Studies of the development of fabrics in some naturally deformed limestones
The purpose of this investigation has been to study in detail the development of deformation fabrics in some naturally deformed limestones, with a view of testing the geological applicability of experimentally deduced origins of such fabrics in these materials. Flexurally folded limestones have...
| Summary: | The purpose of this investigation has been to study in detail the development of deformation fabrics in some
naturally deformed limestones, with a view of testing the
geological applicability of experimentally deduced origins
of such fabrics in these materials. Flexurally folded
limestones have been used to make comparisons between experiment and nature, by taking advantage of the approximately known character and variation of strain in these structures and using this information to deduce theoretically the deformation fabrics from place to place to compare with those actually observed in the structures. Two kinds of fabric problems are treated, ones dealing with gross changes in crystal orientation accompanying large strain, and those treating the origin of twinning lamellae in carbonate rocks in relation to applied stress, the so-called dynamic analysis. An extension of this analysis is made which allows quantitative information as to rock strain due to twinning (and translation gliding) to be obtained from thin sections.
Current theoretical treatments used in predicting fabric changes with strain in marble are found inadequate for predicting fabric changes with strain in flexure folds.
Exact derivations of these fabrics for folds have not been
made. Instead, an implication of a more general theory
treating development of fabrics in metal aggregates is used
to derive approximately the changes in c-axis orientations
with large strain by analogy with fabrics obtained from
experimental deformed Yule marble. Well defined fabric
changes with large strain involving both twinning and
translation gliding in individual crystals have not been
observed in the folds studied.
Predicted results for the dynamic analysis of an aggregate with isotropic c-axis distribution are derived with special reference to one of the folds studied. The stress distribution in plane strain is calculated for the structure starting with an already partly folded unconfined
layer of circular cross-section, and assuming it to be
loaded elastically with simple compressive forces applied
in the limbs directed normal to the axial plane. Twinning
deformation in individual crystals is treated by assuming
that the law of maximum resolved shear stress determines an
active twin set in each grain. The results obtained are
compared favorably with those observed in a natural fold.
Deformation fabrics from two small folds are given.
The first fold occurs in a large anticlinorium in western
Washington County, Maryland, in thinly bedded limestones
and shales of the Silurian McKenzie Creek formation. The
second is a drag fold on the eastern limb of a north-trending anticline in Carboniferous limestones and shales
located in upper Darwin wash, Darwin Hills, Inyo County,
California. In the Maryland fold, c-axis fabrics obtained
from the axial region show no preferred orientation due to
deformation, but a dynamic analysis of the twinning lamellae
is in good agreement with that expected in theory. The dynamic analysis is shown to be sensitive in depicting
small changes in twinning deformation throughout portions
of the body examined. The strains due to twinning are
compatible with bending in part of the structure. More
quantitative comparisons of the observed and expected deformation have shown that under a derived system of stress
at the axial plane, twinning deformation in 80% of the
grains in the aggregate has followed the law of maximum
resolved shear stress. The amount of twinning strain within
individual crystals varies with their orientation in the
stress field. The calculated visible (twinning) strains of
about 0.01 are considerably less than the strains computed
from the geometry of the fold of 0.25, and much of this
discrepancy may be due to fracturing (slip on planes parallel to bedding) during folding.
In the fold from Darwnn wash, observed fabrics cannot be related simply to the megascopic deformation in the fold. Preferred orientations of c-axes are thought to be partly due to veining in t he rock. Fabric changes due to twinning are however qualitatively correlated with shortening in the fold perpendicular to the axial plane, and a shear similar to that necessitated by the relation of the drag fold to the major anticline with which it is associated. Calculated visible strains are considerably less than those approximately deduced from fold geometry, but can be partially correlated with the observed deformation in the structure. Analysis of the deformation in nonhomogeneously strained individual crystals of these aggregates (Appendix II) shows that in addition to e{0112} twinning, translation gliding has occurred on most types of glide planes deduced for calcite from laboratory experiments. Microscopically these rocks appear to have undergone large deformation, exhibiting local cataclastic texture, much twinning and warping of individual crystals. However, well defined formation fabrics due to large strain have not been observed in them.
A second part of this investigation has dealt with the so-called nontwinned lamellae in calcite and dolomite. From these studies it has been concluded that such structure
are extremely thin (a few microns) but otherwise normal twin lamellae parallel to e{0112} in calcite and
f{0221} in dolomite, and are for this reason renamed microtwinned lamellae. In addition to interference colors,
these lamellae exhibit four different types of interference
fringes. A new method is presented, which utilizes the optical properties of the twins, for obtaining the
orientation of lamellae inclined at small angles to the
plane of a thin section. The utility of this technique
lies in the fact that it may be used to eliminate the
central "blind-spot" in twin lamellae fabric diagrams.
Some measurements of twin thicknesses, made using the new
orientation method, are given, together with calculations
which show that only an average and not a cumulative optical
thickness for a stack of lamellae superposed in thin section may be obtained using the method.
|
|---|