A set of scaled analogue models was performed to investigate the role of a décollement layer in the structural style of fold-and-thrust belts. Silicone putty (SGM 36) has been employed to represent the ductile décollement, while frictional material (quartz-sand) has been used to simulate the brittle behaviour of roof- and floor- sequences, situated above and below the décollement respectively. In order to explore a wide combination of strength profiles, models have been deformed under five distinct rates of compression (0.15, 0.3, 0.45, 0.6 and 1.5 cm h-1) employing four different thicknesses for the silicone layer (0.1, 0.2, 0.4 and 0.8 cm). Experimental results indicate that roof-sequences exhibit two distinct styles of deformation: (a) passive-roof duplex geometry (e.g. Banks and Warburton, 1986; PRD) and (b) outward propagation of folding along the décollement (e.g. Davis and Engelder, 1985; OFP). In PRD geometry, the roof-sequence remain relatively stationary being underthrust by a wedge-shaped floor-duplex, while in OFP, displacement along the floor thrusts is transferred horizontally along the décollement layer. Experiments suggest the occurrence of a genetic relationship between shear stresses (at the base of the floor-sequence, tb, and within the ductile layer, td) and the development of PRD or OFP geometry. For a given strain-rate deformation pattern of roof-sequences depends upon the tb/td, ratio as well as the absolute tb and td values. The transition field between OFP and PRD styles has been approximated by curvilinear regression of transition-style experimental data points, with a minimum value of tb, varying between about 112 and 160 Pa, to enter the PRD field. Syntectonic sedimentation in the molasse basin is found to inhibit development of folding into the foreland, but to promote PRD style. Experiments simulate many of the salient characteristics of fold-and-thrust belts developed above a weak basal evaporite décollement, as well as the development of passive-roof duplexes at mountain fronts. Shear stress values inferred for many fold-and-thrust belts correlate well the deformation fields predicted by the present experimental study, allowing to propose a simple model describing the mechanical behaviour of natural roof-sequences underlain by a décollement layer.
Passive roof thrusting and forelandward fold propagation in scaled brittle-ductile physical models of thrust wedges
BONINI M
2001
Abstract
A set of scaled analogue models was performed to investigate the role of a décollement layer in the structural style of fold-and-thrust belts. Silicone putty (SGM 36) has been employed to represent the ductile décollement, while frictional material (quartz-sand) has been used to simulate the brittle behaviour of roof- and floor- sequences, situated above and below the décollement respectively. In order to explore a wide combination of strength profiles, models have been deformed under five distinct rates of compression (0.15, 0.3, 0.45, 0.6 and 1.5 cm h-1) employing four different thicknesses for the silicone layer (0.1, 0.2, 0.4 and 0.8 cm). Experimental results indicate that roof-sequences exhibit two distinct styles of deformation: (a) passive-roof duplex geometry (e.g. Banks and Warburton, 1986; PRD) and (b) outward propagation of folding along the décollement (e.g. Davis and Engelder, 1985; OFP). In PRD geometry, the roof-sequence remain relatively stationary being underthrust by a wedge-shaped floor-duplex, while in OFP, displacement along the floor thrusts is transferred horizontally along the décollement layer. Experiments suggest the occurrence of a genetic relationship between shear stresses (at the base of the floor-sequence, tb, and within the ductile layer, td) and the development of PRD or OFP geometry. For a given strain-rate deformation pattern of roof-sequences depends upon the tb/td, ratio as well as the absolute tb and td values. The transition field between OFP and PRD styles has been approximated by curvilinear regression of transition-style experimental data points, with a minimum value of tb, varying between about 112 and 160 Pa, to enter the PRD field. Syntectonic sedimentation in the molasse basin is found to inhibit development of folding into the foreland, but to promote PRD style. Experiments simulate many of the salient characteristics of fold-and-thrust belts developed above a weak basal evaporite décollement, as well as the development of passive-roof duplexes at mountain fronts. Shear stress values inferred for many fold-and-thrust belts correlate well the deformation fields predicted by the present experimental study, allowing to propose a simple model describing the mechanical behaviour of natural roof-sequences underlain by a décollement layer.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
