Fold and thrust belt occurrence and origin

Occurrence

Orogenic belts of folded sedimentary rock are often found near the edges of orogens, where the layers are duplicated by thrust faults.A belt of unmetamorphosed rocks exposed in a belt that may be hundreds or thousands of kilometers wide typically has folds and reverse faults.

Classic examples occur in:

The Valley and Ridge province of the Appalachians. The Subalpine belt and the Jura Mountains in the Alps. The western part of Taiwan, a fold and thrust belt that is still extremely active and is very earthquake prone

Rock units involved

"Foreland fold and thrust belts" typically involve shelf rocks or a former continental margin (or miogeocline).

They often also involve strata of the early part of the foreland basin that have later been incorporated into the orogen.

Some thrust belts (e.g. the Rockies) involve only the sedimentary cover, but many (e.g. Alps) involve basement metamorphic or igneous rocks, brought to the surface on steeper reverse faults.

Thin-skinned
Thick-skinned

.None is preserved in the Rockies.

Foreland basins

There are some orogens in which sedimentation continues even after folding and thrusting has begun.An influx of immature clastic sediment accumulates in a foreland basin ahead of the thrust sheets.

Geometry

In and adjacent to the external zones of many orogens are sedimentary basins with characteristic form.

History

The history of the foreland basin is best envisaged by imagining the orogen as fixed; the basin moves towards it. Any given point experiences:

Sediment fill

The sediment fill may or may not fill the foreland basin to sea level.

underfilledflyschmolasse

Thrust faults

Fault surfaces

Any low-angle reverse fault is called a thrust fault or simply a thrust. There is no formal definition of what constitutes "low-angle", but most faults that are called thrusts have dip into a steep orientation , or into a flat-lying orientation where the hangingwall has actually moved down (technically a low-angle normal fault)

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In either case, such faults are still called thrusts in most published descriptions, even though they would technically be classified otherwise. Notice that for the most part, such faults still cut bedding at low angles.

There is an alternative definition of thrust (that avoids these ambiguities): A thrust is a fault that puts older rocks on top of younger. This definition works well in arturkpopmuzigi.com that had undeformed stratigraphy in prior to shortening (like the Rockies), but breaks down if there are prexisting steep structures like folds or faults.

Hanging wall, footwall

We can distinguish the hanging wall (above the fault plane) from the footwall (below).

klippeklippenfenster

Ramps and flats

Many thrust faults are non-planar. Most commonly, steeper and more gently dipping segments are present.

Ramps are regions on thrust faults where stratigraphy is truncated at relatively steep angles (typically > 10°)Flats are regions where thrust faults do not cut stratigraphy, or where the strata are cut at a very low angle.

Typically, ramps and flats correlate with stratigraphy.

Massive, competent units usually correspond to ramps Shaly, incompetent units contain thrusts that stay in one unit over large arturkpopmuzigi.com.

Regional flats are known as décollement surfaces. Several stratigraphic units in foothills are décollements.

Notice that although the thrust changes from ramp orientation to flat orientation, the overlying hanging wall strata are may be more or less parallel to the thrust. Hence we need definitions for ramps and flats that make clear whether we are talking about the hanging wall or the footwall. We distinguish

Pine Mountain thrust in the Appalachians shows examples of all four.

Frontal, lateral and oblique ramps, tear faults

Ramps need not be oriented parallel to the overall strike of the orogen.

frontal ramplateral rampoblique ramp

Lateral ramps can be quite steep. Direction of thrusting is typically parallel to the strike, so hangingwall block does not have to climb up the dip slope.

tear faults

Cutoff points and lines

Where thrust faults cut stratigraphic boundaries there are cutoff points in cross-section. Typically, for a given stratigraphic horizon, we can identify a footwall cutoff point and a hanging wall cutoff point. The separation between them is evidence for the amount of displacement - though we need to know the slip direction to be precise about this.

In 3-D each cutoff point corresponds ot a cutoff line: the line of intersection between a bedding plane and a fault.

Fault tips and tip lines

Sometimes thrusts lose displacement in the direction of transport. Typically this occurs when the hanging wall is folded or otherwise internally deformed but the footwall is not. Eventually, the offset may fall to zero.

tip. tip line. blind thrust.

Faults can die out laterally too, producing a lateral tip line where offset dies to zero. Typically another thrust takes up the offset as in the diagram.

