Structural and Petrologic Evolution of Acadian Dome Structures in Southern Vermont

Abstract

Petrologic and thermobarometric studies, coupled with geologic mapping and structural analysis, provide critical evaluation of several different models for Acadian (Late Silurian to Middle Devonian) dome evolution in southern Vermont. Previous models considered diapiric uprise and composite nappestage crustal thickening and subsequent diapirism as likely causes of dome formation. Both of these previous models result in symmetrical distribution of PT values about the dome structures with corresponding coreward increases in temperature, and typically, coreward decrease in associated pressures. Thermobarometric calculations made during this study demonstrate that both P and T increase eastward across the entire region and are not symmetrically distributed about dome axes. The poT data coupled with petrographically derived relative age relationships and available geochronology also suggest that attainment of peak metamorphic conditions and concurrent dome-stage deformation are diachronous and young from west to east. These relationships are consistent with new geologic mapping and structural analysis which show that all of the domes in southern Vermont are low-amplitude fold interference structures. A current tectonic model indicates that Acadian Barrovian metamorphism in this region was a consequence of west-directed crustal thickening of an eastward dipping tectonic wedge, presumably from the Bronson Hill Terrane; an Ordovician arc sequence. The basal surface of this allochthonous mass projects above the present land surface within this area. Accretion of lower-plate rocks (of this study) into the thrust complex and continued west-directed thrusting of the accreted package over a seismically recognizable east dipping ramp structure provided the necessary geometry and mechanism for dome-stage fabric development, calculated uplift rates (1.2 to 1.7 km/m.y. and west to east younging of Acadian structural and metamorphic evolution.