Dynamics of the North American continent

The forces that cause deformation of western North America have been debated for decades. Recent studies, primarily based on analysis of crustal stresses in the western United States, have suggested that the deformation of the region is mainly controlled by gravitational potential energy (GPE) variations and boundary loads, with basal tractions due to mantle flow playing a relatively minor role. We address these issues by modelling the deviatoric stress field over western North America from a 3-D finite element mantle circulation model with lateral viscosity variations. Our approach takes into account the contribution from both topography and shallow lithosphere structure (GPE) as well as that from deeper mantle flow in one single model, as opposed to separate lithosphere and circulation models, as has been done so far. In addition to predicting the deviatoric stresses we also jointly fit the constraints of geoid, dynamic topography and plate motion both globally and over North America, in order to ensure that the forces that arise in our models are dynamically consistent. We examine the sensitivity of the dynamic models to different lateral viscosity variations. We find that circulation models that include upper mantle slabs yield a better fit to observed plate velocities. Our results indicate that a model of GPE variations coupled with mantle convection gives the best fit to the observational constraints. We argue that although GPE variations control a large part of the deformation of the western United States, deeper mantle tractions also play a significant role. The average deviatoric stress magnitudes in the western United States range 30-40 MPa. The cratonic region exhibits higher coupling to mantle flow than the rest of the continent. We find that a relatively strong San Andreas fault gives a better fit to the observational constraints, especially that of plate velocity in western North America.

Peak Last Glacial weathering intensity on the North American continent recorded by the authigenic Hf isotope composition of North Atlantic deep-sea sediments

We have retrieved radiogenic hafnium (Hf) isotope compositions (ɛHf) from authigenic Fe–Mn oxyhydroxides of deep northwest Atlantic sediments deposited over the past 26 ka to investigate the oceanic evidence of changes in dissolved weathering inputs from NE America during the last deglaciation. The extraction of seawater-derived Hf isotopic compositions from Fe–Mn oxyhydroxides is not a standard procedure. Comparisons between the Al/Hf ratios and Hf isotopic compositions of the chemically extracted authigenic phase on the one hand, and those of the corresponding detrital fractions on the other, provide evidence that the composition of past seawater has been reliably obtained for most sampled depths with our leaching procedures. This is endorsed most strongly by data for a sediment core from 4250 m water depth at the deeper Blake Ridge, for which consistent replicates were produced throughout. The Hf isotopic composition of the most recent sample in this core also closely matches that of nearby present day central North Atlantic seawater. Comparison with previously published seawater Nd and Pb isotope compositions obtained on the same cores shows that both Hf and Pb were released incongruently during incipient chemical weathering, but responded differently to the deglacial retreat of the Laurentide Ice Sheet. Hafnium was released more congruently during peak glacial conditions of the Last Glacial Maximum (LGM) and changed to typical incongruent interglacial ɛHf signatures either during or shortly after the LGM. This indicates that some zircon-derived Hf was released to seawater during the LGM. Conversely, there is no clear evidence for an increase in the influence of weathering of Lu-rich mineral phases during deglaciation, possibly since relatively unradiogenic Hf contributions from feldspar weathering were superimposed. While the authigenic Pb isotope signal in the same marine sediment samples traced peak chemical weathering rates on continental North America during the transition to the Holocene a similar incongruent excursion is notably absent in the Hf isotope record. The early change towards more radiogenic ɛHf in relation to the LGM may provide direct evidence for the transition from a cold-based to a warm-based Laurentide Ice Sheet on the Atlantic sector of North America.

A check list of mammals of the North American continent, the West Indies and the neighboring seas.

v.6(1905)

The American gazetteer : exhibiting a full account of the civil divisions, rivers, harbours, Indian tribes, [et]c. of the American continent, also of the West India and other appendant islands : with a particular description of Louisiana /

Sabin,

United States, from the discovery of the North American Continent to the present time /

Index: v. 9, p. 323-410.

A drummer's experience, by J. S. Dearing. 103 half-tone illustrations representing beauty spots and noted scenes of the North American continent.

Mode of access: Internet.

The history and geography of the Mississippi Valley. To which is appended a condensed physical geography of the Atlantic United States, and the whole American continent.

Vol. 2 has title: The United States and the other divisions of the American continent. By Timothy Flint ... vol. II.

Arctic travels, or, An account of the several land expeditions to determine the geography of the northern part of the American continent.

"First published in 1824," according to Osborne, cited below.

History of the United states : from the discovery of the American continent /

Mode of access: Internet.

History of the United States from the discovery of the American continent.

Engraved armorial bookplates of C.L. Peirson.

A History of the United States, from the discovery of the American continent /

Imprint varies.

The Field dog stud book : an authentic register of names, colors, ages, pedigrees, sex, winnings and owners of field dogs on the American continent.

Mode of access: Internet.

The American gazetteer, exhibiting a full account of the civil divisions, rivers, harbors, Indian tribes, &c. of the American continent: also of the West-India, and other appendant islands: with a particular description of Louisiana.

Atlas issued separately. cf. Sabin, Bibl. Amer., 50939.

History of the United States, from the discovery of the American continent.

Mode of access: Internet.