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41 Cards in this Set

  • Front
  • Back

Rifting

The process by which continental lithosphere undergoes extension. If thinning of crust is great, you will transition from rifting to spreading.

Delamination vs Pure shear model

Delamination: Asymmetric model, single master fault that goes listric, creating a detachment fault and displaces and offsets the Moho.




Pure shear: Symmetrical, stretching the crust, detachments trend towards each other.

Is the Basin and range (over 100% extension) the site of future spreading?

No because the previous contraction shortened and thickened the crust.

What is the old school understanding of normal faults?




What are low angle normal faults?

That they always formed at dip angles of greater than 45.




Detachment faults (younger on older, low grade on high grade)


What are grabens and horsts?

Grabens are down dropped blocks, horsts are up thrown.

Rigid domino model (4)

- No block internal strain


- Faults and layers rotate simultaneously


- Faults have the same total offset and dip


- Faults and layers are planar



Rollover anticline

On a listric fault, the hanging wall rolls over.

Antithetic fault




Synthetic fault

Dips into master fault




Soles into master fault

If there is rotation of the hanging wall rocks...

The fault is listric.

Normal faults usually have...

Associated sedimentation.

What do low-angle normal faults accommodate?

Extension.

Evidence against (2) and for low-angle faults (4)

Against: 1) Rock mechanics, 2) scarcity of low-angle normal fault earthquakes




For: 1) Least-work principle, 2) geologic reconstructions, 3) thermobarometric & paleomagnetic data, 4) seismic reflection profiles.

Rolling hinge model (3)

- An initial listric normal fault


- Extensional faulting drives isostatic rebound and abandonment of some faults


- Uplift and erosion results in exposure of the metamorphic core

Inversion tectonics

Normal faults become thrust faults

START HERE FOR QUIZ 6:




Drivers of of contractional deformation (4):

- Continent-continent collision


- Collision of island arcs


- Subduction of seafloor features


- Rapid convergence between continental and subduction oceanic plates

Means of accommodating shortening:

1) Volume loss: Dissolution of parts of the rock.


2) Pure Shear: Shorten in one direction -> thicken in other direction.


3) Buckling


4) Imbrication, thrusting

Thrust Sheet/Nappe

Rock packages that make up the upper plate of a thrust fault.

Hinterland


Foreland

- The region in the center of an orogen


- The marginal portion of orogen

Autochthonous vs Allochthonous

- Rocks that are in place


- Rocks that have been displaced

Klippe vs Fenster

- An erosional outlier of a thrust sheet


- An exposure of the footwall created by localized erosion of the thrust sheet

Flat vs Ramps

- Parallel to bedding: HW flats, FW flats


- Cuts across bedding (fault cut up-section)

Why do thrust faults have ramp-flat geometry? What does this mean?

Thrust faults take advantage of preexisting planes of weakness. Instead of moving continually up an inclined fault plane, the fault cuts up-section in a stepwise manner.

Lateral Ramp

Similar to a footwall ramp, but oriented such that it ramps up in the strike direction.

Blind thrusts

Thrusts that don't make it to the surface.

Cutoffs

Both the hangingwall and footwall have cutoffs and have matching cut off angles. Cutoff lines mark the intersection of the thrust with the stratigraphic horizon it cuts.

Horse

Thrust sheets tend to stack on each other as "horses".

Drag Fold

Footwall rocks that are deformed into synclines.

Fault trace

Where a fault intersects the surface of the Earth.

Branch lines

Where new faults are breaking off

What is a back thrust and how is it different?

Regular thrust faults tend to propagate toward the foreland, when they propagate away from this, they are back thrusts.

Imbricate fan

Where thrust faults daylight. (Several of them, looks like a fan)

Thin skinned (low-angle thrust faulting)

Lots of basement involved.

Duplex

A system of imbricate thrust faults that branch off from a floor thrust and curve upward to join a roof thrust at a branch line.

Basal Decollement

Thrusts root into basal decollement, which is the main detachment surface at depth, usually at brittle-ductile transition.

Mechanics of Thrust Systems (3)

1) Mechanical paradox of moving large thrust sheets


2) Thrust belt evolution: Critical Taper theory


3) Foreland basins

Two possible explanations for thrust belts moving uphill?

1) Fluid pressure decreases the stress required to move a thrust belt


2) Thrust sheets do not move all at once, a wave of slip moves though a thrust sheet (like a caterpillar).

The wedge model of thrusting


1) Material will not slide until wedge shaped.


2) Once a wedge forms, the mass can slide but only after it has attained its critical taper.

Critical taper

Geometry function of the material strength and basal friction. Critical taper angle is controlled by the friction.

Equilibrium

Critical taper angle maintained

Sub-critical




Super-critical

- Growing the wedge


- Need to thin out the wedge

Foreland basins

A "hole" created by thrust belts because of loads that flex the lithosphere.