SHEAR SENSE MICRO-ANALYSIS OF MELANGE IN THE CAUCOMGOMOC INLIER, NORTHERN MAINE

This research examines the microscopic textures found in highly foliated rocks from the Hurd Mountain and Avery Brook formations (mélange) in the Cambro-Ordovician Caucomgomoc Lake inlier of northern Maine. The Hurd Mountain is a complex meta-sedimentary unit with numerous blocks, including dismembered mafic lavas, while the Avery Brook Formation is a massive greenschist that is locally pillowed. These units are in fault contact, with the Avery Brook Formation west of the Hurd Mountain Formation. The mélange is similar to that found in the Lobster Mountain Anticlinorium (Hurricane Mountain Formation) that is roughly along strike, interpreted to be a suture between Boundary Mountains and Gander Terranes.

25 oriented thin-sections of mélange were examined to determine their shear sense. Shear sense indicators occur at meso- and microscopic scales and include asymmetric folds, mica fish, porphyroclasts ( and ), and C-S fabrics. Most foliations in the inlier strike NNE and dip to the west. 18 of the slides examined show normal sense (down-to-the-west), 2 show reverse sense (up-to-the-east), and the remainder are indistinct or inconclusive. These data are consistent with meso-scale shear sense indicators observed in the field. Based on the present orientation of foliations, this movement is of normal sense. However, nearby Devonian cover rocks have been folded by several tens of degrees, suggesting that the original orientation of older mélange-related foliations may have been east-dipping, in which case deformation within the mélange could have been east-over-west reverse motion. Overall this suggests that motion within the mélange brought rocks from the east (Hurd Mountain Formation) over rocks to the west (Avery Brook Formation) and the suture dips to the east, consistent with evidence from the Hurricane Mountain Formation.