Interplay between extension and magmatism

This is an ongoing investigation aimed at understanding the interplay between magmatic and extensional processes in such places as the Basin and Range province, the Arctic Alsaka terrane, and on the Tibetan plateau. My work in this field together with my students, post-docs, and outside collaborators has involved extensive geologic mapping and sample collection, petrography, chemical and isotopic analyses, and 40Ar/39Ar dating as well as some numerical modeling. Our work in a number of areas in the Basin and Range provincehas demonstrated a close spatial and temporal association between extensional faulting and volcanism. A typical progression of tectonic-magmatic events consists of:

1. Widespread eruptions of mafic to intermediate volcanic rocks prior to the onset of the major episode of extension. These early eruptions show clear evidence for mixing between a mantle-derived (basaltic) end-member and silicic crustal melts, though the mantle source region(s) for the basalts remain an important unresolved question.

2. The onset of rapid, large-magnitude extension, often coinciding with and immediately following voluminous silicic eruptions. Large extensional strains appear to be concentrated within the immediate vicinity of the major eruptive centers, and are characterized by closely spaced normal faults and large block rotations. Strain rates of > 1 cm/year within these areas have been documented.

3. Abrupt waning of volcanic activity once extension begins.

These relations strongly support an "active" rifting model, wherein a flux of mantle-derived basalt into the crust and accumulation of silicic magma into large shallow chambers thermally weakened the crust and thereby focused upper crustal strains within active magmatic centers. Once extension began, a positive feedback situation developed wherein faulting and thinning of the brittle layer decompressed mid-crustal magma chambers leading to catastrophic eruptions.

This general evolutionary history is nowhere better documented than in the Eldorado Mountains of southern Nevada. My work here has demonstrated that after 4 Ma of precursor eruptions, the area was rapidly extended by a factor of > 2.0 in less than 1 million years, and this extension immediately followed the caldera-forming eruption of a voluminous ignimbrite. Large-scale eastward tilting associated with extension has exposed the normal faults as well as the magmatic system from the paleo ground surface to mid crustal depths. Our current NSF proposal for this area will continue our work on magmatic tectonic interactions and look specifically at the petrogenesis of the mafic lavas as a consequence of progressive extension, at fault-dike interaction in the brittle crust, and at the subsurface geometry of a large silicic caldera system formed in an extensional envoronment.

SELECTED PUBLICATIONS AND ABSTRACTS

  • Gans, P. B., and E. L. Miller. (1983) Style of mid-Tertiary extension in east-central Nevada. Utah Geol. and Mineral Survey Special Studies 59, p. 107-160.

  • Gans, P. B. (1987) An open-system, two-layer crustal stretching model for the eastern Great Basin. Tectonics, v. 6, p. 1-12.

  • 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, 53 p.

  • Miller, E. L., Gans, P. B., J. E. Wright, and J. F. Sutter. (1989) Metamorphic history of the east-central Basin and Range province: Tectonic setting and relationship to magmatism. In Ernst, W. G., ed., Metamorphism and Crustal Evolution, Western Coterminous United States, Rubey Volume VII: Englewood Cliffs, New Jersey, Prentice-Hall, p. 649-682.

  • Dumitru, T. A., Gans, P. B., Foster, D. A., and Miller, E. L. (1991) Refrigeration of the western Cordilleran lithosphere during Laramide shallow angle subduction. Geology, v.Ê19, p. 1145-1149.

  • Amato, J. M, Wright, J. E., Gans, P. B., and Miller, E. L. (1994) Magmatically induced metamorphism and deformation in the Kigluaik gneiss dome, Seward Peninsula, Alaska. Tectonics, v 13, p. 515-527.

  • Faulds, J. E., Feuerbach, D. L., Reagon, M. K., Metcalf, R. V., Gans, P. B., and Walker, J. D. (1995) The Mt. Perkins block, northwestern Arizona: An exposed cross section of an evolving, pre- to syn-extensional magmatic system, Journal of Geophysical Research, v. 100, p. 15,249-15,266.

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

  • 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

Abstracts
  • 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., Miller, E. L., and Lee, J. (1993) The Mesozoic-Cenozoic tectonic evolution of the eastern Great Basin: A revised history, Eos Transactions, AGU, v. 74, p. 608.

  • 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.

  • Hopson, C. A., Gans, P. B., Baer, E., Blythe, A., Calvert, A., and Pinnow, J. (1994) Spirit Mountain pluton, southern Nevada: A progress report, Geological Society of America Abstracts with Programs, v. 26, n. 2, p. 60.

  • 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., Landau, B., and Darvall, P. (1994) Ashes, ashes, all fall down: Caldera-forming eruptions and extensional collapse of the Eldorado Mountains, southern Nevada, Geological Society of America Abstracts with Programs, v. 26, no. 2, p. 53.

  • 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 EXTENSION AND MAGMATISM

  • Cenozoic extension and magmatism in Sonora, Mexico
  • The lower Colorado River extensional corridor - a failed Miocene rift
  • Cretaceous - early Tertiary evolution of Arctic Alaska and Northeast Russia
  • Origin if gneiss domes in southern Tibet
  • Cenozoic-Mesozoic evolution of the eastern Great Basin

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