Overthrusts, and fold and thrust belt structures

Content:

False color satellite image of Valley and Ridge in Pennsylvania. This basically a locality where in a traverse across the fold structures we see the same stratigraphy repeated over and over again as one crosses pairs of anticlines and synclines. The Valley and Ridge province is the Alleghanian foreland fold-thrust belt associated with the Appalachian orogen and is thin-skinned. Interestingly, while the fold-thrust belt is dominated by folds in this area, when traced down to the south thrust faults become better expressed. Image source: http://geomaps.wr.usgs.gov/parks/province/applandsat.html.

Reading: Chapt. 16 - Fossen, Structural Geology

Term list

 



Historical perspective on overthrusts (thrusts with a significant subhorizontal section)

While the recognition of vertical tectonism was very early (and included Leonardo de Vinci), the recognition of significant horizontal transport required detailed geologic mapping and a knowledge of the stratigraphy.

Fold and thrust belts:

Some examples:


Thrust fault geometries and map patterns

klippe and windows:

tear faults: subvertical faults with a significant strike-slip component that link thrusts.

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

backthrusts and wedge insertion.

Portion of the USGS geologic map of the Montana fold-thrust belt by Reynolds and Brandt, 2005. A lot can be learned from reading this map. The red lines represent the axial traces of folds that formed above the the sub-horizontal Black Canyon Decollement, which is the basal detachment in this frontal part of the fold-thrust belt. The imbricate splay thrusts occur to the north, and the dominant transport direction is to the the NE here. The Black Canyon Decollement is the floor thrust for both the folds and imbricate splays. The red blobs represent igneous rocks that clearly post-date the folding and thrusting.

Another portion of the same map, but more to the hinterland. The Moor Mountain Thrust fault is folded here so that one sees underlying younger strata in the underlying plate. Several small windows exist in the northern third of the map where the bluish unit is surrounded by a thrust fault with the 'teeth' on the outside and surrounding older rocks. A klippe occurs just right of the center of the image where the brownish unit is surrounded by the younger lower plate rocks. These windows and klippes are due to the interplay of folding of the thrust faults and erosion.

Cross section through the Montana fold-thrust belt showing its thin-skinned character with the basal Black Canyon detachment, a foreland dipping duplex (described below), and the structural window created by the Moors Mountain Thrust Fault. Link to full USGS map of Montana fold-thrust belt map.

large scale wedge geometry. Taper angle of the wedge is a critical descriptor. Upper surface not preserved in old and eroded wedge, and hence wedge angle not directly measurable, but in an active wedge it can be measured (more on wedges below) .

basement involved thrusts vs. thin-skinned detachments.

duplexes - all sorts of duplexes.

Cross section showing earthquake activity and thrust-fold development in the Seattle area. Note the compartmentalization (dashed lines) depicted associated with the fault bends. Image source: http://geomaps.wr.usgs.gov/pacnw/psv1/index.html .

Duplexes

Thrust ramp evident in mountain side truncating the cliff-forming, light colored strata (from North Alaska) Thrust transport is to the right in this view. Image from USGS site : http://energy.usgs.gov/arctic/ .

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

Fault duplexes:

Photo to right is of a antiformal stack delineated in the Brooks Range of Alaska, a classic fold-thrust belt. Image source is a Wikipedia site that has taken the image from a USGS source.

Fold duplexes:

Cleavage duplexes:

Animation of duplex formation from Allmendinger.

Interpreted seismic image of a backthrust as a roof thrust to the leading edge of a duplex structure in the Mackenzie Mountain front in the Canadian Cordillera. This style has been termed subcutaneous wedge insertion by some, and is common near the leading edge of some fold-thrust belts. Image source: Virtual Seismic Atlas, Adriana Taborda , Deborah Spratt , "Triangle zone model for Mackenzie mountain front for AMOCO060-91-10:, http://seismicatlas.org/entity?id=4cc4ef98-c4e7-4554-8ade-d826de2c66e9 . The Virtual Seismic Atlas is well worth a visit.

Seismic section of imbricate thrust system. The upper bounding surface here is likely not a roof thrust, but instead a buried surface, with the thrusts daylighting. This is on the continental slope in deep waters of Namibia, and represents the toe of a gravitational driven system. Image source: Virtual Seismic Atlas, Rob Butler, "A high resolution version of the thrust belt, Namibia" - http://seismicatlas.org/entity?id=8f3d5439-6dc2-4c68-9664-e6839b496277 .


Thrust kinematics

foreland propagating sequence:

out-of-sequence faults:

foreland migration of foreland basins:


Mechanical paradox of overthrusts

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 slopes is Paleozoic carbonate and underneath are Eocene basin fill sediments. the overthrust contact is semi-horizontal.


Two simple models for emplacing overthrusts, tectonic push and gravity sliding.

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 for 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 - not geologically likely. 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 or history.

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

pr g hr tan ø = (pr g hr - pw g hw) tan µ

tan ø = (1 - (pw hw/ pr hr))tan µ = (1-lambda) tan m

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, the ratio between pore pressure and the fault normal traction can approach one. Mechanisms for producing overpressures?

Analysis of gravity sliding versus tectonic push:


Wedge model and orogenic topography

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. What controls that 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.

Diagram to right shows the basic parameters for the wedge model. Note that this is for a submarine wedge (e.g. an accretionary wedge). The details for a foreland fold-thrust belt will be different, but the basic idea is the same. Image from http://en.wikipedia.org/wiki/File:Critical_taper_wedge.svg

This model has been widely applied.

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

USGS seismic section 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 .

U-tube video of development of a bivergent wedge.

Laboratory demonstration:

Fold and thrust belts versus accretionary wedges associated with subduction zones. - what are the differences and similarities?


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