UNDERSTANDING TECTONIC STRAIN AND SYN-TECTONIC FLUID FLOW PROCESSES USING MAGNETIC FABRICS IN DIAMAGNETIC ROCKS: EXAMPLE FROM THE EAST KAIBAB MONOCLINE, SOUTHERN UTAH

Tectonic and diagenetic fabrics in the N-S-trending East Kaibab Monocline in southern Utah are investigated using magnetic fabrics in the Navajo Sandstone. The study examines the direction and magnitude of tectonic strain in deformation bands, and the possible direction and relative timing of fluids forming dense iron-oxide concretion packages. The study methods comprise anisotropy of magnetic susceptibility (AMS) and stepwise AF/thermal demagnetization, X-ray diffraction, thin section and SEM analyses.

We sampled deformation bands and densely clustered concretions for signatures of tectonic and fluid processes at three locations along an N-S transect that totaled 180 cores from 12 sites. The deformation bands have a diamagnetic signal, and the concretions have a dual antiferromagnetic and paramagnetic signal. Mineralogical data support that hematite and goethite are the leading magnetic carriers of the signal.

Deformation bands vary in thickness and ratio of band to matrix, which consists of zones of deformed grains from cataclasis and non-deformed grains, respectively. The least-dispersed AMS data are from the thickest bands with the highest band density. AMS results from 6 of 10 sites show a Kmax axis that trends NE-SW, sub-parallel to the N 40° regional mean shortening direction of the Laramide orogeny. AMS results show that deformation bands in the Navajo Sandstone provide quantifiable tectonic strain directions. The highest strain is at the southern site, closest to the fault tip of the reactivated, reverse-slip, right-lateral Butte Fault.

Concretions show AMS results with their Kmin perpendicular to bedding, possibly indicating fluids followed bedding. Paleomagnetic fold tests will be used to assess the relative timing of the concretions.

Our results highlight the limitations of working in diamagnetic rocks, as the weak magnetic signal in the Navajo Sandstone is from being quartz-rich, reducing the ability to quantify tectonic strain in deformation bands. Comparatively, iron oxide concretions have a strong enough ferromagnetic signal to use in a fold test.

This study demonstrates the feasibility of using paleomagnetic tools to quantify the following; possible fluid directions, structural fabric strength and orientation in monoclines on the Colorado Plateau formed during the Laramide orogeny.