Geometry and kinematics of extensional fault systems

The geometry, kinematics, and mechanics of normal faulting in continental rift regions remains an outstanding problem in structural geology and tectonics. There is as yet no consensus on such basic issues as: (1) What is the range of initial fault geometries(e.g., planar vs. listric, low-angle vs. high-angle)?, (2)How do systems of normal faults interact and evolve geometrically?, and (3) What are the driving forces for regional extension? My work has concentrated on Oligocene to Recent extensional fault systems in the eastern Great Basin and in the lower Colorado River extensional corridor. This work has involved extensive geologic mapping (approximately fifteen 7.5' quadrangles), interpretation of geophysical data, extensive high-precision 40Ar/39Ar geochronology, and uplift studies using fission track thermochronology.

A detailed study of the structural evolution and thermal history of the Kern Mountains/Deep Creek Range in east-central Nevada has illustrated for the first time how carefully designed thermochronologic transects in concert with geologic mapping and structural studies can be used to reconstruct the geometry and rates of fault slip on extensional fault systems. Continued work on the "detachment faults" in the metamorphic core complexes of east-central Nevada has led to significant revisions to our understanding of how these enigmatic low angle normal faults form and evolve. We now have unequivocal evidence that movement on these regionally extensive faults was distinctly episodic (from Eocene to mid-Miocene) and that the final "detachment fault" is a composite surface that includes several distinct segments that were active at different times. These exciting discoveries have gone a long way towards solving the mechanical paradox posed by detachment faulting.

An entirely new extensional structural style has been documented in the Eldorado Mts. region of southern Nevada - one I believe may be widespread, but overlooked. This style involves closely spaced normal faults that begin as near vertical tensile fractures and then rotate very rapidly "domino-style" to low angles during progressive extension. This structural style suggests a runaway phenomena, wherein elevated pore-fluid pressures and low differential stresses of a thin brittle crust results in localized extension fracturing and then rapid fault block rotation - a sort of large scale strain softening. A review of other areas that display this type of geology suggest it may be due to the presence of shallow magma chambers in an extensional environment.

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.

  • Miller, E. L., Gans, P. B., and J. Garing. (1983) The Snake Range decollement: An exhumed mid-Tertiary ductile-brittle transition. Tectonics, v. 2, p. 239-263.

  • Gans, P. B., E. L. Miller, J. McCarthy, and M. L. Ouldcott. (1985) Tertiary extensional faulting and evolving ductile-brittle transition zones in the northern Snake Range and vicinity: New insights from seismic data. Geology, v. 13, p. 189-193.

  • Gans, P. B., and E. L. Miller. (1985) Comment on "The Snake Range decollement interpreted as a major extensional shear zone" by John M. Bartley and Brian P. Wernicke. Tectonics, v. 4, p. 411-415.

  • Howard, K. A., Gans, P. B., John, B. E., Davis, G. A., and Anderson, J. L. (1995) A guide to Miocene extension and magmatism in the lower Colorado River region, Nevada, Arizona, and California, U.S. Geological Survey Open File Report, OF 94-0246.

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

  • Gans, P. B., Calvert, A. T., Brown, R., and Lister, G. (in preparation) Thermal evolution of extensional fault blocks. Journal of Geophysical Research
Abstracts
  • Gans, P. B. (1982) Geometry of Mid-Tertiary extensional faulting, northern Egan Range, east-central Nevada. Geological Society of America Abstracts with Programs, v. 14, p. 165.

  • Gans, P. B. (1983) Low-angle faults and extensional tectonics - insights from east-central Nevada. Geological Society of America Abstracts with Programs, v. 15, p. 403.

  • Gans, P. B., Clark, D. H., Miller, E. L., Wright, J. E., and Sutter, J. F. (1986) Structural development of the Kern Mountains and N Snake Range. Geological Society of America Abstracts with Programs, v. 18, p. 108.

  • 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., Miller, E. L., Clark, D. H., and Rodgers, D. (1990) Geologic relations in the Kern Mountains-Deep Creek Range, Nevada-Utah and the origin of metamorphic core complex detachment faults. Geological Society of America Abstracts with Programs, v. 22, p. 24.

  • Gans, P. B., Miller, E. L., Houseman, G., and Lister G. (1991) Assessing the amount, rate, and timing of tilting in normal fault blocks: A case study of tilted granites in the Kern-Deep Creek Mountains, Utah. Geological Society of America Abstracts with Programs, v. 23, p. 28.

  • 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 Landau, B. (1993) Initially vertical normal faults and runaway block rotations in the Basin and Range province: Some geological and mechanical observations, Geological Society of America Abstracts with Programs, v. 25.

  • Miller, J. M., John, B. E., and Gans, P. B. (1996) Extensional strain models evaluated by coeval sedimentary record, Colorado River extensional corridor, Geol. Soc. America Abstracts With Programs, v. 28 , p. 511.

CURRENT RESEARCH INVESTIGATIONS ON NORMAL FAULTING

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