Significance of dextral reactivation of an E-W transfer fault in the formation of the Pennsylvania orocline, central Appalachians

Three hypotheses for orocline development including (1) primary plate boundary shape, (2) arc rotation, and (3) arc overprinting were evaluated for the formation of the Pennsylvania orocline, Maryland, USA, using foliation intersection/inflection axes preserved within porphyroblasts (FIA). The distribution and the timing of sequences of FIA data measured from garnet porphyroblasts within the Loch Raven formation are statistically the same across the Pennsylvania orocline, suggesting that all subregions of the arc have experienced the same deformation history. This may suggest that the geometry of the paleocontinental margin controlled the basic shape of this orocline. However, if the orocline is treated as three separate regions, defined by the SW-WE-NE trending portions of its slightly staircase-shaped outline, a much higher proportion of the NNE-SSW trending FlAs (set II) are preserved in the W-E trending region than those to either side. This geometric relationship could have resulted from WNW-ESE directed bulk stress causing a zone of dextral shear along what is now the W-E trending portion of this orogen. It appears that what could have been an early formed W-E trending sinistral transform shear zone was preferentially dextrally reactivated during the Taconian orogeny.