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Experimental deformation of synthetic magnetite-bearing calcite sandstones: effects on remanence, bulk magnetic properties, and magnetic anisotropy
Full text linkNSF EAR 88-04820 and NSERC A6861Jackson, Mike; Borradaile, Graham; Hudleston, Peter; Banerjee, Subir. (1993). Experimental deformation of synthetic magnetite-bearing calcite sandstones: effects on remanence, bulk magnetic properties, and magnetic anisotropy. Retrieved from the University Digital Conservancy, 10.1029/92JB01028
Magnetic fabrics from sheeted dikes reveal regional magma flow patterns, and the spacing and dimensions of ophiolite magma-chambers, Troodos ophiolite, Cyprus
Full text linkOrientation-distributions of crystals were determined for 1289 specimens of the
Sheeted Dike Complex of the Troodos ophiolite, located on the island of Cyprus. These
were inferred from anisotropy of magnetic susceptibility (AMS). From these data the
dispersion of magmatic flow fabrics with a mild tectonic overprint were recognized.
The study area (~400km2) is located to the east of Mt. Olympus, and adjacent to a
fossil transform fault (STTFZ) that was responsible for shearing of dikes and a change in
their orientation from predominantly north-south to east-west as the fault is approached.
The predominantly magmatic AMS fabrics blend a flow-aligned paramagnetic
component from mafic silicates with a ferromagnetic component from titanomagnetites.
The inclination of magma-flow axes varies from near vertical to near horizontal
throughout the area with predominantly steep magma flow regions separated from
regions with predominantly shallow magma-flow. Fast Fourier Transform (FFT) analysis
of steep-flow region spacing shows that the magma chambers that fed the dikes were
point-source with minimal along-axis extent, and very short lived. FFT wavelength
calculations suggest that they may have been spaced approximately every 4km along the
ridge, and every 100,000 to 250,000 years in time. These results imply localized magma
chambers, thereby supporting the slow-spreading origin of the Troodos crust, and refine
models for slow-spreading ridge processes to include a point-source magma delivery
system between magma reservoirs and the sheeted dike complex
The magnetic fabrics and strain history of the Achean Seine Group metasedimentary rocks near Mine Centre, Northwestern Ontario
Full text linkThe Seine Group metasedimentary rocks are situated between two converging major dextral transcurrent faults, the Quetico Fault to the north and the Seine River Fault to the south. To the west lies the Bad Vermillion Intrusive Complex (B.V.I.C.) The Seine Group shows one major period of metamorphism resulting in major F. folds. Regional metamorphism of chlorite to biotite zone greenschist facies was synkinematic with the deformation. Later minor deformations include localised crenulation cleavage and faults and shear zones
Dimensional and crystallographic fabric development in experimentally deformed synthetic aggregate and natural rocks
Full text linkCalcite Portland-cement aggregate samples were deformed
triaxially at 25 deg. with confining pressures of 200 Mpa. The
samples were deformed under experimental approximations of
pure shear (dry and wet experimental conditions),
transpressional shear and simple shear. The pore fluid
pressure during the wet pure shear test was less than 195 MPa.
Extensive grain rotation accompanied by twinning of the
calcite grains occurred.
Optical analyses of calcite crystallographic fabrics have
been used to infer the orientation of the maximum principal
compressive stress. Stress orientations in the deformed
specimens agree well with the externally imposed stresses. A
new method has been successfully used to determine the a,
orientation. The method uses contouring of the lamellae index
associated with the compression direction determined from
Turner's Dynamic analysis method.
In pure shear, preferred dimensional orientation (PDO) of
the calcite grains are produced more efficiently in the
presence of a pore fluid pressure. In dry specimens,
transpressional shear is more effective in producing a PDO in
the calcite grain than either pure shear or simple shear.
Grain shape fabrics do not conform to the symmetry of the bulk
deformation when extensive rotation of calcite grains is
involved. Mean grain alignment is perpendicular to the
shortening in pure shear, initially inclined and later
parallel to the shear zone wall in transpressional shear, and
inclined to the shear zone wall in simple shear. The mean
orientation of the grain-alignment fabrics is, therefore, a
reliable kinematic indicator under the conditions
investigated. Transpressional shear and dry pure shear exhibit
higher lamellae indices than either wet pure shear or simple
shear.
