Crustal Heating and Lithospheric Alteration and Erosion Associated With Asthenospheric Upwelling Beneath Southern New England (USA)

The Northern Appalachian Anomaly (NAA), a region of exceptionally low seismic velocities in the asthenosphere beneath southern New England and easternmost New York State, has been interpreted as a site of mantle upwelling. We synthesize a combination of new and previously published data that indicates the following: (1) The upwelling has eroded or delaminated the lithosphere in a localized region centered in southern Vermont that we call the "Green Mountains Anomaly". Forty-second period Rayleigh wave phase velocities, which have peak sensitivity at lithospheric depths, are slow in this region, and S wave receiver functions are dominated by shallow (60-km depth) mantle structures indicating reduced velocities. Thermal springs and sites with anomalously high concentrations of mantle-derived helium-3 are concentrated at the borders of the Green Mountains anomaly, perhaps due to stress concentrations produced by geologically recent, uncompensated delamination of the lower lithosphere. (2) S wave receiver functions indicate intense (>10%), shallow (60-km depth) short wavelength structures along the southern and western edges of the NAA, indicating that the lithosphere there is being intensely altered, possibly by a combination of shearing and introduction of volatiles. And (3) Notwithstanding the localized thinning and alteration, the NAA lithosphere as a whole does not appear to have experienced pervasive heating, for the compressional wave quality factor of Q(P) = 870 inferred from the decay rate of Po waves is very significantly above the Q(P) approximate to 60 value previously reported for the NAA asthenosphere. Plain Language Summary At depths between 100 and 300 km (60-180 miles), the Earth beneath southern New England and easternmost New York State (USA) is unusually hot, so much so that the rock is at or near its melting point. This region has been nicknamed the Northern Appalachian Anomaly, or NAA, for short. One previously published explanation is that convection currents in the Earth are bringing up hot material from deeper depths. We follow up on this idea by examining whether the upwelling is heating up and eroding the lithosphere, the roughly100-km-thick (60-mile-thick) layer of cool rock just below the Earth's surface, and causing pieces of it to fall off (delaminate) and sink. We find evidence for erosion and delamination only beneath a region in and around the Green Mountains (Vermont, USA). There, seismic waves are unusually slow and spring temperatures are unusually warm, both of which are signatures of heat being close to the Earth's surface. The lithosphere above the rest of the NAA is less affected by the upwelling, except near its southern and western edges, where some alteration may be beginning.