TRANSPRESSION IN DUCTILE SHEAR ZONES UNDER CONSTANT-VOLUME CONDITIONS: ESTIMATES FROM ANALOGUE EXPERIMENTS

Ductile shear zones can simultaneously undergo shortening normal to the shear zone boundary, a phenomenon described as transpression. Several workers have used kinematic models to show the patterns of internal deformation in transpression zones (Sanderson and Marchini, 1984; Fossen & Tikoff, 1993). However, there have been few attempts for estimating the degree of transpressive movement possible in a ductile shear zone (Robin & Cruden, 1994). This study aims to simulate ductile shear zones in analogue experimental models, and investigate the amount of transpressive movement under a given stress condition. Analogue experiments were conducted with a PDMS (viscous) layer sandwiched obliquely between rigid blocks. Under uniaxial stresses the rigid blocks had a tendency to slide past each other, giving rise to shear localization in the viscous layer. Using these experiments we address the following questions: 1) Does transpression occur during shearing under constant-volume condition and if so, what can be its variation with progressive shearing and 2) what are the factors controlling the degree of transpression?

Our experiments suggest that ductile shear zones can undergo transpression depending on their aspect ratio (Df) and the inclination, α, of the shear zone normal to the bulk compression axis. For low values of Df and small α, a ductile shear zone show transpression at the initial stage. We use the ratio (S_r) of shear zone normal shortening to shear parallel sliding as a measure of transpression. S_r = 0.77, when Df = 5, which decreases almost to zero when Df = 12, implying dominance of simple shear in ductile shear zones. For a given Df, S_r tends to decrease with increasing α. Based on the experimental results, we finally conclude that shear zones with large aspect ratios cannot undergo large transpressive movement, irrespective of their orientations to the regional compression direction.