科研之友
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摘要
Lower greenschist-facies metasedimentary rocks of the Middle Cambrian Conanicut Group occur in and around Beavertail State Park, Rhode Island. Detailed structural mapping (1:1000-scale) and petrology of these rocks indicate an early fold generation (F-1) and axial planar metamorphic foliation (S-1). F-1 is folded by a more prominent, E-verging, NNE- to NNW-trending, non-coaxial fold generation (F-2) and an associated pressure solution-enhanced crenulation cleavage (S-2). A third map-scale fold generation is inferred from NNE-trending broad folding of F-2 and S-2. N-S extension resulted in boudins that deformed S-2 on a scale of 1-10 m, whereas late planar quartz veins indicate NW-SE extension. All structures are cross cut by faults striking N- to NE- and ENE- to ESE that show dominantly normal motion with minor sinistral or dextral components. Kink bands associated with faulting trend NNE to ENE with WNW to NNW side up. The vertical Beaverhead shear zone juxtaposes the Cambrian rocks with Pennsylvanian rocks of the Narragansett Basin, and deflects S 2 in a dextral sense, consistent with motion recorded elsewhere. The Cambrian rocks record the same deformation and metamorphism as the adjacent Narragansett Basin rocks. No evidence was found for pre-Alleghanian deformation or for northwest- or north-directed thrusting and accretion of a Meguma-like terrane during the Alleghanian orogeny. If the Beaverhead shear zone was a preexisting terrane boundary within Avalonia, both the Cambrian and Pennsylvanian Narragansett Basin sediments were deposited after terrane accretion.
- 出版日期2013
