QUANTIFYING THE VARIABLE DEVELOPMENT OF AN EARLY ACADIAN L1 PSEUDOANDALUSITE MINERAL LINEATION

Coarse-grained, pseudoandalusites exposed in schists of the Devonian Littleton Formation show variable L1 lineation development throughout the Presidential Range, N.H. The L1 lineation lies within the S1 axial planar schistosity and is hinge-parallel to early Acadian D1 nappes. Transects along Osgood and Chandler Ridges each containing a variety of structural positions along D1 nappes were examined. To quantify the lineation development, the value for the longest possible petal of 5º bin rose plots, rotated through a 5° range, of trends of 50 pseudoandalusite long axes were recorded at numerous sites. For Chandler Ridge, 32 sites were measured along a 3 kilometer long transect, through an inverted limb, a hinge zone, an upright limb, a second hinge zone and a lower inverted limb. Longest petal values ranged from a low of 8.3% for weakly aligned pseudoandalusites, to a high of 30.8 % for those with strong alignment, with a mean value 16.9%. Lineation development was lowest (<10% to 15%) in the two D1 hinge zones positions, intermediate (15% to 20%) in the upright limb, and intermediate to high (15% to 30%) in the inverted limbs. The highest lineation development was in a thin, cm-scale bedded, schist-quartzite couplet. For Osgood Ridge, 42 sites were measured along a 3.5 kilometer long transect containing a D1 hinge zone region and an inverted limb region. Longest petal values ranged from a low of 8.0% for weakly aligned pseudoandalusites, to a high of 26.0 % for those with strong alignment, with a mean value 15.2%. Lineation development was low to high (<10% to >25%) in the inverted limbs and intermediate (12% to 20%) in the hinge zone region. Bands of similar lineation development, whether high or low, were generally parallel to lithologic contacts. The results suggest that lineation development is equally variable and similar in range of longest petal % along the two transects with Chandler Ridge having slightly greater lineation development. Lineation development does not appear to be related to D1 structural position, but instead influenced by local rheologic variations manifest by differing thicknesses (viscosities) of schist and quartzite couplets. Where lineation development is low strain tends toward oblate with strong S>>L tectonites developed and where lineation is strong strain tends toward prolate with strong L>>S tectonites developed.