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A mass spectrometer allows precise measurements of isotopes contained in minerals. Their ratios determine their age.

In LITHOPROBE, geochronology is important in providing the detailed age control for surface rocks and intrusives that is essential to unravelling their local geological history. Isotopic dating is critically important where organic material and fossils are absent (as in most Precambrian settings) and is the only means that can securely establish timing of the intrusions and metamorphism that are associated with tectonic activity.
Dating of surface exposures is required to connect the observed rocks with the geophysically-determined geometry and domains at depth.

Isotopic geochronometric studies are planned along all transects to date the emplacement of all significant granitic and volcanic suites and the times of their metamorphism and deformation. Dates by several methods, including fission track counting, are used to establish times and rates of uplift of the metamorphic and plutonic rocks and basin deposits. World-class facilities for these studies exist at a number of Canadian universities, the GSC and the Royal Ontario Museum.

The techniques of paleomagnetism are based on measuring the directions of magnetization "frozen" into rock formations having minute quantities of iron at their time of origin (time of solidification) or at subsequent times when they have been reheated or metamorphosed. By making careful corrections for the present attitude of a rock formation (how it has been folded or tilted) and comparing these directions with the known magnetic field at the time of their magnetization, it is possible to calculate the original latitude of the rocks. The unravelling of the movements through geological time of terranes and continents from the record of the Earth's magnetic field contained in appropriate rocks is a complex process. However, paleomagnetic studies have had remarkable success in providing the framework for measuring continental drift and plate tectonic movements and contributing to the ultimate proof that such movements have occurred.
In recent years, a major achievement of paleomagnetism has been the measurement and confirmation of the complex assemblage processes that have occurred in the western Cordillera. These are now seen to have involved major latitudinal shifts (indicating up to thousands of kilometres of movement) and block rotations. Although fossil assemblages and facies have been important in providing other geological evidence for these movements, paleomagnetism offers the opportunity for direct measurement. Determination of the latitude of origin of rock formations and, in particular, that certain rock formations now juxtaposed may have had origins many hundreds or thousands of kilometres apart, is critically important in understanding the geotectonic developments within a transect and thus the evolution of the continent.

Paleomagnetic field work involves field sampling of the selected rock types with close geological control (rock unit, age, attitude). Knowing the age of the rock, either from geochronology or fossil control, is critical. Notice how the different disciplines must interact to be effective.

Physical Properties of Rocks

All geophysical observations relate in some way to the physical properties of rocks - seismic to compressional and shear velocities, gravity to density, magnetic to magnetic susceptibility, electromagnetic to porosity and electrical conductivity, geothermal to thermal conductivity and heat production, and paleomagnetism to various types of magnetization associated with rocks. Some of these properties must be determined as a requirement of the method itself; others are determined independently. Geophysical logging of boreholes made available by the mining industry can provide valuable information where this is applicable.
In many cases, it is important to determine physical rock properties under in situ conditions of temperature, pressure and fluid content. This requires specialized and complex equipment so that only a few laboratories in universities have appropriate capabilities. The appropriate rock samples are selected in the field usually with the collaboration of a mapping geologist, or are obtained from cores or cuttings from boreholes of opportunity from either the mining or petroleum industry. Particular success has been achieved in relating physical rock properties derived from borehole studies to seismic and electromagnetic data in mining areas.

For LITHOPROBE and other research, access to information about physical properties of a wide variety of rock types under varying conditions is highly valuable.

Summary on Earth Science Disciplines
Now that you’ve had a “crash course” on earth science methods, you’ll probably appreciate what efforts we must make to try to understand the structure and evolution of the very continent we live on. On its own, not one of these disciplines could provide sufficient information to even begin our understanding. But put them all together, along with the knowledge of the scientists who apply the methods, and then real scientific progress and understanding can be developed.
We hope that you have found our story on “Probing the Lithosphere” a fascinating one, and that in so doing you have even learned something. LITHOPROBE is acknowledged as the premier multidisciplinary earth science program operating on any continent or in any country. Canadians can be proud of our contribution to earth science and to understanding the very evolution of the land on which we live.

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