Lindley Hanson's List: Colorado Plateau: Parks and Geology

Geologic Provinces of the United States: Colorado Plateau Province

The sculptured beauty and brilliant colors of the Colorado Plateau's sedimentary rock layers have captured the imaginations of countless geologists. This is a vast region of plateaus, mesas, and deep canyons whose walls expose rocks ranging in age from billions to just a few hundred years old.

Geology; November 2008; v. 36; no. 11; p. 835-838; DOI: 10.1130/G25032A.1 2008 Geological Society of America

Model for tectonically driven incision of the younger than 6 Ma Grand Canyon
Karl E. Karlstrom1,*, Ryan Crow1, L.J. Crossey1, D. Coblentz2 and J. W. Van Wijk2

1 1Department of Earth and Planetary Sciences, University of New Mexico, Albuquerque, New Mexico, 87131, USA
2 2Geodynamics Group, Los Alamos National Laboratory, Los Alamos, New Mexico, 87545, USA

Correspondence: *E-mail: kek1@unm.edu.

Accurate models for the incision of the Grand Canyon must include characterization of tectonic influences on incision dynamics such as active faulting and mantle to surface fluid interconnections. These young tectonic features support other geologic data that indicate that the Grand Canyon has been carved in the past 6 Ma. New U-Pb dates on speleothems are reinterpreted here in terms of improved geologic constraints and understanding of the modern aquifer. The combined data suggest that Grand Canyon incision rates have been relatively steady since 3–4 Ma. Differences in rates in the eastern (175–250 m/Ma) and western (50–80 m/Ma) Grand Canyon are explained by Neogene fault block uplift across the Toroweap-Hurricane system. Mantle tomography shows an abrupt step in mantle velocities near the Colorado Plateau edge, and geodynamic modeling suggests that upwelling asthenosphere is driving uplift of the Colorado Plateau margin relative to the Basin and Range. Our model for dynamic surface uplift in the past 6 Ma contrasts with the notion of passive incision of the Grand Canyon due solely to river integration and geomorphic response to base-level fall.

SA Bulletin; March 2003; v. 115; no. 3; p. 259-270; DOI: 10.1130/0016-7606(2003)115<0259:MOGEIC>2.0.CO;2
© 2003 Geological Society of America

Mechanics of graben evolution in Canyonlands National Park, Utah
P. Walsh{dagger},1 and D.D. Schultz-Ela{ddagger},1

1 Bureau of Economic Geology, The University of Texas at Austin, University Station, Box X, Austin, Texas 78713–8924, USA

Results of numerical models and field observations of regularly spaced grabens in Canyonlands National Park, Utah, demonstrate that salt flow beneath a brittle overburden accommodated recent and ongoing westward gravity spreading. Erosion of the Colorado River canyon differentially loaded the underlying viscous salt. In our models, the overlying brittle strata flexed downward toward the canyon, initiating faults near the surface that propagated downward toward the salt contact. Modeled grabens developed sequentially away from the canyon (eastward) as salt was expelled from beneath undeformed strata. After their eastern boundary faults broke through, horst blocks tilted in the opposite direction of initial flexure, resulting in increased symmetry of older grabens closer to the canyon. Continued extension formed a reactive diapir beneath each graben.

Field observations show that multiple faults bound grabens, indicating reactive diapirs beneath them. Topographic profiles and surveyed points along a stratigraphic layer show that horst blocks subsided as salt migrated toward the river canyon and into the diapirs. Field data from less evolved horsts imply that individual horst blocks responded to differential loading by progressive flexure and tilt, similar to the models. Horst-block flexures also vary along strike, and localized folds and faults formed where fault displacement changes abruptly.

Key Words: Canyonlands National Park • extension • faults • grabens • numerical models • salt tectonics

In the late nineteenth century, the American Southwest
emerged as one of the most important regions for geological
study in the world. When geologists first encountered the
canyon country of the Colorado Plateau, and the Grand
Canyon in particular, they lacked vocabulary, methodology,
and theory to describe and analyze the myriad stratigraphic
and structural features and geomorphological processes that
confronted them. Consequently,