On the Nature of the Boulder Batholith of Montana

Klepper, M.R.; Robinson, G. D; Smedes, Harry Wynn

doi:10.1130/0016-7606(1971)82[1563:otnotb]2.0.co;2

Cited by 24 publications

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“…The next-largest body of Mesozoic granitic rock is the Boulder batholith in Montana, which covers about 6,000 km2 and lies between Butte and Helena. This body is a composite of 18 or more rock types ranging from mafic to felsic but consisting domlnantly of granitic rocks (Klepper, et al, 1971;Smedes, 1973). This composite body was emplaced during a span of about 10 million years --78 to 68 million years ago (Tilling, et al, 1968).…”

Section: Suitability For Repository Sitesmentioning

confidence: 99%

National survey of crystalline rocks and recommendations of regions to be explored for high-level radioactive waste repository sites

Smedes¹

1983

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It ias beea reprsiiisca frsm tiii besS waliable etpf to permit the fireailast passiijie atailaiiiif. !. ":B i NOTICE TU READER The Department of Energy (DOE) is evaluating various geographic regions and rock types in the conterminous United States to determine their potential suitability for deep geologic disposal of commercial high-level radioactive wastes. The Office of Crystalline Repository Development (OCRD) of Battelie Memorial Institute's Project Management Division is responsible to DOE for coordinating the work of a number of National Laboratories and subcontractors with geological and environmental expertise in studies of crystalline rock. The evaluation process is one of screening successively smaller units of land containing potential host rock types. Eventually, one or more repository sites may be selected from a number of potentially suitable alternatives. Salt domes, bedded salt, tuff, and basalt are being considered for the nation's first repository. Status of the investigations in crystalline rock will allow their consideration for a second or subsequent repository. This report is being issued to describe the geologic reconnaissance that has been completed in the national survey of crystalline rocks and the decision process applied in evaluating and recommending regions for further study. This document supersedes the earlier draft ONWI-50 entitled "Crystalline Intrusives in the United States and Regional Geologic Characteristics Important for Storage of Radioactive Waste". It is not simply a revision of draft ONWI-50, but a document which provides additional information and logic to support the regional recommendations resulting from the national reconnaissance and evaluations. As such, the document incorporates some of the draft ONWI-50 data base as well as other relevant data from more recent sources. It is responsive to and includes resolutions of, pertinent issues and substantive comments raised through state reviews of draft ONWI-50, This document will serve as the basis for the regional phase of activities for the Crystalline Rock Project of the National Waste Terminal Storage (NWTS) program. In issuing this document, DOE is completing the national phase of reconnaissance and evaluations for crystalline rocks. The regional surveys that characterize the regions of crystalline rock recommended in this report are nearly complete and will be released in draft form for review by involved states in May 1983. iii ACKNOWLEDGEMENTS This report was 5£epa£ed by Harry W. Smedes (under subcontract No. E514-099-00 with BPMD/ONI of Battelie Memorial Institute) and BPMD/OCRD geotechnical staff (under contract No. DE-AC02-83 CH 10140 with the U.S. Department of Energy) with the technical supervision and aid of G. L. Stirewalt and M. E, Shea of BPMD/OCRD. The report draws upon data collected and compiled In 1979 by Dames and Moore under subcontract No.

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Section: Suitability For Repository Sitesmentioning

confidence: 99%

National survey of crystalline rocks and recommendations of regions to be explored for high-level radioactive waste repository sites

Smedes¹

1983

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“…The nature and mode of emplacement of these plutons, especially the Butte pluton, have been debated for over 75 years (Lawson 1914;Hamilton andMyers 1967, 1974;Klepper et al 1971Klepper et al , 1974Hyndman et al 1975). This paper considers the mode of emplacement based on the shapes of the plutons and their spatial and temporal relationships to known faults.…”

Section: Introductionmentioning

confidence: 98%

Syncompressional Emplacement of the Boulder and Tobacco Root Batholiths (Montana‐USA) by Pull‐apart along Old Fault Zones

Schmidt

Smedes²,

MichaelO'Neill

1990

Geological Journal

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The 15 major plutons comprising the 6000 km' Late Cretaceous Boulder batholith were emplaced at mid-to upper-crustal levels (<20 km) during active thrusting in the Montana thrust belt. The Tobacco Root batholith, the largest satellite pluton of the Boulder batholith ( -300 km') was emplaced into uplifted Archean basement rocks immediately southeast of the frontal margin of the thrust belt.Emplacement of the Boulder batholith and its satellite plutons was controlled principally by three long-lived and deeply penetrating regional fault sets. Two of these sets (a NE-trending set and an E-trending set) appear to have controlled the emplacement of the main mass of the batholith (Butte pluton). A third (NW-trending) fault set controlled the emplacement of the Tobacco Root batholith and many of the smaller plutons of the Boulder batholith. The intersecting fault sets give the Butte pluton a distinctly rhomboid shape, elongate in a northeasterly direction. Geophysical data suggest that the pluton may have a flat floor at the level cf the basal decollement of the thrust belt ( -17 km).One mode of pluton emplacement consistent with a rhomboid pattern and with B compressional tectonic setting is the filling of the pull-apart regions of shear zones. We propose that the main mass of the Boulder batholith was emplaced along a pull-apart within a segment of a thin-skinned thrust sheet during ENE translation in the thrust belt. Sinistral slip on the E-trending faults at the north and south margins allowed the rhomboid-shaped cavity to form within the thrust sheet above the basal dkcollement and to progressively fill with magma below a roof of cogenetic volcanic rocks.Late Cretaceous movement on the NW-trending fault set was oblique with roughly equal components of sinistral slip and reverse slip. The main mass of the Tobacco Root batholith is elongate in a northwesterly direction between two of these faults and has a crudely rhomboid map pattern. We suggest that it, too, was emplaced by filling a sinistral pull-apart between these faults.

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Seismic Reflections From the Basal Contacts of Batholiths

Lynn¹,

Hale²,

Thompson³

2013

1989, Granites and Rhyolites

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Although seismic reflection data in batholithic terranes are scarce, datasets available to us from three different regions show surprising similarities. All three profiles, one across Precambrian rocks in Texas, and two across Mesozoic batholiths in California and Nevada, show strong subhorizontal reflections at depths of 6-10 km. These reflections are interpreted to originate from the base of the batholiths, and indicate that these batholiths are tabular in shape. The large amplitudes of the reflections require a marked contrast in acoustic impedance (product of velocity and density) between the batholith and the underlying rocks. Two of the profiles show reflective zones having appreciable thicknesses (0.5-2.5 km), while the third profile shows a single continuous reflection. The four most favored candidates for sub-batholithic reflections are (1) underplated gabbro, (2) cumulate layers, (3) migmatites, and (4) thrust faults. Cumulate layering is the preferred interpretation where reflectors constitute a thick package and are laminated in appearance.

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On the Nature of the Boulder Batholith of Montana

Cited by 24 publications

References 0 publications

National survey of crystalline rocks and recommendations of regions to be explored for high-level radioactive waste repository sites

National survey of crystalline rocks and recommendations of regions to be explored for high-level radioactive waste repository sites

Syncompressional Emplacement of the Boulder and Tobacco Root Batholiths (Montana‐USA) by Pull‐apart along Old Fault Zones

Seismic Reflections From the Basal Contacts of Batholiths

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