Contractional tectonic regimes Flashcards
What four factors make up contractional tectonic regime shortening?
Volume loss, pure shear (horizontal shortning with vertical thickening, buckling, movement on contractional faults with folding related to the movement
Define thrust fault
Dip-slip fault where the upper block, above the fault plane, moves up and over the lower block
Where is most shortening in contractional orogenic belts localised?
On thrust faults
Describe thin-skinned tectonics
Deformation in the foreland where thrust systems are restricted to the cover rocks, the basement is not involved
Describe thick-skinned tectonics
Deformation in the higher metamorphic grade hinterland where the thrust systems are thicker and the basement is involved
Describe wrinkle ridges
Local contraction features
Formed during basin-localised flexure
What are lobate scarps indicative of?
Global contraction due to net cooling of the planetary interior
The surface expression of antiforms formed by the tips of reverse faults that break the surface
Define thrust
A predominantly low-angle fault or shear zonewith a predominantl reerse dip-slip sense of movement
Describe a thrust in rocks that young upwards
The thrust beings older rocks over younger rocks
Define a thrust nappe or thrust sheet
The hanging wall of a low-angle, large displacement thrust that is wide compared to its thickness
When is a thrust nappe discontinuously exposed?
When it has been eroded through
What structures are formed by the erosion of a nappe?
The erosional remnant of a nappe is a klippe and the erosional hole exposing the unts benearth the thrust is a tectonic window
Are the thrust sheet and footwall allo/autochthonous?
The thrust sheet is allochthonous because it is transported and the footwall is autochtonous because it is close to its orginal location
Describe the positions of flats and ramps in the series of steps as a low-angle thrust fault passes up through the stratigraphy
The flat is where the fault runs parallel to the layering and the ramp is where it cross-cuts and truncates the layering
For each hanging wall ramp there must be a corresponding footwall ramp
See diagram in book
Define frontal ramps
Ramps that form perpendicular to the main thrust transport direction
Define oblique ramps
Ramps that are oblqiue to the transport direction and are oblique-slip reverse faults
Define lateral ramps
Parallel to the movement direction and are strike-slip faults
They transfer slip from one frontal ramp to another, therefore they are transfer faults or tear faults when sub-vertical and cutting through the hanging wall
Define an imbricate fan in relation of thrusts
Where individual thrust sheets overlap, they are a set of listric faults with the same sense of slip
The merge down on a sole or floor thrust or up onto a roof thrust, a duplex is formed when they are bound by both
Define a decollement or basal detachment
The major floor thrust at the base of a stack of thrust nappes and/or imbricate system
Describe a leading imbricate fan
Where the maximum slip is on the frontal fault
Describe a trailing imbricate fan
Where the maximum slip is on the most internal fault
Describe a horse
An internal block of an imbricate fan that is bound completely by faults
In which direction do thrust duplexes form and how do they form?
The form sequentially and usually towards the foreland
A horse is formed in the footwall, it gets displaced forwards, becomes inactive and then a new horse is formed in the footwall
In which direction do the ramps binding the horses dip?
Towards the hinterland
Describe a break-back sequence
When horses nucleate in the hanging wall and propagate towards the hinterland
Describe an out-of-sequence thrust
A thrust that may be a newly initated fault or a reactiveted earlier fault that develops at a position behind the thrust front
What happens when there is greater drive on the roof thrust than the floor thrust?
Foreland-propagating thrust duplexes develop, stacking the horses, and forming an antiform in the overlying thrust sheet
This is an antiformal stack
Describe a foreland-dipping thrust duplex
Where horses are added to the back of the duplex and ramps binding the horses dip towards the foreland
Describe back-thrusts
Thrusts that form in the opposite vergence to the thrust sheet that is climing over the ramp
What are pop-ups and triangle zones?
Structures formed from a combination of thrusts and back-thrusts with the same floor thrust
Describe a passive roof thrust and passive roof duplex and give the type of process that forms them
Formed when a duplex underthrusts the foreland and the overlying units are not laterally displaced
This is a tectonic delamination process
Give three structures to include in the cross section of a thrust system that indicate how faults join up
Tip-lines (where fault displacement dies out), branch-lines (where a fault splits into two (trailing)) or two faults merge (leading)), cut-off lines (intersection of a contact with the fault surface)
Describe a fault-bend fold
Formed the hanging wall climbing over the ramp
Their shape is determined by the geometry of the ramp because they are geometrically-necessary folds and therefore their shape can be predicted from ramp geometry
What are three results of fault-bend folding?
Cleavage produced in lower flat, cleavage steepens and intensifies in ramp, cleavage shortened and crenulation developed in upper flat
Describe fault propagation folds
The asymmetric antiform-synform pair that verges towards the foreland as a result of deformation being accommodated in a more ductile fashion ahead of a thrust tip
How do fault-propagation folds differ from fault-bend folds?
They move with the propagating fault tip rather than being stationary at the location of the ramp
Describe the formation of a hanging wall antiform and a footwall synform
Formed when thrusts breach a fault-propagation fold, cutting through the steeply dipping intermediate limb
They can also arise from drag or progressive shearing
What determines the orientation of folds that develop around the margins of a thrust sheet?
Orientation of ramps in the underlying footwall
En echelon arrays are formed over lateral and oblique ramps (sense of oblquity indicates transport direction)
When do detachment folds form around the margins of a thrust sheet?
When layers above a decollement shorten more than their substrate
They tend to be upright and form over very weak layers
Calculate amount of shortening of a detachment fold
Ax = A = t0 ΔL
Ax = measuring area
t0 = initial stratigraphic thickness
ΔL = amount of shortning
Describe the geometry of fold and thrust belts in cross-section
Overall wedge-shaped, deformation occurs until it reaches a constant critical taper
What forces are appled in the wedge model?
Gravity and push from behind
What does the angle of taper (α+β) in the wedge model depend on?
Relative magnitudes of frictional resistance at the base and the strength of the wedge material
Increasing friction increases the angle of taper
Increasing wedge material strength decreases the angle of taper
What are α and β that add to give the angle of taper?
α = surface slope
β = basal slope
How does the wedge adjust to thickening?
Lengthening (e.g. imbrication and duplex formation at its toe) or thinning
How does the wedge adjust to thinning (e.g. by erosion?
Surface slope is maintained by uplift by internal shortening (e.g. reverse faults, folding)
Define flysch
The syntectonic sedimentation the accompanies adjustments to thickening/thinning
Redistribution of matter of the surface of the wedge to maintain a stable surface slope
Describe what happens when the centre of the orogen is weak during orogenesis, what is this an example of?
The thickened and elevated crust collapses under its own weight and thrust nappes are pushed out onto the foreland
This is gravity-driven deformation
What can we expect to see in gravity-driven deformation?
A significant component of pure shear coaxial deformation
The orientation of the strain ellipsoid would reflect the extent of any lateral constraints
Give the three ways in which strains can be distributed through a thrust sheet
Gravity-gliding, push-from-behind, gravity-spreading
Describe gravity-gliding
Thrust sheet does not shorten or length in the slip direction (simple shear)
Describe push-from-behind
Thrust sheet shortens in the slip direction
Describe gravity spreading
Thrust sheet extends and thins in the slip direction
What can happen to the sole fault in the hinterland?
A: return to surface so horizontal shortening in one part of crust is balanced by extension in another
B: merges into high grade metamorphic root zone of ductile deformation
C: forms a subduction zone with the thrust belt
What two criteria should a balanced cross section meet?
Admissable and restorative
What three assumptions are made when constructing a balanced cross section?
No volume change, plain strain (all deformation is in the plane of the section), layer thickness is preserved
