The exhumation of gneiss domes in bivergent wedges: Geometrical concepts and examples from the Himalayan syntaxes
Published:January 01, 2004
Alan G. Whittington, 2004. "The exhumation of gneiss domes in bivergent wedges: Geometrical concepts and examples from the Himalayan syntaxes", Gneiss Domes in Orogeny, Donna L. Whitney, Christian Teyssier, Christine S. Siddoway
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Gneiss domes are typically bounded by shear zones that accommodated differential exhumation relative to their surrounding host rocks. Consideration of possible structural geometries shows that two arrangements are particularly effective at exhuming deep crustal rocks. One consists of sub-parallel dipping shear zones, where an upper normal shear zone overlies a lower reverse shear zone. The other is a bivergent wedge, bounded by conjugate reverse shear zones, where vigorous erosion is required for substantial exhumation to occur.
For the bivergent wedge to remain stable, material lost through erosion must be balanced by a combination of accretion via downward migration of the bordering shear zones and advection of material into the wedge. Symmetric wedges can only exhume deep rocks if either buckling or diapiric flow is significant. Asymmetric wedges are bounded by a dominant and a subordinate antithetic shear zone and can always exhume deep rocks. The petrologic signature of the asymmetric bivergent wedge is a thick zone of penetratively sheared rocks on the dominant side. If the antithetic shear zone becomes inactive, it may be eroded entirely, and the resulting structure may resemble a simple thrust nappe.
The western and eastern Himalayan syntaxes are experiencing rapid exhumation. The western syntaxis contains an asymmetric bivergent wedge that formed in response to continued shortening after development of a crustal-scale antiform. In the eastern syntaxis, a bivergent wedge is found in one limb of a major antiform. This may represent a reactivated mid-crustal duplex, implying a different structural evolution than for the western syntaxis.