Geometry and kinematics of thrust-related folds

Symmetry relationships of folds and thrusts

In many thrust belts the folds are asymmetric (S-folds or Z folds), and also show vergence toward the foreland. In this case we recognize

backlimbsforelimbs:

Early descriptions of fold and thrust belts adopted a terminology in which thrust were described in relation to folds

This is a purely geometric classification. It suffers from the disadvantage that the classification of a given thrust depends on the erosion level. A backlimb thust might pass into an out-of-the-syncline or forelimb position as it is traced up or down-dip.

Kinematic fold models

More recently another type of classification of folds in thrust belts has been developed, in which folds are related to thrusts (rather than the other way round). This classification is not purely geometrical; it has a relationship to thrust kinematics which we will explore more fully in the next section.

Folds are almost always associated with thrusts and we can recognize a number of different fold styles that are characteristic of thrust belts. Thrust belts almost always combine ductile strains with brittle faulting, and most analyses of thrust-related folding require some assumptions about the type of strain. Some common assumptions are:

In addition, many fold models assume:

We will meet these assumptions again when we consider section balancing later in this unit.

Because of these assumptions, it is turkpopmuzigi.comiest to construct diagrams of thrust-related folds using a kink-fold geometry. Kink folds are characterized by:

Note that although this construction is a matter of convenience, real folds do not necessarily (or even commonly) conform to this geometry. Nonetheless, a curved fold may often be approximated by a large number of kink-like hinges.

Fault bend folds are folds formed at bends in fault surfaces, typically changes from ramp to flat or vice versa. When movement occurs on a curved fault, rotation and/or distortion of the wall rocks must occur to preserve strain compatibility. Typically, for a large-scale fault with ramps and flats, the hangingwall is folded to accommodate to the shape of the footwall ramps.

The conspicuous fold in the hanging wall McConnel thrust at Mount Yamnuska is probably an example of a fault-bend fold.

Idealized animations of folds

Because ramps typically dip at ϕ = tan − 1 < − sin ( b − θ ) < sin ( 2 b − θ ) − sin θ > cos ( b − θ ) < sin ( 2 b − θ ) − sin θ > − sin b >

but if we assume a simple geometry where one part of the fault is a true flat then the relationship is simpler

φ = θ = tan − 1 ( sin 2 γ 2 cos 2 γ + 1 )

Detachment folds

Detachment folds are folds formed above a flat or décollement surface, to accommodate a change in slip. Some examples are particularly conspicuous in cross-sections of the Jura Mountains, in the thrust belt of the Swiss Alps, where the underlying décollement is located in a layer of Triassic evaporites.

Fault propagation folds

Fault propagation folds occur at ramps, as the tip of a fault climbs up a ramp during fault propagation. Strata at the base of the ramp are shortened by thrusting. Strata above the tip of the fault are shortened entirely by folding. In between, strata are shortened by a combination of folding and faulting.

Animation

Once again, it"s possible to predict the inter-limb angle of a fault propagation fold based on the angle of the ramp, subject to general "balancing" assumptions and kink geometry.

2 sec θ − cot θ = < 1 − 2 cos 2 γ sin 2 γ >

It is possible for the fault tip to propagate beyond the area of folding, leaving the fold truncated in the hangingwall.

There may be a continuum of geometries between "classic" detachment and "classic" fault-propagation folds.

Arrays of thrust faults

Thrusts do not occur alone. There are a many of characteristic geometries of multiple thrust faults. Because they are much studied in oil exploration, there is a great variety of terms surrounding thrust fault arrays.

Imbricate thrusts

In general, thrust faults that have en echelon arrangement in cross-section are described as imbricate.

A series of imbricate thrust faults that branch out of a single, deeper "floor" thrust is known as an imbricate fan.

The points on a cross-section where the traces of two thrusts meet is called a branch point.

In 3-D each branch point corresponds to a branch line.

The branch points at the floor of an imbricate fan, where two thrusts branch when traced toward the foreland, are called trailing branch points.

Sometimes we can find other branch points where two thrusts join into one as they are traced toward the foreland. These are called leading branch points.

In 3-D we recognize leading and trailing branch lines.

Duplexes

Where a series of thrusts connects both with a floor thrust below and a roof thrust above, the configuration is known as a duplex. The individual thrust-bounded slices in a duplex are called horses.

Photo of outcrop-scale duplex at Crowsnest Pass

There are multiple trailing branch points at the floor of a duplex, at the hinterland end of each horse.

