Centrifuge modelling of deformation of a multi-layered sequence ...

474 Int J Earth Sci (Geol Rundsch) (2012) 101:463–482Fig. 8 Model 52. A singlecompetent layer is placed in thecentre of the ductile substrate.See text for discussion.a Undeformed, b 10%shortening, c 25% shorteningFig. 9 Model 59. a Photographof a slice through Model 59after 42% shortening. Theinterface between the ductilesubstrate and layered sequencecontains a 0.5 mm thick layer ofpolydimethylsiloxane across theentire model. A thick layer ofDC 3179 with thin internalmarker layers of plasticinerepresents a ductile décollementwithin the layered sequence.b Sketch of photographhighlighting key shorteningfeaturesactive, buckle folds with different geometries that vary as afunction of the thickness of layers in the upper sequence,the presence or absence of a low-viscosity and densityductile layer at the cover sequence–substrate interface, andthe presence of internal thick ductile horizons.Layer thicknessThe mechanical properties of the layered sequence have afundamental impact on fold styles and geometries producedduring buckle folding where the dominant wavelengthof folds in a layered sequence is controlled by thethickness of individual layers, the space between competentand incompetent layers, ease of slip between layers,the rheology of those layers, and the total bulk shorteningimposed on the model (e.g. Biot 1961; Ramberg 1967;Smith 1977, 1979). Models 52 (Fig. 8), 60 (Fig. 4), and 61(Fig. 5) are each composed of the same materials (but ofdifferent colours) in the same proportions and experienced25–37% bulk shortening but contain layers differing inthickness. Although the dominant wavelength betweenindividual models is similar (10–20 mm), the thinnestlayeredmodels show the greatest variety of fold styles,amplitudes, and wavelengths (Fig. 5). In general, thickerlayers promote a more regular fold train with a relativelymore consistent wavelength and amplitude (Fig. 8).Presence/absence of a weak ductile layerThe presence of a weak layer (PDMS) between the layeredsequence and ductile substrate promotes different foldstyles and geometries than the half of the same model123