Spatial and temporal patterns of extension and magmatism in western North America

This project is endeavoring to more accurately determine Cenozoic extensional strain rates and magmatic eruptive rates in different parts of the North American Basin and Range province. A knowledge of how extensional strain and volcanic activity was distributed in space and time throughout the province is absolutely essential if we are to ever understand the ultimate cause and driving forces for Basin and Range extension and the role of magmatism from a regional perspective. Detailed geologic mapping and high precision geochronology and thermochronology is being carried out in four distinct areas:(1)Êeast-central Nevada, (2)Êwest-central Nevada (Yerington district and environs), (3)Êthe lower Colorado River extensional corridor (the Mojave, Black, Sacramento, and Eldorado mountains), and southern Sonora in the Mexican Basin and Range province I am trying to constrain paleostrain rates in these areas by two independent methods. One involves high precision 40Ar/39Ar dating of key volcanic units that bracket different parts of the local normal faulting and tilting histories. The other involves determining the temperature-time history of the deeper seated rocks in these same areas and then using these extension related cooling and uplift histories to estimate fault slip rates and strain rates.

Results from east-central Nevada indicate a surprisingly episodic extensional history, with significant episodes of rapid extension in the late Eocene - associated with extension, in the mid Miocene -with no associated volcanism, and in the Quaternary to Recent related to movement on the present range-bounding faults. Similarly, new gechronologic data from west-central Nevada indicate several distinct episodes of extension from the late Oligocene to the present (Dilles and Gans, in press). Perhaps the most exciting results have come from the lower Colorado River region, where our structural studies and extensive geochronologic work has demonstrated that large magnitude extension and voluminous mafic to silicic volcanism are intimately related, but that the inception of magmatism consistently preceded the inception of extension by up to a few million years. Both extension and magmatism then migrated northwards within a narrow corridor at a rate of 3 cm/yr from the latitude of Parker, AZ at 22 Ma to the latitude of Las Vegas at 12 Ma. These data suggest that the lower Colorado River extensional corridor may be a superbly preserved example of a failed propagating rift analogous to the Asal Rift in NE Africa. These studies have illustrated how, by combining detailed structural and stratigraphic studies with high precision 40Ar/39Ar geochronology of syntectonic volcanic rocks, it is possible to evaluate fault slip rates, magma eruption rates, etc., on time scales of less than 100k years, even for rocks tens of millions of years old.

Finally, new data from the Sonoran Basin and Range province indicate that the magnitude of extension in northwest Mexico is considerably greater and the timing is older than was previously thought. The entire area between the Sierra Madre Occidental and the Gulf of California appears to have been highly extended during the late Oligocene to middle Miocene, well before subduction had ceased at this latitude. This new data strongly suggests that extension was driven mainly by intraplate (body) forces rather than plate boundary effects, and has led to a new model for the evolution of northwest Mexico, outlined in the paper in press for Tectonics.

SELECTED PUBLICATIONS AND ABSTRACTS

  • Faulds, J. E., Gans. P. B., and Smith, E. (in preparation) Spatial and temporal patterns of extension in the Colorado River extensional corridor. To be submitted to Tectonics

  • Gans. P. B., Darvall, P., and Faulds, J. E. (in preparation) Magmatic-tectonic interactions in a Miocene continental rift: The Eldorado Mountains and environs, Colorado River extensional corridor, Geological Society of America Special Paper

  • Gans, P. B. (in press) Large-magnitude Oligo-Miocene extension in southern Sonora: Implications for the tectonic evolution of northwest MexicoTectonics

  • Dilles, J. H. and Gans, P. B. (1995) The chronology of Cenozoic volcanism and deformation in the Yerington area, western Basin-and-Range and Walker Lane. Geological Society of America Bulletin, v. 107, p. 474-486.

  • Gans, P. B., G. A. Mahood, and E. Schermer. (1989) Synextensional magmatism in the Basin and Range province: A case study from the eastern Great Basin. Geol. Soc. America Special Paper 233, p.53

Abstracts
  • Gans, P. B. and Mahood, G. A. (1989) The interplay between Cenozoic extension and magmatism in the Basin and Range province. IAVCEI General Assembly on Continental Magmatism, Santa Fe, New Mexico. New Mexico Bureau Mines Technology Bull., v 131, p. 101

  • Gans, P. B. (1990) Space-time patterns of Cenozoic N-S extension, N-S shortening, E-W extension, and magmatism in the Basin and Range province: Evidence for active rifting. Geological Society of America Abstracts with Programs, v. 22, p. 24.

  • Gans, P. B. (1991) Magmatically induced flow in the lower crust of the Basin and Range province and its influence on the geometry and kinematics of upper crustal extension. Proceedings from International School of Solid Earth Geophysics, 7th Course: "Modes of crustal deformation from the brittle upper crust through detachments to the ductile lower crust", Erice, Italy, pp. 39-41.

  • Gans, P. B., Darvall, P., and Faulds, J. (1992) Miocene extension in the Colorado River extensional corridor: A comparison with slow-spreading ridges. Geological Society of America Abstracts with Programs, v. 25, p. 108.

  • Gans, P. B. and Miller, E.L. (1993) Extension in the Basin and Range Province: Late orogenic collapse or something else? Proceedings, Int. Conf. on Late Orogenic Extension in Mountain Belts, Montpellier, France.

  • Faulds, J. E., Gans, P. B., and Smith, E. I. (1994)Spatial and temporal patterns of extension in the northern Colorado River extensional corridor, northwestern Arizona and southern Nevada, Geological Society of America Abstracts with Programs, v. 26, no. 2, p. 51.

  • Gans, P. B. (1996) Large-magnitude Oligo-Miocene extension in southern Sonora: Implications for the tectonic evolution of northwest Mexico, EOS Transactions, AGU, v.77 p. 641.

CURRENT RESEARCH INVESTIGATIONS ON SPACE-TIME PATTERNS

Phil's home Phil's Home