At the roof, thrusts join as they are traced toward the foreland; the foreland limit of each horse is marked by a leading branch point.

Some duplexes exhibit very regular geometry, but in others the amount of offset between horses varies greatly. In some cases the horses are piled on top of each other to form an antiformal stack. If the displacement is higher for higher horses, they may have moved right over the antiformal stack to produce a foreland dipping duplex.

Triangle zones and tectonic wedges

In the above examples all the thrusts have the same sense of offset (e.g. west-over-turkpopmuzigi.comt), but there are arturkpopmuzigi.com in thrust belts where thrusts of opposite sense - conjugate thrusts - are developed.

Foreland-vergent thrustsHinterland vergent thrusts

A tectonic wedge is a body of rock that has moved between a pair of oppositely vergent thrusts.

A triangle zone is a wedge in which a third, foreland-vergent thrust completes a triangle in cross section.There is a particularly prominent triangle zone at the turkpopmuzigi.comtern margin of the Alberta foothills. Cross section of triangle zone at Oldman River

Passive roof duplex - a duplex that fills a tectonic wedge. The prominent duplex of Belly River sandstone (yellow) has roof and floor thrusts with opposite vergence; the duplex has thickened the contents of the triangle zone while simply raising its roof.

Also, note that the passive roof duplex becomes very thin towards the turkpopmuzigi.comtern tip of the triangle zone. This type of thin passive-roof structure is sometimes called an intercutaneous wedge.

Fold and thrust belt kinematics

Rules for thrust development

Several rules for thrust kinematics have been proposed, based on early experience with petroleum exploration particularly in the Cordilleran foothills. These rules are variously known as "Dahlstrom"s rules" or "foothills rules". Although these rules are much quoted, many exceptions are known, so they must be used with caution.

Thrusts cut up-section in the direction of transportThrusts place older over younger rocksHigher thrusts, closer to hinterland, develop first; later thrusts form progressively lower, closer to foreland

There are many exceptions to rule 3. Thrusting that obeys rule 3 is knows as "in-sequence" thrusting. If a thrust develops above an already existing thrust it is an "out of sequence thrust"

Constraints on thrust direction and distance

.Our classroom time may not allow us to cover this in the lectures.

In many method of kinematic analysis, a cross-section parallel to the transport line is required.How do we determine the direction the transport takes?

The ideal would be this.We can precisely calculate the displacement vector if a linear feature penetrates the thrust in both the wing wall and footwall.This linear feature could be:

These features, as well as slickenlines on fault surfaces, chatter marks, and Riedel shears, can be used to show direction.A feature of this type shows only part of the deformation history; if slip direction varies, it can be misleading.

.The usual method is to plot poles to bedding using an equal area projection.

This method is subject to uncertainty if there are many lateral ramps or other oblique structures in the belt.

In flat-lying stratigraphy, thrusts always cut up section in the direction of transport.This also applies to oblique ramps.

Constraints on thrust distance

Note: this section is mostly a recap of material covered in turkpopmuzigi.com 233 and many elementary turkpopmuzigi.com on structure. We may skip this in the lectures if we are short of time.

If transport direction is known, other criteria can indicate the distance that a thrust sheet has moved.

The most reliable method is to match corresponding cutoff lines on ramps in the hanging wall and footwall.

It"s sometimes also possible to match the ramps and flats themselves between hanging wall and footwall.

Sinuosities in the thrust front, fenster, and klippen

Sinuosities in the thrust front, together with fenster and klippen, may provide a minimum distance of thrust displacement

Balancing sections

Balancing concept

In many thrust belts, conventional cross-section construction cannot constrain the deep structure. For any set of field and well observations, there are many solutions to the subsurface structure. To supplement traditional methods, we use section balancing to constrain our models. This is a set of techniques designed to weed out "wrong" answers.

A balanced cross-section is actually a pair of cross-sections:

Section balancing is typically an iterative process, involving trial-and-error modification of both sections.

To balance a section, we need to be able to make the following assumptions:

Transport directionStrain compatibilityPlane strain

The net effect of these assumptions is that each unit must have the same area in the present-day cross-section and in the restored section (area balancing).

In addition, we may make the following additional assumptions where warranted by the overall structural style:

template

When trying a section for the first time, you will typically encounter some inconsistencies at step 4.The process will be repeated until all inconsistencies are resolved.

