On the development of gneiss domes

Abstract

The New England Appalachians contain some of the first documented gneiss domes. The classic domes of southeast Vermont are typical of these structures in that they appear to have formed by doming of both the gneissosity in basement gneisses and the dominant matrix schistosity in the overlying rocks, after these foliations had formed. However, the three matrix foliations (other than bedding) present in these rocks, which include the one parallel to all compositional layering, post-date dome development. The domes formed as upright anticlines after deposition of the overlying Lower Palaeozoic sediments and volcanics on a Precambrian granitic or gneissic basement. This occurred during orogenesis driven by horizontally directed compressional forces with interspersed periods of relatively coaxial gravitational collapse, rather than through early nappe development followed by folding or diapiric granite emplacement, or crustal extension and upwards bowing of a necked portion of the crust. In spite of numerous subsequent deformations, the domes are very old structures that have been little modified at the level of current crustal exposure. The dominant process during each deformation was reactivation of the compositional layering or gneissosity on at least one limb of the dome. This destroyed earlier developed crenulation cleavages that lay oblique to the compositional layering and commonly prevented new ones from developing. Consequently, a schistosity parallel to bedding in the overlying rocks, and the gneissosity below, formed and was intensified by reactivation on at least one limb of the dome during all subsequent deformations. Nappe development, which has commonly been proposed as the origin of foliation parallel to bedding in the rocks overlying gneiss domes, is not necessary and there is no evidence for such a deformation history preserved within either the bedding or matrix foliations that have been folded around the Chester and Athens Domes of southeast Vermont. Subhorizontally dipping foliations that formed during collapse stages of orogenesis have mainly been the product of relatively coaxial deformation rather than of the highly non-coaxial deformation that accompanies nappe development. The fold to the west of the Chester and Athens Domes, which has classically been regarded as the nose of the major nappe, formed with an upright rather than shallowly dipping axial plane.