PERI-GONDWANAN TERRANE ASSEMBLY AS REVEALED IN THE NEW BEDROCK GEOLOGIC MAP OF THE ESSEX QUADRANGLE, CONNECTICUT

The assembly of the rocks of the Neoproterozoic Gander and Avalon terranes against the Ordovician Bronson Hill (BH) and Silurian Merrimack (Hebron Fm., Sh) terranes during the Alleghanian orogeny in southern New England produced complicated interfering structures. Previous mapping in these rocks was made difficult by the high-grade metamorphism, by the abundance of ‘look-alike’ plagioclase gneisses common to all terranes, by complicated overprinting foliations, and by the abundance of migmatites and pegmatites that dominate many outcrops. New 1:24,000-scale mapping in the Essex quadrangle identified crosscutting schistose blastomylonites (bm) containing many boudins and tectonic inclusions of pegmatite and amphibolite, and some containing garnet or sillimanite. Misidentification of these fault rocks in the past as Ordovician metasediments led to complicated stratigraphic interpretations rejected by the new mapping. Geochemical analyses of granodioritic and tonalitic gneisses reveal two map-scale orthogneisses and one paragneiss complex within the BH terrane. Our mapping also identifies a clock-wise rotation of shortening directions from the WNW to NNW to NNE all at anatectic metamorphic conditions.

Major ductile fault zones composed of well-developed bm foliations (D1) mark the boundaries of all terranes. These are strongly folded to produce overprinting WNW dipping axial plane foliations (D2). D1 and D2 fabric is transposed into slabby fabrics (D3) trending NNW. All of these fabrics are in turn transposed again by N and NNE dipping ductile faults (D4) containing bm schists and gneisses with penetrative NE-plunging sillimanite and anthophyllite lineations. The ductile faults, interference folds, and possible km-scale sheath folds record a D4 period of high strain in these rocks.

A wedge of Ediacaran Avalonian granodioritic orthogneisses displaced the Sh along the Honey Hill fault separating Gander cover rocks above from Gander ‘basement’ rocks structurally below in the Lyme dome. This wedging thinned the Sh between the Avalon and BH terranes, leaving a sinuous N-S belt of overturned Sh separating Ordovician from Neoproterozoic rocks. The wedge also overrode some Sh rocks such that slivers of Sh are present between Ganderian and Avalonian rocks.