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Understanding Tectonic Strain Using Magnetic Fabrics In Diamagnetic Rocks: Example From The East Kaibab Monocline, Southern Utah



This poster highlights the main findings and interpretations of the tectonic and structural part of my master's thesis project. This part of my thesis explored if magnetic fabrics in deformation bands can be used as a proxy for tectonic strain in terms of relative deformation amount, timing and direction.
1. 140 cores from 10 sites along three transects in the N-S
trending East Kaibab Monocline (EKM), Utah, are presented as a
pilot study for understanding magnetic fabrics in deformation
bands (DBs), in the Navajo Sandstone. DBs are small faults
formed in this area by Laramide compression and used as a proxy
to quantify tectonic strain and direction (Tindall & Davis, 1999).
Trace amounts of ferromagnetic and paramagnetic minerals (< 4-5
wt.%) are believed to be enhancing the dominantly diamagnetic
signal. This weak, diamagnetic signal is caused by high
abundances of quartz and K-feldspar grains in the Navajo Ss., a
world class aeolian deposit. DBs in the Navajo Ss. occurred when
strain caused cataclasis in this brittle, well sorted, porous
sandstone, breaking and redistributing its minerals (Fossen et al.,
2007). The Laramide orogeny; a flat slab, thick-skinned,
lithospheric basal shearing event, with a mean shortening direction
of N040°E, reactivated basement faults across western North
America (Bird, 2002). The reactivation of basement faults formed
the numerous uplifts across the Colorado Plateau, mainly as fault
propagation folds or basement cored uplifts, with folding starting
in the EKM ~80-76 Ma (Tindall et al., 2010). This study captures
the feasibility of using magnetic fabrics in DBs, in the Navajo Ss.,
to quantify the EKMs regional or local tectonic history, and
improving this technique for future research across this region.
Ballas, G., Fossen, H., & Soliva, R. 2015. Factors controlling permeability of cataclastic deformation bands and faults in porous
sandstone reservoirs. Journal of Structural Geology,76, 1-21.
Bird, P. 2002. Stress direction history of the western United States and Mexico since 85 Ma. Tectonics,21(3), 5-1.
Fossen, H., Schultz, R. A., Shipton, Z. K., & Mair, K. 2007. Deformation bands in sandstone: a review. Journal of the Geological
Society,164(4), 755-769.
Tindall, S. E., & Davis, G. H. 1999. Monocline development by oblique-slip fault-propagation folding: the East Kaibab
monocline, Colorado Plateau, Utah. Journal of Structural Geology,21(10), 1303-1320.
Tindall, S. E., Storm, L. P., Jenesky, T. A., & Simpson, E. L. 2010. Growth faults in the Kaiparowits Basin, Utah, pinpoint initial
Laramide deformation in the western Colorado Plateau. Lithosphere,2(4), 221-231.
Zuluaga, L. F., Fossen, H., & Rotevatn, A. 2014. Progressive evolution of deformation band populations during Laramide fault-
propagation folding: Navajo Sandstone, San Rafael monocline, Utah, USA. Journal of Structural Geology,68, 66-81.
i. To understand why deformation bands might be recording different
magnetic fabrics on a site by site basis and how they correspond or differ
to one another.
ii. To understand if the magnetic fabrics in deformation bands are related to
either the two regional tectonic events during the EKMs evolution
(Laramide & Sevier) or solely associated with local deformation.
iii. To try correlate magnetic fabrics to a relative timing of events.
By Rhys Park
Understanding Tectonic Strain Using Magnetic Fabrics In
Diamagnetic Rocks: Example From The East Kaibab
Monocline, Southern Utah
Anisotropy of Magnetic Susceptibility (AMS)
Determines the average alignment degree and shape of magnetic minerals.
Hysteresis Loops
Determines magnetic signal strength and magnetic behavior.
Thin Sections
Determines paragenesis, mineralogizes and relative alignment of minerals.
X-Ray Diffraction (XRD)
Acquires mineralogical data and the relative abundance in a sample.
Scanning Electron Microscope (SEM)
Mineral identification and relative alignment of minerals with AMS data.
Well defined DBs and the relatively undeformed zones (RUZs) have similar
mineral abundances: quartz > 90 wt.%, some K-feldspar, clays and few iron
oxides from XRD and SEM analysis. Specifically, the alignment and distribution
of K-feldspars and clays in both the defined DBs and RUZs match well with the
Kmax direction. Possibly indicating these minerals are contributing to the
magnetic signal.
The AMS is defined by three
tensors Kmax, Kint and Kmin.
This provides information on
the degree of magnetic mineral
anisotropy, lineation, foliation
and ellipsoid shape. This study
correlates the patterns shown
on equal area lower hemisphere
projections and the values from
the various AMS parameters to
strain directions, bedding
orientations and relative
degrees of deformation.
DB shortening directions: 8/10 sites have NW-SE directed shortening
directions, appearing to show 8/10 sites sub-parallel to the shortening direction
of the Sevier orogeny. 2/10 sites have NE-SW directed shortening directions and
appear sub-parallel to the mean shortening direction of the Laramide orogeny.
