Idaho Batholith and Its Southern Extension

Abstract

Distribution of minerals and rock types, emplacement structures, and postconsolidation history are more complex within the Idaho batholith than most published descriptions suggest. Hornblende occurs in some areas many miles within the batholith, and planar structure also is present in some of its interior regions. In the vicinity of the Cascade Reservoir in west-central Idaho, field relationships and modal data for granitic rocks are inconsistent with the conclusion (Schmidt, 1964) that the bedrock systematically and gradationally changes from schist and gneiss to directionless granitic rock along 35-mi traverses west to east across the border and interior of the batholith. Major fault blocks within the Idaho batholith invalidate the concept (Hamilton and Myers, 1966) of a resistant mass that defied internal deformation during the Cenozoic evolution of western North America. Gabbro and norite typically occur west of the batholith. Granitic intrusions near the batholith in western Idaho are characterized by megascopic crystals of epidote. Interstitial zeolites also occur in satellite masses rather than in granitic rocks of the batholith. Granitic rocks of southwest Idaho are correlated with the Idaho batholith primarily by the location and trend of gneissic border rocks on either side of the Snake River Plain. The trend of S. 20° W. in the gneissic border zone north of the Snake River Plain also is present in rocks lying between 40 and 55 mi to the south-southwest where gneissic granitic rocks reappear in the westernmost exposures of pre-Tertiary rocks south of the Snake River. Gross mineralogical characteristics of granitic rocks near the Snake River in southwest Idaho closely resemble those in the west part of the batholith just north of the Snake River Plain. Southwest structural trends in granitic rocks in southwest Idaho near the Snake River begin to deviate to the southeast about 25 mi due south of Marsing. Farther to the south, trends for 28 mi in the most westerly exposures of the batholith are about S. 20° E. Southeast trends also occur near South Mountain in pre-Tertiary country rocks west of the southernmost exposures of the batholith. The southeast trends within and outside the batholith indicate that a significant change in structural direction occurs in southwest Idaho in the region near South Mountain. The locations of the south and southeast contacts of the Idaho batholith are uncertain, but some inferences regarding the position of the batholith are possible from isolated occurrences of Ordovician sedimentary rocks south of Twin Falls and from exposures of pre-Tertiary sedimentary and igneous rocks near the Idaho-Nevada state line. Northward continuity of the Sierra Nevada batholith to the Nevada-Oregon boundary is well established. The trend of the batholith bends toward the northeast before the batholith disappears under Cenozoic volcanic rocks in southeast Oregon and northern Nevada. The distribution and composition of the plutonic rocks near the Nevada-Oregon border suggest that the quartz diorite boundary line is about 160 mi east of the inferred location (Moore, 1959) in northern California. If the Idaho and Sierra Nevada batholiths are connected, the Idaho batholith southeast of South Mountain must veer sharply west beneath Cenozoic volcanic rocks. Any connecting link between the two batholiths must be confined to a narrow belt that extends east-northeast for about 75 mi near the Idaho-Nevada and Oregon-Nevada boundaries. An appreciable change in the relative position of the Idaho and Sierra Nevada batholiths has occurred since Oligocene time. If the suggested magnitudes of displacement from normal faulting, dike intrusion, and right-lateral faulting are approximately correct, the east-west change in the alignment of the batholiths is as much as 50 mi. Gravity and seismic data considered in terms of surface geology and the distribution of granitic rocks are consistent with the interpretation that models of crustal structure should include a granitic layer underlying nearly all of southwest Idaho.