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.