IMPLICATIONS OF VERY LONG BASELINE INTERFEROMETRY MEASUREMENTS ON NORTH-AMERICAN INTRA-PLATE CRUSTAL DEFORMATION

Very Long Baseline Interferometry (VLBI) experiments over the last 1 3 4 years between Owens Valley, California, and Haystack, Massachusetts radio observatories suggest an upper limit of east-west crustal deformation between the two sites of 1 cm/yr over a disstance of 3929 km. The baseline between the two sites traverses most of the major geological provinces of the United States. The most active region traversed is the Basin and Range Province, estimated to be expanding in an east-west direction at a rate of 0.3-1.5 cm/yr. The Colorado Plateau and Rocky Mountains system also appear to be expanding, but at a somewhat lower rate. East of the Rocky Mountains, the predominant stress appears to be compressional, nearly horizontal, and east to northeast trending. Possible explanations for this low rate of change of distance between the two VLBI observatories are: 1. (1) The North American Plate, as assumed by continental drift theory, is perfectly rigid. 2. (2) The North American Plate is sufficiently elastic to absorb intra-plate regional tectonic motions but, overall, behaves as a nearly rigid plate. 3. (3) Intra-plate crustal motions are episodic and this is a quiet period. 4. (4) Motions in the various geologic provinces counteract one another. While the available data are not sufficient to resolve the above explanations, it is suggested that the last one is the most plausible explanation and that opposing motions between various geological provinces do exceed the 1 cm/yr limit. © 1979.