STRAIN VARIATIONS OF A HINGE-PARALLEL LINEATION, PRESIDENTIAL RANGE, NEW HAMPSHIRE

A pseudo-andalusite porphyroblast lineation (L1) in Devonian Littleton Formation schists of the Presidential Range, New Hampshire has been examined in order to evaluate its strain history. L1 lineations are an early Acadian deformational (D1) structure because they are hinge parallel to F1 nappes and lie within the S1 axial plane schistosity. These lineations are enigmatic in terms of strain because they require hinge parallel extension during D1, which has historically been considered to create a structural room problem. L1 shows extreme variations in the degree of alignment at both outcrop and, more importantly, regional scales of observation. To evaluate this partitioning of L1 strain, lineations were measured on S1 foliation surfaces in nineteen 50 cm x 50 cm quadrats along a transect from the summit of Mount Washington 2 km down to tree line along Chandler Ridge. Additionally, the patterns of the deformed L1 lineation, as affected by F4 folding, were analyzed for five outcrops where a single S1 containing a L1 lineation was folded by D4. It has been deduced from the deformed lineation patterns that D4 deformation did not significantly reorient L1 lineations. This means that the L1 lineations when unfolded about F4 hinge lines are in their original L1 orientation. Variations in L1 alignment have been quantified down transect. Statistical analysis showed a reverse correlation between degree of L1 alignment and thickness of quartzite beds in the schist/quartzite units. Strain variations have been determined for many structural domains along the transect. In domains where L1 is not developed, the strain is typical of S tectonites where S>>>L, L=0 and the strain is oblate. Domains where L1 is the most intensely developed are characterized by L>>>S, the rocks are L-S tectonites and the strain is within the prolate field. One of the factors linked to these strain variations may be position within D1 nappe. For example, the F1 Horn nappe hinge zone, is dominated by oblate strain with no lineation present. The inverted limbs of the Horn nappe and Tuckerman Ravine syncline are characterized by strong and moderate L1 lineations respectively, and prolate strain.