Gravitational collapse origin of shear zones, foliations and linear structures in the Neoproterozoic cover nappes, Eastern Desert, Egypt

摘要

The Um Esh-Um Seleimat area lies to the west of the Meatiq Core Complex (MCC), in the Central Eastern Desert (CED), Egypt, which forms part of the Neoproterozoic Arabian-Nubian Shield in NE Africa and Western Arabia. The study area is a NW-trending zone of intensely foliated ophiolitic melange and molasse sedimentary rocks. There is a single regional foliation, S_1, defined mainly by low- to very low-grade metamorphic phases, though grade increases to amphibolite facies in the areas bordering the MCC. S_1 is associated with shearing and passes directly into the mylonites of the MCC sheared carapace. The foliations and mylonites together define an originally subhorizontal thick ductile shear zone of regional extent. The sense of shearing is top-to-the-NW, parallel to NW-SE trending stretching lineations, L_1 . S_1 is folded by open rounded symmetrical mesoscopic F_2 folds with NW-SE trending subhorizontal hinges and variably dipping axial planes. F_2 folds are folded by coaxial (i.e. NW-SE trending) but non-coplanar close to tight macroscopic folds (F_3). Subhorizontal S_1 foliation formed continuously during F_2 folding and perhaps also into the early stages of F_3 folding. This reflects top-to-the-NW shearing under laterally confined conditions produced by the onset and gradual dominance of NE-SW shortening. SW-ward thrusts and NW-SE trending sinistral brittle faults are late stage structures. The NW-ward shear translation of the ophiolite and molasse cover nappes results from gravitational collapse following arc-collision and crustal thickening. A gliding-spreading nappe emplacement mechanism is most consistent with the field evidence. The steep metamorphic gradient from low-grade cover rocks downwards into gneissic rocks is interpreted as a result of vertical thinning of the ductile shear zone during collapse. Amphibolite facies conditions are found at the base of other top-to-the-NW low-angle major shear zones associated with gneissic complexes in the CED (e.g. El-Sibai, El-Shalul complexes) suggesting that the crustal level of the shear zone may be determined by thermally controlled rock rheological factors.