WHAT'S HAPPENING UNDER THERE? KINEMATICS AND VERTICAL AXIS ROTATION OF CRUSTAL BLOCKS IN OBLIQUE DIVERGENCE AND CONVERGENCE

Fundamental to investigations of kinematics in the deforming lower crust and mantle at plate boundaries is the quantification of the rates and senses of vertical axis rotation of continental crustal blocks. In order to relate the rotation of rigid crustal blocks to strain and kinematics below them, we conducted physical experiments using a latex sheet to distribute deformation in an overlying layer of viscous gel with elongate, buoyant blocks embedded on top of the gel. In particular, we investigated vertical axis rotation in oblique divergence and convergence (i.e. transtension and transpression). Our results for the rotation of single blocks are consistent with purely kinematic predictions (i.e. Jeffery, 1922; Ghosh and Ramberg, 1976). However, we found that arrays of blocks do not conform to these purely kinematic predictions. Instead, they respond to both kinematic boundary conditions and dynamic topography created by strain incompatibility between the rigid blocks and the ductile gel. Using our results to interpret paleomagnetic data from field studies is challenging. First, most crustal blocks occur in arrays, but no predictive model exists to quantify the influence of dynamic topography on their rotation. Second, crustal block rotation may be driven either by forces in the upper crust (i.e. a top-driven system) or by flow in the lower crust or mantle (the bottom-driven system that our experiments mimic). Third, although the orientation of each boundary of a deforming zone is well known in experiments, this orientation is often ambiguous in field studies. For example, although results from our geologic and paleomagnetic study of crustal block rotation during Cenozoic extension in Long Valley (Idaho) constrain the kinematics of the underlying deformation that caused rotation, we can not identify the exact boundary conditions of divergence in this case. In summary, although knowledge of the rates and senses of vertical axis rotation of crustal blocks is insufficient to predict the kinematics of deformation in the lower crust and mantle at diffuse plate boundaries, these data are very useful for testing clearly defined hypotheses.