MIGRATION OF THE TACONIC DEFORMATION FRONT ACROSS THE MID-ATLANTIC PIEDMONT: EVIDENCE FROM DETAILED PORPHYROBLAST ANALYSIS

Researchers have debated the characterization, relative timing, and absolute timing of Paleozoic deformation and metamorphic events in the mid-Atlantic Piemdont more than a century. Advances in zircon and Ar/Ar geochronology have generally provided valuable timing information about provenance of these metasedimentary rocks, the timing of metamorphic events, and cooling history. But, it is difficult to sort out most published ages because they are not reported in the context of textural evidence for poly-metamorphism or in the context of deformation fabrics. This investigation re-examined deformation and metamorphic fabrics within the Octoraro Formation, one of the most geographically extensive suites of metasedimentary rock in the entire mid-Atlantic Piedmont. Earlier researchers identified multiple metamorphic and deformation fabrics in these rocks (e.g. Fraser, 1880), and a general diachronous relationship exists between metamorphism and the primary deformation fabrics (e.g. Valentino, 1994). In effect, the relative timing of deformation and metamorphism are not the same everywhere within the Octoraro Formation, and documenting the details of porphyroblasts relative to deformation fabrics has provided insight into the tectonic evolution. From southeast to northwest, the relative timing between deformation and metamorphism as revealed through porphyroblast inclusion fabrics, shows that northwestward deformation was superimposed on progressive low-grade Barrovian metamorphism that increased with structural depth. This means that rocks were deformed prior to metamorphism in the hinterland, and the deformation outlasted early metamorphism toward the foreland. The transition from pre-kinematic, syn-kinematic and post-kinematic porphyroblast growth have been delineated geographically, and there does not seem to be a direct correlation with the regional metamorphic zones previously mapped (Valentino and Faill, 1994). Although they occurred in a broad sense together, the metamorphism and deformation of the Octoraro Formation appear to be very much independent. The entire transition between these domains is preserved within the core of the Octoraro Formation, and future detailed zircon geochronology in the context of deformation fabrics and porphyroblast generation, may provide rates of deformation front migration at the scale of the orogeny.