GEOLOGY OF THE HANOVER QUADRANGLE, NEW HAMPSHIRE-VERMONT

The Hanover quadrangle, in west-central New Hampshire and east-central Vermont, is underlain predominantly by three metamorphosed eugeosynclinal formations, all of probable Ordovician age. The Waits River formation, with an exposed thickness of 4000 feet, and the Gile Mountain formation, with a thickness of 7500–8500 feet, are representative of the so-called Vermont sequence. In areas to the north and in part of the Hanover quadrangle, this sequence is separated from the New Hampshire sequence (here represented by the Orfordville formation, with a thickness of 6000 feet) by the Monroe fault. This fault apparently dies out in this quadrangle, and the two sequences are in sedimentary contact. Although the evidence is not unequivocal, it is concluded that Vermont rocks underlie the New Hampshire section and that the Gile Mountain formation is a probable equivalent to the lower part of the Orfordville formation and the upper part of the Waits River formation. A revision of the originally described stratigraphic sequence of the Orfordville formation is necessitated by the geologic relations of the Hanover area.

Two pre-metamorphic faults, the Monroe and Northey Hill, and the post-metamorphic Ammonoosuc thrust, disrupt the regional structures. The areal geology is dominated by two domal uplifts, the Lebanon dome and the Pomfret dome. The former is a synkinematic granite intrusive which has been forcibly injected from the northwest toward the southeast, overturning the beds south of the pluton. Considerable granitization has accompanied the intrusion of this massif. The Pomfret dome is a cleavage dome produced by a late-stage or second-deformation upward welling of previously deformed plastic schists. Granite or migmatite working its way upward appears to be the active tectonic element in the formation of the cleavage dome. Most of the minor structures of the Hanover area are tectonically related either to the Lebanon dome or to the Pomfret dome.

Regional metamorphism has affected all the rocks, and the intensity of metamorphism increases toward the domal structures. The latter are evidently the major heat sources in the metamorphic reconstitution of the interbedded sedimentary and volcanic rocks. Soaking of quartz, albite, and, to a lesser extent, tourmaline occurs in schists remote from Lebanon granite. Sedaclase tonalites, occasionally in lit-par-lit structures in these metasomatized rocks, possibly represent albitized granites, but their origin is conjectural.