Nature of the Boulder Batholith of Montana: Discussion

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

doi:10.1130/0016-7606(1974)85<1953:notbbo>2.0.co;2

Cited by 13 publications

(1 citation statement)

References 0 publications

Supporting

Mentioning

Contrasting

Order By: Relevance

“…Hamilton and Myers (1974) argued that the ca. 70-80 Ma Boulder Batholith was emplaced as a tabular sheet under the Elkhorn Mountain volcanics and thus questioned earlier interpretations by Klepper et al (1971Klepper et al ( , 1974, who interpreted the Boulder Batholith as a steep-sided, thick intrusion. Some emplacement mechanisms involved lateral magma fl ow from the Idaho Batholith along major décollement surfaces beneath Sevier-style thrust plates (Hyndman et al, 1975) or emplacement into a pullapart space formed by intersecting fault sets in the Helena Salient .…”

Section: Introductionmentioning

confidence: 93%

Magnetic fabric, paleomagnetic, and 40Ar/39Ar geochronologic data bearing on the emplacement of the Late Cretaceous Philipsburg Batholith, SW Montana fold-and-thrust belt

2010

View full text Add to dashboard Cite

The Late Cretaceous (ca. 75 Ma) Philipsburg Batholith in SW Montana provides an excellent setting to examine the emplacement mechanisms of large-volume silicic plutons in the Sevier fold-and-thrust belt of the western U.S. Cordillera. Magnetic fabrics, determined using the anisotropy of magnetic susceptibility from 119 sites and anisotropy of anhysteretic remanent magnetization data from a subset of these sites, suggest that the Philipsburg Batholith is a tabular body and that its emplacement involved subhorizontal magma fl ow controlled by local thrust faults, including the Georgetown-Princeton thrust, which served as conduits for magma ascent. Field relations and magnetic fabric data, which are typically well characterized at the site level and dominantly subhorizontal except near major thrust faults, suggest that the Philipsburg Batholith was emplaced within a fault-bend fold at the top of a ramp in the Georgetown-Princeton thrust system. New 40 Ar/ 39 Ar age spectrum data indicate that the entire Philipsburg Batholith was emplaced rapidly at ca. 74.8 ± 0.1 Ma. Paleomagnetic directional data indicate that the Philipsburg Batholith has experienced ~9°-16° ± 8° of west-side-down tilt (tilt estimate and associated error are based on a new approach described herein) since emplacement in the Late Cretaceous. The data do not support any internal deformation of the batholith since emplacement and suggest that Sevier-age thin-skinned deformation in the Flint Creek Range had ceased by ca. 75 Ma. The modest magnitude of observed tilt could have occurred during subsequent Cenozoic extension. The inferred mode of emplacement of the Philipsburg Batholith may typify how large volumes of silicic magma are emplaced in shallow, fold-and-thrust belt settings.

show abstract

Section: Introductionmentioning

confidence: 93%

Magnetic fabric, paleomagnetic, and 40Ar/39Ar geochronologic data bearing on the emplacement of the Late Cretaceous Philipsburg Batholith, SW Montana fold-and-thrust belt

2010

View full text Add to dashboard Cite

show abstract

The Roots of Ash Flow Calderas in Western North America: Windows into the Tops of Granitic Batholiths

Series

1984

Collected Reprint Series

View full text Add to dashboard Cite

Large-volume ash flow eruptions and associated caldera collapses provide a direct link with subvolcanic granitic plutons of batholithic dimensions. The eruptive history, structural features, and petrologic evolution of ash flow calderas provide data on early stages of the evolution of an associated subvolcanic magmatic system. Broadly cogenetic, erosionally unroofed granitic plutons provide a record mainly of the late stages of emplacement and crystallization of silicic magmas. This review summarizes features of well-studied calderas and ash flow volcanic fields in western North America, exposed at advantageous levels where both remnants of a volcanic sequence and upper parts of the cogenetic intrusion are preserved, in comparison with similar rocks elsewhere in the world. Primary examples include San Juan, Mogollon-Datil, Marysvale, Latir-Questa, Chiricahua-Turkey Creek, Challis, and Boulder Batholith-Elkhorn Mounfains. Most ash flows have erupted from sites of preceding volcanism that records shallow accumulation of caldera-related magma. Structural boundaries of calderas are single ring faults or composite ring fault zones that dip vertically to steeply inward' outward dipping boundary faults favored by some models have not been identified in North American calderas. The area and volume of caldera collapse are roughly proportional to the amount of erupted material. Pyroelastic eruptions of relatively small volume (less than 50-100 km 3) may cause incomplete hinged caldera subsidences or structural sags; larger systems are bounded by complete ring faults. Few ash flow vent structures have been related to major calderas' vent geometry, as determined by size analyses of pyroelastic materials, may shift complexly during 9aidera collapse. Scalloped topographic walls beyond the structural boundaries of most calderas are due to secondary gravitational slumping during subsidence. Most exposed floors are a structurally coherent plate or cylinder bounded by a ring fault or dike, indicating pistonlike caldera collapse' chaotically brecciated floors predicted by models of piecemeal collapse have not been identified. Deviations from circular shape commonly reflect influence of regional structures; some calderas in extensional terranes are elongate in the direction of extension. Large calderas (greater than 100 km 3 of erupted material) collapse concurrently with eruption, as indicated by thick intracaldera ash flow fill and interleaved collapse slide breccias. Volumes of intracaldera an d outflow tuff tend to be subequal' correlation between them is commonly complicated by contrasts in abundance and size of phenocrysts and lithie fragments, degree of welding, devitrification, alterati øn, and even chemical composition of magmatic material. Postcollapse volcanism may occur from varied vent geometries within ash flow calderas' ring vent eruptions are most common in resurgent calderas, reflecting renewed magmatic pressure. Large intrusions related to resurgence are exposed centrally within some calderas; ring dikes and other ...

show abstract

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

View full text Add to dashboard Cite

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.

show abstract

Nature of the Boulder Batholith of Montana: Discussion

Cited by 13 publications

References 0 publications

Magnetic fabric, paleomagnetic, and 40Ar/39Ar geochronologic data bearing on the emplacement of the Late Cretaceous Philipsburg Batholith, SW Montana fold-and-thrust belt

Magnetic fabric, paleomagnetic, and 40Ar/39Ar geochronologic data bearing on the emplacement of the Late Cretaceous Philipsburg Batholith, SW Montana fold-and-thrust belt

The Roots of Ash Flow Calderas in Western North America: Windows into the Tops of Granitic Batholiths

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

Contact Info

Product

Resources

About