Northeastern Section - 53rd Annual Meeting - 2018

Paper No. 29-2
Presentation Time: 8:20 AM


THOMPSON, Peter J., New Hampshire Geological Survey, Post Mills, VT 05058 and STRAUSS, Justin V., Department of Earth Sciences, Dartmouth College, HB6105 Fairchild Hall, Hanover, NH 03755

Recent detailed mapping in west-central New Hampshire demands a reevaluation of regional nappe-stage structures in the Bronson Hill anticlinorium (BHA). In the Smith Pond area, between the Mascoma and Croydon domes, schist layers formerly interpreted as infolds of Littleton and Partridge Fms alternating with Clough Quartzite are more likely schist intervals within the Clough, and a single F1 isocline of Fitch Fm. Samples have been collected for U-Pb detrital zircon analysis to test this hypothesis. The 1.5-km-amplitude syncline of Fitch opens westward and is deformed by a large overturned F2 anticline plunging SE, by upright F3 dome-stage folds, and locally by minor kinks associated with the nearby Mesozoic Grantham fault. The F1 fold appears to be a minor fold on the upright limb of the much larger westward-opening, recumbent Garnet Hill syncline (GHS), above the autochthonous Bronson Hill sequence, and not the “root zone” of the Skitchewaug nappe. A closer look at other purported extremely attenuated repetitions between Clough and interbedded schist units (e.g. Croydon Mountain, Surry Mountain) is warranted.

The Bethlehem Gneiss, presumably part of the Fall Mountain thrust nappe (FMTN), cuts obliquely across nappe-stage folds. The upper, overturned limb of the F1 GHS is preserved farther west as the lower limb of the Cornish nappe (CN), where it was dropped down along the Northey Hill shear zone. Facies changes between Littleton, Gile Mountain and Waits River Fms must occur beneath the CN. South of the Croydon dome, and east of the Grantham fault, the overturned limb of a westward-closing anticline is also preserved, there called the Skitchewaug nappe (SN), truncated above by the Brennan Hill thrust and Bethlehem Gneiss in the FMTN. The implication of these structural relationships is that CN = SN, rooted east of the BHA and below the FMTN. The upright limb of this nappe is preserved in a small graben at Skitchewaug Mountain and also north of the Lebanon dome. Farther north, the nappe is truncated above by the Piermont allochthon in a position analogous to the FMTN. Rocks NW of the Keene dome, supposedly in a deeper Bernardston nappe, may be a case of metavolcanics within the Littleton having been misinterpreted as a tight isocline of Ordovician units. Samples for U-Pb geochronology have been collected to resolve this question.

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