Geometry and significance of internal windows and regional isoclinal folds in northeast Saih Hatat, Sultanate of Oman

Abstract

Saih Hatat contains tiered sets of low-angle structural breaks that reflect a complex stacking of sheets, which requires multiple upward movement of units within an unstable orogenic wedge in a convergent setting. Early recumbent folds within the Saih Hatat window are subparallel to, but cut by, a major low-angle contractional fault (décollement) that separates lower plate high-pressure rocks (some eclogites) exposed in two windows (Hulw and As Sifah windows) from high-pressure upper plate rocks containing carpholite and lawsonite. Parts of this décollement had been mapped, or speculated upon, to explain the variation in metamorphic grade across Saih Hatat. Upper plate fold-nappes are controlled by the mapped décollement surface. Below the décollement sheath-like lower plate closures formed independently in an intense zone of top-to-the-northeast shearing. Eclogites occur within mafic boudins at the structurally lowest level of the retrograde lower plate shear zone. This zone was responsible for the partial exhumation of the lower plate very early in the observed structural history and has a normal movement sense. The two lower plate windows are essentially foliation domes reflecting fold interference between regional-scale, sheath-like, recumbent isoclines and more open, upright northwest-, north- and northeast-trending fold sets. The early upper plate fold-nappes reflect a major top-to-the-north or -northeast movement of the rocks away from the Oman margin at the same time the ophiolite was being emplaced over the top in the opposite direction (top-to-the-southwest or -south). South-directed faulting and shearing is superimposed on the early recumbent folds in the mountain range. Mapped relationships and structural observations agree with Bureau de Récherches Géologiques et Minières mapping that recognised fold-nappes in this part of the Oman Mountains. All of the mapped structures are associated with exhumation of the high-pressure rocks making reconstruction of the margin during high-pressure metamorphism problematic.