Strain analysis of calcite grains by Robin's method
(1977) , the linearization method (Yu and Zheng, 1984) and
Harmonic mean method (Lisle, 1977) yields overestimates of the
experimental bulk strain in wet pure shear. These methods fail
to take into account interparticle motions that occur in the
presence of a high pore fluid pressure.
The triaxial deformation of the Ancaster oolitic limestone
was preformed with a confining pressure of 200 Mpa, a natural
strain rate of 10-5/s and at a temperature of 135°C. The
samples were deformed under dry and wet experimental
conditions. The pore fluid pressure, during the wet test, was
less than 60 % of the confining pressure.
The deformation process of ooids in the dry experimental test is rigid rotation of the ooid particles. In the case of
wet experimental conditions, it appears that the pore fluid
pressure produces particulate flow in the fine grained ooid
matrix.
Due to a viscosity contrast, between ooids and cement
matrix, strain analysis on the ooids exhibits an overestimate
of strain compared to the experimental bulk strain. This holds
true for both wet and dry experimental conditions.
Experimental triaxial deformation was conducted on the
China Beach sandstone by pure shear for dry experimental
conditions. The temperature was held constant at 25°C, with
computer controlled natural strain rates of 10-5/s and a
confining pressure of 200 Mpa.
Mechanical heterogeneities in the grains of the China
Beach sample play an important role in the development of
cleavage. Altered feldspar grains and lithic fragments deform
by ductile processes, while unaltered feldspar and quartz
grain deform by rigid rotation and brittle processes. Strain
analysis of each grain type in the China Beach sandstone yield
a range of strain estimates depending on the deformation
process compared to the experimental bulk strain.
Comparison of Robin's method, the linearization method and
Harmonic mean method suggest that Robin's method generates the
best estimates of the bulk experimental strain ratio
Rock magnetic and structural investigation of the Moss Lake stock and local area : western Shebandowan belt
Full text linkThe thesis area is located 120 km west of the City of Thunder
Bay, Ontario, straddling the contact of the Quetico and Wawa
subprovinces of the Archean Superior Province.
Metasedimentary rocks of the Quetico Subprovince with basic and
granitic intrusions occupy the northwestern portion of the study
area and are in contact to the southeast with metamorphosed mafic
and felsic metavolcanic rocks of the Wawa Subprovince.
Tectonic compression has shortened the metasedimentary rock
layers a minimum of 80%. Bedding dips steeply to the northwest and
a later cleavage, developed by transpression, is sub-parallel to
bedding. Graded beds young predominantly to the northwest. Some
graded beds young to the southeast and may represent pre-cleavage
folding. No large-scale folds are present. Metamorphic grade
increases northwest from greenschist to amphibolite facies over a
distance of about 10 km.
Magnetite is the predominant magnetic component of the Moss
Lake stock; hematite is present in trace amounts. Magnetic
susceptibility of the stock is high ( 12,000 X10-6 SI), making the
intrusion amenable to anisotropy of magnetic susceptibility (AMS)
work.
AMS study of the Moss Lake stock shows that individual
directions of maximum susceptibility have been reoriented, in most
cases sub-parallel to the local planar fabric strike of the Quetico
Subprovince. Magnetic fabric parameters show that the rock magnetic
fabric of the intrusion is deformed. Vestiges of original magmatic
fabric are evidenced by prolate (constricted) magnetic fabric
associated with the central long axis of the stock but magnetic
fabric parameters confirm that the intrusion margin is more
deformed than the interior. The predominant oblate (flattened)
magnetic fabric of the Moss Lake stock is the product of northwestsoutheast
tectonic compression.
Alternating field and thermal demagnetization of oriented rock
specimens confirm that the Moss Lake stock is deformed by tectonic
compression. Separation of magnetite and hematite magnetic
contributions, by blocking temperature, reveals that primary
natural remanent magnetization (NRM) orientations from hematite are
parallel to the local planar fabric strike of the Quetico
Subprovince. Minor preservation of primary magmatic fabric is
indicated by the mean principal component analysis (PCA)
orientation for magnetite which corresponds closely in trend to the
mean ~ e orientation for the region, obtained by previous
investigators. Original Moss Lake stock magnetic fabric is
overprinted by compression and shearing.
Comparison of magnetic studies (AMS and NRM) of the Moss Lake
stock to structural data of country rocks argues in favor of a
common tectonic control
Tectonic magnetic fabrics in pure and simple shear : experimental investigations
Full text linkTriaxial compression tests were preformed such that
changes in the magnetic susceptibility anisotropy with strain
would be represented by experimental approximations of simple
shear and pure shear. Both types of tests were performed on
artificial materials of high magnetic susceptibility at room
temperature and atmospheric pore fluid pressure. Experimental
displacement-rates and strain-rates were computer controlled
during testing.
