Pure shear-dominated transpression and vertical extrusion in a strike-slip fault splay from the Itapirapuã Shear Zone, Ribeira Belt, Brazil

Abstract

The Itapirapuã Shear Zone is a major NE-trending dextral transcurrent zone from the southern Ribeira Belt (Brazil), which was active during the Neoproterozoic Brasiliano-Pan African Orogeny, related with West Gondwana assembly. Macroscopic sigmoid-shaped blocks in its northernmost splay termination present a NE-trending subvertical tectonic foliation and subvertical stretching lineation suggesting a transpressional tectonics. This paper investigates this splay termination using integration of data from geological mapping, thermodynamic modeling, microtectonics, quartz c-axis fabrics, kinematic vorticity and finite strain analysis. Distinct rock assemblages are placed in tectonic contacts along the splay termination, including Statherian sillimanite-biotite paragneiss and granitic orthogneiss (Apiaí-Mirim Complex), Statherian phyllites (Bairro dos Prestes Formation), Calymmian chlorite-epidote-actinolite metabasites (Fazenda Velha Suite), Ediacaran phyllites, metalimestone and quartzite (Itaiacoca Group) and Ediacaran granite (Três Córregos Suite). Isochemical phase diagrams constrain synkinematic metamorphic conditions of 490–510 °C and 11–12.5 kbar (chlorite-biotite-phengite phyllite), 520–560 °C and 9–10 kbar (biotite-phengite phyllite) and 680–690 °C and 5–6 kbar (sillimanite-biotite paragneiss), with isothermal decompression paths. Microstructural evidence suggests that increase in metamorphic temperatures were accompanied by changes in the dominant deformation mechanism in quartz aggregates from subgrain rotation to grain boundary migration recrystallization. Significant contribution of dissolution-precipitation creep indicates water-abundant deformational conditions. Strain ellipsoids are oblate-shaped and present NE-trending subvertical XY planes and subvertical X-axes in agreement with field tectonic fabric. Quartz c-axis fabrics in XZ sections of the strain ellipsoid are dominated by symmetrical crossed girdles, while the fabrics present dextral asymmetry in horizontal YZ sections, consistent with macroscopic structural deflection, indicating a subvertical vorticity axis. Vorticity results indicate 48–59% of pure shear contribution. The data indicate that strain in the Itapirapuã Shear Zone northern termination was accommodated by a pure shear-dominated transpression with vertical extrusion at mid-crustal conditions, which contrasts with a transtensional deformation in the southeastern block.