Partitioning of deformation along an orogen and its effects on porphyroblast growth during orogenesis

An identical succession of foliation inflection/intersection axis trends in porphyroblasts (FIAs) is present in three separate areas affected by Acadian deformation and metamorphism along 130 km of the Appalachians from East Central Vermont to North Central Massachusetts. The FIAs trend successively NW-SE, SW-NE, W-E, NNW-SSE and SSW-NNE and appear to have formed during the same succession of deformations, which have been dated in SE Vermont as follows; not yet dated, pre-425, 425-404, 404-385 and 385-360 Ma, respectively. The two areas that are furthest apart, in east central Vermont and north central Massachusetts, have strikingly similar total distributions of FIAs in garnet porphyroblasts that show a somewhat inverse relationship to those observed in the Chester-Athens dome area of SE Vermont that separates them. That is, the number of samples containing the first formed FIA set is greatest for the two regions lying furthest apart and decreases in younger sets, whereas the number of samples containing the first FIA set is the least in the central region and increases for more recently developed sets. Partitioning of deformation into zones of progressive shearing and shortening has been demonstrated to control the sites of development of differentiated crenulation cleavage versus porphyroblast growth and sites of porphyroblast growth on one limb of a regional fold versus the other. We argue that partitioning of the deformation around competent feldspathic gneiss bodies at an orogen scale produced the variation in the number of samples containing the various FIA sets between the central and outer areas described above.Porphyroblasts most commonly grew during SW-NE directed shortening to the north and south of the Chester-Athens dome region of the Appalachians, but only rarely grew in the latter region. This suggests that deformation at this time partitioned around an outlier of competent basement feldspathic gneiss that underlies the Chester-Athens dome region, and this protected the rocks above and in the strain shadow to the NW and SE from the effects of the great bulk of the deformation that occurred over the period of time during which NW-SE-trending FIAs formed. We attribute the increase in the number of samples containing progressively younger FIAs in the Chester-Athens dome area during subsequent periods of NW-SE-, N-S-, WSW-ENE-, and WNW-ESE-directed shortening to the rise in temperature and pressure recorded in the rocks above the Chester-Athens dome with peak conditions occurring during the development of the NNW-SSE oriented FIA set between 404 and 385 Ma. As the temperature and pressure increased, the gneiss below would have become less competent, allowing the deformation associated with the succession of FIA sets to partition more pervasively through the dome core. This provided more sites for porphyroblast nucleation and growth and hence the number of samples recording these FIA sets increased. The concomitant decrease in the number of samples in which porphyroblasts grew in the rock within the areas to the north and south with successive FIAs possibly resulted from a progressive increase in competency of these rocks as the number of porphyroblasts increased.