The exhumation of gneiss domes in bivergent wedges: Geometrical concepts and examples from the Himalayan syntaxes

Abstract

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 fl ow is signifi cant. 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.

Carbonate rocks in northern of West Jiwo Hills Bayat: The indication of thrust belt development in southern Central Java

“…The cross-section interpretation in Figure 15 shows that the older rock (metamorphic rock) is on top of the younger rock (carbonate of Oyo Formation) due to thrust fault growth. The structural geology analysis in the study shows a stacking system called an imbrication thrust stack (Whittington, 2004). This phenomenon causes the exhumation and uplifting of West Jiwo Hills (Figure 16).…”

“…Figure16. Modified imbricate thrust stack model byWhittington (2004) as a suggested model to explain the exhumation of West Jiwo Hills, Bayat. This model also proves that Jombor Swamp was a piggyback product of thrust fault.…”

Dynamic model for the exhumation of the Tutak gneiss dome within a bivergent wedge in the Zagros Thrust System of Iran

“…Various mechanisms have been proposed for the dynamic causes and kinematic development of gneiss domes by Yin (2004) and Whittington (2004). The latter suggested that bivergent wedges may develop by two conjugate shear zones, that on a smaller scale are known as "pop-up" or "flower" structures, in a compressional deformation regime.…”

“…2) (Getty and Gromet, 1992;Amato et al, 1994;Whittington, 2004). The Tutak gneiss dome provides a structural window for studying deformation and polyphase orogenic events in the Zagros Orogenic Belt of Iran.…”

“…Fault geometries accommodating exhumation: (A) Metamorphic core complex (univergent extension; erosional denudation of tilted fault blocks produces stippled sediments filling basins); (B) imbricate thrust stack (univergent compression; requires erosion or exhumation); (C) parallel normal and reverse faults; (D) conjugate normal shear zones (horst and graben); (E) conjugate reverse shear zones (bivergent wedge; requires erosion for exhumation); (F) bivergent wedge as for E but including a significant component of buckling(Whittington, 2004).…”