IGNEOUS PROVINCES IN WESTERN UNITED STATES 



563 



displaced the Plain downward relative to the highlands on the north. 

 At least 5000 feet of movement occurred between the early and middle 

 Pliocene, and progressively diminishing movements amounting to 4000 

 have occurred since. 



The crustal break implied by the gravity measurements is possibly expressed 

 by a line of earthquake epicenters that extends diagonally from Puget Sound, 

 across the Columbia River Plateau, along the northern boundary of the 

 western Snake River Plain, and thence across the plain to northern Utah. In 

 Idaho, these earthquakes originate principally at average depths of 61 and 38 

 km (38 and 24 mi), the shallower earthquakes being near the base of the 

 crust (6). The displacement calculated from the gravity measurements there- 

 fore ranges from one-tenth to one-third of the local crustal thickness (Malde, 

 1959). 



The geology of northern Utah hardly permits the extension of the fault 

 zone into this region. The writer believes, rather that a more logical 

 projection is eastward under the Snake River volcanic field to Yellow- 

 stone Park. It is thus shown on Figs. 31.21 and 31.22, where its tectonic 

 significance is discussed. It is interpreted chiefly as a zone of distention, 

 and if so, seems to afford a natural channelway for the lavas from the base 

 of the silicic crust and from the basaltic subcrust. See Fig. 31.25. The 

 transcurrent nature of the Snake River volcanic field is thus better under- 

 stood. Also, the fissure effusion of great volumes of basalt from the sub- 

 i crust may be accounted for. 



! PROVINCES OF THE MIOGEOSYNCLINE AND SHELF 



I 



^General Characteristics 



The tectonic provinces of the Rocky Mountains stand apart from the 

 '(Pacific marginal provinces in several respects; their mountains, plateaus, 

 and basins were developed by late Mesozoic and Tertiary orogeny and 

 epeirogeny in the Paleozoic miogeosynclinal and shelf regions and also 

 ton the miogeosynclinal-type sediments of various Jurassic, Cretaceous, 

 and Tertiary basins. Thick late Precambrian sandstone and shale se- 

 quences underlie part of the Paleozoic miogeosyncline and shelf areas, 

 and in other areas, particularly in Colorado and Arizona only a very thin 

 sedimentary veneer existed on the crystalline rocks of the Precambrian 



basement at the time of late Cretaceous and Tertiary orogeny. Such is the 

 general tectonic setting for the eventful and diversified igneous history 

 of the Rocky Mountains which began in Cretaceous time and continued 

 from place to place to the present. 



The igneous rocks of the Rocky Mountains, like the sedimentary rocks 

 and structures, stand apart fairly distinctly from those of the eugeo- 

 synclinal and batholithic belt to the west; in particular they are generally 

 more alkalic. Rasalts and andesites are present and in places abundant, 

 and the orogenic type basalt-andesite-dacite-rhyolite association is promi- 

 nent, and therefore a similarity exists with this overwhelmingly pre- 

 ponderant kindred of extrusive types in the Pacific marginal regions. But 

 where present the intermediate latitic differentiates are most abundant in 

 contrast to the dominant andesites of the Pacific marginal belts. The 

 Rocky Mountains are characterized especially by the classical kindreds 

 of calc-alkalic olivine basalt-trachyte-phonolite and alkalic leucite basalt- 

 trachybasalt-trachyte. The nepheline syenites are intrusive accompani- 

 ments in places. For the fractional crystallization associations an olivine 

 basalt is generally considered the parent magma, but assimilation or 

 fusion of small or appreciable amounts of calcic or alkalic country rock 

 such as limestone, amphibolite, granite, or mica schist by the olivine 

 basalt magma is postulated, or at least admitted as possible, to produce 

 the melts from which the high calc-alkalic or alkalic fractional crystalliza- 

 tion kindred resulted. 



Trachyte and Phonolite Provinces 



Extent of Provinces. Igneous rocks containing a high amount of either 

 sodium or potassium or both are characteristic of large areas in the Rocky 

 Mountains. 



Three high alkalic kindreds are generally recognized on a world-wide 

 basis, the leucite basalt-trachyte, the olivine basalt-phonolitc, and the 

 nepheline syenite (Turner and Verhoogen, 1951). The first two are classed 

 as nonorogenic assemblages and the last, which is, of course, an intrusive 

 type is regarded as a low-temperature, high alkalic residue of an evolu- 

 tionary series in which volatiles played an important role. The phonolites, 

 trachytes, and syenites appear as minor end members of an olivine basalt 



