ABSTRACT
Over the past 12 years, many different computational methods or
variations
of existing methods have been proposed for determining paleostress
tensors
from fault populations and their slip directions. These methods
are all
based upon well-known relationships between stress and shear and
use iterative,
non-linear mathematical algorithms which seek to minimize the
angles between
the calculated maximum shear stress direction and the observed
movement
directions on each fault plane in a population. The solution
returned is
the best-fit paleostress tensor for the population.
By taking the Coulomb failure criterion into account, several
paleostress
analysis programs have been able to use linear, rather than
non-linear,
methods to solve for a paleostress tensor. The advantages of using
linear
equations is that they are less computationally-intensive and are
far easier
to solve.
A major problem with computational methods of paleostress analysis
is that very little work has been done on evaluating their
effectiveness
and/or possible limitations. If the techniques return results
consistent
with other methods of estimating paleostress directions, or with
various
kinematic analysis methods, they are often used by geologists. If
not,
an attempt may be made to explain why, but geological explanations
are
usually sought rather than criticizing the paleostress analysis
methods.
This study is an attempt to formulate the problem and to begin
systematically
examining it.
For my thesis project, I obtained several working versions of
paleostress
analysis computer programs. After much work, I decided to test two
of the
methods – those developed by Angelier and Reches. Artificial fault
populations
were created for these tests with a slip vector calculation
program which
I wrote specifically for that purpose. The artificial fault
populations
were created using exactly the same initial assumptions that the
paleostress
analysis programs used.
An artificial fault population is a set of fault orientations and
their
associated slip directions consistent with a predetermined stress
field.
For all of the fault populations created, the most compressive
principal
stress axis was vertical with a relative magnitude of +1.0 and the
least
compressive principal stress axis was oriented north-south with a
relative
magnitude of -1.0. Entering these populations into a paleostress
analysis
program should have, theoretically, returned the same orientations
for
the principal stress axes.
With this in mind, I chose to create several different types of
artificial
fault populations to test possible limitations in paleostress
analysis.
I used randomly-oriented fault populations, special-case fault
populations,
and fault populations which had data added or removed from them.
The results of these tests are that the two paleostress analysis
programs
I examined may work sufficiently well for certain types of
well-constrained
fault populations, but often give large errors when examining
special types
of fault sets such as conjugate faults, orthorhombic symmetry
faults, and
fault populations where all of the faults have very similar
orientations.
The paleostress analysis programs may also be sensitive to
measurement
errors and/or extraneous data depending upon several factors,
including
the orientations of the faults in question.
In conclusion, much more work is currently needed to further
examine
this topic and to begin to formulate general guidelines for
applying paleostress
analysis methods to fault populations gathered by geologists in
the field.
Schimmrich, S.H., 1991. Evaluation of computational methods of
paleostress
analysis using fault-striation data. Unpublished MSc.
thesis, State
University of New York at Albany. 394 pp., +xix
University at Albany Science Library call number: SCIENCE
Oversize
(*) QE 40 Z899 1991 S35
MS thesis digital
text pdf - 6.3MB pdf file
Return to MS Theses completed in the
Geological
Sciences Program, University at Albany