STRAIN RECORDED IN DIFFERENT METASEDIMENTARY ROCK TYPES IN THE SAME PALEOZOIC SHEAR ZONE SYSTEM, SOUTHEASTERN PENNSYLVANIA PIEDMONT

Metasedimentary rocks of the SE Pennsylvania Piedmont have recorded multiple metamorphic episodes during orogenic shear (lower Paleozoic) along the Laurentian margin. The E end of the 070-trending Chester Valley, west of Philadelphia, and adjacent sub-parallel ridges contain, from N to S, foliated quartzite and marble of the Laurentian Cambro-Ordovician passive-margin, and phyllite of the Iapetus ocean basin. The Taconic Orogeny suture is found along the south margin of the valley as the contact between the marble and phyllite, known as the Martic Line. This suture and foliation were transposed during the Alleghanian orogeny to 070-striking. Previous mapping suggests two Alleghanian episodes of dextral transpression during the same event. Early deformation was at amphibolite facies conditions where all rocks were recording strain relatively homogeneously. Strain continued as temperature waned to greenschist facies conditions, resulting in strain partitioning where deformation localized into 4 separate, sub-parallel, narrower zones containing rock type contacts, including the Martic Line.

We examined the different rock types across strike to document the transpressive fabric differences. We reconnaissance mapped a 4.5 km across-strike traverse from Valley Forge (Mount Misery) in the NNW to Crabby Creek Park (S of the Martic Line). From 20 outcrops, along with field data, we collected 5 specimens of the quartzite (1), marble (2), and phyllite (2) and produced thin sections for microanalysis. Fabrics for each rock are strongly preferred along the suture (generally 069-striking). Foliation is steeply SSE-dipping and mineral lineation is subparallel to strike. Quartzite has a quartz grain-shape preferred fabric. Marble fabrics vary, but our marble specimen from near the Martic Line is micaceous where calcite aggregate layers are separated by foliated Ms and opaque minerals. The phyllite has a phylonnite fabric defined by Bt+Ms+Chl, quartz veins, and porphyroclasts of K-feldspar with strain shadow tails. Overprinting is in the form of crenulation and type II S-C fabrics. More micaceous rocks have more intense fabrics, suggesting mica focused strain more efficiently vs. rocks with lesser (or no) phyllosilicates. Therefore, rock type controls fabric variations at the same deformation conditions.