56 NORTH AMERICA/Southern Cordillera tectonostratigraphic terranes not yet completely attached to the continental margin During the Middle and Late Jurassic, magmatism spread eastward to include areas well within Laurentian crust (e.g., the Ruby Mountains of north-eastern Nevada) Also, during this time the Sundance Sea occupied an incipient foredeep in the Western Interior Basin and important terrane accretion took place in the western Sierra Nevada, Klamath Mountains, and Blue Mountains The geological history of the Southern Cordillera during the early part of the Cretaceous Period (i.e., Neocomian) is poorly understood Exposures of Neocomian magmatic and sedimentary rocks are rare in the western North American Cordillera The time interval $140–125 Ma is commonly referred to as ‘the Early Cretaceous magmatic lull’ Global sealevel was low during this time, and in the Rocky Mountain region and environs a regional unconformity exists between the chiefly fluvial deposits of the Upper Jurassic Morrison Formation and overlying mid-Cretaceous strata of the Western Interior Basin In contrast, widespread Late Cretaceous magmatism in the Sierra Nevada and shortening in the foreland fold-and-thrust belt (i.e., Sevier Orogenic Belt of south-eastern Idaho, western Wyoming, and northern Utah) were synchronous with the culmination of seaway advancement The fold-and-thrust belt is characterized by sled-runner thrust faults that are directed eastward and merge into a de´ collement near the top of the crystalline Precambrian basement Major thrust systems are younger to the east, and the shortening history extends from $119 to $52 Ma The Late Cretaceous culmination of magmatism in the Cordilleran Magmatic Arc ($100–85 Ma) coincides with intense shortening in the fold-and-thrust belt One tectonic model argues for a close tie between Cretaceous magmatism in the California arc and continental lithospheric underthrusting beneath the arc (so-called A-type subduction) Although Sevier belt thrusting was initiated in mid-Cretaceous time, earlier (Jurassic) back-arc shortening is well documented in central Nevada (e.g., the Luning-Fencemaker fold-and-thrust belt) While Late Cretaceous magmatism occurred in the Cordilleran Arc and shortening took place in the Sevier Orogenic Belt, regional metamorphism, largescale folding, and the generation of peraluminous granitic rocks (commonly containing two micas, garnet, Ỉsillimanite) characterized the deep crust of the hinterland (Figure 3) The regional thrust faults of the Sevier Belt root into this igneous–metamorphic infrastructure, and remobilized Precambrian basement rocks were involved in both the plastic deformation and deep-crustal anatexis Juvenile magmatic additions from the mantle did not play a significant role in the petrogenesis of the granitic rocks Tectonic thickening coupled with dehydration melting during late decompression were the key processes responsible for metamorphism and anatexis in the Sevier hinterland The deep-crustal rocks of the Sevier hinterland are exposed in windows developed during Tertiary crustal extension Laramide Orogeny Distribution of igneous activity in the Cordillera of the western United States changed dramatically near the end of the Cretaceous Period At $75 Ma, magmatism in the Sierra Nevada batholith ended, and magmatism migrated eastward into the Rocky Mountains and beyond This change in the geographic distribution of igneous activity in the Southern Cordillera is roughly synchronous with the Laramide orogeny of the Rocky Mountains In the Rocky Mountain foreland, the Laramide Orogeny is characterized by the development of basement-involved uplifts and adjacent deep basins during Late Cretaceous through Early Eocene time (Figures and 4) This ‘thick-skinned’ tectonic style is dramatically different from the contemporaneous ‘thin-skinned’ style of the Sevier fold-and-thrust belt Although both deformations may be broadly related to plate tectonic processes near the continental margin, the local decoupling levels for these foreland deformation belts are different The decoupling zone for the Sevier foldand-thrust belt lies above the Precambrian basement except in the more interior (western) parts of the belt, whereas seismic-reflection studies show the decoupling zone for the basement-involved Laramide uplifts to be rooted into the deep crust The typical plate tectonic explanation for both the Late Cretaceous change in geographic distribution of Cordilleran magmatism and development of a thickskinned tectonic style in the eastern Rocky Mountains involves a change to a shallow-dipping subduction mode In this interpretation, subduction occurs at a shallow depth beneath the overriding plate in a wide segment of the orogen that extended from south-western Montana to central New Mexico (Figure 4) Why such segmentation of the subduction system would occur along the strike of the orogen is a major unanswered question in Cordilleran tectonics The Andean orogenic system of South America is commonly proposed as a present-day analogue for the Late Cretaceous–Early Tertiary orogen of western North America In the present Andean system, segmentation of the subduction system may be related to the character of the subducting plate; i.e., the presence of an oceanic plateau or aseismic ridge The age of the subducted oceanic lithosphere may also be important