Physical analog models of basement fault
segmentation and monoclinal folding
Jenesky,
Timothy A., Tindall,
Sarah
E., and
Simpson,
Edward
L., 2007, Physical
analog
models
of
basement fault segmentation and monoclinal folding
[abs]: Rocky Mountain Section - 59th Annual Meeting (7–9 May 2007),
Geological Society of America Abstracts with Programs, Vol. 39, No. 5,
p. 37.
Debate
about the existence of reactivated basement faults underlying some or
all Colorado Plateau monoclines has raised questions about the effects
of basement fault geometry, slip gradient, and segmentation on
monocline surface expressions. Physical analog models are being
conducted to study the relationships among basement fault segmentation,
monoclinal folding, and surface strain patterns. The basic model design
involves a stiff plastic sheet containing pre-cut fault segments
overlain by alternating colored layers of dry sand. The plastic
‘basement' sheet and the sand overburden are shortened at 6.0 cm/hr by
a motor-driven moveable wall, causing the plastic sheet to both flex
and fault along the pre-existing cuts, resulting in deformation of the
overlying sand layers. Variations among experiments involve the lengths
and orientations of pre-cut fault segments, and the strike-parallel and
strike-perpendicular step-over distances between fault segments.
Following deformation, models are moistened and sliced to inspect fault
and fold patterns within the sand overburden.
Model results show that stepped basement fault segments create bends in
the surface expressions of monoclines. In most models conjugate normal
faults form along the crests of monoclines, developing earlier and
showing greater displacement above lateral basement fault steps than
above the centers of basement fault segments. Deformation of surface
markers is consistent with increased extensional strain accommodation
in step-over regions of the models. Cross sectional slices reveal
thrust faults propagating from both the hanging wall and footwall tips
of the basement faults. Surprisingly, some models display the greatest
vertical offset and the largest amplitude of monoclinal folding near
the tips of basement faults while others experience maximum
displacement and fold growth at the middle of each segment.
The normal faults observed above basement fault steps in the physical
models resemble a pattern of faulting associated with eastward bends in
the generally northerly trace of the East Kaibab monocline of southern
Utah. Further physical modeling and field studies of monoclines may
help identify specific links between surface fold and fault patterns
and underlying basement fault characteristics.
