Abstract
The eastern North American rifted margin is a passive tectonic margin that has experienced Paleozoic ocean closure and Mesozoic continent rifting. To understand evolution of this continental margin, we modeled the two-dimensional P-wave and S-wave seismic velocity structure of the crust with a seismic wide-angle reflection/refraction profile located in North Carolina and Virginia. There is a seismic low-velocity zone (LVZ) at 10–12 km depth beneath the western segment of the profile. We infer the LVZ to be the base of a Paleozoic metasedimentary succession beneath the eastern Piedmont and westernmost coastal plain. The P-wave velocity and Poisson’s ratio suggest a felsic composition for the upper and middle crust beneath the seismic profile, and an intermediate composition for the lower crust. Overall, the measured crustal velocities and the lateral homogeneity of the crust, especially the middle and lower crust, indicate that Laurentian middle and lower crust extends beneath the entire coastal plain. The lack of a basal crustal layer with a high seismic velocity indicates that no magmatic intrusions have underplated the eastern Piedmont and coastal plain. The comparison with South China Sea, which is a wide rift, and Kenya Rift, which is a narrow rift, indicates that eastern North American margin has the character of a narrow rift. We infer that narrow rifts and wide rifts may have similar crustal compositions, but show strong differences in crustal thickness and the distribution of basal crustal mafic intrusion. These differences may be related to differences in extensional rate during rifting.
Highlights
Continental rifts and their products, passive margins, define the majority of the Earth’s coastline and are the expression of a fundamental process that continually reshapes our planet’s surface
We present the P- and S-wave velocity structure, Poisson’s ratio, and infer the crustal composition based on a comparison with laboratory measurements of P-wave velocities and Poisson’s ratio
We infer that the upper crustal low-velocity zone (LVZ) is an over-thrust metasedimentary layer based on the P-wave velocity and Poisson’s ratio, and we infer the LVZ is the Evington Group metasedimentary rocks reaching depths of up to 10 km (Pratt et al 1988)
Summary
Continental rifts and their products, passive margins, define the majority of the Earth’s coastline and are the expression of a fundamental process that continually reshapes our planet’s surface. A significant feature of the crustal velocity model (Fig. 7) is the alternate low- and high-velocity bodies obtained by inverting the Pg traveltimes in the upper crust at 0–5 km depths. Poisson’s ratio Poisson’s ratio along the profile was calculated using the final P- and S-wave seismic velocity models (Fig. 7) and mainly ranges from 0.24 to 0.26 for the crystalline crust (Fig. 11) These values are consistent with the previously reported low crustal Poisson’s ratio (< 0.27) of the Paleozoic Appalachian orogenic belt (Zandt and Ammon 1995). We model this anomaly with the density structure of the 10-km-thick upper crust using a block with density 2.61 g/cm, close to the density of granite (2.64 g/cm3) measured by Christensen (1996) This is consistent with the velocity of this region The geological interpretation of this seismic velocity/density structure is discussed below
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