Modeling 3-D crustal velocities in the United States and Canada

A numerical model for three-dimensional (3-D) crustal velocities has been derived for most of the United States and Canada, primarily from repeated geodetic data. This model provides a foundation for a prototype of the TRANS4D software. TRANS4D is being developed to enable geospatial professionals and others to transform 3-D positional coordinates across time. The derived model reveals several macroscopic features of the 3-D velocity field, including the pervasive presence of the glacial isostatic adjustment associated with the past melting of the ice fields that formed more than 19,000years ago during the Last Glacial Maximum. In this study, the present-day 3-D velocity field associated with this melting (as estimated via the recently published ICE-6G_C (VM5a) model) was subtracted from this study's total 3-D velocity field to identify features of the residual velocity field. In particular, this study introduces the NA_ICE-6G reference frame in which residual horizontal velocities have magnitudes that are less than 2mm/yr everywhere east of longitude 104 degrees W and south of latitude 60 degrees N, except in southern Texas. Residual horizontal velocities of greater magnitude are found west and/or north of these two boundaries, and they are due mostly to interactions among tectonic plates with localized pockets due to other geophysical phenomena. Large residual vertical velocities, some with values exceeding 30mm/yr, are found in southeastern Alaska. The uplift occurring here is due to present-day melting of glaciers and ice fields formed during the Little Ice Age glacial advance that occurred between 1550A.D. and 1850A.D.