NEW EXPERIMENTS ON VERTICAL AXIS ROTATION IN OBLIQUE DIVERGENCE

We present physical experiments that explore the vertical axis rotation of long rigid blocks of continental crust during oblique divergence. Experiments use matchsticks embedded in a 2 cm thick layer of silicone gel that overlies a rubber sheet. A new device uses two motors to distribute oblique divergence (transtension) homogeneously in the rubber sheet and silicone gel. During experiments, single matchsticks behave like material lines because they rotate to parallelism with the direction of oblique divergence. Matchsticks in arrays, however, rotate away from parallelism with this direction. The sense and rate of rotation of these matchsticks in arrays depends on the geometry of the array, the scaling of experiments (using the Argand number), and the spacing of the matchsticks. Strain incompatibility between the rigid matchsticks and deformable silicone layer builds relief on the surface of experiments. Gravity-driven flow of silicone away from zones of positive relief causes the instability of the orientation of matchsticks in arrays. These results and interpretations agree with predictions by Lamb (1994). In zones of continental crust that are undergoing oblique divergence, stable orientations of long crustal blocks are not parallel to the direction of divergence, as a simple kinematic analysis predicts.