Structural evolution, metamorphism and restoration of the Arabian continental margin, Saih Hatat region, Oman Mountains

Abstract

170 m.y. of relatively stable passive margin sedimentation along the northern continental margin of Arabia was abruptly terminated during the Cenomanian–Turonian (∼95 Ma) when the Oman continental margin collapsed and subsided rapidly (Aruma basin) to accommodate obduction of the Semail ophiolite complex and underlying thrust sheets (Haybi and Hawasina complexes) in the Oman mountains. The ophiolite was emplaced at least 200 km over the passive margin, probably over 450 km in total, from NE to SW. During the later stages of obduction the leading edge of the continental margin was subducted to depths where carpholite-bearing rocks (6–8 kbar), blueschist (12–15 kbar) and eclogite (ca. 20 kbar) facies metamorphism formed in a ductile deforming NE-dipping subduction zone. Along the northeastern margin of the Saih Hatat culmination in NE Oman the Mesozoic shelf carbonates are deformed by recumbent folding on all scales. The present-day structure is dominated by four SW-vergent thrust slices in carpholite grade rocks at higher structural levels, overlying lower units of crossite-bearing blueschist (Hulw unit), garnet+glaucophane±chloritoid blueschists and garnet+clinopyroxene+phengite+glaucophane eclogites (15–20 kbar). The largest shear zone separates 7–8 kbar Hulw unit blueschists above from ∼12–15 kbar retrogressed eclogites and ca. 20 kbar eclogites of the As Sifah unit below. Further inboard (SW) a major shear zone separates the lower plate Hulw blueschist facies rocks from the upper plate sedimentary rocks (Upper plate–Lower plate discontinuity), which also show carpholite grade HP metamorphism towards the NE. Restoration of the entire Saih Hatat continental margin shows that the Upper plate–Lower plate discontinuity cuts down-section to the SW as far as the Proterozoic Hatat schists. NE-directed extensional crenulation schistosity and NNE oriented stretching lineations in the eclogite and blueschist facies rocks are consistent with SW-directed exhumation of footwall HP rocks. NE facing folds and spectacular sheath folds with greatly attenuated limbs in the upper plate sediments are interpreted as antithetic backfolds, with shortening in the upper plate balanced by the subduction of the lower plate, consistent with a NE-directed subduction of the continental margin rocks beneath the SW-obducting ophiolite, Haybi and Hawasina thrust sheets. Recent suggestions of a nascent SW-directed subduction beneath the Oman margin are not consistent with the sedimentary evolution of the shelf and slope carbonates or the geological structure of Saih Hatat. Two main phases of tectonic uplift of Saih Hatat and Jebel Akhdar are firstly, the latest Cretaceous ramp culmination associated with foreland propagating thrusts in the SW and backfolds in the NE, and secondly, post-late Eocene–Oligocene uplift by ca. 2000 m.