A Plate-Tectonic Model for Late Cenozoic Crustal Spreading in the Western United States
- Robert E. Riecker
Published Online: 21 MAR 2013
Copyright © 1979 by the American Geophysical Union.
Rio Grande Rift: Tectonics and Magmatism
How to Cite
Eaton, G. P. (1979) A Plate-Tectonic Model for Late Cenozoic Crustal Spreading in the Western United States, in Rio Grande Rift: Tectonics and Magmatism (ed R. E. Riecker), American Geophysical Union, Washington, D. C.. doi: 10.1029/SP014p0007
- Published Online: 21 MAR 2013
- Published Print: 1 JAN 1979
Print ISBN: 9780875902142
Online ISBN: 9781118664988
- Cenozoic crustal extension;
- Interpretive kinematic. model;
- Plastic deformation;
- Plate-tectonic model;
The timing, location, and nature of late Cenozoic extension in the western United States are described in order to provide a basis for plate-tectonic interpretation. The model of Atwater (1970) does not account for the variations observed.
Cenozoic extensional strain and related volcanism apparently first began about 30 m.y. (million years) ago, in the Rio Grande rift area. Although stratigraphic evidence suggests that it may have continued episodically throughout the rest of Cenozoic time, a pronounced pulse of uplift and block faulting occurred between 7 and 4 m.y. ago, after a magmatic lull and with a substantive change in style of extensional deformation.
The time of initiation of crustal extension in the southern Basin and Range province is less clear. Extensional block faulting may have begun about 17 m.y. ago, and had certainly begun by 13 m.y. ago, but began to wane about 10 m. y. ago. However, significant earlier deformation involving rocks metamorphosed in latest Oligocene and early Miocene time is interpretable as evidence for plastic extension of the crust. It took place while extensional faulting was occurring in areas both to the east and southwest.
Extension in the northern Basin and Range province, the High Lava Plains of Oregon and California, and the Columbia Plateau of Washington and Oregon apparently began between 16 and 17 m.y. ago, while the Mendocino triple junction lay far to the south. It continues today, mainly in the Great Basin, but in a notably different direction.
The nature of spatial and mechanical relations among the North American, Farallon, and Pacific plates changed substantially during this 30 m.y. period, as did the thickness and temperature of the subducting Farallon plate. As a result, profound, present-day differences of a tectonic, geophysical, and geomorphic nature exist among the various regions of late Cenozoic extension. Actively extending areas, like their oceanic counterparts, are hot and topographically high. Those where extensional activity has waned, or is waning, are not. Possible convective motions in the asthenosphere, ductile thinning and magmatic invasion of the lithosphere, and preexisting tectonic features in the crust, may all have played important roles in shaping the surface patterns of crustal extension. These patterns are believed to have developed approximately as follows:
1. The Rio Grande rift probably opened intially along an old crustal flaw near the east edge of the middle tertiary volcanic arc. The subducting Farallon slab had a low dip then, owing to its relative youth and rapid convergence with the North American plate. The rift valley, where sharply defined, is generally everywhere less than 100 km wide. Many of its characteristics are like those of other major continental rifts.
2. Spreading in the Basin and Range province began ina west-southwest direction in a thermally weakened and partially ductile lithosphere. It occurred first within the broad volcanic arc in the south, then behind a much narrowed arc in the north. Extension probably took place by basaltic diking and by solid-stage flowage of the deep lithosphere, possibly driven by hydrodynamically forced convective motions in the asthenosphere as a result of steepened subduction of the Farallon plate. Initial spreading in the Sonoran Desert section began at a time of slowing and cessation of plate convergence; its direction, initially anti parallel to the convergence vector of relative plate motion, changed later, in the Great Basin, to a direction consonant with the worldwide pattern of east-west spreading, and the extending region broadened to 800 km to form a distributed rift.
3. The brittle upper part of the northwestern U.S. region of the North American plate then began to rotate and open northwestward, owing to lateral traction and regional shear resulting from northwest migration of the Pacific plate. The eastern Snake River Plain, which developed initially as a zone of transform faulting, pulled open ductilely as an elongate sag above a northeast-trending lithospheric fracture whose northeast-propagating tip had served as a major focus of volcanism for more than 10 m.y. Reinvigorated opening of the Rio Grande rift, with uplift and block faulting, took place at this time.