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Shillington, D.J., 2004

The formation and rupturing of continents: Seismic studies of the Aleutian Island arc and the Newfoundland nonvolcanic rifted margin

Bibliographic Reference

Shillington, D.J., 2004, The formation and rupturing of continents: Seismic studies of the Aleutian Island arc and the Newfoundland nonvolcanic rifted margin: University of Wyoming, Laramie, Ph.D. dissertation, 345 p., illust., maps (some color).

Abstract

Island arcs and rifted margins embody the creation and destruction of continental and oceanic crust. Subduction zones consume downgoing oceanic crust, and island arcs are the manifestation of the associated magmatism. At the other end of the spectrum, rifted margins record continental breakup and the inception of seafloor spreading in the resulting new ocean basins. Although island arc magmatism constitutes a source of significant magmatic addition and might serve as a primary continental-crustal building block, debate continues regarding the composition of the middle and lower crust of island arcs and their relationship to the compositions of lavas erupted at the surface. Crustal velocity structure along the central Aleutian island arc suggests that the middle crust is composed of plutons with andesitic to basaltic compositions, and the lower crust is composed of ultramafic-mafic cumulates. The bulk composition of this island arc estimated from velocity is significantly more mafic than average continental crust, thus requiring modifications to be a suitable building block for continental crust. Additionally, variations in crustal velocity show a first-order correspondence to trends in the major element geochemistry of lavas in the central Aleutians. Nonvolcanic rifted margins form by continental breakup accompanied by little synrift magmatism and thus are ideal places to image rifting structures that would otherwise be obscured by synrift magmatism. On many nonvolcanic margins, crust of uncertain affinity has been observed between unambiguous continental and oceanic crust. On the Iberia margin, 'transitional' crust is denuded, subcontinental mantle, exposed during the final stages of rifting. Delineating crustal structure of Iberia's conjugate margin, Newfoundland, can allow us to determine margin symmetry, which can be used to understand continental rupture and initial seafloor spreading. Pretack depth migrations of multichannel seismic reflection data on the Newfoundland margin, together with the work of Van Avendonk et al . [2003] and Nunes [2002], suggest that 'transitional' crust on the Newfoundland margin is oceanic, which implies significant margin asymmetry and a complicated period of seafloor spreading following continental rupture. The 'featureless' appearance of 'transitional' crust on Newfoundland might be due to poor signal penetration beneath postrift sills encountered during Ocean Drilling Program Leg 210.

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