Microstructural Analysis of the Acebuches Metabasites from the South Iberian Shear Zone

Fault zones at depth are not widely investigated in situ because the area of interest is too deep and expensive to probe. At depth, increased heat and pressure cause the style of deformation to change from brittle fracturing to ductile flow, and faults that flow ductilely are referred to as shear zones. Subsequent erosion can cause ancient shear zones to be exposed for study and used as analogues for contemporary faults at depth. One of these exposed zones is the South Iberian Shear Zone (SISZ), which formed due to continental collision following the closing of the Rheic Ocean in the late Paleozoic. The SISZ exposes deformed Acebuches metabasites (north of the shear zone) and Pulo do Lobo schists (south of the shear zone). In this study, we examined the fluid-rock interactions of the Acebuches metabasites. To do this, we analyzed seven Acebuches metabasite samples spanning 188 meters along the Almonaster transect of the shear zone for mineralogy, grain size, and microstructures including solution seams, veins, and undulose extinction. Mineralogy is dominated by amphiboles and plagioclase. Grain size decreases towards the shear zone. Amounts of plagioclase decrease as amphiboles increase closer to the shear zone. Veins and solution seams show no patterns in their location or abundance indicating that fracturing and fluid interaction were dispersed. Undulose extinction is ubiquitous in plagioclase throughout the transect, but becomes more prevalent in the amphiboles towards the shear zone This indicates that 1) amphiboles are stronger than plagioclase and 2) crystal-plastic deformation increases towards the shear zone. The combination of veins, solution seams, and crystal plastic features indicates that deformation was accommodated by ductile processes of diffusion and crystal-plasticity with episodic brittle deformation, likely caused by fluid infiltration