1. Fold-thrust belts
Goonyella coal mine, Australia, courtesy Steve Marshak/Scott Wilkerson

2. 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

3. Fault Related Folds Fault Bend Folds Fault Propagation Folds
Includes Trishear Folds
Decollement Buckle Folds

4. Fault Bend Folds Fault surface exists across entire cross-section
Flat-ramp-flat geometry for fault surface
Fault surface exists across entire cross-section
Fold develops above ramp where limb angles related to fault dip

5. Fault-Propagation Folds
Fault tip propagates thru cross-section
Fold develops above ramp
with uniform forelimb angles
Original FPP has forelimb with
same dip, trishear dips flatten upwards (trishear = reality)
Active axial surface
Passive axial surface
Displacement
Migration of particles

6. 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

7. Duplexes Roof thrust Floor thrust
Earth Sciences Dept., University of Leeds

10. Sawtooth Range, MT
“Shingled” imbricate fan of Paleozoic carbonates

11. Does displacement of individual sheets change along strike?
Displacement transfer, tear faults and lateral ramps (after Dahlstrom, 1970)
Does displacement of individual sheets change along strike?
Yes No

12. Map Patterns in Thrust Belts
(Fermor,
GSAB 1999)
Sinuous traces reflect folded thrusts
Branches reflect merging of faults
Oldest rocks in center of hanging wall
Units that commonly occur adjacent to faults are detachment levels
Plunge allows maps to be viewed like cross-sections
High-angle “tear faults” occur in some belts but origins/kinematics can be difficult to establish 2 3 1 4 5

13. FTB’s create topographic loads that flex the lithosphere
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; wedge-shaped 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 fine-grained, multiple regional unconformities