T77 Deformation Zones in Time and Space I: A Celebration of the Career of Steven Wojtal

Monday, 10 October 2022
8:00 AM - 12:00 PM
207

MULTISCALE STRAIN PARTITIONING IN THE PULO DO LOBO METASEDIMENTARY ROCKS AFFECTED BY THE SOUTHERN IBERIAN SHEAR ZONE, SOUTHWESTERN SPAIN

Paper number: T77-14
Program: Technical Sessions
Time: 11:10 AM – 11:25 AM

Authors

Abstract

The Southern Iberian Shear Zone (SISZ) forms a Variscan suture between the Ossa-Morena and South Portuguese tectono-stratigraphic terranes in southwestern Spain. At the terrane scale, the SISZ exploited the mechanical boundary between two rheologically contrasting units, the Pulo do Lobo (PdL) metasedimentary unit to the south and the Beja-Acebuches metabasite (BAM) to the north. Localized deformation that formed the SISZ produced mylonitic fabrics accompanied by greenschist - lower amphibolite facies metamorphism. The structural geometry of the SISZ within the BAM has allowed interpretation of the SISZ as a simple shear-dominated triclinic transpressional zone with a primarily left-lateral sense of shear. Variations in plunge directions of lineations in different km-scale sectors are attributed to local modifications of the extrusion direction of the coaxial component of deformation. In comparison, SISZ structural fabrics within the PdL are more complex at smaller scales. Within the PdL, rheologic heterogeneities formed by interbedded strong quartzite and weak schist provide first-order constraints on strain localization, but deformation within the schist is also heterogeneous. Anastomosing shear bands at the mm-dm scales locally affect fabric intensities and orientations, and likely cause fabric-induced weakening. The foliation orientation is similar to that in the BAM (striking WNW-ESE, steeply dipping NNE), but the PdL contains only weakly-moderately expressed lineations. Magnetic lineations determined by Anisotropy of Magnetic Susceptibility (AMS) match scarce lineations observed in the field, and provide additional lineation data in other locations. Lineation plunge varies significantly on the m scale in both sets of lineation data. Electron backscatter diffraction (EBSD) mapping of quartz reveals weak flattening fabric crystallographic preferred orientation (CPO) patterns. Asymmetric features and Crystallographic Vorticity Axis (CVA) analysis confirm that, along with the flattening strain, most of the SISZ-affected PdL was deformed by thrust directed motion, but some locations were more affected by left-lateral shearing. These changes in vorticity combined with the variable lineations suggest m-scale kinematic partitioning within the PdL.

Geological Society of America Abstracts with Programs. Vol. 54, No. 6

doi: 10.1130/abs/2022AM-383453

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