BEHAVIOR OF THE BRITTLE CRUST IN WIDE PLATE BOUNDARY ZONES

In wide and active continental plate boundary zones, ductile flow in t he deeper and strong parts of the lithosphere may control crustal defo rmation. This is likely if average resistive shear stresses on faults in the brittle crust are much less than 10(8) Pa and the underlying bu lk effective viscosity is much greater than 10(21) Pa s. In this case, a simple model of distributed deformation, referred to as the floatin g block model, may be useful. This treats the crust as an array of rot ating and translating rigid blocks, which are floating on an underlyin g continuous flow with a constant rheology. The model is analyzed in d etail in this paper because it has the potential to link detailed obse rvations of crustal deformation with the large-scale flow. Crustal blo cks are defined by at least two sets of faults. The kinematics of crus tal deformation can be described in terms of the motions of these bloc ks. Both the relative motion on block boundaries (faults) and block ti lting about a horizontal axis can be described in terms of the underly ing flow and block rotation about a vertical axis. However, rotations about a vertical axis, which are an important component of the crustal deformation, will depend not only on the underlying flow but also on the shape, orientation and arrangement of the crustal blocks. The aver age rotation rate about a vertical axis, over finite rotations, will b e significantly different from that predicted at any instant. Also, th e rotation history is considerably complicated if, as is likely, the u nderlying flow field, or block shape, has changed with time. These asp ects of crustal deformation are discussed with reference to real zones of active deformation in New Zealand, Greece and western North Americ a.