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New shear-wave splitting measurements at permanent broadband seismic stations in the south-central United States reveal the orientation and degree of polarization of mantle fabrics, and provide constraints on models for the formation of these fabrics. For stations on the stable North American craton, correspondence between observed polarization direction of the fast wave and the trend of Proterozoic and Paleozoic structures associated with rifts and orogenic belts implies a lithospheric origin for the observed anisotropy. The largest splitting times (up to 1.6 s) are observed at stations located in the ocean-continent transition zone, in which the fast directions are parallel to the Gulf of Mexico continental margin. The parallelism and the geometry of the keel of the craton beneath the study area suggest that asthenospheric flow around the keel of the North American craton, lithospheric fabrics developed during Mesozoic rifting, or a combination of these factors are responsible for the observed anisotropy on stations above the transitional crust. [ABSTRACT FROM AUTHOR]

Folded Quaternary deposits across the south-central Puget Lowland, an area just south of the Seattle fault that extends across the Seattle uplift and its boundary with the adjacent Tacoma basin, provide increased resolution of the character and rate of crustal deformation. They also constrain alternative, and partly incompatible, views of crustal structure previously suggested by geophysical investigations. Tectonic deformation has been progressive for at least the past few hundred thousand years: older sediments display greater deformation than the youngest exposed deposits in the study area. Strain rates across the Seattle uplift have probably been between 0.25 and 1.0 mm/yr during this period, accounting for ∼10% of the total strain shortening of the western Washington crust. The Seattle uplift displays Quaternary deformation across its full north-south extent and has structural discontinuities at both its northern and southern boundaries. Previous workers have already established the faulted nature of its northern boundary; exposed Quaternary strata across its southern boundary display intense folding, the location of which generally corresponds to the projection of a "Tacoma fault" suggested by prior geophysical studies. [ABSTRACT FROM AUTHOR]

Examines the paleoproterozoic bimodal volcanic rocks in central Colorado. Indication of an extensional continental setting caused by the bimodality of the rocks; Growth of Laurentia by arc accretion; Identification of a potential continental crusts.