Three studies used magnetic fabric measurements directly as a strain
gauge in deformed rocks. Hirt et al. [1993] studied the layered
breccia of the Onaping Formation in the Sudbury Basin, Ontario,
Canada and developed a correlation between strain, measured by
R
/
analysis [ Ramsay and Huber, 1983] of devitrified
glass clasts, and AMS intensity. This correlation allowed Hirt et al. to
extend their geologic strain measurements to over 500 samples. The
regional strain pattern that emerged indicates that the Sudbury Basin
may have initially had a circular outcrop pattern, hence a meteorite
impact origin for the basin cannot be ruled out. More importantly, the
study indicates the care and detail needed to document a local, rather
than a universal, relationship between strain and magnetic fabric so
that a magnetic strain gauge can be applied confidently.
In a study of a strain gradient from relatively undeformed
meta-anorthosites to highly deformed ultramylonites of the Parry
Sound shear zone, Housen et al. [1994] used quantitative
petrologic measurements to determine the geothermometry of the shear
zone. From this information they inferred that the dominant magnetic
mineral in the rocks, magnetite, would have plastically deformed,
rather than rigidly rotated, during progressive deformation. The
change in AMS fabric orientation can then be used to quantify the
amount of simple shear strain in the rocks. This AMS-strain
correlation could be applied to other mylonites in the shear zone to
extend the geologic strain measurements, much like in the Hirt et
al. study. Housen et al. point out they were able to measure the
very high shear strains in the Parry Sound shear zone
(
=9-13, see Ramsay and Huber [1983] for definition
of 
) because the magnetite plastically deformed, rather than
rigidly rotated.
Richter et al. [1993] also used AMS to extend rock strain
measurements in the red beds of the Quartenschiefer Formation of the
Central Alps in Switzerland. In this case strain was measured by X
ray pole figure goniometry. Richter et al. documented the effects
of increasing metamorphism (from undeformed shales to
amphibolite-facies schists) and deformation on the AMS of the
Quartenschiefer and found that increasing deformation had a greater
effect in controlling magnetic fabric development than metamorphism.
Richter et al. used a combination of numerical modeling, high
field hysteresis measurements and low-temperature (liquid N
)
treatments to identify the mineralogic carriers of susceptibility.
Apparently paramagnetics control the AMS of low deformation
samples and magnetite controls the AMS in high grade rocks.
Using a strain gradient, but on a much more local scale, to document magnetic fabric development in rocks was the goal of Housen and van der Pluijm's [1991] remanence anisotropy study of the Martinsburg Formation shale to slate transition at Lehigh Water Gap, Pennsylvania. Previous work by Housen and van der Pluijm [1990] from the last quadrennium indicated that AMS did not record the development of magnetic fabric, but rather the dissolution and new growth of paramagnetic chlorite with increasing deformation since there were no intermediate AMS fabrics observed in the transition zone. In this study [ Housen and van der Pluijm, 1991] measurement of anisotropy of anhysteretic remanence (AAR) avoided the effects of paramagnetics and showed that magnetic fabric carried by magnetite underwent a transition from a bedding-parallel foliation in shales to a cleavage-parallel foliation in slates. However, the heterogeneity of the fabric in the transition zone did not allow the authors to distinguish between grain rotation and growth of secondary magnetite as the cause of fabric development.
Detailed studies involving rock magnetic, strain and microscopic examination of magnetite-bearing rocks (Precambrian Thomson Fm. [ Johns et al., 1992]) and hematite-bearing rocks (Cambrian Welsh slates [ Jackson and Borradaile, 1991]) were conducted to determine the mineralogic controls on AMS. In the Thomson Formation varying mixtures of paramagnetic chlorite and ferrimagnetic magnetite could cause either positive or negative correlations between anisotropy and bulk susceptibility. In the Welsh slates several generations of hematite contributed to the AMS fabric, thus giving a weak magnetic fabric despite high finite strains for the rocks. These two studies show the power of employing detailed SEM examination and rock magnetic measurements in unraveling the mineralogy controlling the AMS of deformed rocks.
Several studies used magnetic fabric measurements of remagnetized Paleozoic carbonates to shed light on the relative ages of different magnetic grain populations. Lu and McCabe [1993] isolated the magnetic fabrics of coarse and fine-grained magnetite in carbonates from the Nashville and Jessamine Domes in the Southern Appalachian Basin using anisotropy of anhysteretic remanence (AAR) and anisotropy of isothermal remanence (AIR). The coarser magnetite appears to carry a predeformational fabric while the finer magnetite has an Alleghanian (secondary) fabric. Two studies compared the AAR fabric in remagnetized carbonates with calcite twinning strains. In one study [ Sun et al., 1993] remagnetized carbonates only had a very weak magnetic fabric whereas in the other study [ Sierra et al., 1993] a supposedly post-folding age magnetite apparently carries a syn-folding age tectonic fabric. The results of this study suggest either a re-thinking of the deformation history for the Hudson Valley, New York or our understanding of AAR.
Finally, although they did not directly measure magnetic fabric Stamatakos and Kodama [1991b; 1991a] did document a strong correlation between bulk rock fabric and remanence direction in deformed Paleozoic red beds (Silurian Bloomsburg Fm. and Mississippian Mauch Chunk Fm.) from the central Appalachians. These results were used as evidence for physical rotation of hematite grains during simple shear strain associated with flexural flow/slip folding and have relevance to fabric development in sheared hematite-bearing rocks.