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

Abstract

AbstractThe 15 major plutons comprising the 6000 km2 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 km2) 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 of the basal décollement of the thrust belt (∼17km).One mode of pluton emplacement consistent with a rhomboid pattern and with a 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 décollement 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.