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The AAPG/Datapages Combined Publications Database

AAPG Bulletin


Volume: 65 (1981)

Issue: 5. (May)

First Page: 1014

Last Page: 1014

Title: Structural Variations in Wissahickon Group: ABSTRACT

Author(s): William Ferrell McCollough

Article Type: Meeting abstract


Four phases of deformation have been recognized along a 16-km transect perpendicular to strike from Mt. Airy to Sykesville in the Maryland Piedmont. Axial plane foliations, S1, S2, S3, S4, and associated lineations, L1, L2, L3, L4, are the result of successive superposition of four deformations, D1, D2, D3, D4. The three phases, D1, D2, D3, vary systematically in orientation across the region. The D4 phase occurs only locally.

The oldest deformation, D1, developed a major foliation across the region. The major foliation is represented by a schistosity in the southeast and a slaty cleavage in the northwest. Primary bedding is isoclinally folded, with the major foliation parallel to the limbs of these folds. The folds were most likely rendered isoclinal owing to flattening during successive phases of deformation. In the southeast, the S1 surface dips steeply southeast and then flattens northwestward to become part of a recumbent fold system. The associated L1 lineations plunge gently to the southwest or northeast.

The D2 deformation developed a near-vertical foliation which remains uniform in orientation across the region. In the southeast, the S2 surface is a crenulation cleavage while northwestward, it changes into a fracture cleavage, locally obliterating the major foliation, S1. The preexisting S1 surface was cylindrically folded coaxial with F1, about an axis that plunges gently southwest. The F2 folds are commonly the best developed and vary from open to tight.

The D3 deformation is characterized by a northwest-dipping crenulation cleavage, S3, and associated crenulation lineations, L3. The degree of development of the crenulation cleavage is locally variable, but, in general S3 becomes better developed to the southeast.

The D4 deformation is not continuous across the region. This can be attributed either to its overall weak development or to its parallel orientation with the preexisting structures. In the extreme northwest, kink bands grade into broad folds that disappear in the extreme southeast.

Studies of the microfabric across the region allowed fold styles to be characterized for each phase of deformation, as well as the determination of metamorphic grade. Although the metamorphic grade increases to the southeast, the peak of metamorphism at any location was coincident with D1.

D1 utilized nearly horizontal flow, while D2 and D3 were dominated by near-vertical movements. The D1, D2, and D3 phases were probably generated during the Taconic and Acadian orogenies. The D4 structure was probably a result of the Appalachian orogeny.

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