16 foldthrustbelts 2015 full

Systems:

Fold-Thrust Belts (FTBs)

Earth Structure (2nd Edition)

W.W. Norton & Co, New York

Ben van der Pluijm

© WW Norton; unless noted otherwise

Fold-Thrust Belts © EarthStructure 24/12/2015

Fold-Thrust Belts

Mt. Kidd (Alberta)

Glarner Thrust (Swiss Alps)

Canadian Rockies (Jasper, Alb.)


Thrust-related Folds

fault-bend folds

fault-propagation fold


Fold-Thrust Belt Refresher

Crowsnest Klippe, Alberta’s

(Eocene) Lewis Thrust

Gerald Dahl


Fault Ramp Structures

Note that number of hanging-wall ramps exactly

matches the number of footwall ramps.

3D block diagram illustrating

different types of fault ramps

(hanging wall removed). Tear

faults are vertically dipping

lateral ramps.

ramp anticlines and ramp synclines


Tectonic Setting of FTBs: Continental Collision


Imbricate Fan Type

Imbricate fan: relative small

displacements.

Break-forward (“piggy-back”)

thrusting. Successively

younger thrusts cut into

footwall, and older faults and

folds become deformed by

younger structures.


Fault-propagation Fold

Fault-propagation fold in Lost River Range,

Idaho, showing asymmetric fold dying out

updip.

Progressive development of a fault-

propagation fold.


Thrust Duplex Type

Duplex: relatively

large displacements.

Flat-roofed duplex

develops by

progressive break-

forward faulting.

Roof thrust

undergoes a

sequence of folding

and unfolding.

R. Allmendinger


Fault-bend Fold

Fault-bend fold above McConnell Thrust, Alberta.

Paleozoic strata moved 5 km vertically and 40 km

horizontally, and now lie above Cretaceous

foreland basin deposits. (mirror image)

Progressive stages during development of fault-

bend fold. Dashed lines are traces of axial

surfaces.


Sevier Thrust belt

FTB Mechanics


Thrust Paradox

Sliding block:

100 x 10 x 5 km block

r = 2600 kg/m3

sn = F/area = (a.m)/area

= 9.8 x (2600x1E5x1E4x5E3)

/1E5x1E4

= 127 x 10E6 Pa = 127 MPa

sf = m x sn , m = 0.7

so σl ~90 MPa

Strength of natural rock on same

order, so fracturing at front instead of

sliding

σn is stress from loading, σf is frictional

resistance (=σs), σl is boundary load

at end of thrust sheet, PH2O is pore

pressure


Effective Friction: Fluid Pressure Scenario

sf = C + m (sn – Pf); fracture

or

sf = m (sn – Pf); friction

So, meffective = m (1 – Pf/sn)

meffective ≤ m ;

Note: Pf < s3

(a) Pushed from rear

(b) High fluid pressure at basal detachment

σn is stress from loading, σf is frictional

resistance (=σs), σl is boundary load at end of

thrust sheet, PH2O is pore pressure (Pf)


Low Friction: Lubricant Scenario

(a) Pushed from rear

(b) Low friction material at basal detachment

σn is stress from loading, σf is frictional resistance, σl is boundary load at end of

thrust sheet

Low friction

material

Sliding block:

100 x 10 x 5 km block

r = 2600 kg/m3

sn = F/area = 127 MPa

sf = m x sn

m = 0.2, so σl ~25 MPa

Strength of natural rock

greater, so sliding


Gravity-driven: Sliding and Spreading

“Dry” sliding: dip is 35-40o (angle of repose)

“Wet” sliding: l = Pf/Pl

l = 1 (Pf=Plith), dip is ~0o

l = 0.8, dip ~10o

(a) Gravity Sliding. A block slides down foreland-tilted slope.

(b) Gravity sliding partly downslope and partly upslope.

(c) Gravity spreading. Before spreading, the wedge had shape indicated by

dashed line (“warm brie”).


Thrust Wedge

Snowplow analogy:

Wedge of snow extends with

continued shortening; younger

thrusts initiate in hinterland to

foreland progression.

While new thrusts are adding

material at toe of wedge,

hinterland portions are developing

penetrative strain, normal faults

and slumps.

184/12/2015

Wedge Mechanics: Critical Taper Theory

Critical taper (angle φc) is sum of surface

slope angle (α1) and detachment slope

angle (β).

a) Stress acting on a wedge, partly

horizontal boundary load caused by

backstop (σbs) and partly caused by

gravity (σg).

b) If backstop moves, wedge thickens, so

surface slope increases, and taper (φ)

eventually exceeds φc.

c) Wedge slides toward foreland and new

material is added to toe, and extension

of wedge occurs so that surface slope

decreases.

φc

Example: Taiwan wedge


Critical taper (φc) is sum of surface

slope angle (α) and detachment slope

angle (β): 4.7o

F = meffective = st /sn = 0.1

(weak wedge)

Suppe, 2007

16 foldthrustbelts 2015 full

Documents

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foldthrust beltsmt

thrust paradoxsliding

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asymmetric fold

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