LATE ACADIAN FOLD TRAIN SHORTENING AND STRAIN PARTITIONING

Shortening calculations of well-exposed D4 fold trains in the Presidential Range of New Hampshire were done at the micro-, meso-, and macro-scales to quantify the strain and to evaluate the strain partitioning of this last widespread Acadian deformation (380-365 ma). D4 folds are parallel, open, flexural slip to flexural flow, east vergent, fold all previous fabrics (S0, S1 and L1) and have a variably developed S4 crenulation cleavage. Transects along Osgood and Chandler Ridges containing well-exposed D1 nappes refolded by D4 were examined. Shortening was calculated using the equation e = (lf-lo/ lo) * 100, where lf = the hinge to hinge straight length of the fold train and lo = the length of the folded layer. For Chandler Ridge the following shortening calculations were made: macro-scale = 10.3%; 49 meso-scale fold trains varied systematically by domain over a macro-scale D4 Dome, ranging from 1.1% on the flank to 47.8% on the apex with a mean value of 14.2%; micro-scale shortening on 6 thin sections, with triplicates, varied non–systematically from 3.0% - 11.6% with a mean of 7.0%. The total shortening at all scales was 28.7%. The results for Osgood Ridge were very similar: macro-scale = 10.2%; 32 meso-scale fold trains varied systematically by individual fold train over several macro-scale D4 anticlinal synforms and synformal anticlines, ranging from 5.9% on the limbs to 32.3% on the crests or troughs, with a mean value of 16.4%; micro-scale shortening on 13 thin sections, again with triplicates, varied non-systematically from 6.1% - 20.9% with a mean of 12.2%. The total shortening at all scales was 32.5%. Collectively, these results suggest that meso-scale strain is preferentially partitioned on the crests and troughs of macroscopic folds by parasitic meso-scale folding, that macro-scale and micro-scale strain is fairly uniformly distributed across this part of the orogen, and that the total D4 shortening is approximately 30%. Osgood and Chandler Ridges are both approximately 3 km in length and would have been about 1 km longer prior to D4. Depending on the duration of the D4 event (10 mya - 1 mya), shortening or convergence rates would range from .1cm/yr to .01 cm/yr during the Late Acadian.