SUPERPOSITION OF DUCTILE STRUCTURES IN THE MONTPELIER QUADRANGLE: CENTRAL VERMONT

Metasedimentary and metaigneous rocks of Ordovician through Devonian age are mapped in the Montpelier Quadrangle. A major unconformity (RMC) that is locally faulted separates Ordovician rocks (Cram Hill+Moretown fms) to the NW from Silurian-Devonian rocks (Shaw Mt, Northfield,+Waits River fms) to the SE. Within the Ordovician section, truncation of lithologies and truncation of the F1/F2 composite fold structure mapped in the northwest portion of the quadrangle suggest a fault contact (Shady Rill fault) between western and eastern sections of the Moretown Fm. The Montpelier Quadrangle can be divided into three discontinuity-bounded structural domains (NW, central, and SE) which reflect the dominance of Taconian structures, an overlap domain, and Acadian structures, respectively.

In the NW domain, bedding is folded by steeply-plunging asymmetric tight folds (F1). Within the phyllitic units, well-developed closely spaced foliation and a “pinstripe”(S1) is parallel to the axial plane of the F1 folds and bedding is transposed. Bedding is preserved in the massive quartzites. Quartz rods (L1) are colinear with F1 fold axes. In the NW domain, S0 commonly trends nearly east-west whereas S1 trends NNE.

Moretown and Cram Hill Fm rocks of Central Domain are bounded by the Shady Rill fault (west) and the RMC (east). The dominant foliation in the Central domain is an S1/S2 composite and S2 intensifies close to the RMC. F2 folds are isoclinal with severely attenuated limbs and preferentially develop in certain zones. Steeply-plunging lineations (L1A) on the F2 cleavage planes are, in most cases, the F1 fold axes; however, steeply-plunging stretching lineations (L1B) are also present. The F2 overprint obscures F1 structures. Bedding is only locally preserved in isolated fold hinges.

The structures in the Silurian and Devonian lithologies of the eastern domain are related to the Acadian Orogeny; they consist of tight to isoclinal, moderately north-plunging folds (F2) with a steeply west-dipping axial planar spaced cleavage (S2). S2 is locally deformed by asymmetric, tight folds (F3) with sub-horizontal axes and a moderately west-dipping crenulation cleavage (S3). F2 folds and the northwest striking more open F3 folds occur both east and west of the RMC. F1 is not recognized east of the RMC.