Fluid-rock interaction in deformed diamictites, Willard thrust fault, Sevier orogenic belt, Utah (USA): Microstructures and fluid pathways

W. Adolph Yonkee: Weber State University; ayonkee@weber.edu

Dyanna M. Czeck: University of Wisconsin-Milwaukee; dyanna@uwm.edu

Spenser Pantone: Weber State University; SpenserPantone@weber.edu

Amelia C. Nachbor: University of Wisconsin-Milwaukee; anachbor@uwm.edu

Kimberly R. Johnson: University of Wisconsin-Milwaukee; krj2038@gmail.com

Christine Barszewski: University of Wisconsin-Milwaukee; barszew2@uwm.edu

Detailed micro-structural and micro-geochemical analyses of deformed diamictites along the Willard thrust fault in northern Utah (USA) are underway to determine interrelations between deformation mechanisms and fluid-rock interaction. The Willard thrust was active from ~ 130 to 100 Ma during the Sevier orogeny and transported a 10- to 15-km-thick sequence of miogeoclinal strata >60 km eastward, synchronous with internal deformation and greenschist-facies metamorphism (Yonkee, 2005). Studies are focused on heterogeneous deformation patterns in Neoproterozoic diamictite in the base of the hanging wall (Fremont Island area) and the western part of the footwall (Antelope Island area). Diamictite consists mostly of pebble- to boulder-size granitic and paragneiss clasts derived from basement rocks and quartzite clasts that sit within a sandy to micaceous matrix. Strain magnitude is overall greater in the footwall (X-Z ratios ~2:1 to ~10:1) compared to the hanging wall (X-Z ~2:1 to ~4:1). Within the footwall, strain magnitude increases with increasing alteration of feldspar to mica, abundance of quartz veins, water content within quartz grains, reduction in grain size, and development of shear bands, reflecting reaction softening, hydrolytic weakening, and textural softening. Within the hanging wall, strain variations are more subtle, feldspar is less altered, veins are relatively rare, and clasts better preserve protolith textures. At a given outcrop, different clast types behaved differently; clasts containing more mica deformed the most, whereas quartzite clasts deformed the least.

Patterns of fluid influx and evolving deformation processes over a range of scales are being evaluated from petrography (Fig. 1), SEM x-ray mapping and microanalysis (Fig. 2), SEM back scatter electron imaging (Fig. 3A) and cathodoluminescence (CL), and synchrotron infrared (IR) spectroscopy (Fig. 3B). Microstructures and alteration vary between areas across the Willard thrust, along strain gradients between sites, and between different clast types and matrix. Within the footwall, quartz grains in lower strained clasts display microcracks, patchy undulose extinction, and deformation lamellae. Grains are relatively large with polygonal shapes related to protolith textures. Feldspar displays microcracks along which mica alteration is concentrated. Quartz grains in higher strained clasts are stretched, display subgrains and recrystallization, and include fibers in strain shadows. Feldspar is mostly altered to mica. Microveins filled with calcite, quartz, mica, and Mn-oxides record repeated influx of fluids and sealing. Disseminated carbonate grains in some samples indicate fluids had minor CO2 that helped buffer pH during feldspar alteration. Variations in concentrations of Fe-Ti-oxides record varying local volume loss from quartz dissolution to volume gain from quartz precipitation in different microdomains, which are approximately balanced at a cm-scale. CL imaging reveals multiple healed and sealed microcracks in quartz grains that provided fluid pathways and decreased distance for water to enter crystal lattices. Microcracking appears enhanced by stress concentrations in polyminerallic clasts. IR imaging shows water contents are greater along some (but not all) grain boundaries and healed microcracks. Preliminary data suggest greater water content in higher strained clasts, and in paragneiss clasts compared to granitic clasts. Microstructures in diamictite clasts are similar to those found in shear zones in the underlying granitic basement, and record complex interactions between diffusive mass transfer, dislocation creep, fracturing, and fluid processes. Within the hanging wall, quartz displays widespread undulatory extinction, subgrains, and local recrystallization. Feldspar is partly altered to fine-grained muscovite along microcracks and to Fe-rich biotite. Selvages and strain caps enriched in mica and oxides, with limited precipitation of quartz fibers in strain shadows, record mass transfer deformation with net volume loss. Overall, there is less alteration of feldspar to mica, more widespread plastic deformation of quartz, and less variation in strain magnitude in the hanging wall.

www.jpgvvv.jpguuuu.jpg Figure 1. Photomicrographs of typical microstructures for lower (left) and higher (right) strained granitic clasts from the footwall. fd- feldspar partly altered to mica, db- deformation bands, dl- deformation lamellae, mc- microcracks, rx- recrystallized quartz, qf- quartz fibers. Field of view is ~3.7 mm wide.

Figure 2. Example X-ray maps for Na (from albite), K (from muscovite), and Mn (from oxides) for a moderately strained granitic clast from the footwall. Field of view is ~1mm wide.

Figure 3. A) BSE image of moderately strain granitic clast from footwall. Q-quartz (darker), M-muscovite (lighter), ox- oxide (bright), cv- calcite vein; inset shows details of alteration along microcracks. Field of view is 9 mm wide.

B) Photomicrograph and synchrotron IR image (warm colors indicate higher water content) of quartz grain from moderately strain clast. Field of view is 1.4 mm wide.

Reference:

Yonkee, W.A., 2005, Strain patterns within part of the Willard thrust sheet, Idaho-Utah-Wyoming thrust belt: Journal of Structural Geology 27, 1315-1343.





Return to publications.