Skip to Main Content
Book Chapter

Belt-Purcell Basin: Template for the Cordilleran magmatic arc and its detached carapace, Idaho and Montana

By
James W. Sears
James W. Sears
Department of Geosciences, University of Montana, Missoula, Montana 59812, USA
Search for other works by this author on:
Published:
September 01, 2016

The bedding-plane anisotropy and structural configuration of the Mesoproterozoic Belt-Purcell Supergroup guided a narrow magmatic salient >350 km eastward from the Salmon River suture of Idaho to the foreland basin of central Montana, along a deep graben within the southern part of the Belt-Purcell Basin. The magma assimilated anatectic melt from the lower Belt-Purcell Supergroup in the western half of the graben, where the Lemhi subbasin had intersected and deepened the graben by several kilometers. The magma stepped across the stratigraphic section as it intruded eastward along the graben, spread laterally as it climbed into the overlying Paleozoic and Mesozoic strata, and eventually erupted into the foreland basin.

This paper develops a model in which the magma formed a thick, east-tapering wedge beneath the Belt-Purcell carapace. The wedge elevated and tilted its lid, which failed along the trend of the graben to a terminus in the Crazy Mountains basin of the Helena structural salient, much like a tectonic-scale landslide. The carapace failed in two main phases between ca. 100 and 75 Ma. It slid ~100 km during the first failure phase, and ~40 km during the second, when the Boulder batholith and its volcanic cover filled a large pull-apart structure within the carapace. Slaty cleavage, tectonic slides that omit strata, and a nested series of hairpin-shaped allochthons characterize the failed carapace.

Shear zones and nappes bound the carapace; the sinistral Lewis and Clark line bounds it on the north, and the dextral southwest Montana transverse zone bounds it on the south. The Lewis thrust fault and associated structures of the Rocky Mountain fold-and-thrust belt overprinted and displaced the magmatic salient and its carapace from ca. 74 to 59 Ma. The magma crystallized, cooled, and generated hydrothermal ore deposits from Late Cretaceous to middle Eocene time. Eocene extension overprinted the system from 53 to 39 Ma and exhumed its infrastructure in core complexes. Those exposures, together with regional structural tilt, enable reconstruction of a deep cross section of the magmatic wedge and its carapace.

You do not currently have access to this article.

Figures & Tables

Contents

GSA Special Papers

Belt Basin: Window to Mesoproterozoic Earth

John S. MacLean
John S. MacLean
Department of Physical Science, Southern Utah University, 351 W. University Boulevard, Cedar City, Utah 84720, USA
Search for other works by this author on:
James W. Sears
James W. Sears
Department of Geosciences, University of Montana, 32 Campus Drive #1296, Missoula, Montana 59812-1296, USA
Search for other works by this author on:
Geological Society of America
Volume
522
ISBN print:
9780813725222
Publication date:
September 01, 2016

References

Related

Citing Books via

Related Articles
Close Modal
This Feature Is Available To Subscribers Only

Sign In or Create an Account

Close Modal
Close Modal