Western craton (Cordilleran) was a passive margin Late Proterozoic to Early Paleozoic.

Beginning in the Middle Paleozoic - an island arc formed off the western margin of the craton.

Antler Orogeny -- collision of island arc with craton -- Late Devonian/Early Mississippian.

















                                          Laramide orogeny

  --   took place as the Farallon plate, buoyed up by a mantle plume subducted beneath North America
  at a decreasing angleand igneous activity shifted inland

                                    Change to Shallow Subduction

*      By Early Tertiary time,

  --   the westward-moving North American plate

  --   had overridden the part of the Farallon plate,

  --   above the head of the mantle plume

*      The lithosphere

  --   immediately above this plume

  --   was buoyed up,

  --   accounting for a change

  --   from steep to shallow subduction

                                      Igneous Activity Ceased

*      With nearly horizontal subduction,igneous activity ceased and the continental crust was
deformed mostly by vertical uplift

                                      Renewed Igneous Activity

*      Disruption of the oceanic plate by the mantle plumemarked the onset of renewed igneous
activity









                                          Colorado Plateau

                                      Basin and Range Province

*      Generalized cross section of the Basin and Range Province

  --   ranges are bounded by faults



                                          Basin and Range

*why is there extension in the Basin and Range in the first place?

*extension started about 25 Ma - same time as the beginning of subduction of the East Pacific Rise



                                         Cordillera Evolved

*      After Laramide deformation, Cordillera continued to evolve with large-scale block-faulting, 
extensive volcanism and vertical uplift and deep erosion

      --  Basin and Range

*      During about the first half of the Cenozoic Era, a subduction zone was present along the
entire western margin of the Cordillera, but now most of it is a transform plate boundary

                                           Pacific Coast

*      Before the Eocene,

  --   the entire Pacific Coast was a convergent plate boundary

  --   Farallon plate was consumed at a subduction zone

  --   stretched from Mexico to Alaska

                                       Change from Subduction

*      As the North American Plate overrode the Pacific--Farallon Ridge, its margin became  transform
faults

    *   the San Andreas

    *   and the Queen Charlotte

  --   alternating with subduction zones

                                  Extending the San Andreas Fault

*      Further overriding of the ridge extended the San Andreas Fault and diminished the size of the
Farallon--Plate remnants

*      Now only two small remnants of the Farallon plate exist

  --   the Juan de Fuca and Cocos plates