Some Experiments on Block Rotation in the Brittle Upper Crust

Abstract

We present three series of experiments showing rotations of fault blocks about vertical axes. Experimental materials have yield criteria or flow laws similar to those estimated for the continental lithosphere. Thus we use a Coulomb material (dry sand) for the upper crust and viscous fluid layers for the lower crust, and most of the mantle. From nature to experiment, strengths are scaled down in proportion to lengths and densities: this ensures that the ratio of gravitational forces and surface forces is preserved. The first series of experiments concentrates upon brittle deformation in the upper crust. Sandpacks are subjected to horizontal plane-strain via lateral boundaries. The resulting pattern of strike-slip faults and block rotations depends upon the deformation history. Coaxial stretching produces conjugate faults and small block rotations. Coaxial stretching, with some simple shearing, results in conjugate domino domains. Coaxial stretching and simple shearing in nearly equal proportions result in a single domino domain synthetic with the imposed shearing. Rotations are always clockwise in left-lateral dominos, counterclockwise in right-lateral dominos.

The second series of experiments deals with brittle deformation above a viscous detachment. Double layers of sand upon silicone putty are subjected to right-lateral basal wrenching via two sliding baseplates. A jog in the plate boundary produces an area of pull-apart deformation in the overlying layers. Sigmoidal fault traces in the sand show strike-slip motions at their ends and dip-slip motions in their centers. Intervening fault blocks rotate clockwise, separate to produce rift valleys, and show right-lateral offsets.

Finally, the third series of experiments deals with continental indentation. Sand and silicone layers (representing the lithosphere) rest upon glucose syrup (the asthenosphere). Northwards indentation by a rigid piston results in a complex pattern of faults and block rotations. If the lateral boundaries of a continent are rigidly confined, indentation produces crustal thickening and arcuate thrust faults. Rotations of 20° or more occur near the ends of the thrusts. If there is less confinement at the eastern margin, indentation produces a major left-lateral shear zone (with internal counterclockwise block rotations) as well as clockwise rotation of a large escaping eastern block. Finally, if confinement is even smaller, indentation produces a major left-lateral shear zone with a system of wrench-rifts that rotate by a domino mechanism.