Archean Crustal Provinces once separated
                            Canadian Shield assembled from small cratons













































































































   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