Bitterroot dome-Sapphire tectonic block, an example of a plutonic-core gneiss-dome complex with its detached suprastructure

Abstract

The Bitterroot dome-Sapphire tectonic block appears to be a well-developed example of a plutoniccore gneiss-dome complex or infrastructure separated from the adjacent suprastructure by a gently dipping zone of mylonitic shearing or an "Abscherungszone." The suprastructural Sapphire block, on the order of 15 km thick, 100 km long, and 70 km wide, apparently moved eastward about 60 km, bulldozing rocks of the eastern Flint Creek Range ahead of it. Movement of the block must have occurred about 75 or 80 m.y. ago during late stages of consolidation of the Idaho batholith which, along with sillimanite-zone regional metamorphic rocks, makes up the infrastructure under the mylonitic detachment zone. Timing of movement in the Sapphire block matches that in the Bitterroot dome.The Bitterroot dome must have risen after off-loading of the Sapphire block, because the shear foliation and lineation that formed during movement completely cross the dome; this indicates that the block must have moved eastward across the whole of the area now occupied by the dome rather than radially down the flanks of an existing dome. The shear lineation maintains its eastward trend even at the south end of the dome where the foliation dips southward. The lineation and shear foliation are strongest along the eastern flank of the dome, over which the greatest thickness of the block would have passed.

Rocks and structure of the southern Sapphire Mountains, Granite and Ravalli counties, western Montana

“…The amount of tectonic shortening that accompanied emplacement of the Sapphire thrust plate may be much greater than earlier estimates by Hyndman (1980) and Wallace (in Ruppel and others, 1981), because these interpretations did not account for large amounts of shortening among stacked thrust sheets. Hyndman's hypothesis proposed that shortening occurred along the decollement that bounded the "Sapphire tectonic block" ~nd _that the "tectonic block" was shortened internally by hstnc thrusts of relatively small displacement that joined the decollemellt at depth (Hyndman, 1980, fig.…”

“…Alternatively, Desmarais (1983) suggested that the southern boundary of the Sapphire plate may be just south of the mapped area, and that the boundary may be defined by a northwesttrending zone of ductile shear, which is expressed as a prominent foliation in syntectonic Cretaceous plutons and in metamorphic pendants of Middle Proterozoic rocks. The two-fold subdivision of the Sapphire thrust plate used in this report identifies a terrane of stacked thrust sheets and a frontal zone of imbricate thrust faults (Lidke, 1985;Lidke and others, 1987); these terranes cross boundaries of earlier lithotectonic subdivisions proposed by Hyndman (1980) and by Wallace (in Ruppel and others, 1981). Hyndman (1980) defined two lithotectonic elements in the area of the Sapphire thrust plate as (1) a western part, the "Sapphire tectonic block," which consisted of allochthonous Middle Proterozoic rocks, and (2) an eastern part, the "bulldozed" terrane, which was composed mostly of parautochthonous Paleozoic and Mesozoic rocks.…”

“…The two-fold subdivision of the Sapphire thrust plate used in this report identifies a terrane of stacked thrust sheets and a frontal zone of imbricate thrust faults (Lidke, 1985;Lidke and others, 1987); these terranes cross boundaries of earlier lithotectonic subdivisions proposed by Hyndman (1980) and by Wallace (in Ruppel and others, 1981). Hyndman (1980) defined two lithotectonic elements in the area of the Sapphire thrust plate as (1) a western part, the "Sapphire tectonic block," which consisted of allochthonous Middle Proterozoic rocks, and (2) an eastern part, the "bulldozed" terrane, which was composed mostly of parautochthonous Paleozoic and Mesozoic rocks. Wallace (in Ruppel and others, 1981) divided the Sapphire thrust plate into three allochthonous subplates: (1) the Rock Creek subplate, in the western part, consisting of Middle Proterozoic rocks; (2) the Garnet Range subplate, in the northern and eastern part, consisting mainly of imbricated Paleozoic and Mesozoic rocks; and (3) the Flint Creek subplate consisting of klippen of Middle Proterozoic rocks above the Garnet Range subplate.…”

“…We suspect the fault is deformed because a C! c;;outhwest to northeast direction of thrust movement is atypical of the general west-to-east movement of the Sapphire plate (Hyndman and others, 1975;Hyndman, 1980;and Ruppel and others, 1981) and because other thrusts in this region are folded (Emmons and Calkins, 1913;Poulter, 1956;Lidke, 1985).…”

“…Although these early interpretations of lithotectonic units were useful working hypotheses, modern geometric analysis of structures of the Sapphire thrust plate suggests that (1) the Rock Creek subplate is composed of at least three regionally extensive thrust sheets, (2) the Flint Creek subplate represents only one thrust sheet within the Rock Creek subplate, and (3) the Garnet Range subplate consists partly of the imbricate frontal zone and partly of thrust sheets. Because some of the thrust sheets and the imbricate frontal zone of thrust faults cross both the tectonic subdivisions proposed by Hyndman (1980) and those proposed by Wallace (in Ruppel and others, 1981 ), the previous terminology is not used in this report; the Sapphire thrust plate is here divided into a terrane of thrust sheets and a terrane of frontal imbricate thrusts (Lidke and others, 1987). The study area is in the southwestern part of the thrust sheet terrane (fig.…”