Fluid-rock interaction in deformed diamictites, Willard thrust fault, Sevier orogenic belt, Utah (USA): Macroscopic strain and alteration patterns

 

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

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

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

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

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

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

 

Deformed diamictites record strain accumulation and fluid-rock interaction during deformation along the Willard thrust fault in northern Utah (USA). The Willard thrust was active from ~ 130 to 100 Ma and forms a dominant, aerially extensive thrust sheet in the Sevier orogenic belt. The thrust sheet comprises a 10- to 15-km-thick sequence of miogeoclinal strata that were thrust >60 km eastward over cratonic strata and basement rocks (Yonkee, 2005). Strata at the base of the thrust sheet include a thick interval of Neoproterozoic diamictite; a much thinner interval of diamictite is locally preserved in the western part of the footwall. Diamictite consists of a variety of pebble to boulder sized clasts, including granitic and paragneiss clasts derived from basement rocks (similar basement rocks of the Farmington Canyon Complex are locally exposed in the footwall), quartzite clasts, and reworked intrabasinal clasts, which sit within a sandy to micaceous matrix.

 

Strain varies regionally across the Willard thrust, locally between sites that experienced differing fluid flow, and between various clast types at a single site. Strain was estimated for multiple clast types (granitic, paragneiss, and quartzite) and matrix along traverses in both the hanging wall (Fremont Island area) and footwall (Antelope Island area). Granitic and paragneiss clasts contain varying amounts of quartz, feldspar, and mica related to differing protoliths and extent of alteration of feldspar to mica. For strain analysis, ~20 to 60 clasts of each type were measured on 3 subperpendicular outcrop faces. Two-dimensional strains (shape fabrics) were estimated using the Rf/Phi method for clasts and from photomicrographs of matrix. Two-dimensional strains were combined from 3 faces to determine a best-fit 3-D strain ellipsoid. Regionally, 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); alteration of feldspar to mica is also overall greater in the footwall. Within the footwall, strain magnitude varies locally, with the highest strain at sites having the densest arrays of quartz veins (Fig. 1). Strain ellipsoid shapes plot near the plane strain line, and microstructures indicate only minor extension/shortening in the Y direction, consistent with little to no net volume change. Within the hanging wall, local variations in strain are subtle and veins are relatively rare. Strain ellipsoids have oblate shapes and microstructures indicate widespread, but minor extension in the Y direction, consistent with 10-30% volume loss. Strain variations between different clast types and matrix indicate a competence hierarchy that matches qualitative observations of clast strain shadows and caps, with quartzite > granitic > paragneiss matrix. Strain does not appear to vary significantly with clast size at most sites, except larger clasts have higher strain than smaller clasts at the highest strain sites, associated with development of shear bands in larger clasts.

 

Bulk chemistry for clast and matrix samples was determined using X-Ray Fluorescence analysis. Results were compared to analyses of basement granitic rocks, paragneiss, and shear zones of the Farmington Canyon Complex. The basement shear zones formed during the Sevier orogeny, related to concentrated influx of reactive fluids that altered feldspar to mica and resulted in strain softening (Yonkee et al, 2003). Fluid influx was extensive in the sedimentary cover, resulting in varying alteration of feldspar to mica and local precipitation of quartz veins. Granitic protoliths have relatively consistent compositions with ~68-72 wt% SiO2, 11-12% Al2O3, 2-4% Na2O, and <1% MgO. Paragneiss protoliths range from quartzo-feldspathic with similar composition as granitic protoliths to pelitic (with Al2O3 > 20% and SiO2 < 60%). Within the footwall, consistent TiO2, SiO2 and Al2O3 contents between deformed diamictite matrix, clasts, and corresponding basement protoliths indicate that diamictite underwent only limited volume change (Fig. 2). Diamictite matrix and clasts record Mg gain and Na loss, similar to patterns in basement shear zones (Yonkee et al., 2003), consistent with infiltration of fluids from shear zones into the diamictites. Within the hanging wall, chemical characteristics of diamictite are different. Significant increases in Al2O3 and decreases in SiO2 indicate ~20% volume loss. Compared to the footwall and basement, Na is higher and Mg is lower, reflecting less alteration of feldspar, neocrystallization of Fe-rich biotite, and net dissolution of quartz. Differences in geochemical patterns across the fault indicate differences in flow patterns, consistent with overall downward (up-temperature) flow in the hanging wall, overall upward (down-temperature) flow in the footwall, combined with concentrated flow parallel to the main fault zone.

 

Fig. 1 Cross-sectional view of the footwall with X-Z strain ellipses for different clast types and matrix. Xf- Farmington Canyon basement, Zmf- Neoproterozoic diamictite. Strain increases to WNW, associated with increased quartz veins.

 

Fig. 2 Examples of whole-rock XRF data for diamictite clasts and matrix from Antelope Island area (footwall). TiO2 vs Al2O3 plot indicates little/no volume change.  SiO2 vs Al2O3 plot indicates little/no net change in Si. Na2O vs Al2O3 plot shows Na loss. Approximate range of corresponding basement protolith granitic rocks and paragneiss (varying from pelitic to quartzo-feldspathic) shown by dashed lines.

 

References:

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.

Yonkee, W.A., Parry, W.T., Bruhn, R.L., 2003. Relations between progressive deformation and fluid-rock interaction during shear-zone growth in a basement-cored thrust sheet, Sevier orogenic belt, Utah. American Journal of Science 303, 1-59.





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