A TEST OF THE ACADIAN THRUST-NAPPE MODEL IN NEW ENGLAND USING 40AR/39AR COOLING AGES OF MUSCOVITE

New ⁴⁰Ar/³⁹Ar data collected from the southern edge of Thomas' (1977) Quebec embayment challenge current tectonic models for western New Hampshire and eastern Vermont which hold that westward thrusting of nappes from New Hampshire over Vermont was responsible for the Acadian metamorphism in Vermont. Our new data from ~thirty white micas in southwestern New Hampshire suggest final emplacement of west verging thrust nappes took place after the early Permian.

The New Hampshire series consists of sillimanite and higher-grade east dipping thrust nappes identified from east to west as the Chesham Pond (structurally highest), the Fall Mountain, the Skitchewaug and the “Big Staurolite” nappe (structurally lowest). Spear and others (2003) proposed the Big Staurolite nappe had been metamorphosed by the emplacement of the other structurally higher nappes onto it. However, existing cross sections through all four nappes suggest the overall thickness of the upper three nappes in the west is less than a few kilometers, while loading to staurolite grade conditions would probably require an additional 10-20 kilometers depending on the starting temperature of the lower nappe.

Our new ⁴⁰Ar/³⁹Ar data identify two age discontinuities that suggest any thrusting that may have taken place in southwestern New Hampshire occurred during the late Paleozoic. In one 200 m section at Fall Mountain, muscovite ages of the higher nappe are ~335 Ma. These rocks lie immediately upon Skitchewaug and Big Staurolite nappe rocks with ages of ~315 Ma. This 20 million year age discontinuity can not be explained by conductive cooling (e.g. Eusden and Lux, 1994), an inference confirmed by one-dimensional thermal modeling. Across a second age discontinuity, identified east of the Alstead dome, rocks with muscovite ages from 265-288 Ma in the Fall Mountain nappe lie directly on the Skitchewaug nappe with muscovite ages ranging from 298-315 Ma. These two age discontinuities, located across regionally significant thrust faults mapped parallel to Thomas' (1977, 2006) Laurentian boundary, show that final nappe emplacement must have occurred during the Permian or later. Moreover, the Laurentian boundary may have acted as a buttress providing a rigid backstop along which deformation was concentrated during the Alleghanian orogeny.