FoldThrust Belts

1
Fold-Thrust Belts
Zagros Mts., Iraq and Iran
2
Fold-Thrust Belts
Fig. 18.2, text
FTB region where upper crust shortening is
accommodated by a system of faults and related
folds
3
Fold-Thrust Belt Settings

• Andean-type margin (Andes Mts., Sevier/Laramide
  orogens)
• Accretionary prism (Taiwan)
• Collisional orogen (Appalachians, Alps, Himalaya)
• Inverted rift basins (Pyrenees between Spain
  France Uinta Mts., UT)
• Restraining bends along strike-slip faults
  (Transverse Ranges, CA)

Restraining bend
Inverted rift basin
Figs. 18.4 and 18.6, text
4
Mechanical Stratigraphy

• Geometry of FTB is a function of rock strength
  between layers (e.g., strong sandstone and
  limestone layers and weak shale, coal, and
  evaporite layers)

Faults utilize weak layers as bedding-parallel
detachments or flats Faults cut across strong
layers (ramps)
Fig. 18.3, text
5
FTB Geometry

• Thrusts have a ramp-flat geometry in cross
  sectiona consequence of thrusts moving toward
  foreland through a well-stratified section of
  sedimentary rocks

Flat
Ramp
Ramp anticline
6
FTB Geometry
Footwall geometry (thrust sheet or hanging
wall removed)
7
Features of a Thrust Sheet
Crows Nest Mt., Alberta (klippe)
8
Thrust Systems

• 1. Imbricate Fan-series of overlapping listric
  faults dipping in the same direction (e.g.,
  southern Appalachians Sevier FTB)

Sevier orogeny 150-75 Ma
9
Thrust-System Development
Break-forward sequence (break-backward
possible, but probably limited)
10
Thrust Systems

• 2. Duplex-imbricate thrusts that branch from
  lower to upper detachment (e.g., central
  Appalachians)

Floor and roof thrusts
11
Thrust Systems(central Appalachians)
Duplex
Duplex
Note repetition of strata!
Note the 2 duplexes one in the
Cambrian-Ordovician carbonates, the other in the
Devonian strata The roof thrust of the C-O
duplex forms the floor thrust for the Devonian
duplex
12
Fault-Related Folds

• Thrust movement produces all types of folds

Trishear folding
Jura Mts., Switzerland
Detachment folds
Ramp anticline
13
Map-View Geometry of FTBs

• Most FTBs are curved in map view many reasons

Fault-displacement gradient
14
Map-View Geometry of FTBs
Zagros and western Himalaya
Andes
Appalachians