Fault-controlled pluton emplacement in the Sevier fold-and-thrust belt of southwest Montana, USA

Problems associated with syncompressional pluton emplacement center on the need to make room for magma in environments where crustal shortening, not e xtension, occurs on a regional scale. New structural data from the Pioneer and Boulder batholiths of southwest Montana, USA, suggest emplacement at th e top of frontal thrust ramps as composite tabular bodies at crustal depths between 1 and 10 km. Frontal thrust facilitated pluton emplacement was acc ommodated by: (1) a magma feeder zone created along the ramp interface; (2) providing 'releasing steps' at ramp tops that serve as initial points of e mplacement and subsequent pluton growth. and (3) localizing antithetic back -thrusts that assist in pluton ascent. A model of magma emplacement is prop osed that involves these elements. This model for syntectonic ramp-top empl acement of plutons helps explain how space is made for plutons within fold- and-thrust belts. (C) 2001 Elsevier Science Ltd. All rights reserved.