Abstract

Mesoscopic structures and microtextures in pelitic schist, psammitic gneiss, and amphibolite of the Wissahickon Formation near Philadelphia reveal a complex history involving five structural events and two independent(?) episodes of metamorphism, M1 and M2. The dominant folds, F2, are overturned to the southeast. They probably developed during uplift of the basement gneiss complex along vertical faults during the main phase of deformation, D2. The F2 folds deform older D1 structural elements and M1 synkinematic, metamorphic fabrics, including L1 mineral lineations and S1 schistosity. S1 is generally parallel to bedding, but locally it is parallel to axial planes of isoclinal, similar F1 folds.

The F2 folds are overprinted by horizontal crenulation and fracture cleavages. S3, which are parallel to axial planes of local, open F3 folds. The M2 episode of metamorphism was prekinematic, synkinematic, and postkinematic with respect to D3 and produced the staurolite, kyanite, and sillimanite zones of metamorphism marginal to a zone of granitized schist. D3 structural elements are overprinted by late M2 crenulation cleavage, S4, which dips southeast and is parallel to axial planes of local, open F4 folds. F4 folds become more intensely developed northwest and are probably the dominant folds near the Martic line. They are probably correlative with late Paleozoic structures in the Appalachians. D5(?) structural elements consist of steeply plunging folds and mullions, in a narrow cataclastic belt along the northern margin of the granitized zone, and weakly developed crenulation lineations that rake steeply in schistosity farther north.

Evidence of mineral growth during D2 has not been found, which suggests that D2 was a low temperature deformation, perhaps late Taconic in age, separating early Taconic events D1 and M1 from Acadian events D3 and M2. Although amphibolite faces metamorphism occurred during both orogenies, the profound recrystallization, granitization, and present zones of metamorphism reflect the Acadian orogeny.

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