Outline for Today Thrust belts systems Geometry

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Outline for TodayThrust belts/
systemsGeometry!
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The architecture of many fold-thrust belts
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Himalayas
Strain can accommodate MAJOR shortening Stress
s1 is horizontal
Principal stress directions?
s1 is horizontal, s3 is vertical
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Canadian Rockies
Thrusts root into a basal decollement, below
which shortening is accommodated by a different
mechanism decoupling between upper and lower
crust
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may be higher-grade rocks and ductile shear zones
in hinterland
hinterland
foreland
low-grade rocks and brittle faults in foreland
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thrust systems generally propagate toward the
foreland
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Thrust system Geometry and Kinematics
There are 2 fundamental arrangements of
thrusts 1) Imbricate fans marked by a sole
(basal) thrust that has several branching splays
off of it - the fan opens upward, and 2)
Duplexes like an imbricate fan, but instead of
the faults splaying upward, they are cut by
another fault - the roof thrust. In this case,
the basal thrust is called the floor thrust.
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Geometry- Imbricate fans
"thin-skinned" deformation
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imbricate fan
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Geometry- Duplexes
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Development of duplexes

• Boyer Elliot (1982)

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(No Transcript)
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development of duplexes
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map pattern of a duplex
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Duplex terminology
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Duplex terminology
Horses fault-bounded slices of rock, carried
along and within the fault zone.
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Duplex Geometry

• Normal duplexes.
• 2) Antiformal duplexes.
• 3) Forwarded-dipping duplexes.

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duplex outcrop-scale
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Where is the duplex?
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What is it?
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The observation that faults do not continue
around the entire Earth suggests that they must
terminate Generally, a gradual decrease in slip
toward fault termination
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Younger and structurally deeper faults lead to
rotation of older faults
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Summary Thrust systems 1. Accommodate
significant crustal shortening 2. Basal
detachment decoupling within the crust 3. Faults
have ramp and flat geometries 4. Fault 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
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What is it?
1. breached anticline
2. tip lines
4. klippe
7. blind thrust
5. window
6. lateral ramp
8. branch lines
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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?
• Why are almost all faults dipping one way when
  rock mechanics predict equal chance for both?

Next lecture (1) thrust belt mechanics (p.
336-339)(2) Discuss fault project
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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
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What is it?
1. breached anticline
2. fault tips/ terminations
4. klippe
6. lateral ramp
7. blind thrust
8. branch lines
5. window
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• Important terminology/concepts
• hinterland vs. foreland
• foreland propagation of thrusting
• thin-skinned vs. thick-skinned deformation
• blind thrust
• duplexes, their development, and map pattern
• roof thrust, floor thrust, and horses
• basal decollement
• hinterland vs. foreland deformation
• ramp-flat thrust geometries and terminology
• klippe vs. window
• allochthonous vs. autochthonous
• imbricate fan
• lateral ramp
• tear faults
• footwall synclines
• pop-up structures and backthrusts
• fault tips
• rotation of old faults during slip along younger
  faults