The Pennsbury-Concord shear zone: A deformed ductile thrust, central Appalachian Piedmont
- Temple Univ., Philadelphia, PA (United States). Dept. of Geology
The contact between the Baltimore Gneiss and the Wissahickon Group in southeastern Pennsylvania is a ductile shear zone. The contact extends from just west of Pennsbury to just east of Concord, Pennsylvania. The strike of the shear zone contact varies along its length from a general orientation of N60--65E in the west to N75--80E near Concord and then to N30E adjacent to the Rosemont Shear Zone. Microstructural analysis reveals deformation in units on both sides of the contact that is characterized by plastic deformation mechanisms associated with depths below the brittle-ductile transition. Retrograde metamorphism and grain-size reduction as well as continuity across the zone indicate that this deformation was later than the metamorphism of the individual units. The contact has experienced at least two phases of deformation. The early phase was ductile thrusting juxtaposing the Wissahickon Group gneiss on top of the Precambrian Baltimore Gneiss and producing a high temperature pervasive planar and linear fabric. This fabric is most preserved in the west where foliation is shallowly dipping with a lineation trending S54E and plunging 35[degree] to the SE. The later phase was characterized by transposition of the ductile thrust fabric to a more strike-parallel orientation. Along the contact zone to the east, strike of the foliation gradually changes 10--15[degree], dip becomes steeper and the orientation of lineation changes to S85E with plunge of 17[degree] to the east. The deformation of the contact therefore confirms that the ductile thrusting is earlier than the major transcurrent shearing of the Appalachian Piedmont. Named the Pennsbury-Concord Zone, this deformed ductile thrust could be a root zone of brittle thrust faulting to the west in the fold-and-thrust-belt of central PA.
- OSTI ID:
- 5805642
- Report Number(s):
- CONF-9303211-; CODEN: GAAPBC
- Journal Information:
- Geological Society of America, Abstracts with Programs; (United States), Vol. 25:2; Conference: 28. annual Geological Society of America (GSA) Northeastern Section meeting, Burlington, VT (United States), 22-24 Mar 1993; ISSN 0016-7592
- Country of Publication:
- United States
- Language:
- English
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