To unravel the mysteries surrounding the present structure
and past evolution of the continent requires the application
of the diverse methods by which geoscientists study the Earth.
The great success of LITHOPROBE has been its coordinated,
multidisciplinary approach. By bringing together geologists,
geochemists, geophysicists, and focussing their knowledge
and energy on major geotectonic problems, the scientific
study of those problems is enhanced far beyond the level
of individual contributions.
Some of the techniques used are very briefly described below:
- Geological Mapping
- Mapping the geology exposed on the Earth's surface is the foundation upon
which all LITHOPROBE transects are based.
- Structural Geology
- Structural studies involve determining the geometry
and style of folding that has affected the rocks. In
geometry and nature of the faults that cut the
Earth are established. These observations permit some
the direction of paleostresses.
- Igneous and Metamorphic Petrology
- The study of how rocks form and evolve when exposed
to high pressures and temperatures deep in the Earth.
- Stratigraphy and Sedimentology
- The study of how sediments and structures form near
the Earth's surface and evolve over time.
- Seismic Reflection
- A technique used to image boundaries between different
rock types and structures. Energy, usually generated
mechanically using large "vibroseis trucks",
is sent into the Earth. When the waves pass a boundary between different
materials, some of the energy is reflected back to the surface. These
are used to construct a high-resolution (e.g., 100 metres) map of
lithospheric structures. Linked with surface geological
maps, these reflection profiles
spearhead the detailed multidisciplinary interpretations.
[Click here for more information]
- Seismic Refraction
- Refraction surveys image subsurface structure at a
much lower resolution (e.g., 1 kilometre) than typical
However, refraction surveys provide detailed
velocity information that is essential for interpreting
data and for
determining the composition and state of the
lithosphere. Refraction surveys also record reflection
a large range of angles, providing another suite
of data to interpret
interface structure and composition. Refraction
surveys utilize large explosive sources to generate
propagate upwards of 600 km along a profile.
LITHOPROBE reflection and refraction surveys image
structure to depths
greater than 70 km.
[See the WWW pages describing the recent SNORE97
- Gravity and Magnetics Studies
- Measurements of the spatial variation of gravity can
be used to constrain the density of structures in
the Earth's crust.
Interpretations of density distributions are
often carried out in conjunction with seismic refraction/reflection
since density and seismic velocity provide a
constraint on structural modeling.
- The magnetic field measured at or above the Earth's
surface is dependent upon the magnetization and
iron content of the
rocks making up the crust. Magnetic anomaly data,
derived after subtraction of time variations and
can be a powerful interpretive tool for establishing
the geometry and nature of subsurface rock formations.
- Electromagnetic Studies
- Electromagnetic (EM) studies investigate the electrical
conductivity of the subsurface. Conductivity is a
physical property that
is independent of velocity and density. Instead,
it is extremely sensitive to composition, texture
and fluid content within
and between rocks. Highly conductive materials
include saline water in interconnected pores/fractures,
and silicate partial melts.
- Heat Flow and Geothermal Studies
- The Earth's interior heat drives tectonic processes.
Temperature dependent rheological properties control
zones of strength
and weakness in the crust and thus depths at
which tectonic motions take place. In addition, heat
for the formation of mineral deposits and the
maturation of hydrocarbons in sedimentary basins. Therefore,
measuring the temperature gradient in the crust and
thermal conductivity of rocks can provide useful
information for interpreting Earth structure and
- The techniques of paleomagnetism are based on measuring
the directions of magnetization "frozen" into rock
formations at their time of origin 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 determine the original latitude of the
rock. This information plays an important role in unravelling
the movements through geological time of terranes and continents
(ie. continental "drift").
- Physical Properties
- All geophysical observations relate in some way to
the physical properties of rocks -- seismic to compressional
velocities, gravity to density, magnetic to magnetic
susceptibility, electromagnetic to porosity and electrical
heat flow to thermal conductivity and heat production,
and paleomagnetism to various types of magnetization.
studies of such properties are a key to correct
of field data.
The processes involved in the formation and modification of the Earth's
lithosphere (igneous processes, erosion and formation of sediments, tectonic-metamorphic
processes) tend to have unique chemical signatures. Thus, geochemistry
another important sector of information used to form a more complete
model of the processes of crustal generation and evolution.
Geochronometry is a special branch of geochemistry, related
to isotope physics, that involves determining the time
of formation of rocks, minerals and fossils. Many different
techniques and isotope combinations are used depending
the specific target age and material. However, most
utilize the principle that radioactive isotopes present at
the "birth" of
a mineral will decay at a certain fixed rate. Measurement
of relative abundances of the "parent" and "daughter" isotopes
can determine the age of the rocks. Dating of surface
exposures of rocks is required to connect the observed
the geophysically-determined geometry and domains at depth.