THRUST TECTONICS IN CRYSTALLINE DOMAINS - THE ORIGIN OF A GNEISS DOME

Structural geological field work, microscopic and magnetic fabric stud ies have been applied in order to assess the structural origin of a gn eiss dome, based on a regional example from the Neoproterozoic Pan-Afr ican Belt of NE Africa, the Wadi Hafafit Culmination (WHC). The culmin ation is dominated by a number of major shear zones, which form both t he boundaries between the gneissic core and surrounding low grade succ essions as well as those of minor structural units within the gneisses . These shear zones form a linked fault system, which, based on shear criteria, fault-bend fold and overall geometric interrelationships, ca n be classified as an antiformal stack. The relative age sequence of t he shear zones/thrusts with the highest thrust oldest and the lowermos t youngest points to a forward-propagating thrust system. This, togeth er with the shear criteria; exclude an origin of the WHC as a metamorp hic core complex, where the highest shear zone should be youngest. The geometry of the WHC antiformal stack is documented by maps and sectio ns as well as section balancing and restoration. Microscopic work show ed brittle deformation in feldspar and dynamic recrystallization in qu artz ribbons. The asymmetry of the fabric confirmed the macroscopicall y determined shear sense. However, there is one example of an earlier, perhaps extensional shear movement. Mylonitic foliation and transport -parallel lineation have also been determined by magnetic fabric studi es. The observations suggest that thrusts may cut across both previous ly folded crystalline rocks as well as homogeneous granitoid plutonic bodies. According to the regional tectonic picture the large-scale str ucture of the gneiss dome originated after a phase of (late-orogenic) extensional collapse. It is speculated that during late-orogenic cooli ng the upper part of the lithosphere was sufficiently strong to allow brittle thrusting whilst the lithosphere as a whole was still weak eno ugh to allow large-scale compressional deformation, perhaps in a trans itional stage from late-orogenic to intra-cratonic deformation.