattern of epicentres and focal depths is poorly known in Atlantic Canada. Attempts to correlate epicentres with known geological features have generally failed because of the uncertainty in the epicentral positions. Nevertheless, several hypotheses have been advanced to explain the earthquake pattern in Atlantic Canada: 1 ) movement on faults, 2) glacial rebound, 3) association with igneous intrusions.
An analysis of these explanations, shows that the five major known episodes of faulting all happened between Precambrian and early Tertiary times and are not at present active. Furthermore, glacial rebound in detail cannot be related to the distribution of earthquakes. Association with igneous intrusions is a working hypothesis, but recent evidence from Quebec and New England suggest a mid-crustal focal depth for those earthquakes for which a good solution is available. It is therefore suggested that earthquakes are not associated with the intrusions as such, but possibly only with deep channels that led the magma to the upper crust. The repeated motion on these deep magmatic conduits is provisionally attributed to the cooling of the existing thermal high that underlies New England and Atlantic Canada.
Virtually all recent attempts to relate plate tectonics and mineral deposits have adopted a similar approach whereby major plate boundary regimes (e.g., spreading, subducting, transform faults, continental collision, etc.) are first described in broad terms. This is followed, or accompanied by, descriptions of the mineral deposits considered to be associated with each plate tectonic regime. A conventional approach such as this produces a list of plate-related deposits impressive in number and diversity. Closer examination of these documentation attempts, however, reveals that apart from porphyry coppers, volcanogenic massive sulphides and perhaps carbonatites, the deposit-types so considered are relatively small and inconsequential.
A different perspective is attained when instead of examining mineral deposits from the plate tectonic viewpoint (the conventional approach), plate tectonics are examined relative to a list of major deposit-types. When this is done, it is evident thai many deposits cannot readily be assigned to plate tectonic regimes or processes (e.g., sandstone Cu-U-V; Precambrian banded iron-formations; Kupferscheifer Cu, etc.). Part of the problem is that many major deposit-types occur in the Precambrian for which plate tectonic processes can be documented only with difficulty, if at all. Some major Precambrian deposit-types apparently unrelated to modern-style plate tectonics would be: banded iron-formation, layered gabbroic (Sudbury) and komatiitic (e.g., Western Australia) Ni deposits, anorthosite Ti, layered mafic complexes (Cr, Pt), and conglomerate U-Au. Other deposits occur in stable regimes and, indeed, seem to require the absence of plate tectonics (e.g.. sandstone Cu-U-V; Mississippi Valley Pb-Zn; stratiform barite and phosphorite). Thus correlation of plate tectonics and mineral deposits is hampered by: a) the difficulty in "pushing" plate tectonics into the oldest rocks where many of the world's major deposits occur, and/or b) the fact that many major deposit-types occur on the (continental) plate, not at the margins, and therefore must be considered the antithesis of plate tectonic-related deposits. With all these difficulties, one can only conclude, from the perspective of a spectrum of the world's major deposit-types, that plate tectonic theory is of limited use in understanding the origin and distribution of mineral deposits.
A program of integrated geophysical surveys has been developed within the Department of Energy, Mines and Resources in response to requests from Atomic Energy of Canada Ltd. for assistance in verifying the concept of deep underground storage of radioactive waste and in selecting suitable sites for a disposal vault. Both well established and innovative airborne, ground and borehole techniques are being tested for their usefulness in determining overall structure, lithological variations, rock quality, the character of specific fracture systems and long term stability of selected areas. Preliminary results from the Chalk River Nuclear Laboratories property illustrate the use of electrical, seismic and other methods in analysing complex fracture systems.
Earthquakes, whether natural or induced, pose a significant risk to the disposal of toxic wastes by burial or fluid injection in the crust. Methodology exists for assessing the ambient seismic risk at moderately low levels of probability, but for greater degrees of conservatism the assessment is essentially deterministic. Such estimates are considered appropriate for periods comparable to that of the available seismic history but an improved understanding of the nature of currently active seismic zones is required for the estimates to be applied with confidence to periods of time measured in thousands of years. The potential hazards associated with this natural seismic risk can be mitigated by appropriate engineering design and practice. Induced seismicity associated with mining excavations, thermally induced stresses or fluid injection can be controlled by appropriate engineering design and operational procedure.
Safe, permanent disposal of radioactive wastes requires isolation of a number of elements including Se, Te, I, Sr, Cs, Pd, U, Np, Pu and Cm from the environment for a long period of time. The aquatic chemistry of these elements ranges from simple anionic (I", 10"3) and cationic (Cs+, Sr++) forms to multivalent hydrolyzed complexes which can be anionic or cationic (Pu(OH)^,Pu(OH)t Pu02(C03)(OH)', Pu02C1+ etc.) depending on the chemical environment. The parameters which can affect repository safety are rate of access and compostion of groundwater, stability of the waste container, stability of the waste form, rock-water-waste interactions, and dilution and dispersion as the waste moves away from the repository site. Our overall research program on radioactive waste disposal includes corrosion studies of containment systems, hydrothermal stability of various waste forms, and geochemical behaviour of various nuclides including solubilities, redox equilibria, hydrolysis, colloid formation and transport, ion exchange equilibria and adsorption on mineral surfaces, and irreversible precipitation reactions. This paper discusses the geochemistry of I, Se, Te, Cs, Sr and the actinide elements and potential mechanisms by which the mobility could be retarded if necessary.
Atomic Energy of Canada Limited (AECL) in conjunction with other federal agencies, universities and commercial firms has undertaken a program to demonstrate that the burial of nuclear wastes deep into carefully selected geological formations is a viable and safe method of disposal. During the next three years, approximiately eight formations m the Canadian Shield, representing various classes of plutons, will be investigated in detail. This paper describes a case history of the investigation of one of these, the Lac du Bonnet batholith, Manitoba.
If granite bodies are to be used as receptacles for toxic waste materials, the presence or absence of barren fractures and the virgin stresses in the granite are of fundamental importance. Unfortunately, very little is known regarding the incidence of fractures, or stresses, which exist at depths (of about 1 km) in granite bodies. A simple analysis is presented of a hypothetical intrusion which indicates the magnitudes of stresses and the possible fracture development which may be expected in such bodies.