Two different shear zone materials were employed for the
"simple shear" testing, a sand-cement mixture and a
calcite-cement mixture. Three series of simple shear tests
were conducted on the sand-cement material at various
confining pressures; Series A, at 0.689 kbars, Series B at 1.0
kbar and Series C at 1.5 kbars. Two series of simple shear tests
were conducted on the calcite-cement material, Series 1, at
1.0 kbar confining pressure and Series 2 at 1.5 kbars Pc. For
both materials a constant axial displacement-rate of 5.0x10-6
inches, s-1 (corresponding to a slip displacement-rate on the
shear zone walls of 8.7x10-6 inches, s-1) was employed
Neo-tectonic and rock magnetic study of the Circum Troodos Sedimentary Succession, Cyprus
Full text linkThe Circum Troodos Sedimentary Succession (Late Cretaceous to Recent)
overlies the Troodos ophiolite of Cyprus, located in the Eastern Mediterranean.
The pattern of neo-tectonic deformation was investigated through magnetic analysis
of the tectonic petrofabncs. Anisotropy of magnetic susceptibility (AMS), anisotropy
of anhysteretic remanence magnetization (AARM) and hysteresis loop parameters
were determined in order to define the magnetic fabric and the magnetic
mineralogy.
The study area extends over approximately 1000 km2 mainly to the south of
the exposed Troodos ophiolite. The sample suite includes 432 oriented hand
samples, predominantly of the Lefkara and Pakhna Formations
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
An investigation of the use of anisotropy of complex magnetic susceptibility to analyse strain in experimentaly deformed materials and massive sulphides
Full text linkAnisotropy of magnetic susce:ptibility (AivJS) has become a widely
accepted method of fabric analysis in rocks, especially those
which have been deformed tectonically. The use of anisotropy of
complex magnetic suscepti bi 1 i ty (ACf"lS) is a new potential method
of fabric analysis in which the imaginary, or out of phase A.C.
component of an induction coil used for the measurement of
magnetic susceptibility is used to delineate rock fabric. Complex
magnetic susceptibility is a function of of electrical
conductivity, thus making it potentially useful in the analysis
of highly conductive sulphide-rich rocks, some of which are not '
suitable for AMS analysis.
Preliminary measurements were performed on highly conductive
l ~inum test specimens of differing shapes to determine the
relationship between grain shape anisotropy and ACI"lS. A
relationship was found in which shape anisotropy and resistive
ACf"lS fabrics were of the same sense, but there was no
quantitative correlation. Pure and simple shear deformation
exper-iments performed on plasticene containing numerous small
aluminum disks exhibited a correlation between ACMS fabric
anisotropies and strain in most cases, as the ACMS fabr-ics were
controlled by the distribution of the disks, which became wellaligned
as flattening proceeded. Although t.here was no
quantitative relationship between strain and ACMS, they tended to
increase together.
Triaxial deformation studies on loose pyrrhotite aggregates and
pyrrhotite plus talc mixtures were performed at confining
pressures of 150 MPa. The ACMS fabrics developed in these
specimens were compared to Af"JS fabrics and strain analysis data
to determine if the ACMS fabrics change as a function of strain.
As e f~ te oblate resistive ACMS fabr-ics developed dur-ing these
pure shear deformations. The pyrrhotite aggregates exhibited a
complex relationship in which ACMS increased with strain, at
least up to a critical strain value, after which ACMS appeared to
decrease. The pyrrhotite plus talc mixtures exhibited an
unmistakable inct·ease in ACMS with increased strain probably
influenced by the presence of the talc matrix. The ACI"lS fabrics
developed in.these experiments were undoubtedly tKe e ~lt of
grain alignment and distribution within the aggregates, with
insignificant contributions from crystallographic resistive
anisotropy.
Measurements performed on specimens of massive pyrrhotite
revealed ACMS fabrics compl8tely diffr:r-ent fr-om those observed in
the loose pytThoti te aggregates, with ambiguous relationships
between str a i n and ACivJS. This is because the massive specimens
behave electrically as a single grain and anisotropy is almost
exclusively ct·ystallographically controlled. Thus the ACI"lS
properties of single minerals must be understood before ACMS
fabrics in massive sulphides can be interpreted
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