Faults III Lecture 29 Gly 326

Compression and deformation In areas of compression, we have several options for deformation Dependent on lithology, rock strength, confinement, fluid occurrence, etc.

Reverse/Thrust faults Reverse fault recap: Hanging wall moves up relative to the footwall If shallow angle (<30°), we use the term thrust fault Characteristic of regional compression  leads to shortening Often with associated drag folds and more Vocab: Detachment/Décollement: A subhorizontal gliding plane Base of a listric fault Flat of a thrust fault

Thrust faults: Horses and Wedges A thrust fault is a reverse fault in which the dip of the fault plane is very low… (<30o)

Boyar and Elliot, 1982

Thrust systems Imbricate fans Repeated series of thrusts sharing a dip direction Faults flatten out into the basal thrust (décollement) Fault dip depends on erosion level Blind thrust: No surface expression Normal sequence of imbrication thrusts: younger in transport direction

Thrust systems Duplexes Form when sub- horizontal thrust propagation “sticks” Bound by floor and roof thrusts May contain many individual horses (recall: Horse = fault bounded rock unit)

Duplexes Hinterland dipping duplex (Normal duplex) Normal thrust progression Horse bounding ramps have small overall displacements (slice length > fault slip) New horse formation tilts older horses back (hinterland-ward) Most common duplex structure

Duplexes Antiformal duplex Greater displacement of thrust ramps (slice length = fault slip) Horses piled one upon another overall anticlinal geometry from fault rollover folding

Duplexes Foreland dipping duplex (Forward dipping duplex) Out of sequence thrust formation Ramp displacement > length of horse (older horses travelled farther than younger)

Thrust systems Stacked geometry: Floor of an upper level duplex or imbricate sequence may be the roof of a lower duplex Back thrusts: Antithetic faults in the thrust system. Associated with pop-up or triangle zone Blind thrust: Tip point does not break the surface or merge w/ another fault

Thrust faults – associated folds Fault bend fold “Drag” folds in a fault zone Fault propagation and drape fold Detachment folds

Thrust faults – associated folds Fault-bend fold: Hanging wall sheet moves over a ramp Thrusting begins, beds flex into kink-like folds as they move from flat to ramp, & from ramp to flat Anticline grows in height & width, width of kink-like fold increases

Thrust faults – associated folds Multiple Ramps  Multiple Fault Bend Folds

Thrust faults - associated folds Fault propagation folds Hanging wall folded during faulting Fault and fold form at same time As thrusting takes place, the fault tip migrates steadily upwards. Beyond the tip the fault is replaced by an overturned syncline. Above the fault tip in the hanging wall there is a complementary fault propagation fold – an overturned anticline whose axial plane dips more steeply than the thrust. Footwall relatively undeformed

Thrust faults - associated folds Fault propagation fold

Thrust faults - associated folds Fault propagation folds (several faults)

Thrust faults - associated folds Fault propagation folds (several faults)

Thrust faults - associated folds Detachment folds Occurs above flats in bedding parallel to the flat Folds expressed in competent (mechanically strong) layers Incompetent layers flow to fill in the gaps beneath mechanically strong layers, changing unit thicknesses

Thrust faults - associated folds Detachment fold

Thrust faults – associated folds Continued detachment folding may result in fore and back thrusts as compression continues Pop-up geometry: Faulted top of a detachment fold

Thrust systems Flats and ramps in 3D!

Lateral ramps act like transform faults  also called tear faults Thrust systems Lateral ramps act like transform faults  also called tear faults

Thrust faults – large scale geometry Klippe: Erosional remnant of a thrust nappe. Sometimes the only indication of minimum displacement distance on a nappe Fenster/Window: Erosional exposure of the footwall to a nappe

Keystone Thrust, Nevada Paleozoic Jurassic (View to northwest)

HIMALAYAN FRONTAL THRUST Himalayan Frontal thrust- active thrust slicing up foreland basin Flat on ramp geometry here in poorly consolidated sediments

The McConnell thrust, Alberta. View to North The McConnell thrust, Alberta. ramp

Example: Lewis Thrust Fault

Example: Lewis Thrust Fault fenster klippe

Lewis Thrust Fault (cont'd) Source: Breck P. Kent pre-Cambrian Limestone over Cretaceous Shales!!!

Thrust faults – large scale geometry Horse Horse Horse