Reading:

Chapt. 18 - Fold-thrust belts in Earth Structure, 2nd edition, van der Pluijm and Marshak, p. 444-470.

Term list

• thin-skinned overthrust style
• klippe and windows
• Heart Mntn. thrust in Wyoming
• examples of fold-thrust belts
• mechanical enigma and solution to large overthrusts
• ramps and flats
• detachments and decollements
• ramp antiforms (fault bend folds)
• oblique ramps and tear faults
• wedge geometry and dynamics (critical taper angle)
• foreland and hinterland
• foreland propagation
• duplexes
• out-of sequence thrusts
• backthrusts
• fault propagation folds
• balanced cross sections

Historical perspective on overthrusts:

• 1841 - Arnold Escher Van der Linth desribed Glarus thrust in Switzerland and gave it double fold interpretation, in order to minimize required movement - still required 15 km of transport on a subhorizontal surface.
• Marcel Bertrand - reinterpretation as on N directed thrust with a minimum of 30 km of movement.
• Many other similar structures soon documented.
• Overthrusts: sheets emplaced on subhorizontal surfaces.
• Gretner (reference on handout) gives vital statistics:
  • 3-6 km thick, up to 150 km wide, 10s of km of transport.
  • this does not included hinterland crystalline thrust sheets which are often larger (e.g. the Appalachians Piedmont thrust sheet which is > 10 km thick

Fold and thrust belts - typically found in orogenic forelands or in rear-arc position. There are exceptions.

Some examples:

• Valley and Ridge, Appalachians and Ouachitas - formed ca. 320-275 Ma.
• southern Canadian Rockies - formed ca. 80 Ma.
• Spitsbergen fold-thrust belt - formed 60-40 Ma.
• western Taiwan, actively forming.

Fold and thrust belts versus accretionary wedges - what are the differences and similarities.?

klippe and windows:

• erosional: Heart Mountain a classic example
• structural: due to antiformal stacks and folding of thrusts by underlying foreland propagation.

tear faults: strike-slip faults that link thrusts.

oblique and lateral ramps: strike of ramp is at a significant angle to the transport direction

backthrusts and wedge insertion.

large scale wedge geometry. Taper angle of the wedge is a critical descriptor.

basement involved thrusts vs. thin-skinned detachments.

duplexes - all sorts of duplexes.

Duplexes are mechanisms by which slip is transferred from one detachment horizon to another.

Fault duplexes:

• parts: flats, ramps, roof thrust, floor thrust, horses.
• hinterland dipping duplex.
• antiformal stack: characterized by folded thrusts.
• foreland dipping duplex.
• horse transport (ht) versus horse width (hw):
  • if ht < hw then get hinterland dipping duplex.
  • if ht = get antiformal stack.
  • if ht >> hw then get hinterland dipping duplex.

Fold duplexes:

• note decreasing displacement gradient towards tip lines.
• with either fault propagation folds or with detachment folds.

Cleavage duplexes: likely a lower strain rate type of duplex.

Animation of duplex formation from Allmendinger.

foreland propagating sequence:

• seen in laboratory models and in active fold and thrust belts.
• basically a history of footwall plucking.
• most common type of history.

out-of-sequence faults:

• suggest change in dynamics of fold-thrust belt.
• recognized by decapitated structures.

migration of foreland basins:

• foreland basins form due to flexural loading and isostatic depression of the foreland.
• as wedge tip propoagates basin depocentre migrates.

cross section balancing: already addressed.

An introduction to the paradox - Heart Mountain, Wyoming:

To start out with it is important to realize that this is not typical, but a highly unusual situation.

In the view above of Heart Mountain above the unit forming the steeper slops is Paleozoic carbonate and underneath are Eocence basin fill sediments. the overthrust contact is semi-horizontal.

1) tectonic push - simple model of boundary force applied to rear of rectangular thrust sheet.

For a given h the normal stress is constant and independent of Ft, but the greater the block length the greater Ft needed to produce a high enough shear stress for slip to occur. The higher Ft the higher internal stresses in the block produced by Ft. Critical limit: stress within block > rocks internal strength, block will break up internally, i.e. there is a maximum size of block you can push from behind. Computed reasonable values - 1 km thick by 8 km wide (in transport direction) or 5 km thick by 18.4 km. Actual thrust sheets exceed notably the restrictions imposed by this model.

