CONVERGENCE, COLLISION AND THE FORMATION OF CONTINENTAL TRANSFORMS: IMPLICATIONS FOR THE DEVELOPMENT OF TRANSTENSIONAL DOMAINS

Temporal and kinematic characteristics of fault sets that bound transtensional basins in western North America distinguish the effects of extensional episodes that may be correlated with the initiation and subsequent evolution of plate margin transforms. Along the transforms, a process zone denoted by simple shear strain is generally prominent. In continental rocks inboard of the faults a transition from principally simple to principally pure shear may be recognized. On a regional scale the pattern of fault sets is rather simple and may be compared to a model of pure shear strain in which the plane of maximum shear stress, i.e. the transform, is wide and well developed.

In western North America continental margin transforms have formed in response to collision that impeded subduction and led to coupling of the overriding continental plate with the subducting oceanic slab. As the coupled plates are dragged against the buttress-like North America plate interior, they fracture systematically and deform along normal and strike-slip faults. Soon a thru-going continental transform develops along a plane of maximum shear stress. The orientation of plate boundary transforms may be determined by assuming that: 1) the direction of maximum principal stress is parallel to the strike of normal faults that bound pull-apart basins at releasing steps along strike-slip faults; 2) the transform coincides with a plane of maximum shear stress, 2) over a broad region, the transform has a simple, i.e. Andersonian, relationship (30-45O) to the ä1 axis, and 3) the movement sense of the transform is kinematically compatible to coeval normal faults. Simple shear along the transform accommodates continuing convergence while concurrent lateral displacement ferries away the subduction-clogging lithosphere (i.e. microplates) of both plates.

Transmission of the plate tectonic forces into the adjacent continental crust of the overriding North America plate has resulted in the abrupt development of:1) Late Middle to Late Jurassic, 2) Eocene to lower Oligocene; 3) Early Miocene and 4) Late Miocene transtensional domains.