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A gravity network of 700 stations over an area of approximately 4,500 km2 in central and southern New Hampshire, and mapped intrusive contacts and density determinations for rocks of the New Hampshire Plutonic Series, were used to deduce the structural relations of these Acadian-age intrusives. The bodies occur principally as subhorizontal sheets no thicker than 2.5 km, some of them once probably continuous or semicontinuous over much of the area examined.

Forcible injection of the earliest members of the series, the Bethlehem Gneiss and Kinsman Quartz Monzonite, occurred during a cycle of nappe development. Next, biotite quartz monzonite and the Spaulding Quartz Diorite were forcibly emplaced during a cycle of predominantly horizontal compression, but the magmas were channeled beneath and along the margins of the earlier intrusives.

Posttectonic Concord Granite was irrupted after the temperature of the region had cooled to the point where brittle fracture was possible, and the intrusive mechanism may have involved forcible injection, cauldron stoping and possible displacement of roof zones.

Elsewhere in New England the only Acadian-age plutons which have vertical dimensions greater than 2 or 3 km are those which occur outside the sillimanite zone of regional metamorphism. We suggest that the rheidity and the pre-existing foliation of the metasediments dictated the formation of thin plutonic sheets at the level of the infrastructure, and that a necessary condition for the formation of a thick stock or batholith is emplacement in the brittle superstructure.

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