4: Regional magnetic patterns in part of the Cordillera in the Western United States

Abstract

A residual aeromagnetic map of Idaho, western Montana, western Wyoming, southwestern Oregon, Nevada, Utah, western Colorado, and northern Arizona illustrates magnetic patterns that are related to regional geology. The magnetic map provides useful information on the development of the crust in this region of the western Cordillera since Precambrian time. A major feature of the map is a broad zone extending from southern Nevada to northern Idaho, where magnetic anomalies from basement rock are not apparent. This feature is here named the " quiet basement zone." To the west of this zone abundant magnetic anomalies are produced by Phanerozoic intrusive and extrusive igneous rocks. West-trending zones of magnetic anomalies in western Utah and Eastern Nevada contain abundant igneous rocks and most of the known mineral resources of this region. The major magnetic anomalies in the area east of the northern part of the Basin and Range province and east of the overthrust belts in southeastern Idaho and western Montana reflect lithologic contrasts in the Precambrian basement with prominent northeast and northwest trends. 93on July 2, 2015 memoirs.gsapubs.org Downloaded from 9 4 M a b e y a n d O t h e r s A magnetic high in north-central Nevada and another over the western Snake River Plain suggest north-northwest-trending Miocene rifts. The eastern Snake River Plain and the Yellowstone caldera are part of a much more extensive northeast-trending feature here called the Humboldt zone. In east-central Idaho, Tertiary intrusive and extrusive igneous rocks produce large magnetic highs. The Idaho batholith has little magnetic expression, but the Boulder batholith and related volcanic rock of western Montana produce a major magnetic high.

Structural Fabric of the North American Continent, as Defined by Gravity Trends

“…The western part of the Snake River magnetic high, with similar trend and also associated in part with Miocene basalts, has also tentatively been linked to a rift (Mabey et al, 1978). Tbese two areas of rifting, now dextrally offset roughly 130 km along the gradient lineament, were thought to have formed a continuous feature 17 to 14 Ma, but have become displaced by progressive transform displacement (Christiansen and McKee, 1978).…”

“…Tbe western section cIosely follows the boundary between the High Volcanic Plateaus and the Basin and Range tectonic provinces, and yet is distinctly different in trend (W25"S) from the boundary and the northeastern margin ofthe Humboldt magnetic zone which trend W43°S. Tbe Humboldt zone is interpreted as a major structure of the Cordillera, which may be influenced by structure in the Precambrian basement (Mabey et al, 1978). The gradient lineament is also distinct in trend and position from other southwestward trending lineaments apparent in maps of faults (Stewart, 1978) and Tertiary sedimentary rocks less than 17 Ma old, and Quaternary sediments (Christi ansen and McKee, 1978).…”

“…South of the gradient lineament, Mabey et al (1978) have attributed a north-northwest trending magnetic high produced in part by Miocene basalt dykes and flows to a rift-like structure named the Cortez rift. The western part of the Snake River magnetic high, with similar trend and also associated in part with Miocene basalts, has also tentatively been linked to a rift (Mabey et al, 1978).…”

Cenozoic transverse zones and igneous belts in the Great Basin, Western United States: Their tectonic and economic implications

“…The genetic relationship of the axis to the quiet zone is unclear (Blakely, 1988). Geologic features west of the axis that may reflect processes that formed the bilateral symmetry include several north-northwest-striking rifts (Stewart et al, 1975;Mabey et al, 1978;Zoback and Thompson, 1978;McKee and Noble, 1986;Blakely, 1988;Blakely and Jachens, 1991). The most prominent of these is the northern Nevada rift (Fig.…”

The Yellowstone “hot spot” track results from migrating basin-range extension

“…The ESRP-Y zone, functioning essentially as a transfer zone between regions of differential extension (Christiansen and McKee, 1978;Christiansen and Yeats, 1992;Payne et al, 2008), lies at a profound change in lithospheric structure between thin, hot, extending lithosphere to the south and thick, cold lithosphere underlying the North American craton to the north. This zone is also marked by a regional aeromagnetic anomaly that runs along the axis of the ESRP from Nevada, northeastward through Montana and on to Canada (Eaton et al, 1975;Mabey et al, 1978); 15. Numerous Precambrian geologic and geophysical alignments that parallel the ESRP-Y zone suggest deep-seated lithospheric structural control.…”