Eye-to-eye with a mega-sheath fold: A case study from Wadi Mayh, northern Oman Mountains

Sheath folds are highly curvilinear folds, typically considered to develop by hinge rotation toward the transport direction during intense deformation. Most sheath folds are recognized on the centimeter to meter scale, with very few detailed studies of larger kilometer-scale structures. We here present evidence for a megascale sheath fold superbly exposed in the northern Oman Mountains. The Saih Hatat culmination consists of an exhumed high-pressure subduction zone with deep-level eclogites and garnet blueschists (As Sifah unit), epidote blueschists (Hulw unit) and carpholite-bearing metasediments (part of the Upper plate). Sheath folds are present at a variety of scales throughout the As Sifah, Hulw, and Upper:plate tectonic packages. Curvilinear fold hinges along a 15-km-long profile outline the Wadi Mayh mega-sheath fold, which closes to the SSW and faces to the NNE. It consists of at least four secondary sheath folds with cross-sectional (y-z) eye folds measuring 200 m stacked on top of one another within Permian and Triassic shelf carbonates. These eye folds display highly elliptical geometries (Ryz > 5) and cat's-eye fold patterns consistent with simple/general shear. The sheath fold is enveloped by Ordovician and older rocks that do not,show equal amounts of internal deformation. The upper bounding envelope is a detachment that has been subsequently sheath folded. The lower boundary of the sheath fold is the Upper plate-Lower plate detachment, which forms a major high-strain shear zone showing little vertical displacement but significant horizontal displacement. The sheath fold displays a SSW elongation (x axis) of at least 15 km and possibly up to 25 km, coupled with significant amounts of vertical flattening and WNW-ESE shortening.