Transcript FoldThrustBelts2016_shortx

Fold-thrust belts
Goonyella coal mine, Australia, courtesy Steve Marshak/Scott Wilkerson
Canadian Foothills and Front Ranges
•Thrust faults are low-angle and can have substantial displacement, stack above each other
•Faults tend to cut upsection toward craton, flattening at intermediate levels (e.g. shales)
•Low structural relief in external zones
•Older rocks (higher structural relief) typically uplifted in internal zones
•Many faults are folded above underlying folds
•Backthrust/triangle zone at E terminus
•Overall system is wedge-shaped above gentle basement dip toward hinterland
Fault Related Folds
Fault Bend Folds
Fault Propagation Folds
Includes Trishear Folds
Decollement Buckle Folds
 Flat-ramp-flat geometry for fault surface
Fault Bend Folds
 Fault surface exists across entire cross-section
 Fold develops above ramp where limb angles related to fault dip
Fault-Propagation Folds
Fault tip propagates thru crosssection
Fold develops above ramp
with uniform forelimb angles
Active axial surface
Original FPP has forelimb with
same dip, trishear dips flatten
upwards (trishear = reality)
Passive axial surface
Displacement
Migration of
particles
Mechanical Stratigraphy
Competent layers
•Carbonates, sandstones, basement
•Strong, stiff
•Tend to maintain layer thickness
•Ramps form across these
•Tend to fault before folding
Incompetent layers
•Shales, evaporites
•Weak, ductile
•Can “flow” into or out of fold hinges
•Detachments (“flats”) localize in these
•Tend to fold before faulting
Duplexes
Roof thrust
Floor thrust
Earth Sciences Dept., University of Leeds
Sawtooth Range, MT
“Shingled” imbricate fan of Paleozoic carbonates
Displacement transfer, tear faults and
lateral ramps
(after Dahlstrom, 1970)
Does displacement of individual
sheets change along strike?
Yes
No
Map Patterns in
Thrust Belts
1. Sinuous traces reflect folded
thrusts
(Fermor,
GSAB 1999)
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2. Branches reflect merging of faults
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3. Oldest rocks in center of hanging
wall
4. Units that commonly occur
adjacent to faults are detachment
levels
5. Plunge allows maps to be viewed
like cross-sections
6. High-angle “tear faults” occur in
some belts but origins/kinematics
can be difficult to establish
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Foreland basins
FTB’s create topographic loads that flex the lithosphere
“An elongate region of potential sediment accommodation that forms on continental
crust between a contractional orogenic belt and the adjacent craton, mainly in
response to… subduction and …the resulting fold-thrust belt.” DeCelles and Giles (1996)
DeCelles and Giles,
1996, Basin Research
Wedge-top depozone:
•Coarse-grained alluvial
and fan-delta deposits
with well-developed
growth structures
•Is part of the orogenic
wedge
Foredeep depozone:
•Increased sediment
accommodation; wedgeshaped package;
~continuous deposition,
fine- to coarse-grained
Forebulge depozone:
•Decreased sediment
accommodation;
unconformities, (poss.
major, >10Ma)
condensed sections
Back-bulge depozone
•Sediments may be
derived from both the
orogenic wedge and the
craton; typically finegrained, multiple
regional unconformities