Crustal Velocity and Moho Topography in Central New-Hampshire

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Journal of Geophysical Research-Solid Earth
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Sep 10
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The White Mountain Batholith in central New Hampshire is a major plutonic, topographic and gravimetric feature, superimposed on the NE-SW trending regional pattern of accreted terranes in northeastern North America. Compressional and shear-wave arrival times from a two-dimensional seismic array experiment are used to constrain a one-dimensional crustal velocity model and Moho topography for central New Hampshire. Our deployment, a ''piggyback'' on the Ontario-New York-New England refraction experiment (ONYNEX), recorded 10 explosive sources using 20 three-component digital seismometers. The crust can be modeled as 34.5 km thick, and with three layers having linear gradients without midcrust discontinuities. For this model, P-velocities are 6.1-6.2 km/s in the upper crust, and 6.2-6.8 km/s in the lower crust. S-velocities are 3.5-3.7 km/s in the upper crust, and 3.7-3.9 km/s in the lower crust. Because Moho reflections comprise our principal constraints on lower-crustal velocities, there is a tradeoff between lower-crustal velocity and crustal thickness. In particular, an average crustal thickness of 39-40 km, reported elsewhere in New England, is not precluded by our data. The Poisson's ratio for the crust varies between 0.23-0.26, in the lower range of values reported for crustal minerals. Our dataset is consistent with less-than-or-similar-to 2% lateral variation in the upper crust underlying the New Hampshire and White Mountain Plutonic Series, but larger variations deeper than 15 km. If modeled in terms of Moho topography, PmP traveltime residuals and PmP - P residuals suggest thicker crust (36-37 km) under the White Mountain batholith, with thinner crust (33-34 km) to the east and southwest of the White Mountains. Such topography of the Moho correlates well with both the surface elevation and the Bouguer gravity anomaly in the region. Gravity data do not favor the simplest alternative model: a thick low-velocity granitic lower crust beneath the batholith. Using a simplified density model, roughly 30-35% of the gravity anomaly in the White Mountain region can be modeled as the result of the inferred Moho depression, requiring only a 5 km low-density root for the White Mountain batholith. The short wavelength of the Moho topography is consistent with a very low flexural rigidity for the lithosphere (almost-equal-to 10(21) Nt-m), suggesting that the plutons were intruded into a weak lithosphere. A rapid transition from positive to negative PmP residuals falls along a line that parallels the onshore extension of the Kelvin Seamounts, and may correspond to the ''line of weakness'' hypothesized by Sykes (1978) to have been tectonically active during the opening of the North Atlantic Ocean at 100-200 Ma.


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