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

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

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

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

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

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

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