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SummaryThe EarthScope Transportable Array (TA) deployment has

provided dense array coverage across the continental US and with it, the opportunity for high resolution 3D seismic veloc-ity imaging of both the lithosphere and asthenosphere in the continent� Building upon our previous work, we present a new 3D isotropic and radially anisotropic shear wave model of the North American (NA) lithospheric mantle, using full waveform tomography and shorter-period (40 s) waveform data� Our model exhibits pronounced spatial correlation between major tectonic localities of the eastern NA continent, as evidenced in the geology, and seismic anomalies� This suggests recurring epi-sodes of tectonic events are not only well exposed at the surface, but also leave persistent scars in the continental lithosphere mantle, marked by isotropic and radially anisotropic velocity anomalies that reach as deep as 100-150 km� In eastern North America, our tomographic images distinguish the fast velocity cratonic NA from the deep rooted large volume high velocity blocks which are east of the continent rift margin, and extend 200-300 km offshore into the Atlantic Ocean� In between is a prominent narrow band of low velocities that roughly follows the south and eastern Laurentia rift margin and extends into New England� The lithosphere associated with this low veloc-ity band is thinned likely due to combined effects of repeated rifting processes along the rift margin and northward extension of the Bermuda low–velocity channel across the New England region� Deep rooted high velocity blocks east of the Laurentia margin are proposed to represent the Proterozoic Gondwanian terranes of pan-African affinity, which were captured during the Rodina formation but left behind during the opening of the At-lantic Ocean�

Full waveform inversionWe present a new high-resolution 3D tomographic model of

shear velocity in the cratonic North American mantle (Figure 2�3�1), developed using long-period full waveform inversion� The new inversion shares many of the methodological features of our previous continental scale time-domain 3D waveform tomographic inversions for isotropic and radially anisotropic structure (Yuan et al., 2011)� We largely mirror the processing performed in the global SEM (spectral element method)-based hybrid modeling approach (Lekic and Romanowicz, 2011), ex-cept the global SEM synthetic code is replaced by RegSEM (Cu-pillard et al., 2012)�

Tomographic ResultsThe cratonic root has been affected by the recurring episodes

of tectonism, however its eastern margin seems largely intact

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2000 4000

Topo

grap

hy (m

)

Figure 2�3�1: Source and station distribution for the new North American inversion� The black triangles show the seismic stations and the red stars show 136 local events in addition to our global and NA regional/teleseismic dataset� The thick black line indicates the mod-el boundaries used for RegSEM forward modeling� The background shows the topography

west of the Grenville deformation front� The Archean litho-sphere may have extended further outboard of the Proterozo-ic Grenville deformation front, as indicated by a high velocity Archean-like structure that extruded beyond the deformation front� Across the continental margin, thinning of the lithosphere is evident in our new tomographic images as well as discontinu-ity–sensitive receiver functions (Rychert et al., 2007) and local Rayleigh wave dispersion inversion (Li et al., 2003)� A thinning process is likely further facilitated in New England by the pas-sage of the Bermuda low velocity channel, which connects to the Bermuda hotspot in the Atlantic Ocean, and extends farther east in to the Atlantic Ocean�

Our new tomographic image reveals deeply rooted Gondawa-nian blocks that were captured during the North American/Af-rican collision that closed the Iapetus Ocean, but stayed behind when the present Atlantic Ocean was formed (e.g., Nance and Murphy, 1994)� These exotic terranes are clearly separated from the Laurentia core along the east continent margin, and extend greatly outboard into the Atlantic Ocean, reaching out to the oldest Atlantic oceanic crust� The presence of large segments of fossil continents off cratonic margins is thus not uncommon,

3 Shear Wave Velocity Model of the Eastern North America Upper Mantle Using Full Waveform Tomography

Huaiyu Yuan, Scott French, Paul Cupillard and Barbara Romanowicz

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and may suggest greater roles in the global crustal-growth mod-els (e.g., Griffin et al., 2011)�

ReferencesGriffin, W�L�, Begg, G�C�, Dunn, D�, O’Reilly, S�Y�, Natapov, L�M�,

Karlstrom, K�, Archean lithospheric mantle beneath Arkansas: Con-tinental growth by microcontinent accretion, Geological Society of America Bulletin 123, 1763-1775, 2011�

Li, A�, Forsyth, D�W�, Fischer, K�M�, Shear velocity structure and azimuthal anisotropy beneath eastern North America from Rayleigh wave inversion, J. Geophys. Res. 108, 24 PP, 2003�

Nance, R�D�, Murphy, J�B�, Contrasting basement isotopic signa-tures and the palinspastic restoration of peripheral orogens: Example from the Neoproterozoic Avalonian-Cadomian belt, Geology, 22, 617-620, 1994�

Rychert, C�A�, Rondenay, S�, Fischer, K�M�, P-to-S and S-to-P imag-ing of a sharp lithosphere-asthenosphere boundary beneath eastern North America, J. Geophys. Res. 112, B08314, 2007�

Yuan, H�, Romanowicz, B�, Fischer, K�M�, Abt, D�, 3-D shear wave radially and azimuthally anisotropic velocity model of the North American upper mantle, Geophys. J. Int. 184, 1237-1260, 2011�

dlnVs 150 km dlnVs 200 km

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SMA

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Mid−continent rift Rift margin Grenville/LLano Front Appalachian Front Hotspots

dlnVs 60 km dlnVs 120 km

dlnVs (%) −100 − 90°° − 80 − 70°° ° − 60

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Gulf of Mexico

West Avalo

nia terra

nes

ECMA

Figure 2�3�2: 3D isotropic Vs and radial anisotropy structure in eastern North America� The depth of each map is indicated� The Grenville/Llano deformation front is coded in cyan and continental rift margin in red� Green dots are hotspot locations; the Bermuda hotspot is near the lower right corner� The top four subpanels are for isotropic Vs; the bottom two are for radial anisotropy, ξ� Note the eastern edge of the high velocity craton closely follows the eastern continental rift margin (red line)