Abstract

The Chatham fault separates the structurally lowest Giddings Brook slice of the Taconic allochthon from the next higher Chatham slice in New York. Slivers of the autochthonous shelf sequence within the Chatham fault zone were attributed by Zen (1967) to a tectonic mélange that separated two structurally independent slide masses in Middle Ordovician time. The zone of inclusions and the fault thus represented a primary boundary between two allochthonous paleotectonic units.

Restudy of the Chatham fault shows that it is a major postmetamorphic thrust fault that clearly postdates emplacement of the allochthonous rocks assigned to the Giddings Brook and Chatham slices. A sliver of Grenville basement gneiss with attached miogeoclinal shelf-sequence rocks and other slivers suggests dip-slip movement in excess of 3.5 kin on the east-dipping fault. Geometric relationships and textural evidence indicate that the tectonic mélange in the (Chatham fault zone postdates Taconic dynamo-thermal metamorphism and further suggest that the Giddings Brook and Chatham slices may have been continuous before Acadian? imbrication. The western contact of the Rensselaer Plateau slice with the Giddings Brook slice may also be a zone of late imbrication rather than a primary Taconian contact.

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