Post-obduction deformations of the northwestern end of the Hatta Shear Zone, El Rawdah area, Northern Oman Mountains

摘要

The NW-trending Masfut-El Rawdah structural ridge lies at the western end on the Hatta shear zone, UAE, and is dating to the time of the obduction of the Semail Ophiolite at the end of the Cretaceous. This ridge is draped over by the Campanian-Lower Eocene carbonate rocks which were thrusted and folded later during the Tertiary Alpine deformation event. The folds are represented by two large plunging; NW oriented anticlines that are alternating with a box-shape syncline, in addition to, other two small NE and E-W trending fold pairs. The large middle syncline is characterized by a Z-shape that may be controlled by the displacement over NW trending, en echelon thrust faults bounding it. The segments of southwestern limb of this syncline are separated by two bends which forms the two NE oriented small folds. The northeastern limb involves the other two E-W trending folds that developed in the relay space between the en echelon thrust faults. The E-W regional compressive stress that predated the Late Cretaceous obduction of Semail Ophiolite was adopted in syn- and post-obduction phases to NE-SW orientation. This NE-SW compression is responsible for the origin of the NW oriented thrust faults and the related parallel El Rawdah folds, which reflect shortening values between 14% and 29.4%. The post-obduction deformation took place during three events which were commenced in the pre-Campanian. The second is renewed in the Late Campanian to continue during the Maastrichtian-Paleocene times. While, the final event was in post Early Eocene. The rock mechanical properties and bed thicknesses controlled the folding mechanisms. Whereas, buckling mechanism developed open folds in the relatively thick and brittle beds of the Campanian-Maastrichtian units. But flexural slip folding mechanism produced tight folds in the thin and ductile beds of the overlying rock units.