Adding line-length balancing to folds is needed when they are parallel or kinked.Thicknesses remain constant in these situations.It is, therefore, only necessary to maintain bed-length to maintain area (because in cross-section the area of a layer equals thickness x bed-length).

.In light of this line, bed-lengths can be measured.

A marker line perpendicular to the bedding is sometimes useful in thrust sheets as well.This is known as a loose line.To monitor thrust sheets, loose-lines are used.

In general, for a section to be regarded as viable, loose-lines in long, tabular panels of strata should stay straight. If a loose-line has to be deformed, the sense of shear should be constant through the entire thrust unit.

If folds are not parallel or kink-style, and bed-thicknesses vary, then it is necessary to use area-balancing. This involves more tedious calculations and is less highly constraining on the final section. We mturkpopmuzigi.comure the area of units on the deformed section, and draw equivalent arturkpopmuzigi.com on the restored section.

Some features to check

Note that the fact that a section balances doesn"t mean it"s right. There may be several viable solutions to the same data.

In particular, kink-style section constructions have become very popular because they are relatively turkpopmuzigi.comy to balance. Line-length balancing around folds works "automatically" in such sections. However, this does not make these sections correct. Remember that any simplified geometry is likely to be no more than a convenient approximation to the truth!

Dynamics of thrust sheet motion

The dynamics of thrust sheet motion is one of the has been a concern to structural geologists ever since major thrust sheets were first described in Europe in the 19th century. Examples in the Alps, Caledonides of Scotland showed distances of overthrusting of 10s of km. Some Scandinavian examples showed 100s of km, based on klippen and fenster, and yet thrust sheets are often only a kilometre or two thick.

It"s not mechanically possible to push a sheet of these dimensions. In order for a sheet to move up an inclined surface, sufficient force has to be applied to overcome:

You can calculate both types of force.Generally, it can be shown that if enough force is applied to the back of a block to overcome friction at the base, the block will then fracture or crumple right where the force was applied.The emplacement of large thrust sheets appeared to be a major mechanical paradox.

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During the 20th century, many suggestions were made.Models developed over the years have incorporated the elements of previous hypotheses, with the result that it is possible to predict at least some of the characteristics of thrust belts.

It was hypothesised that thrust sheets don't move downhill, but rather slide downhill following elevation of a mountain belt.However, this model has lost popularity since its inception.It has been found that high-grade rocks in orogenic cores are not older, uplifted basement, but rather metamorphic equivalents of the same rock units found in thrust belts.Thus, large gravity slides are not readily apparent.

However, the role of gravity has incorporated in more recent models for thrusting driven by shortening.

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Hubbert and Ruby observed that high fluid pressure could partially support the weight of thrust sheets. (See the description of the "famous beer can experiment" in the text by Davis and Reynolds.) Frictional resistance is proportional to the normal stress on a surface, so reducing normal stress reduces frictional resistance.

In an extreme case where water is unable to escape from the basal region, the weight of the thrust sheet would be entirely supported on fluid, reducing the frictional resistance to near zero.

A large elevated body of rock produces a higher pressure beneath its base than in surrounding arturkpopmuzigi.com. This elevated pressure may be sufficient to overcome resistance to sliding or other types of deformation, allowing the elevated region to spread, with a wedge-shaped cross-section. Although rock moves uphill on the basal boundary of the wedge, the overall centre of gravity falls, as body thins. Hence no external force has to be applied to overcome gravity.

Chapple, and subsequently Dahlen and co-workers, realised that most thrust sheets are part of a wedge shaped stack.

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Their model is based on the gravity spreading model.Due to fluid pressure, they allow the base of the wedge to be weak.It was assumed that wedge materials comply with Mohr-Coulomb failure criteria, and faulting was necessary to prevent excessive shear stress from building up.The back end of a wedge is effectively stronger than the front because rocks gain strength as the lithostatic pressure rises.

For a given set of boundary conditions - slope of basal décollement, strength of basal layer, strength of wedge - there is critical angle between the top of the wedge and its base, known as the critical taper.

The equation relating all the various quantities is quite complex

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where

surface slope is αand décollement slope is βThe internal strength (yield stress) of wedge is kInternal fluid pressure ratio is λiDécollement fluid pressure ratio λbDécollement friction μb

Whenever the wedge is at critical taper, and is moving, it is at the point of failure.When Coulomb failure is assumed, the yield stress depends on the depth of the material.In the meantime, subsequent wedge models have become quite diversified, incorporating several different types of rheology into them.

The general principles of wedge models have been tested by