U
nlike the laboratory-bound physicist,
structural geologists usually make measurements and gather data in the field,
where, because of poor exposures and the limited
view that erosion provides, structures may seem
chaotically arranged. Yet, when each measurement is tied to a particular geographic location
and represented on a map, and when sets of measurements are organized by orientation and
plotted on an appropriate graph, systematic relationships and patterns emerge that can be interpreted. In making such interpretations we have
achieved what Haskie Jim was alluding to, we have
found where to stand to view the structures to
bring order to what otherwise appears to be a
random phenomenon.
In pursuit of this objective we begin this
chapter by “finding our bearings” in terms of the
coordinate systems commonly employed to locate
outcrops in the field and to construct maps of
geological structures. This includes an introduction to the geographic coordinate systems used
for the Global Positioning System (GPS), which is
becoming the standard tool for structural
mapping (see frontispiece to Chapter 2). Also, we
introduce the Universal Transverse Mercator
(UTM) projection, used to project geographic
information from the curved surface of the Earth
onto a flat piece of paper. Structural geologists
should be capable of measuring the location of
key exposures where field data are taken, and
then constructing the map projections that
record the spatial distributions of these data.
Structural maps should convey the geometry of
structures to other geologists in a form that is
quantitatively precise and readily visualized and
analyzed.
A position vector uniquely determines the
location of every exposure and every point at
which structural data are collected in the field.
Vectors have many uses in structural geology,
from locating an exposure, to describing the
shape of folded strata, to representing quantities
such as displacement and velocity in the physical
laws that underlie the modeling of tectonic
processes. The position vector is introduced here
along with some of the vector concepts and notations useful for structural mapping. Position
vectors are defined with reference to a particular
coordinate system. UTM coordinates may not be
the best choice during a mapping campaign, and
usually they are not the most convenient for modeling and data analysis. To change from one coordinate system to another one uses transformation
equations, which make use of vector concepts and
operations.
Because much of the structural information
recorded at exposures can be reduced to the orientations in space of planar or linear elements, we
introduce the techniques to measure, record, and
analyze these orientations. Special projections are
used to depict a set of orientations on a flat piece
of paper – a common one being the stereographic
projection. Here, the basic procedures for plotting
orientation data on a stereogram are described in
such a way that they can be implemented on a
computer. Combinations of structural maps and
stereograms are used to visualize, respectively, the
spatial distributions and the orientations of geological structures. Examples are provided to illustrate how this can be done.
Finally we describe a modern mapping campaign that utilizes GPS technology to create a
precise structure contour map in a region where
four sets of faults intersect to disrupt and fold the
surrounding strata. With new technology the
mapping was accomplished in a fraction of the
time required using traditional methods, with
much greater control on the shapes of the
deformed strata and on the fault offsets and slip
directions because of the abundance of quantitative data. The GPS technology is rapidly evolving
so we anticipate a new generation of structural
maps that will provide the impetus for more
specific modeling of tectonic processes and the
data better to constrain those models.
2.1 Geographic coordinates and
map projections
2.1.1 Geographic coordinates: the
Global Positioning System (GPS)
The first order of business for any mapping project
is to locate oneself on Earth’s surface, either to
26
STRUCTURAL MAPPING TECHNIQUES AND TOOLS
nlike the laboratory-bound physicist,
structural geologists usually make measurements and gather data in the field,
where, because of poor exposures and the limited
view that erosion provides, structures may seem
chaotically arranged. Yet, when each measurement is tied to a particular geographic location
and represented on a map, and when sets of measurements are organized by orientation and
plotted on an appropriate graph, systematic relationships and patterns emerge that can be interpreted. In making such interpretations we have
achieved what Haskie Jim was alluding to, we have
found where to stand to view the structures to
bring order to what otherwise appears to be a
random phenomenon.
In pursuit of this objective we begin this
chapter by “finding our bearings” in terms of the
coordinate systems commonly employed to locate
outcrops in the field and to construct maps of
geological structures. This includes an introduction to the geographic coordinate systems used
for the Global Positioning System (GPS), which is
becoming the standard tool for structural
mapping (see frontispiece to Chapter 2). Also, we
introduce the Universal Transverse Mercator
(UTM) projection, used to project geographic
information from the curved surface of the Earth
onto a flat piece of paper. Structural geologists
should be capable of measuring the location of
key exposures where field data are taken, and
then constructing the map projections that
record the spatial distributions of these data.
Structural maps should convey the geometry of
structures to other geologists in a form that is
quantitatively precise and readily visualized and
analyzed.
A position vector uniquely determines the
location of every exposure and every point at
which structural data are collected in the field.
Vectors have many uses in structural geology,
from locating an exposure, to describing the
shape of folded strata, to representing quantities
such as displacement and velocity in the physical
laws that underlie the modeling of tectonic
processes. The position vector is introduced here
along with some of the vector concepts and notations useful for structural mapping. Position
vectors are defined with reference to a particular
coordinate system. UTM coordinates may not be
the best choice during a mapping campaign, and
usually they are not the most convenient for modeling and data analysis. To change from one coordinate system to another one uses transformation
equations, which make use of vector concepts and
operations.
Because much of the structural information
recorded at exposures can be reduced to the orientations in space of planar or linear elements, we
introduce the techniques to measure, record, and
analyze these orientations. Special projections are
used to depict a set of orientations on a flat piece
of paper – a common one being the stereographic
projection. Here, the basic procedures for plotting
orientation data on a stereogram are described in
such a way that they can be implemented on a
computer. Combinations of structural maps and
stereograms are used to visualize, respectively, the
spatial distributions and the orientations of geological structures. Examples are provided to illustrate how this can be done.
Finally we describe a modern mapping campaign that utilizes GPS technology to create a
precise structure contour map in a region where
four sets of faults intersect to disrupt and fold the
surrounding strata. With new technology the
mapping was accomplished in a fraction of the
time required using traditional methods, with
much greater control on the shapes of the
deformed strata and on the fault offsets and slip
directions because of the abundance of quantitative data. The GPS technology is rapidly evolving
so we anticipate a new generation of structural
maps that will provide the impetus for more
specific modeling of tectonic processes and the
data better to constrain those models.
2.1 Geographic coordinates and
map projections
2.1.1 Geographic coordinates: the
Global Positioning System (GPS)
The first order of business for any mapping project
is to locate oneself on Earth’s surface, either to
26
STRUCTURAL MAPPING TECHNIQUES AND TOOLS