Approximately 240km2 of pre-Beltian metamorphic rocks are exposed in the southern portion of the Highland Mountains, Montana. At least six periods of deformation are indicated for these rocks, two of which are believed to have been accompanied by metamorphism. Three major lithologic units, quartzo-feldspathic gneiss, garnetiferous gneiss, and micaceous gneiss, serve as a crude stratigraphic sequence and help delineate two macroscopic structures, the Rochester antiform and the Gilfoy Hills synform. The axes of both of these folds trend NE-SW. The absence of a more detailed stratigraphy and the scarcity of exposures make structural analysis the most effective means of evaluating the deformational styles and orientations present in the map area. Lineation and foliation diagrams were prepared by computer using fabric element orientation data from over 1800 stations. Differences in structural style and orientation indicate that the two types of quartzo-feldspathic gneiss, leucocratic gneiss and gray gneiss, have undergone different deformational histories. The leucocratic gneiss, which is the lowermost unit exposed in the Rochester antiform, exhibits a similar style of folding, many reversals in dip direction, and a statistical axis of folding which plunges to the east-northeast. This geometry is attributed to a metamorphic/deformational event (M1/D1) which occurred prior to the accumulation of the gray, garnetiferous, and micaceous gneisses. These overlying units exhibit a concentric style of folding and a statistical axis of folding which plunges to the southwest. A subsequent metamorphic/deformational event (M2/D2) is believed to have produced this geometry. Mineral assemblages in the garnetiferous and micaceous gneisses indicate that temperatures between 650-700°C and pressures between 3/5kb were reached during M2/D2. Later deformational events, D3, D4, D5, and D6, all involved faulting of some type and are not believed to have been accompanied by metamorphism. A Laramide-age, metamorphic event, M3, is indicated by the occurrence of cordierite near contacts with the Laramide-age Hells Canyon pluton and Boulder batholith. A northeast-southwest trending fold system is present throughout most of the Precambrian metamorphic terrain in the Highland Mountains, the Tobacco Root Mountains, the Ruby Range, the Gravelly Range, and the Madison Range. The structural styles present within these areas indicates that the northeast-southwest trending fold system in southwestern Montana was produced by a primary stress coming from the southeast. Nappe-like overthrusting is suggested as a possible explaination for the anomalous structural sytle present in the northern Tobacco Root Mountains.

The 8th International Conference on Basement Tectonics was held in Butte, Montana, August 8-12,1988. Historically, basement tectonics conferences have focused on such topics as reactivation of faults, the influence of basement faults on metallogeny and hyrocarbon accumulation, and the use of geophysical and remote sensing techniques to interpret subsurface and surface geology. The 8th Conference diverged from past conferences in that a unifying theme was selected. Because ancient major terrane or cratonic boundaries are often postulated to be fault zones which are subsequently reactivated, the conference was organized to examine all aspects of ancient continental margins and terrane boundaries and to compare younger (Mesozoic) ones, about which more is known, with older (Paleozoic and Precambrian) ones. Moreover, because the 8th Conference was held in the northwestern United States, a greater emphasis was placed on the Mesozoic margin of western North America and the North American shield. The seven oral sessions and four poster sessions all dealt with aspects of the conference theme: characterization and comparison of ancient continental margins. The organizers extend their thanks to those individuals who graciously consented to serve as moderators for the oral sessions: John M. Bartley, Mark S. Gettings, M. Charles Gilbert, John M. Guilbert, Donald W. Hyndman, William P. Leeman, Robert Mason, and A. Krishna Sinha. The program with abstracts volume was prepared by S. E. Lewis and M. J. Bartholomew.

The Pony-Sappington area is located in the northeast foothills of the Tobacco Root Mountains and between the Madison and Jefferson Rivers. Structural and lithologic maps have been prepared for approximately forty-five square miles of pre-Beltian metamorphic rocks exposed in the area. The rocks are predominantly quartzofeldspathic, pelitic, and basic gneisses of sedimentary origin, interlayered with numerous then metabasalt sills. During pre-Beltian time, the rocks were strongly deformed into a series of large, subisoclinal folds. The axial planes of most of the subisoclinal folds now trend between N55W and N80W, with axes plunging slightly east of north at about 50°. During the subisoclinal deformation, axial plane foliation developed in most of the gneisses. In some of the amphibolites and other competent layers, however, deformation occurred partly by slippage along previously-existing bedding or layering surfaces, resulting in foliation parallel or subparallel to these surfaces. Foliation and lineation attitudes for eight subareas have been plotted on Schmidt equal-area lower hemisphere projections and show consistent girdle-type distributions of poles to foliation. Linear structures generally parallel the axes of the subisoclinal folds. During the subisoclinal deformation, the pre-Beltian rocks of the Pony-Sappington area were subjected to regional metamorphism of almandine amphibolites and granulite facies rank. The distribution of the two facies in the Pony-Sappington area is somewhat irregular, with a trend toward increasing rank to the southeastern portion of the Pony area. Compositional and physical variations of amphiboles and pyralspite garnets and structural variations of potash feldspars have been studied in relation to differences in metamorphic rank. Deformation and metamorphism were accompanied by instrusion of ultra-basic sills and stocks, potassium metasomatism, and locally strong metamorphic differentiation and aluminum, magnesium, and iron metasomatism. As load pressures and temperatures decreased, the rocks were subjected to greenschist facies retrograde metamorphism and intruded by numerous quartz-microcline perthite pegmatites. In later pre-Beltian time, the rocks were concentrially folded into open, northward-plunging folds and intruded by basalt and diabase sills and dikes. Concentratic folding of the older structures have resulted in the girdle-type distributions of poles-to-foliation on the equal-area projections. The area remained moderately stable until the Laramide Orogeny, during which major folds and northwest-trending strike-slip faults were produced in the Carmichael and London Hills areas, and numerous northeast-trending faults were formed throughout the Pony-Sappington area. Andesite, latite porphyry, monzonite, and basalt rocks intruded the pre-Beltian metamorphic rocks during the Cretaceous and Tertiary Periods.