UVM Theses and Dissertations
Format:
Print
Author:
McNiff, Christine Marie
Dept./Program:
Geology
Year:
2012
Degree:
M.S.
Abstract:
Two well-known, east-dipping thrust faults that formed during the Ordovician Taconic Orogeny divide the bedrock geology of Vermont's Champlain Valley into lithotectonic slices. On the west, the Champlain Thrust placed Middle CambrianMiddle Ordovician sedimentary rocks over Late Cambrian-Middle Ordovician sedimentary rocks. Farther east, the Hinesburg Thrust placed Late Proterozoic-Early Cambrian low-grade metamorphic rocks over rocks of the upper plate of the Champlain Thrust. Subsequent deformation during the Devonian Acadian Orogeny resulted in the folding of both faults and other Taconic structures. This study documents deformation at the meso-and microscopic scales in the upper and lower plates of the Hinesburg Thrust and compares and contrasts deformation in both the upper and lower plates and documents along and across-strike variations.
Outcrops from the Champlain Valley belt and Green Mountains belt were grouped into domains that coincide primarily with lithology and spatial/structural relation to the Hinesburg Thrust. Three generations of folds are observed in the study area and correlated to regional structures that are attributed to orogenic events: tight to isoclinals F₁ folds that developed pre-and syn-thrust propagation (Taconic), open to closed asymmetric F₃ folds that deform the Hinesburg Thrust (Acadian), and open, asymmetric F₄ folds that deform F₃ (Acadian). Each fold has an associated axial planar cleavage (S₁, S₃ and S₄, respectively).
A west-dipping (S₂) cleavage is documented in other studies though no foliations that can be correlated to S₂ are documented in this study. F₂ folds are also not found in the current study area of the Hinesburg Thrust. In other studies, the second generation of folding deforms S₂ and all older fabrics and is, thus, referred to as F₃. The F₃ and F₄ folds have associated crenulation lineations (L₃ and L₄, respectively) that were used to infer the angular relationship between the two fold sets and, in some cases, infer the presence of F₄. The angular relationship between the F₃ and F₄ fold sets in the study area is consistently orthogonal (85-100°) both along strike of the Hinesburg Thrust and across it.
Ductile structures in the upper plate of the Hinesburg Thrust record all three folding events. The Hinesburg Thrust footwall anticline exhibits F₁ and F₃ structures; the youngest F₄ folds can only be inferred from changes in the plunge of F₃ fold axes or L₃ from one outcrop to another. Ductile fabrics were only observed locally in the carbonate and siliciclastic sequence in the lower plate of the Hinesburg Thrust; however, some fracture sets within these units show a remarkable similarity in orientation to that of cleavages associated with F₃ and F₄. Acadian structures are observed in the upper and lower plates of the Hinesburg Thrust.
Outcrops from the Champlain Valley belt and Green Mountains belt were grouped into domains that coincide primarily with lithology and spatial/structural relation to the Hinesburg Thrust. Three generations of folds are observed in the study area and correlated to regional structures that are attributed to orogenic events: tight to isoclinals F₁ folds that developed pre-and syn-thrust propagation (Taconic), open to closed asymmetric F₃ folds that deform the Hinesburg Thrust (Acadian), and open, asymmetric F₄ folds that deform F₃ (Acadian). Each fold has an associated axial planar cleavage (S₁, S₃ and S₄, respectively).
A west-dipping (S₂) cleavage is documented in other studies though no foliations that can be correlated to S₂ are documented in this study. F₂ folds are also not found in the current study area of the Hinesburg Thrust. In other studies, the second generation of folding deforms S₂ and all older fabrics and is, thus, referred to as F₃. The F₃ and F₄ folds have associated crenulation lineations (L₃ and L₄, respectively) that were used to infer the angular relationship between the two fold sets and, in some cases, infer the presence of F₄. The angular relationship between the F₃ and F₄ fold sets in the study area is consistently orthogonal (85-100°) both along strike of the Hinesburg Thrust and across it.
Ductile structures in the upper plate of the Hinesburg Thrust record all three folding events. The Hinesburg Thrust footwall anticline exhibits F₁ and F₃ structures; the youngest F₄ folds can only be inferred from changes in the plunge of F₃ fold axes or L₃ from one outcrop to another. Ductile fabrics were only observed locally in the carbonate and siliciclastic sequence in the lower plate of the Hinesburg Thrust; however, some fracture sets within these units show a remarkable similarity in orientation to that of cleavages associated with F₃ and F₄. Acadian structures are observed in the upper and lower plates of the Hinesburg Thrust.