COEVAL GLOBAL EXTENSIONAL EVENTS – AN INDICATION OF TRANSMISSION OF CHANGES IN PLATE MOTIONS IN RESPONSE TO COLLISION, COUPLING, AND CAPTURE

Episodes of Tertiary extension in western North America are widespread, affecting 1000s of km² of crust from the Pacific Ocean to the Rocky Mountains. The episodes commonly are considered to record “post-orogenic collapse”, perhaps driven by gravity. However, the regional structural domains in western North America (NA) - including Basin and Range, and core complexes along the Colorado River and in southern British Columbia - are characterized by systems of strike-slip faults commonly linked by normal faults that accommodate temporally distinct (~15-0 ma, ~31-20 ma, 55-42 ma) extensions with remarkable overall consistency in the directions of tectonic transport (~275^o; 240^o, 285^o). The strike-slip faults include large continental-scale “transforms”, which obey E. M. Anderson kinematics (σ₁< 45^o to plane of maximum shear stress). The large faults are generally coeval and kinematically compatible with shorter, right- and left-lateral strike-slip faults, along which normal faults at releasing-steps may result in development of footwalls exposing formerly deep crystalline rocks, i.e. core complexes.

The characteristics of the extensional domains support the hypothesis that convergence followed by collision, coupling, and plate capture may lead to extension. In this model, arrival of buoyant oceanic lithosphere - commonly part of a long segment of spreading center - impedes subduction and leads to collision with the overriding continental plate. During collision, the formerly converging plates couple as the buoyant lithosphere binds against the base of the overriding plate. The length of the sutured former plate boundary will vary depending upon the length of the collided ridge segment. The length of the segment of plate boundary that must be coupled in order to induce crustal stretching is uncertain. Coupling of a ridge segment to the overriding plate leads to "capture", after which movements of the newly coupled plates are integrated. In western NA coupling of the Pacific plate to the North America plate generally adds a northwesterly component that affects the southwestward-moving North America plate. The net effect of the 15 ma and younger integration of the Pacific and NA plate motions has been westward extension accommodated by the San Andreas fault, and kinematically compatible normal and lateral faults. The western NA extensions are interpreted to be driven by movement of the Pacific plate as it and the margin of western NA are pulled toward western Pacific trenches into which the Pacific plate subducts. The distinct extensional events last until the buoyant oceanic lithosphere cools and begins to sink anew, generally a period of 10 to 15 Ma.

The similarity of ages of Tertiary extensional events in western NA to the multiple episodes of extensional deformation and related plutonism, and metamorphism within the zone of collided, coupled plates extending from the Himalaya to the Alps is remarkable. The correlation leads to the speculation that changes in Pacific-North America plate motions may be recorded at the coupled edges of major plates where collision has taken place (e.g. the Tethys collision belt). This conclusion infers that the suture zone is susceptible to reactivation caused by shifts in plate motions following collision, coupling, and capture.