Why Study Thrust Faults

• Host the largest, and potentially most destructive earthquakes (at subduction zones). Low dip requires that faults have large surface area in brittle "seismogenic zone" and that this surface area is close to ground surface where we live.
• Associated with mountain building and collisional tectonics. Low-angle faults with big displacements have been known since 1800's.
• Large displacements and mechanical paradoxes.
• Influence positions of ore deposits and hydrocarbons.

Tectonic Setting of Thrust Systems

• Occur in zones of plate convergence
• Both subduction thrusts and "foreland thrusts"
• Thrust systems especially well developed in collision zones
• Also at the toes of landslides, whether on land or in the deep sea

Two Major Types of Thrust Faults: Thin verses Thick-Skinned

• Thick: Faults cut into basement
• Thin: Basal or sole fault (decollement) of thrust system glides above basement. Sedimentary cover is involved in faulting and basement slides beneath but is uncut by the thrusts. But since shortening must be taken up somewhere, basement is involved (unless the thrust system is due to a landslide). Mostly we discuss thin-skinned systems

Thin-Skinned Thrust Systems or Fold and Thrust Belts

A complex and well studied system, Thin-skinned thrust systems consist of a basal detachment or decollement, or sole thrust. and more steeply dipping thrusts or imbricate thrusts (combined into imbricate fans) that branch off from the basal thrust.

• Bedding-step thrusting: Faults consist of flats parallel to bedding (surface of weakness) and ramps where the fault cuts across the bedding.
• As thrust system continues to form it converges on an adjacent sedimentary basin (foreland) and incorporates the basin by "in sequence" thrusting. The thrusts tend to break forward towards the basin because it is composed of sediment weaker than that incorporated in the thrust system.
• l
• Out-of-sequence thrusts are thrusts that form later in the interior of the wedge of previously thrust-faulted material.
• Duplex: In thrust systems, thrust sheets bounded by a roof thrust and sole thrust, more generally a block bounded by faults. (Run duplex movie)

Remarkable internal imbrication or "duplexing of a single layer (turbidite sequence somewhere in Middle East, from AAPG Bulletin)

Duplex in sedimentary sequence deformed under partially consolidated conditions, Pliocene tuffaceous mudstones and sandstones, south of Tokyo Japan.

Messy duplexing of sst layer in Tertiary turbidites of Olympic accretionary prism, Washington. Note how at least three imbricates have been sheared off from and shoved under the continuation of the layer above.

Folds Associated with Thrust Faulting

These concepts are best developed in association with thrusting but can occur in any fault system.

• Fault Bend Folds: Form when layers move from flat to ramp or visa versa. Because layers must conform to changing fault geometry, a fold forms. (Run fault-bend fold movie)
• Drag and Drape Folds b & d above
• Detachment Folds: Folding of a layer due to shortening above a detachment or decollement
• Fault-Propagation Folds: As fault moves through section, layers in advance of propagating tip are folded. Slip in advance of fault accommodated by folding.
• Blind thrusts, thrusts that don't reach the surface, form beneath fault-propagation folds and fault-bend folds
• 
• Detachment fold formed by sedimentary slumpling, Quaternary deposits, Mono Lake area, eastern CA. Note undeformed horizons above and below that constrain slumped horizon

• Nappes or Large "overturned" Folds associated with thrust faults:
• Fig 18.15. Nappes from Alps: "down plunge projection"-- a way of turning a map into a cross section.

Glarus Thrust Swiss Alps--example of a large scale thrust with related large fold

Glarus thrust along ridge line, near Elm, Swiss Alps

Detail of Glarus Thrust showing sharp contact

Detail of thrust surface below: Triassic over a sliver of Cretaceous over Tertiary

Cross Section of Alps showing fold verses thin-skinned thrust interpretation. Thrust interpretation generally accepted now

3-D aspects of thrust systems: Tend to look at in 2D but 3D variation in geometry and displacement cause complications. Also have tear faults and lateral ramps.

Balanced Cross Sections

A cross section with internally consistent geometry: Concepts of balancing cross sections applies to all kinds of faulting and deformation, but was first developed in thrust systems.

Requirements:

• Balance occurs when bed lengths and cross-sectional area are equal in both deformed and undeformed state (except with volume loss due to consolidation and dissolution
• .
• Structures drawn on section should be those observed in outcrop or limited to geologic structures expected in a particular environment.
• Cross section must be able to be restored to undeformed state. That is, removal of deformation should result in group of undeformed sedimentary rocks with expectable geometry.
• 
• 

Approaches

• Line-length and area balancing
• Depth to detachment calculation--an outcome of area balancing

Value

• Fault displacement established
• Section may be correct

If it doesn't balance it can't be right. If it balances the cross section a possibly correct, acceptable solution.

Mechanics of Thrust Faults

How to transport thin sheets of rock over long distances--approaching 100 km? (see both Chap. 8 and 18)

Problem:

• Rectangular block fails internally because frictional forces on base of block exceed internal strength of the rock.

Possible Solutions:

• Decrease Friction on Base: Normal stress controls friction or s_f = C + m s_n or Frictional Stress = Cohesion + Coefficient of Friction * Normal Stress. Effective Normal Stress, that is stresses on grain contacts may be decreased by increasing fluid pressure, as it offsets total stress, reducing effective stress. s_n* = s_n - P_H2O. Shear stress on base of upper thrust plate must balance frictional stress for thrust to move. High fluid pressure is the main factor that decreases effective normal stress and therefore friction. Different materials may also have different coefficients of friction. For example some clays are very weak--smectite clays, altered volcanic ash beds may have a coefficient of friction of 0.2 or 0.3. Quartz may be 0.6, more than twice as strong.
• Gravity sliding: Use body forces to cause movements (as in landsliding). However, the real world has no mountains high enough for thrust sheets to slide off of and be displaced 10s to 100 km.

• Wedge Shape and Allowing Internal Deformation: A simple wedge shape allows greater pushing force relative to mass and therefore resisting frictional force. Consider that end being pushed is thicker and stronger, and frontal tip is thinner with less normal stress, therefore less friction. Also, internal deformation allows wedge shape to steepen if force on base is too great to allow sliding--this can lead to the development of out-of-sequence thrusts.
• 
• Bottom Line: Friction is decreased on base by high fluid pressure and thrust masses are wedge shaped and deform internally. If imbricate thrust are added at basinward side of thrust wedge, this added length of the wedge must be accommodated for by thickening, for example by duplexing at depth or out-of-sequence thrusting.

Examples of Thrust Faulting

Discrete thrust in Paleocene accretionary prism, Kodiak Islands Alaska. Note quartz vein filling thrust.

Bedding step thrust in Paleocene turbidites, Prince William Sound Alaska. Note how horizontal layers below feet of geologist on right become inclined and "cut off" layers in foot wall that the geologist on the left is standing in front of. This is an example of a footwall cutoff.

Footwall cutoff, Tertiary turbidite sequence, SW Japan.

Footwall cutoff with development of a small drag fold, Tertiary turbidite sequence of accretionary prism of SW Japan

Thrust fault at Ano Nuevo

Sketch of thrust outcrop at Ano Nuevo. Note offset of Qt or terrace deposits. Why is a thrust fault emplacing younger over older deposits here, that is Monterey on top of Vaqueros?

Map of Ano Nuevo area by Jerry Weber. Note thrust fault and shallow dips to east.Also not that thrust faults and graben structure to right have different orientations, consistent with their development in the right-lateral strike slip regime of the San Gregorio Fault.