POST-RIFT TECTONIC EVOLUTION OF THE CHAMPLAIN VALLEY REVEALED BY U-PB DATING OF CALCITE IN FAULTS AND FRACTURES

Recent seismic, geochronologic, and geochemical data make clear that Atlantic passive margins are sensitive recorders of deep lithospheric and asthenospheric processes. Better understanding of these processes may help explain the persistence of high relief and young volcanism along many margins. Our study thus presents ~70 new U-Pb ages on calcite from veins and faults of the Champlain Valley (VT-NY) and the Hartford Basin (CT). These ages record a continuum of ages from ~ 133 to 5 Ma, within which we identify three discrete periods of tectonism in the Champlain Valley. First, a period of primarily sub-vertical mode-1 fracturing, often associated with high fluid pressure, from ~ 115 to 98 Ma. Second, a period dominated by roughly ENE-WSW extension, associated with faulting from ~88 to 76 Ma. Third, a period of fluid flow, fracture reactivation, and E-W extension on high angle extensional faults in the Champlain Valley from ~15-5 Ma. The first period of mode 1 calcite veins overlaps a final stage of New England-Quebec (NEQ) volcanism observed primarily in the Champlain Valley, including the Barber Hill Pluton (~111 Ma), the Cuttingsville Pluton (~103-98 Ma), and the Oka Carbonatite complex (~145-98 Ma). However, it post-dates a period of kimberlite emplacement in NY and Canada centered at ~150-133 Ma, and a period of widespread NEQ plutonism centered at ~128-111 Ma. The second period of ~ENE-WSW extensional faulting in the Champlain Valley post-dates NEQ volcanism by at least 10 Myr and overlaps in time with the youngest episodes of cooling and/or exhumation recorded by thermochronology in the southeast Adirondacks and White Mountains. This raises the possibility that some late Cretaceous cooling could have been driven by tectonic unroofing and erosion. The nature of the third ~15-5 Ma period is yet to be determined, but seems associated with both fluid flow and minor faulting, perhaps recording tectonic adjustments responsible for uplift of modern topography of northern New England during the Miocene. Taken as a whole, our results suggest that the physiography of the northeastern US margin (and similar margins) are the product of a series of tectonic and magmatic events, very likely involving prolonged volatile-driven melting near the base of the lithosphere and perhaps also by far field-stress changes in the late Cretaceous and Cenozoic.