MAPPING SMALL, THINKING BIG IN NEW HAMPSHIRE: REFINING A STRUCTURAL FRAMEWORK FOR STUDIES IN GEOCHRONOLOGY, STRATIGRAPHY AND METAMORPHIC PETROLOGY

New Hampshire has long served as a testing ground for the interpretation of polydeformed rocks, starting with the classic story of nappes draped across the Bronson Hill anticlinorium (BHA). Detailed bedrock mapping in recent decades has allowed recognition of four distinct but correlative depositional sequences of Silurian and Early Devonian rocks. The latter were deposited as flysch that advanced westward in front of the Acadian tectonic front. The four sequences were juxtaposed by three west-directed Acadian thrust nappes, from west to east: (1) the Skitchewaug/Cornish nappe floored by the Monroe thrust, (2) the Fall Mountain nappe and Piermont allochthon, rooted in the Brennan Hill thrust (BHT) and carrying the Monadnock sequence (Monadnock/Kearsarge D1 syncline) and (3) the Chesham Pond thrust (CPT) carrying the Central Maine sequence.

Sheet-like syntectonic intrusions followed the faults as they moved: the Bethlehem Gneiss along the BHT and the Kinsman Granite along the CPT. The faults cut obliquely at a low angle, structurally downward, across the nappes toward the west. Toward the north the Bethlehem Gneiss seems to have escaped the BHT to intrude the D1 syncline below it. The stacking of hot rocks above cooler rocks set up conditions that produced metamorphic facies in inverted order. Subsequent backfolding and doming has deformed this stack such that the present erosion surface exposes deeper structural levels in anticlines and domes.

Retrodeformation of this complex geology aids in understanding depositional settings of the stratigraphic sequences, especially for the Silurian units. The Cornish and Skitchewaug nappes, for example, are one and the same structure, rooted to the east of the BHA, implying that the Clough Quartzite within the nappes was deposited at a site between the parautochthonous Clough on the BHA domes and the Rangeley facies in higher structural levels to the east. Carbonate-rich units overlie these basal Silurian units, from the thick, sandy Waits River Formation in the autochthonous “Vermont sequence”, thin Fitch Formation on the domes, very discontinuous Fitch in the Cornish nappe, distinctive calc-silicate units in the Monadnock sequence (Francestown and Warner Fms), and the much thicker Perry Mountain, Smalls Falls and Madrid Formations farther east. Post-Acadian plutons and shear zones complicate the D1 map pattern, and Mesozoic plutons and normal faults also obscure the older structures. Maps and cross sections at 1:250,000 will be presented to illustrate the ideas outlined above.