Geometry and growth of syn-tectonic plutons emplaced in thrust shear zones: Insights from Abu Ziran Pluton, Egypt

Abstract

Coupling of deformation and magmatism has been reported in several old orogenic belts, particularly along thrust faults and shear zones. The syn-kinematic plutons in exhumed shear zones offer the best opportunity to understand the complex relationship between magmatism and regional deformation. The present paper investigates the geometry and internal structure of granite plutons emplaced in thrust faults and shear zones, and structural control on their emplacement mechanism. Abu Ziran pluton is an example of the intrusions emplaced in active brittle-ductile zones in the Nubian shield, and documents clear evidence on the interaction between magmatism and regional deformation during melt ascent and emplacement. Results from detailed geological mapping, remote sensing, and structural analysis of the pluton and associated highly strained rocks permitted the constraining of the pluton's geometry, emplacement mechanism, and spatio-temporal evolution. Structural analysis of the study area indicates that pluton emplacement was syn- to-late-tectonic. The brittle-ductile fabrics in the wall rock are consistent with a sub-horizontal thrust shear zone with a top-to-NW shear sense. The activity of the shear zone was accompanied by an episode of a calc-alkaline magmatic pulse. Granitic magma ascended upward via non-exposed feeder dykes, or through ramps and flats in the thrust system and emplaced laterally along the shear zone, forming complex sub-horizontal sheet-shaped intrusion. The geometry and extent of pluton emphasize that inherited heterogeneities and regional stress states played important role in the emplacement processes. In addition, localization of pluton along or near the contact between ophiolitic nappes and mylonitic metasediments suggests that the rheological boundaries act as barriers that impedes the rise of ascending magma, causing magma arrest, and triggers lateral spreading and emplacement. The outcomes of this study allowed the reconstruction of the geometry and internal structure of Abu Ziran pluton and an understanding of its evolution in space and time.