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
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