Stress, faults, and folds - Tectonic Geomorphology - page 110

A B C

Fault-Bend
Fold

Fault-Propagation
Fold

Displacement-Gradient
Fold

constant area; constant bed length constant area; changing bed length

constant area; constant bed length

continuous forelimb rotation

i

i

i forelimb angle established

forelimb angle established

ii

ii

changing limb length

synfolding deposition

synfolding deposition

ii

iii

iii

continuous forelimb rotation

note constant forelimb angle

forelimb translation without folding

iii

iv

iv

E

D

Detachment Fold ii. Trishear Geometry ii. Trishear Fold

velocity vectors

growth triangle trishear zone

changing limb length & forelimb angle

hanging wall

velocity of hangingwall
r elative t o footwall

trishear
zone tip line

footwall (fixed)

Fig. 4.36 Models for folds associated with thrust faults.
Each fold is depicted where uplift of the anticlinal crest is slower than the rate of sediment accumulation in the unfolded regions off the flanks of the folds.
This difference in rates creates growth strata that record the development of the fold. Note that the final shape of the deformed bedrock can be very similar
among these different models, but that the geometry of the growth strata differs markedly among them. Reliable reconstruction of step-by-step fold growth is
commonly dependent on analyzing associated growth strata. A. Fault-bend fold. B. Fault-propagation fold. C. Displacement-gradient fold. D. Detachment fold.
E. Trishear fold: (i) geometry of the deforming region of a trishear fold, showing rotation of slip vectors and change in magnitude from the top of the trishear
zone to its base; (ii) model of trishear fold and growth strata above a thrust. Modified after Wickham (1995) and Allmendinger (1998).