Metasedimentary rocks at the E end of the southern Chester Valley (Tredyffrin Township), W of Philadelphia, recorded multiple metamorphic episodes during shear. Foliated marble of the Laurentian Cambro-Ordovician passive-margin are in suture contact (the Martic Line) with Octoraro phyllite, the Early Paleozoic Iapetan trench-fill sediment. Structures in the rocks are subparallel to the suture (070). Previous mappers interpreted the structure as both a thrust fault, and a contact within a Chester-Valley-wide trancurrent shear zone system. We test these conflicting interpretations with new mapping.

A 3km-wide suture-parallel strip (21 km long) was mapped that reveals a system of suture-parallel shear fabrics where discrete greenschist-facies higher-strain zones overprinted a wide zone of distributed transcurrent shear (amphibolite facies). The older shear encompassed the whole area; younger zones are localized, sub-parallel to the suture. Fabrics are strongly preferred along the suture. Foliation is steeply-SE-dipping. Mineral lineations are generally strike-parallel, but locally moderately ENE- and WSW-plunging, illustrating anastomosing structure. Crenulation is moderately NE- and SW-plunging, and quartz veins are boudinage with long axes strike-sub-parallel, or ptygmatically folded at a high angle to these (same strain ellipsoid). Thin sections reveal s-c fabrics, mineral fish and asymmetric boudinage. Phyllite fabric is defined by Bt-Ms-Chl, and all structures are consistently-oriented, but there is some variation in both orientation and fabric strength. Within the phylllite, however, and ~100 south of the suture, is a narrow (~100m-wide) zone where Chl is the dominant fabric forming phase, and where structures are more consistently oriented and grain sizes are relatively smaller (<100 microns). To the north of the suture is a set of elliptically-shaped hills composed of phyllite. The long-dimensions of these hills are generally E-W (suture sub-parallel), and marble drapes the phyllite along the structural grain. Each hill’s phyllite is mineralogically and texturally similar suggesting these hills are map-scale boudins that have recorded a suture-parallel, horizontal stretch. So, all data support an interpretation of suture-parallel transcurrent shear rather than thrusting.