Download More Thrust Faults

Announcements
Field trip this Saturday to Cottonwood Canyon area
7:30 AM at loading dock. We will map some really
cool stuff! Please review map symbols. We may
return after sunset.
Outline for Today
1. More about geometry and kinematics
of thrust systems
2. Forced folds
3. Mechanical "paradox" of moving
large thrust sheets
4. Thrust belt evolution: Critical Taper
theory
5. Foreland basins
6. Two examples
7. Economic applications
the architecture of many fold-thrust belts
"thin-skinned" deformation
development of duplexes
Summary
Thrust systems:
1. Accommodate significant crustal shortening
2. Basal detachment/decollement; decoupling within the crust
3. Faults have ramp and flat geometries
4. Thrusts place older/higher grade rocks over younger/lower grade rocks
5. Faults cut up-section
6. Faults generally propagate (get younger) toward the foreland
7. Younger and structurally deeper faults rotate older faults to steeper
angles
Fold and thrust belts!
Mt Kidd
Forced folds (D&R 413-423)
Free folds: fold profiles are based entirely on physicalmechanical properties of the layers
Forced folds: geometry related to movement over fault
ramps- "they just go along for the ride, and some of the
beds happen to fined themselves in awkward places and
are required to stretch or bend"
2-main types of forced folds:
fault-bend folds
fault-propagation folds
Fault-bend folds
Fault-propagation
folds
monoclines as "drape" folds
"thick-skinned" basement-involved shortening
Colorado Plateau
monoclines may
be related to
thick-skinned
deformation
Major issues
• “mechanical paradox” of thrusting - why
such thin sheets (e.g. 100 km long/2-3 km
thick) can remain intact during faulting?
• What happened to the missing basement?
• “mechanical paradox” of thrusting - why
such thin sheets (e.g. 100 km long/2-3 km
thick) can remain intact during faulting?
Recall Byerlee's Law
Question: How much shear stress is needed to cause movement
along a preexisting fracture surface, subjected to a certain normal
stress?
sc = tanf(sN), where tanf is the coefficient of sliding friction
sc = tanf(sN), where tanf is the coefficient of
sliding friction
Possible explanation- water pressure plays a big role
sc = tanf(*sN), where tanf is the coefficient of
sliding friction and *sN = sN – fluid pressure
What drives a thrust belt??
Old timers thought that decollements beneath
thrust belts dipped away from the elevated
hinterland- and therefore gravity "sliding" was
the main mechanism
But now we know that decollements to thrust belts dip
toward the hinterland. Thrust belts move uphill!
Elevated fluid pressure certainly decreases the
stress required to move a thrust belt.
Gravitational stresses due to elevated
topography also aids sliding.
BUT, a push from the rear is still necessary
Critical Taper
Thrusts belts are
wedge shapedcharacterized by a
topographic slope
(a) and a
decollement dip (b)
Only at some
critical angle (a+b),
will the thrust belt
propagate
The critical taper
angle is controlled
by the coefficient of
friction along the
decollement and the
frictional sliding
strength of the rock
EPISODIC
propagation
Thrust belts
create
topographic
loads that flex
the lithosphere
like a person on a
diving board-
foreland basins!
Important terminology/concepts
role of elevated pore fluid pressure in movement
of thrust sheets
Critical taper theory / wedge theory
foreland basin development
Example 1: Tibet
Geographic Setting
Regional Geologic Setting
Geometry
Footwall rocks include high-pressure
blueschists that formed at depths of >35 km!
Tectonic significance
Example 2: Canadian Cordillera
Roche Ronde
Boulle Range
Roche Ronde
Boulle
Range
Cross-sections
• Thrust faults cut up-section only! (or
section-parallel)
• Every flat or ramp in the FW should
correspond to an equivalent flat or
ramp in the WH
• Bed thickness is preserved
(conservation of volume and mass)
Other than that - it’s all interpretation!
Roche Ronde
Boulle
Range
Relevance to oil exploration
On Thursday:
Normal faults