Thrust tectonics in crystalline domains: The origin of a gneiss dome |
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| Authors: | R O Greiling |
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| Institution: | (1) Geologisch-Palaeontologisches Institut, Ruprecht-Karls-University, INF 234, D 69120 Heidelberg, Germany |
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| Abstract: | Structural geological field work, microscopic and magnetic fabric studies have been applied in order to assess the structural
origin of a gneiss dome, based on a regional example from the Neoproterozoic Pan-African Belt of NE Africa, the Wadi Hafafit
Culmination (WHC). The culmination is dominated by a number of major shear zones, which form both the boundaries between the
gneissic core and surrounding low grade successions 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,
can be classified as an antiformal stack. The relative age sequence of the shear zones/thrusts with the highest thrust oldest
and the lowermost youngest points to a forward-propagating thrust system. This, together with the shear criteria, exclude
an origin of the WHC as a metamorphic core complex, where the highest shear zone should be youngest. The geometry of the WHC
antiformal stack is documented by maps and sections as well as section balancing and restoration. Microscopic work showed
brittle deformation in feldspar and dynamic recrystallization in quartz ribbons. The asymmetry of the fabric confirmed the
macroscopically 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 studies. The observations suggest
that thrusts may cut across both previously folded crystalline rocks as well as homogeneous granitoid plutonic bodies. According
to the regional tectonic picture the large-scale structure of the gneiss dome originated after a phase of (late-orogenic)
extensional collapse. It is speculated that during late-orogenic cooling the upper part of the lithosphere was sufficiently
strong to allow brittle thrusting whilst the lithosphere as a whole was still weak enough to allow large-scale compressional
deformation, perhaps in a transitional stage from lateorogenic to intra-cratonic deformation. |
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| Keywords: | Thrust tectonics gneiss dome Neoproterozoic-Nubian Shield mylonites |
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