Publications
Walsh,
P., and Schultz-Ela, D., in press, Mechanics of graben evolution
in the Needles District of Canyonlands National Park, Utah: Geological
Society of America Bulletin
Schultz-Ela,
D., and Walsh, P., 2001, Modeling of grabens extending above evaporites
in Canyonlands National Park, Utah: Journal of Structural Geology,
v. 24, p. 247-275.
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Modeling of Extensional Systems Detaching on Evaporites
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(1)
Mechanics of graben evolution in the Needles District
of Canyonlands National Park, Utah
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.
(2)
Modeling of grabens extending above evaporites
in Canyonlands National Park, Utah
Grabens
in Canyonlands National Park, Utah, began extending above a layer
of evaporites when the Colorado River cut through the overburden.
Two-dimensional finite-element models simulate the effects of geometry
and rock properties on graben configuration and spacing. Only those
models having a progressively increasing slope or no slope mimicked
the natural upslope graben propagation. Typical rock properties
produced the most realistic fault patterns: an initial friction
angle of 31°, a cohesion of 1 MPa, and strain weakening comprising
cohesion loss and decrease of friction angle to 26°. A tensile
stress limit narrowed the grabens and reproduced the vertical upper
portion of the natural faults. The viscous salt resisted overburden
spreading and controlled its rate. Modeled grabens spread at typical
rates of 1 to 2 mm a-1, for a salt viscosity of 1´1018 Pa
s, and the entire system strained at rates from 6.0´10-14
s-1 to 0.5´10-14 s-1. The faults bounding a graben formed
nearly simultaneously at the top surface and propagated downward.
Salt rose beneath the grabens as reactive diapirs. Overburden adjacent
to the canyon flexed as salt was expelled and formed an arching
horst and graben. A corresponding horst has been found in the field.
The model results scale to larger dimensions, except for the steep
upper part of the faults. Reduced dimensions create vertical or
no faults.
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