2) gravitational sliding - must be overall downslope dip to detachment. Thrusting might then be a response to vertical uplift. Question - what slope is necessary for gravitational sliding?

S will be symbol for stress traction.

Sn = p g h' cos (ø), where ø is slope angle, and p is rock density and h' is vertical thickness of thrust slab.

h' = h/cosø so Sn = pgh, where h is vertical thickness of slab.

St = p g h' sin ø = p g h tan ø, where St is the critical shear stress needed fro slip.

St = K + Sn tan µ , where µ is failure envelope slope angle and K is internal strength

if K close to 0 then we have p g h tan ø = p g h tan µ

µ = 10-45°, so these are appropriate slope angles also.

If overthrust transport has been 30 km then a 5.3 km high source is needed - preposterous. Not only that, but need to produce uplift, gravity sliding, and then tilting to remove the original dip of the slide surface. No evidence for such uplifts.

Possible resolution of paradox - Hubbert and Rubey (1959):

Key is the role of internal fluid, effective stress. Pore pressure will negate normal stress and will not effect shear stresses

where lambda is the ratio of pore pressure to lithostatic pressure.

Implication is that by reducing the normal stress on the shear plane, the shear stress necessary for failure is reduced. In turn the tectonic push or the slope required is also reduced.

Lambda can approach one. Mechanisms for producing overpressures?

• need subhorizontal permeability barrier.
• compaction and dewatering associated with burial and diagenesis
• tectonic loading

Analysis of gravity sliding versus tectonic push:

• mechanism for providing uplift? - batholithic intrusion, utilized for Wyoming fold and thrust belt with the Idaho batholith as the cause.
• major difficulty - most thrust systems have a major detachment that roots at depth and not into the ground.
• tectonic push provided by plate convergence: some thought needed on actually what are the pushing or buttressing elements, but plate tectonics , with its dominance of horizontal motions, naturally incorporated the tectonic push model.

On a large scale accretionary wedges within subduction zones, and fold-and-thrust belts in collision zones can be viewed as having a wedge geometry.

The basis of a theoretical model: Mohr-Coloumb failure criteria with pore pressure considered applied to a wedge in front of a bulldozing element.

• Critical taper angle -"wedge is on the verge of failure under horizontal compression everywhere, including the basal decollement." Davis, Suppe, and Dahlen.
• 3 possible states:
  • a) taper angle less than critical angle = (too thin) internal failure of wedge material and increase in taper angle. Likely associated with out-of-sequence thrusting.
  • b) taper angle equal to critical angle, constant taper. Underplating along the length of the decollement adds material, or wedge slides stably (bending underlying platform) without accretion. May be characteristic of some submarine wedges associated with subduction. Do wedges spend any appreciable time in equilibirum.
  • c) taper angle greater than critical angle = internal extension within the wedge (Platt), a form of gravitational spreading, or, if basal decollement configuration can change, foreland propogation. In case of accretionary wedge, oceanic crust-sediment boundary controls the level of the basal decollement. Super critical state would be expected to produce foreland propagation.
• What is bulldozing element? thickened crust, arc terrane, accreted terrane, crystalline basement
• Applicability extent ? once decollement greater than 15 km depth no longer obeys Mohr-Coloumb failure criteria.
• Actual taper angles - plots in Suppe, range from 4-10 degrees.
• What pertubates the system?
  • erosion and sedimentation: redistribution of load and change in taper angle.
  • change in strength: evolving fluid pressures, development of deformation fabrics (strain weakening), change of material becoming involved.
  • change in convergence rate.
  • magmatic activity.
• Along strike changes in taper angle?

This model has been widely applied.

Spreading wedges - the Absoraka volcanic pile and the Heart Mountain detachment. Other examples of gravitational collapse.

USGS seismic cection with interpretation from the Gulfo of Penas, along the Andean trench showing the accretionary wedge associated with subduction. Image source: http://walrus.wr.usgs.gov/research/sopac.html .

Laboratory demonstration:

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