Polyphase flanking folds
Zusammenfassung der Projektergebnisse
This study analysed a Cambrian deformation sequence in Neoproterozoic metaturbidites of the lower Ugab Domain, Namibia, and specifically focussed on “boudinoids”, fold and boudin like-structures which are common in the area. As a spin-off, a new type of foliation was discovered. The lower Ugab Domain is affected by three phases of deformation related to two main tectonic events. D1 marks an event of sinistral transpressional E-W shortening and forms kilometre scale N-S trending upright to west vergent folds. D2 phase of deformation developed coaxial open folds, mostly in the flat limb of D1 folds, and with an S2 crenulation cleavage that everywhere overprints S1 at a high angle. D3 produced E-W to NE-SW trending minor folds and foliations and is associated to a late N-S shortening event. Two contemporary sets of composite quartz and calcite veins occurring pre-syn D1 are distinguished in the metaturbitites, VA and VB veins. VA are lensoid veins that lie approximately orthogonal to D1 folds axes and which formed by N-S extension during D1. They are strongly deformed due to mainly D1 shearing which produced a characteristic asymmetric shape of boudins and flanking fold alongside the veins, defining a sinistral sense of shear in outcrop. Deformation of boudins by rotation of the boudin neck veins in coaxial and non coaxial flow, as seen in the lower Ugab Domain, was been modelled by finite element analysis. The results show that the flow conditions and the relative competence of the veins and the wall rock can be constrained according to the shape of the resulting reworked boudins. Bone-shaped boudins occur in pure shear flow with shortening perpendicular to the layers where the boudin necks are stronger than the wall rock. They can also form by rotation of tension gashes in coaxial or non-caoxial flow in simple shear, transpressional or slightly transtentional flows. Asymmetric boudins occur where the boudin necks are stronger than the wall rock and the flow is composed of a significant simple shear component. Domino boudins form where the veins are less competent than the wall rock which allows slip to occur between the boudins. A comparison between the simulation results and the asymmetric boudins observed in the Lower Ugab Domain show that the quartz rich boudin necks were stronger than the quartz and mica rich host metapelite during the deformation of the boudin in the greenschist facies. A new type of foliation which occurs at proximity to syntectonic syenite intrusions was named flame foliation (S2f). Flame foliation defines penetrative anastomosing biotite rich selvedges that form at a high angle to S1 late-post D1. The orientation of the biotite perpendicular to the foliation and parallel to S1 suggests that flame foliation formed in extension rather than in compression like the classical foliations. The flame foliation developed in proximity to the contemporaneous syenite intrusions, which suggests that it formed due to devolatilization reactions and fluid overpressure in the metapelite as a kind of synmetamorphic alteration rim along microfractures. Flame foliation is the preferentially nucleation site for the development of S2 crenulation cleavage, which explains its orientation at a high angle to S1 throughout the area.
Projektbezogene Publikationen (Auswahl)
- (2005): Extensional foliation. 15th conference on deformation mechanism, rheology and tectonics (DRT), ETH Zürich, 2-4 May 2005
Maeder, X., Passchier, C.W., Trouw, R.A.J.
- (2005): Syntectonic veins in the Lower Ugab Valley and description of "extensional" foliation, Abstract program. 3th IGCP 485 Symposium, Gao, Mali, 21-28 November 2005
Maeder, X., Passchier, C.W., Trouw, R.A.J.
- 2008. A Damara orogen perspective on the assembly of southwestern Gondwana. Geological Society, London, Special Publications, 294, 257-278
D. R. Gray, D. A. Foster, J. G. Meert, B. D. Goscombe, R. Armstrong, R. A. J. Trouw , C. W. Passchier
- (2006): Sinistral transpressional shearing and formation of asymmetric boudins in the Lower Ugab Domain, southern part of the Kaoko Belt, Namibia. 4th IGCP 485 Symposium, Algiers, Algeria, 2-9 December 2006
Maeder, X., Passchier, C.W.
- (2006): Syntectonic veins in the Ugab Valley. EGU General Assembly, Vienna, 2-4 April 2006
Maeder, X., Passchier, C.W.
- (2007): Flame foliation – evidence for a schistosity formed normal to the extension direction. Journal of Structural Geology 29, p. 378-384
Maeder, X, Passchier, C.W., Trow, R.A.J. Maeder
- (2007): Modelling of boudin neck rotation: constrains of the total strain, the conditions of the flow and the relative viscosity of the host-rock and the veins on the shape of the reworked boudins. 16th conference on deformation mechanism, rheology and tectonics (DRT), Milano, 22 Sep- 02 Oct 2007
Maeder, X, Passchier, C.W., Koehn D.
- 2007. Facies and facies association of the siliciclastic Brak River and carbonate Gemsbok formations in the Lower Ugab River valley, Namibia, W. Africa. Journal of African Earth Sciences 47, 121-134
Paciullo, F.V.P., Ribeiro, A., Trouw, R.A.J., Passchier, C.W.
- 2008. Cambrian orogeny in the Ribeira Belt (SE Brazil) and correlations within West Gondwana: Ties that bind underwater. Geological Society, London, Special Publications, 294, 279-296
Schmitt, R.S., Trouw, R.A.J., Van Schmus, W.R., Passchier, C.W.
