Download Thrust systems: geometry and kinematics

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Talk This Thurs. 4 pm, Rm. Haury Bldg. Rm 216, "Tertiary
structural and stratigraphic evolution of the Greater Tucson
area", by Jon Spencer. Write 1 paragraph summary (+1%
extra credit)
Field trip to Silverbell Mine this Saturday: 7:45 AM to 4 PM
See me after class for information. (+1% extra credit)
midterm
exam
stats
Thrust systems: geometry and kinematics
(D&R: 319-336)
the architecture of many fold-thrust belts
"thin-skinned" deformation
Himalayas
Strain: can accommodate MAJOR shortening
Stress: s1 is horizontal
Principal stress directions?
s1 is horizontal, s3 is vertical
thrust systems generally propagate toward the
foreland
Canadian Rockies
Thrusts root into a basal decollement, below
which shortening is accommodated by a
different mechanism; decoupling between
upper and lower crust
may be highergrade rocks and
ductile shear
zones in
hinterland
hinterland
foreland
low-grade
rocks and
brittle
faults in
foreland
faults cut up-section
thrusts take advantage of preexisting planes
of weakness
In almost all cases, thrusts place older
and/or higher grade rocks on younger and/or
lower grade rocks
Example from the Argentinian Cordillera
Exshaw thrust-hangingwall flat, footwall ramp
imbricate fan
development of duplexes
duplex terminology
also- horses!
duplex: outcrop-scale
map pattern of a duplex
lateral ramps
compartmental faulting and tear faults
footwall rocks are
commonly
deformed into
synclines
Footwall syncline in the Canadian Cordillera
Footwall syncline in the Canadian Cordillera
also, triangle zones
"thick-skinned" basement-involved shortening
Colorado Plateau
monoclines may
be related to
thick-skinned
deformation
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
Younger and structurally deeper faults
lead to rotation of older faults
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
1. breached
anticline
What is it?
2. tip
lines
4. klippe
5. window
6. lateral
ramp
7. blind
thrust
8. branch
lines
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
thetas around sigma 1?
Fold and thrust belts!
Mt Kidd
Next lecture: forced folds and thrust belt
mechanics;
D&R (413-423; 336-339)
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