RELATING STRAIN HETEROGENEITY AND METAMORPHIC HISTORY: OUTCROP AND REGIONAL EXAMPLES FROM THE CENTRAL APPALACHIAN PIEDMONT

High grade metamorphic rocks in the Central Appalachian piedmont of SE Pa. occur in three distinct lithotectonic belts. From NW to SE these belts consist of rocks of Laurentian-, uncertain- and arc-affinity. This paper examines the relationship between strain and metamorphism in mid-crustal rocks, comparing the central belt, which is characterized by basement gneiss-cored nappes and associated shallowly to moderately SE dipping foliation, with the eastern block, which experienced a complex polyphase metamorphic and deformational history. Metasedimentary and metavolcanic cover rocks in the central belt followed a clockwise metamorphic P-T path. In contrast, the eastern belt contains evidence for two major periods of metamorphism: early low-P, high-T sillimanite- and andalusite-bearing assemblages (M1) are overprinted by higher P, kyanite-bearing assemblages (M2). In the central belt, strain is relatively homogenous at the regional scale, the dominant foliation is broadly synchronous with metamorphism and younger deformation is concentrated along terrane-bounding shear zones. The eastern belt is characterized by complex, heterogeneous strain, which is closely related to metamorphic history: in muscovite-poor rocks which attained the highest M1 temperatures, younger deformation, under M2 metamorphic conditions, occurs in discreet meter-scale shear zones, the replacement of sillimanite by muscovite indicates that these zones were likely pathways for fluids. Where M1 conditions were less severe, strain is more broadly distributed.

Strain heterogeneity in the eastern block is illustrated in a series of outcrops which expose the (intrusive?) contact between arc-related orthogneiss of the Wilmington Complex and the metasedimentary Wissahickon Formation. Here, discreet, one to two meter wide right-lateral transpressive shear zones, oriented parallel to the trace of the terrane-bounding Rosemont Fault, occur at a high angle to older foliation. A shear zone present at the contact itself is apparently localized by the lithologic contrast between pelitic and tonalitic gneiss. The initiation mechanism for other discreet shear zones within massive orthogneiss remains enigmatic.