COASTWISE TRANSPORT RECONSIDERED - LATERAL DISPLACEMENTS IN OBLIQUE SUBDUCTION ZONES, AND TECTONIC CONSEQUENCES

Coastwise transport (margin-parallel displacement of arc and forearc slivers) is a common feature of zones of oblique subduction today, but at rates that rarely exceed a few tens of kilometers per million year. Paleomagnetic studies strongly indicate that coastwise transport in the North American Cordillera went on (during Late Cretaceous-Early Tertiary time) at rates several times higher than this. Other studies have shown that obliquity is commonly partitioned between oblique subduction and coastwise transport of a detached sliver of over-riding crust. The analysis developed here indicates that, in the absence of a buttress (a physical impediment to sliver motion at its leading edge), sliver motion should obey the relationship tan (p-phi) = R tan DELTA[1/(1 - R2 sin2 DELTA)]1/2 where phi is the angle of obliquity, DELTA is the angle of subduction, and R = r(f)/r(s) is the ratio of resistance to slip on strike-slip faults cutting the over-riding plate to resistance to slip on the subduction zone. In this equation p is a partitioning factor; 0 < p < 1, where p = 1 implies oblique subduction and no sliver motion, and p = 0 is the case in which the sliver moves at velocity V sin-phi, equivalent to the transverse component of relative motion. Rates of coastwise transport calculated from this relationship are systematically too high, presumably indicating that most modern subduction zones are buttressed. Key factors in the buttress effect are (1) the greater strength of oceanic vs. continental lithosphere and (2) the energy required to thicken continental crust. Large-scale tectonic transport in the North American Cordillera probably was theresult of a combination of favorable factors: long-continued, highly oblique convergence, and a plate-margin geometry that avoided the buttress problem.