Abstract
Detailed field-structural mapping of Neoproterozoic basement rocks exposed in the Wadi Yiba area, southern Arabian Shield, Saudi Arabia illustrates an important episode of late Neoproterozoic transpression in the southern part of the Arabian-Nubian Shield (ANS). This area is dominated by five main basement lithologies: gneisses, metavolcanics, Ablah Group (meta-clastic and marble units) and syn- and post-tectonic granitoids. These rocks were affected by three phases of deformation (D-1-D-3). D-1 formed tight to isoclinal and intrafolial folds (F-1), penetrative foliation (S-1), and mineral lineation (L-1), which resulted from early E-W (to ENE-WSW) shortening. D-2 deformation overprinted D-1 structures and was dominated by transpression and top-to-the-W (-WSW) thrusting as shortening progressed. Stretching lineation trajectories, S-C foliations, asymmetric shear fabrics and related mylonitic foliation, and flat-ramp and duplex geometries further indicate the inferred transport direction. The N- to NNW-orientation of both "in-sequence piggy-back thrusts" and axial planes of minor and major F-2 thrust-related overturned folds also indicates the same D-2 compressional stress trajectories. The Wadi Yiba Shear Zone (WYSZ) formed during D-2 deformation. It is one of several N-S trending brittle-ductile Late Neoproterozoic shear zones in the southern part of the ANS. Shear sense indicators reveal that shearing during D-2 regional-scale transpression was dextral and is consistent with the mega-scale sigmoidal patterns recognized on Landsat images. The shearing led to the formation of the WYSZ and consequent F-2 shear zone-related folds, as well as other unmappable shear zones in the deformed rocks. Emplacement of the syn-tectonic granitoids is likely to have occurred during D-2 transpression and occupied space created during thrust propagation. D-1 and D-2 structures are locally overprinted by mesoscopic- to macroscopic-scale D-3 structures (F-3 folds, and L-3 crenulation lineations and kink bands). F-3 folds are frequently open and have steep to subvertical axial planes and axes that plunge ENE to ESE. This deformation may reflect progressive convergence between East and West Gondwana.