Structures

Structures can be confidently identified as Acadian only in certain places, where deformed Silurian-Devonian rocks are unconformably overlain by Middle to Upper Devonian rocks that are less deformed, are cut by undeformed Middle Devonian to Carboniferous plutons, or contain Devonian metamorphic fabrics that are genetically related to the observed structural features. In places where there are no Silurian-Devonian rocks, Acadian structures can be identified when it can be shown that they developed at the same time as metamorphic fabrics that are identifiably Devonian (Osberg, 1989, p.197).

It is easiest to understand Acadian deformation in New England because Silurian-Devonian sections are present here, displaying many different fold and fault sets. In other places it is often unclear whether structures are a result of the Acadian orogeny or another mountain-building event. The Talladega slate belt in Alabama, however, also contains structural features that can be identified as Acadian (Osberg. 1989, p. 197). There is very little evidence for Acadian deformation in the central Appalachians (Osberg, p. 212).

 

 

 

 

 

 

 

 

Researchers group folds and faults by similarity in orientation and characteristics and work out the sequence of structural deformation by looking at the superimposition of different groups of structures. Osberg, et al. (1989) list seven stages in the sequence of Acadian structural deformation (p. 199):

1	early listric faults (seen in western ME)
2	early recumbent folds and thrusts with westward transport (seen in southern New England)
3	westward-dipping underthrusts along coastal ME and widespread east-verging folds (CT, MA, ME and elsewhere)
4	early north- and northeast-trending upright folds (tight in southwestern New England, open in the north)
5	east-west structural small folds and stretching lineations
6	late north-trending folds and cleavage and associated domes
7	Acadian folds outside of Silurian-Devonian rocks sections (identification of such folds is uncertain)
8	faults that post-date Middle Devonian to Carboniferous plutons, but for which earlier recurrent motion has been suggested (eg., Norumbega fault)

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Beginning in the Late Silurian and continuing into the Early Devonian, Medial New England and Composite Avalon converged along an east dipping subduction zone. Compressive and extensional forces in conjunction with strike-slip faults created areas of both transpression and transtension.

Transpression, compression with strike-slip faulting, associated with collisions between terranes and Laurentia in the Middle Devonian (410-380 Ma) and other tectonic events such as rotation and oblique motion produced both sinistral and dextral strike-slip faulting. (Rast and Skehan, 1993 SP)

In the Late Devonian Composite Avalon thrust sheets were finally emplaced onto Medial New England. By this time the allochthonous Avalonian thrust sheets had traveled many kilometers westward over Medial New England. Regional scale structures including recumbent folds, flattened upright folds, and thrust faults, together accommodated tens to hundreds of kilometers of cumulative shortening in a southeast-northwest direction. Rocks from these thrust sheets occur today as small klippen on rocks of the Central Maine Basin.

The Acadian deformation is characterized by an overall pattern of westward-directed thrusting. Deformation closest to the suture tends to be polyphase, with intense penetrative cleavage. However, deformation farther from the hinterland, is either tight folding with penetrative cleavage or open folding without penetrative cleavage. (Williams, 1993)

The present boundary of Medial New England and Composite Avalon can be seen at the surface in the Senneboc Pond fault, a high-angle fault that cuts the Graham Lake and Clarry Hill thrust sheets (Tucker). This fault marks the end of the Acadian orogeny in coastal Maine. The original suture may be hidden at depth under the Gulf of Maine (Tucker).

 

*Bold face words are defined in the glossary.