Progressive development of lattice preferred orientations (LPOs) of naturally deformed quartz within a transpressional collision zone (Panafrican Orogen in the Eastern Desert of Egypt)

Lattice preferred orientations (LPOs) of quartz were used to establish diff erences in deformation geometry, finite strain, and temperature within a tr anspressional collision zone within the Panafrican Orogen in the Eastern De sert of Egypt. Metamorphic and/or magmatic core complexes in the area are b ordered in the NW and SW by ductile sinistral NW-trending strike-slip zones and low angle normal faults (LANFs). Simultaneous activity of both fault s ystems suggests bulk W-E shortening coeval with orogen-parallel extension. Displacement partitioned into orogen-parallel sinistral strike-slip faults and LANFs. This study compares both quartz-LPOs in shear-zones and normal f aults. From south to north, quartz c-axis data show a continuous evolution along o rogen-parallel strike-slip faults from maxima in Y, with a slight tendency to oblique single girdles at the margins of the Wadi Beitan and Hafafit com plexes, to asymmetric crossed girdles and oblique single girdles along the margins of the Sibai and Meatiq complexes. The NW-directed LANFs to the NW of the Hafafit and the SE-directed LANFs to the SE of the Sibai show maxima in Y. The SE-directed LANF at the SE margin of the Meatiq complex shows sy mmetric crossed girdles, indicating coaxial deformation geometry. Oblique single girdles and maxima in Y occur in the southern part of the or ogen, whereas crossed girdle distributions dominate in the northern part. T he variation in quartz c-axis patterns is explained in terms of decreasing metamorphic grade during deformation from the S (medium to high grade) to t he N (low grade), and decreasing finite strain. This is in accordance with the general progression of transpressional tectonics and exhumation of core complexes from S to N. (C) 2000 Elsevier Science Ltd. All rights reserved.