A detailed structural traverse across the basement rocks of the Eastern Desert of Egypt shows that they consist, apart from intrusions, of four broadly recumbent tectonic units. The lowest, of arkosic metasediments of continental shelf facies, is exposed in a dome. This unit is overlain by an allochthonous ophiolitic mélange containing complete and dismembered ophiolitic masses in a matrix of deep-oceanic graphitic pelites and turbidites. A near horizontal, schistose to mylonitic fabric, most intense near and below the base of the mélange, indicates extensive lateral tectonic transport of the ophiolitic mélange over the shelf metasediments, but the mélange originated as an olistostrome before being tectonically transported. The mélange extends over an area of at least 10 000 km 2 . It is locally overlain by calc-alkaline volcanics, in which deformation is less intense but increases, and schistosity flattens downwards, indicating some translation over the mélange. Unconformable on the mélange and calc-alkaline volcanics is a molasse-facies series, itself also locally strongly deformed. Late tectonic granites preceded, and locally post-date, the molasse-facies sediments. Still later, diapiric peralkaline riebeckite granites locally up-domed the recumbent structures. The tectonic evolution is related to a late Proterozoic subduction zone to the SE of the region.
Mostly the Palaeozoic and Mesozoic basins of North Africa have generally followed, and reworked, earlier basement trends formed by: (1) the NW--SE accretion of continental and oceanic terranes onto a Pan-African nucleus in northeastern Africa, and (2) the collision of this amalgam of accretionary terranes with the West African Craton. During the Upper Precambrian Pan-African Orogeny, the West African Craton formed a rigid block which indented this amalgam of accreted mobile belts to form much of North Africa. Intrusion of this indentor into North Africa caused the expulsion of narrow, triangular-shaped blocks of lithosphere to the north and south in a tectonic style very similar to the Miocene-Pliocene deformation of Tibet. Expulsion reactivated the earlier shear zones to form an anastomosing pattern of steeply dipping shears with left and right lateral sense of displacement. Left lateral shear also affected the northern edge of the West African Craton during this process of indentation.Subsequent rifting of the Pan-African mountain belt resulted in a series of grabens, which were infilled with Upper Precambrian42ambrian molasse. These are the precursor basins for the Palaeozoic sediments which cover much of North Africa. The effects of rifting continued into the CambroOrdovician in the western basins. During the Silurian-Devonian many of the rifts were reworked. A new basin formed in the Atlas and Anti-Arias, related to the growth of the proto-Tethyan Ocean.Basin inversion characterizes the Palaeozoic structures of the western Atlas and Anti-Atlas, producing thickened crust and a large mountain belt during the Carboniferous. Foreland basins formed on either side of this mountain belt and both the mountains and the adjacent basins were compartmentalized by WNW-ESE-trending transfer zones. Pan-African structures, within the African Plate, were reworked with further indentation of the West African Craton into Pan-African crest. The craton was pushed eastward, generating a left lateral shear couple along its northern margin. NW-SE-trending faults were reworked as dominantly left lateral strike-slip faults and N~S-trending fault blocks were rotated slightly in a clockwise sense. There was probably further lateral expulsion of lithosphere, ahead of the NE~SW-trending front of the indentor, reworking earlier N-S-trending shear zones.The North African Palaeozoic basins were inverted during the Hercynian-Appalachian Orogeny. In the Ahnet Basin the shortening was approximately NNE-SSW, perpendicular to the trend of the structures. This inversion was particularly marked in the Ougarta-Ahnet Basin where it produced a series of open to closed, north-south to NW-SE-trending folds above reactivated basement faults.During the Mesozoic, the Hercynian-Appalachian mountain belt underwent extension to produce deep rift basins infilled with continental sediments and some volcanics. The High Arias formed as an arm to the Atlantic Basin. Transfer zones have a WNW-ESE trend, indicating that this was the main extension direction, simila...
The Arequipa Massif, between the Andes and the Pacific, is an extensive pre-Devonian metamorphic complex. The sequence of deformations, metamorphisms and magmatism in this complex has been established. Mollendo, Atico and Marcona events are distinguished by structural and metamorphic methods and dated by Rb-Sr whole-rock isochrons, at about 1918,440 and 392 Ma respectively. The Mollendo event led to partial melting, followed by granulite-facies metamorphism, in sediments buried to about 30 km. Further N W , sillimanite-bearing migmatites and staurolite-andalusite schists are thought to represent the same event. The tectonic trend is uncertain but the structures and metamorphism suggest a collision orogeny which probably pre-dated the Pacific Ocean.The early Caledonian Atico and Marcona events are associated with coast-parallel batholiths, amphibolite-to greenschist-facies metamorphism and penetrative deformations. The Atico and Marcona events are separated by the deposition of the Marcona Formation, which is therefore thought to be Lower Palaeozoic (between about 440 and 392Ma). The early Caledonian deformations are attributed to a subduction zone near the present Pacific margin. There is no penetrative Hercynian or Andean deformation in the Axequipa Massif.Palaeomagnetic study of Jurassic andesites and dykes suggests that there has been no latitudinal motion of the AreauiDa Massif relative to the Brazilian shield during the evolution of --
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