CORDILLERAN TECTONICS: THE ROLES OF LITHOPLATE AND MESOPLATE BOUNDARIES

Mesoplates are a new heuristic for quantifying the motion of lithoplates relative to the mesosphere. The three major mesoplates are Hawaiian (primarily beneath the oceanic plates of the Pacific), Tristan (beneath most of the plates of the Atlantic and Indian Oceans), and Icelandic (beneath Eurasia, the northernmost Atlantic, Arctic Ocean). While lithospheric plate interaction is the principal control on mountain belt evolution, mesoplate interactions also contribute to the overall “style” of deformation.

The principal evidence for mesoplates is distinctiveness of hotspot reference frames for each of the three regions (Hawaiian, Tristan, and Icelandic). While hotspots within each frame show very little relative motion, the three frames demonstrate significant relative motion among themselves over the past 130 to 80 my. Shallowness of the reference frames is indicated by correspondence of stress fields with motions in the hotspot frames, as well as lithospheric thickness controls on the origin of minor hotspot traces in the Pacific and South Atlantic. Because the three reference frames are shallow, interaction with subduction zones is implied. Continuity of deep subduction zones implies that the zones are the probable boundaries between adjacent mesoplates. Where medium-to-deep subduction zones are absent, the boundaries between mesoplates are likely determined by kinematics.

For the North American Laramide (~80 to 48Ma), lithoplate and mesoplate kinematics imply North America moved to the southwest relative to Tristan, while motion between the Hawaiian and Tristan mesoplates was divergent, and relative motion of North America and the Hawaiian mesoplate was minimal. The growing “gap” between the two mesoplates was “filled” by the low-angle subducting Farallon (or Kula) plate.

Pacific plate motion at ~48 Ma (Hawaiian-Emperor bend) changed relative to the Hawaiian mesoplate and North America. Concurrently, the boundary between the Hawaiian and Tristan mesoplate was gradually unconstrained as younger plate began to be subducted, and then subduction progressively ceased. Thus, the mesoplate boundary began to move beneath western North America, contributing to the complex extensional tectonics and uplift of the western United States in the Late Cenozoic.