The role of magmatism during crustal shortening in the retroarc fold and thrust belt of southwest Montana.

摘要

This research is focused on the McCartney Mountain thrust salient of southwest Montana, an area within the Sevier fold and thrust belt that experienced extensive crustal shortening and silicic magmatism during the Late Cretaceous. Thin-skinned folding and thrusting spatially and temporally overlapped with intrusion of large volumes of magma in the Pioneer intrusive suite.; Structural data from the Pioneer intrusive suite, suggest syntectonic pluton emplacement within thrust ramps as shallow (1–10 km depth) composite tabular bodies. A model of magma emplacement is proposed explaining syntectonic ramp-top emplacement of plutons and provides a space-making mechanism for intrusion of plutons within active fold-and-thrust belts.; Detailed mapping and U-Pb zircon geochronology give information for the timing and style of deformation of Late Cretaceous thrusting in southwest Montana. Early frontal thrusts carry Precambrian crystalline rocks, incorporated into thin-skinned thrust sheets through intense imbrication of the basement footwall. The Madigan Gulch anticline and associated Grasshopper Creek growth syncline record basement folding ca. 79 and 75 Ma with shortening rates of 0.5 mm/yr. Later motion of the Ermont thrust ca. 75 and 73 Ma was out-of-sequence with minimum horizontal slip rate of 0.6 mm/yr.; In terms of critical taper models, the Sevier orogenic wedge in southwest Montana shows that pluton emplacement, volcanism, and crustal shortening created conditions that controlled the and geometric development of thrusting. During the culmination of igneous activity, magmatic centers became the focus between a foreland and western hinterland thrust system ca. 75–72 Ma causing changes in wedge behavior marked by thrusting in internal parts of the wedge.; Venusian coronae are explored as analogs for magmatic centers in southwest Montana. As viewed in Magellan radar images, Fakahotu corona shares gross geometric characteristics with the Pioneer batholith and adjacent McCartney Mountain thrust salient. This corona shows evidence of extension within areas dominated by igneous activity contemporaneous with crustal shortening in magmatic centers. Intrusive centers in southwest Montana show similar relationships.