Crust and upper mantle discontinuity structure beneath eastern North America

Abstract

[1] Crust and mantle discontinuities across the eastern margin of the North American craton were imaged using P to S converted phase receiver functions recorded by the Missouri to Massachusetts Broadband Seismometer Experiment. Crustal structure constrained by modeling Moho conversions and reverberations shows a variation of Moho depth from a minimum of 30 km near the Atlantic coast to depths of 44-49 km beneath the western Appalachian Province and 38-45 km beneath the Proterozoic terranes in the west. The variation in crustal thickness is substantially greater than that required for local isostasy, unless lower crustal densities are >3110 kg/m 3 . In the upper mantle, Ps phases corresponding to a discontinuity at depths of 270-280 km were clearly observed beneath the eastern half of the array. Beneath the western third of the array, the receiver function stacks indicate more complex scattering, but weak Ps phases may be generated at depths of roughly 320 km. The transition between these two regions occurs across the eastern edge of the North American lithospheric keel imaged by tomography. The observed phases may be interpreted as conversions from the base of a low-velocity asthenosphere.

Crustal Thickness Variation in the Northern Appalachian Mountains: Implications for the Geometry of 3‐D Tectonic Boundaries Within the Crust

“…The step might represent late Paleozoic differential uplift and exhumation of the orogen (Harrison et al, 1989;Wintsch et al, 2003), or it might reflect changes in crustal thickness due to Mesozoic rifting or underplating (Li et al, 2002). The step might represent late Paleozoic differential uplift and exhumation of the orogen (Harrison et al, 1989;Wintsch et al, 2003), or it might reflect changes in crustal thickness due to Mesozoic rifting or underplating (Li et al, 2002).…”

Deep Earth Structure - Transition Zone and Mantle Discontinuities

“…Bostock (1996b) found evidence for a weak 220 km discontinuity beneath the Canadian part of the North American craton in P receiver functions. Li et al (2002) found from Ps data a discontinuity at 270-280 km depth beneath eastern North America. Li et al (2002) found from Ps data a discontinuity at 270-280 km depth beneath eastern North America.…”

Crust and Lithospheric Structure – Natural Source Portable Array Studies of the Continental Lithosphere

“…Interstation spacing was about 100 km and the array operated for about 16 months. Receiver function determinations of crustal thickness along the profile show thickening of the crust beneath the Appalachians, thinning in the region of the Grenville Front and thickening again beneath the stable Proterozoic platform (Li et al, 2002). The root beneath the Appalachians, thicker than predicted for isostatic compensation, appears to reflect a 'densification' of the lower crust that Fischer (2002) had hypothesized is a common metamorphic process in older orogenic belts.…”

“…The root beneath the Appalachians, thicker than predicted for isostatic compensation, appears to reflect a 'densification' of the lower crust that Fischer (2002) had hypothesized is a common metamorphic process in older orogenic belts. Upper-mantle discontinuities from receiver functions observed beneath MOMA at depths around 270-280 km are visible in the eastern part of the array and have been interpreted as the (rarely observed) base of the asthenosphere beneath eastern North America (Li et al, 2002). Subsequent studies based primarily on extensive data from a handful of permanent stations in eastern United States suggest that the LAB is sharply defined, perhaps no more than 10-12 km in vertical extend, beneath the Phanerozoic (Rychert et al, 2005).…”