20 



STRUCTURAL GEOLOGY OF NORTH AMERICA 



sheets rode eastward from the Yukon to southern Utah, and repeated 

 floods of coarse elastics occurred in this marginal belt. Several phases of 

 deformation are documented in most places. 



East of the thrust belt including the large region from Montana to 

 southern New Mexico and generally in the shelf region of sedimentation 

 anticlinal uplifts, mostly elliptical in ground plan and asymmetrical in 

 cross section rose in latest Cretaceous and Paleocene time. They are 75 

 to 150 miles long and 20 to 50 miles wide. Where the uplift has been 

 great enough to result in erosion exposing the Precambrian rocks, thrust- 

 faulting has occurred on the steep margin. The elliptical uplifts compose 

 the major mountain ranges of the region. Retween are intermountain 

 valleys where, particularly in Wyoming and Montana, considerable 

 amounts of Early Tertiary continental-type sediments were caught. 



The western or Pacific margin of the geanticline continued to shed 

 sediments to the adjacent basins, and no strong disturbance is indicated. 

 The San Andreas fault had probably come into existence and the west- 

 lying block at this time was lodged several hundred miles to the south, 

 but now had started to shift northwestward along the fault, as indicated 

 by the arrows. 



Major deformation of previously deposited Jurassic and Cretaceous 

 sediments occurred in the Greater Antilles with northward thrusting 

 in Cuba. 



Early Tertiary (Plate 14) 



The most conspicuous and probably most significant feature of Early 

 Tertiary time in the western cordillera was magmatic activity, especially 

 volcanic. As can be seen from the map that the Great Rasin region of 

 Nevada, western Utah, and central and southern Arizona, together with 

 the vast region of western Mexico, was mostly covered with volcanic 

 materials. Southern Idaho was also extensively covered. Significant al- 

 though scattered fields occur in New Mexico, Colorado, and Montana. 

 The central Cordillera of Canada developed a large field. Several hun- 

 dred small stocks also were intruded in the Great Rasin, southern Ari- 

 zona, and northern Mexico. All this activity followed the Nevadan and 

 Laramide orogenies and, in places at least, marked the beginning of 



block faulting and rifting that dominated the Late Tertiary activities. 



A eugeosyncline formed in Oregon and Washington, which is made ; 

 up of a very thick mass of sediments and volcanics. Deep but restricted 

 basins between uplifts developed in central and southern California. 

 The San Andreas fault was very active and the west block moved north- 

 ward, but was still considerably south of its present position. The 

 Atlantic and Gulf of Mexico continental margins continued to subside 

 during the Tertiary, but only in one or two places, particularly the 

 Mississippi embayment, did the Tertiary beds overreach the Upper 

 Cretaceous deposits. The Cretaceous and Tertiary sediments form the 

 present Gulf Coastal and Atlantic Coastal Plains. 



As the Atlantic margin of the continent subsided in Late Cretaceous 

 and Tertiary time, the Appalachian orogenic belt arched gently, and 

 successive erosion surfaces record the epeirogenic uplift. 



The Greater Antilles sank and appeared as a belt of islands around 

 which Tertiary sediments accumulated. Florida and the Rahama plat- 

 forms also continued to sink and to be built up by carbonate sediments. 



Late Tertiary and Quaternary (Plate 15) 



Volcanism continued prominent in the Late Tertiary with basalt fissure 

 eruptions in Washington and Oregon building the Columbia River field. 

 To the south in southern Oregon and Idaho another extensive basalt field 

 formed chiefly from vent eruptions. The west margin of these two large 

 basalt fields has been built especially high by additional volcanoes to 

 form the Cascade Range. A row of majestic stratovolcanoes of Quaternary 

 age dominates the Cascades and extends into southern Rritish Columbia 

 beyond the basalt fields. The Cascade volcanics are chiefly andesite. 



Rlock faulting of major proportions spread from the Sierra Nevada of 

 California to the Wasatch Mountains of Utah. It also extended through 

 southern Arizona and southward along the west coast of Sonora, Mexico. 

 An arm of the faulting extended northward through eastern Idaho, 

 western Wyoming, and western Montana to the Rocky Mountain Trench 

 of Rritish Columbia. The block, trench, or rift faulting is believed to be 

 of tensional origin and to penetrate deeply into the crust. 



The San Andreas fault block moved northward to its present position 



