Date of Award

December 2020

Degree Type

Thesis

Degree Name

Master of Science

Department

Geosciences

First Advisor

Dyanna M Czeck

Committee Members

Lindsay McHenry, Barry Cameron

Abstract

The Southern Iberian Shear Zone is a regional scale ductile shear zone that placed Beja-Acebuches metabasites over metapelite and quartzite rocks of the Pulo do Lobo Formation in a transpressional deformation with components of thrust and left-lateral movement. This project evaluates factors that contributed to strain localization at a variety of scales within the Pulo do Lobo formation, including lithologic variations, microstructures, and fluid flow. Additionally, microstructural data are used to test whether kinematic partitioning accompanied strain localization. Grain scale asymmetric indicators of sigma-type clasts, S-C fabrics, and asymmetric folds indicate dominant thrust motion. Strain intensity and metamorphic grade increase towards the shear zone. Within about 170 m, relatively low grade slates with regional slaty cleavage transition to garnet-zone amphibolite metamorphism and closely-spaced, strong schistose foliation. Microstructures indicate a gradual increase in crystal-plasticity towards the shear zone that likely allowed for gradual strain increase. However, fluid interaction promoted additional deformation mechanisms near the shear zone including diffusive mass transfer (most prevalent <50 m) and localized embrittlement (< 10 m). Isocon plots of major element concentrations indicate no significant volume change across the strain gradient. Na2O depletion paired with possible CaO enrichment are likely related to the stability of more Ca rich feldspars at higher metamorphic grade, suggesting that the fluid migration along the shear zone catalyzed metamorphic reactions. MgO enrichment is consistent with metasomatic alteration, potentially with a fluid source from the metabasites. K2O enrichment, previously also documented in the metabasites, is additional evidence of fluid flow along the shear zone. The angle between C and C’ fabrics is used to estimate kinematic vorticity across the strain gradient. In most samples, mean kinematic vorticity numbers between 0.86-0.98 indicate simple shear dominated strain, which did not vary significantly across the strain gradient. Factors that controlled strain localization along the Southern Iberian Shear Zone vary with scale. At the terrane scale, the rheologic boundary between the Beja-Acebuches metabasites and the Pulo do Lobo formation is of primary importance. At the hm scale, fluid channeled along the shear zone further localized strain by enhancing strain softening metamorphic reactions and promoting diffusive mass transfer and embrittlement. At the cm-m scale, contrasting rheologies of quartzites and metapelites controlled strain localization. This study provides an example of how strain localization is accomplished within the mid-lower crust during tectonic events.

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