The contrast between the shortening directions and whether the magnetic fabrics
are truly recording regional deformation is discussed in the conclusion section.
M. Chadima (AGICO Inc)
The EKM is a north-plunging fault propagation fold caused by the reactivation
of the basement Butte fault, seen in x-section in the Grand Canyon. Due to the
direction of regional Laramide compression placed upon the Butte fault during
the EKM’s formation, a dominantly reverse right-lateral oblique slip component
to the EKM formed. 8/10 DB sites are small NE-striking synthetic reverse right-
lateral faults.
4 mm
Site 2 DB
XPL Thin Section
AMS applies a magnetic field through a sample and determines the average
degree of alignment of magnetic minerals in a sample from field collected cores.
15 samples per site on average are used to acquire statistically viable results.
600 µm
Navajo Sandstone
AMS pattern found for a
sedimentary related
magnetic fabric. Seen in
the fluid part of this study
and many others.
AMS pattern found for
transitional magnetic
fabrics in this study.
Formed by initial folding
and relatively low levels
of compressional
AMS pattern found for
intermediate and
tectonic related magnetic
fabrics. Tectonic fabrics
clearly display a well-
defined girdle of Kmax and
Kint susceptibilities along
the foliation plane of the
DB and a tight clustering
of Kmin susceptibilities
close to the DB pole.
Modified from
Zuluaga et al. (2014)
Magnetic fabrics are deduced by finding the difference in angle between the
Kmin and pole to bedding. Δangle < 35° are transitional, Δangle > 35° but <
65° are intermediate and Δangle > 65° are tectonic fabrics. Initially, fabrics
start out as transitional with oblate ellipsoids from compaction related
diagenesis and small amounts of horizontal shortening. Under greater degrees
of deformation and folding, the magnetic fabrics evolve into the prolate
domain as dominantly tectonic fabrics. Interestingly, the shape parameter vs.
the angle between Kmin and pole to bedding plot corresponds well with
Zuluaga et al. (2014) model for DB development, in the Navajo Sandstone, in
a monocline. DBs initially form parallel to bedding but with increased folding
DBs form bed perpendicular. As DBs are grain hardening faults they record
deformation related processes at a certain point in time with the magnetic
fabrics above suggesting the early stage DBs are related to low strain regimes
and the late stage DBs are related to increasing strain regimes.
Only endmember
magnetic fabrics
correlate well.
Intermediate fabrics
are negated from the
Intermediate fabrics
can be explained as
mixing of fabric types
and/or the natural
intermediary stage
between the two
endmember fabrics.
Direction Strike & Dip
Distribution of K-feldspars
appear parallel to the Kmax
direction. They have
crystallographic magnetic
anisotropy but are
diamagnetic. Do they truly
contribute to signal?
Alignment of clays
Structural geologists looking for new ways to quantify tectonic and
structural related deformation processes in folds, with DBs.
Limited rock magnetic study's understand the relationship between
magnetic fabrics and the regional vs. localized strain record.
DBs inhibit fluid flow up to 6 orders of magnitude (Ballas et al., 2015). AMS
can be used to understand DB type and relative timing of DB development.
Mixing of
Is AMS a viable technique for understanding magnetic fabrics in DBs in
the Navajo Sandstone?
In short, yes, the two endmember magnetic fabrics, transitional and
tectonic, appear to have a strong, mutually exclusive relationship with
increased folding and deformation in the EKM. Tectonic fabrics also appear to
have a stronger magnetic signal that overprints the transitional magnetic
fabrics as seen in the AMS intermediate patterns. However, the intermediate
magnetic fabrics are either a natural evolutionary step between transitional
and tectonic magnetic fabrics or a mixing of endmember magnetic fabrics.
The mixing hypothesis is more likely as deformation is shown to be
heterogenous throughout the EKM and the intermediate fabrics are proven to
have a greater amount of RUZs in them, compared to the tectonic fabrics. So,
care must be taken as shown in the figure below, to sample only from DBs
larger than the drill-bit. This reduces mixing of fabric types, improving data.
Are the magnetic fabrics picking up the regional or local strain effects?
The regional strain effects are discounted from this study as being strain
records in the magnetic fabrics: the Sevier orogeny occurred as a far-field
strain and was not a substantial cause for deformation in this area (Bird,
2002). Study bias from sampling 8/10 NE striking synthetic faults also added
to the appearance of a NW-SE shortening direction. 2/10 sites appear to be
sub-parallel to the mean shortening direction of the Laramide. But are
discounted as being regional from the identification of the magnetic fabrics
following local strain patterns associated with individual DB development.
So, magnetic fabrics are classified as being caused by local strain effects,
mostly at the individual deformation band level. However, the transitional
magnetic fabrics may be recording bulk strain at the monocline level. This is
because all transitional magnetic fabrics appear to be perpendicular to the
strike of the EKM, parallel to the inferred local EKM shortening direction and
made up of mostly homogenous RUZs, separate from localized DB formation.
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