MICROPLATE VERSUS CONTINUUM DESCRIPTIONS OF ACTIVE TECTONIC DEFORMATION

Whether deformation of continents is more accurately described by the motions of a few small rigid plates or by quasi-continuous flow has im portant implications for lithospheric dynamics, fault mechanics, and e arthquake hazard assessment. Actively deforming regions of the western United States, central Asia, Japan, and New Zealand show features tha t argue for both styles of movement, but new observations are necessar y to determine which is most appropriate:and at what scale the descrip tion applies. Geologic, geodetic, seismic, and paleomagnetic measureme nts tend to sample complementary aspects of the deformation field, so an integrated observation program can utilize the strengths of each me thod and overcome their separate spatial or temporal biases. Provided the total relative motion across each region is known and the distribu tion of active faults is well mapped, determination of fault slip rate s can provide potentially decisive constraints. Reconnaissance geologi cal studies supply useful slip rate estimates, but precise values depe nd upon detailed intensive investigation of individual sites. Geodetic survey measurements can determine the spatial pattern of contemporary movements and extract slip rate information, but the sometimes elusiv e effects of cyclic elastic strain buildup and relief must be accounte d for in relating current movements to the long-term deformation patte rn. Earthquake catalogs can be applied to determine seismic strain rat es and relative velocities but must be averaged over large regions and are usually limited by the inadequate duration of historical;or instr umental seismicity catalogs. Paleomagnetic determinations of vertical axis rotations provide estimates of block rotation rates hut are often locally variable and averaged over many millions of years. Which of t he two descriptions of continental tectonics is more nearly correct de pends on the local rheological stratification of the lithosphere, espe cially the strength and thickness of the elastic crust relative to the ductile lithosphere, and dynamical models can provide contrasting for ecasts of observable features with testable consequences.