Structural Geology
Geology 341/342    Fall 2004

Old Sample Test

Outline

Topics covered since Test #1

1. Joints, veins and styloliths

Joints :

-Fractures with very small displacements
-Oriented systematically over large geographic areas (joint sets)
-Indicative of the orientation of the stress field during deformation
-Often there is a set of joints parallel to the fold axis (longitudinal joints) and a set perpendicular to the fold axis (cross joints)
-Joints form during:
1) regional deformation
2) uplift and unloading
3) shringking of igneous rocks during cooling.

Veins

-Opening mode fractures
-Formed under high fluid pressure
-The fracture is held open by the fluid pressure and filled with a mineral deposited from the water
-They are important indicators of local stress conditions
-Sometimes they form "en echelon" arrays within a shear zone.
-Can you tell sense of shear of an en echelon vein array?

Stylolites

-Irregular surfaces of dissolution induced by high stress
-They form perpendicular to sigma 1.
-Usually found in limestone and other easily soluble rocks types.

2) Introduction to Faults

-What is a fault?
-Orientation of faults with respect to sigma 1.
-Conjugate fault sets
-How do you determine sense and magnitude of fault slip?

3) Thrust faults

Outline

-Tectonic setting (mainly convergent plate boundaries)

-Components of a fold and thrust belt

-Geometry of thrust faults (flat-ramp geometry)
-Relationship between ramp-flats and anticlines in the hanging wall
-Sequence of faulting (from hinterland to foreland)
-Map patterns of thrust faults
-Critical wedge model of fold and thrust belts ( Bulldozer model)
-Mechanical paradox of large over thrusts
 
Tectonic setting
o Fundamental characteristic: thrust faults accommodate shortening of the earth's crust.
o Most big mountain ranges are fold-and-thrust belts
o Consists of a set of folds and thrust faults that extend for 10s to 100s of kilometers along strike
o Usually shortening takes place near the plate boundary in convergent margins:
1. Continent/continent collision (Himalayas, Alps, Appalachians, Urals)
2. Arc-continent collision (Timor, Brooks Range)
3. Ocean-Continent convergence (Andes)
o Sometimes stress is transmitted far into the continental plate and thrust faults develop far from the boundary (Wind River Range of Wyoming, Tien Shan (China))
o Accretionary prisms are submarine fold-and-thrust belts (Barbados, Taiwan)
o There may be hundreds of kilometers of shortening (negative extension) across a fold and thrust belt.
Example: Restoration of a cross section of the Canadian Cordillera.

Components of a Thrust System
o Individual fault blocks are typically broad relative to their thickness.
- They are called a thrust sheets or a nappes
o Allochthon is a thrust sheet that has moved a long way with respect the underlying basement (the autochthon).
o Foreland is the area in front of the thrust belt
o Hinterland is the area behind the thrust belt
o Decollement or detachment fault separates the fold and thrust belt from the basement
- Found in "thin-skinned" thrust belts, where the sedimentary cover is scraped off of the basement, like a crumpled rug.
-In thick-skinned thrust belts the basement is involved in the shortening
o Imbricate fan: Individual thrust sheets overlap like roofing tiles
o Duplex: system of imbricate thrusts that branch off from a single fault below and merge with a thrust fault above. The rock body bounded by fauots above and below is called a horse.
 
Shape and displacement of thrust faults

o Ramp-flat geometry
- Faults will generally form along bedding planes in weak layers. This is the "flat"
* shale
* salt
- Then it may cut upwards through the strong layers in the direction of displacement to form a ramp
- Ramp will end in another weak layer
- Why do they form? Is it easier to continue deforming along the same layer
or is it mechanically easier to cut upwards toward the surface?
- The spacing between ramps tends to be fairly uniform . This is controlled by factors such as basal friction on the detachment, thickness of the competent layers, dip of the ramp
o Movement on the ramp forms folds in the thrust sheet
- This is are fault-bend folds
- Ramp anticlines
o Eventually it becomes easier to cut a new fault in front = forward propagation sequence
o Blind thrusts are common (don't breach the surface) may form "fault-propagation folds"
o The map trace of thrust faults is generally irregular
o Thrust faults are typically low-angle
o Erosion of a thrust sheet can leave an isolated part of the hanging wall, = Klippe
o If erosion creates a hole in the thrust sheet, we get a window into the footwall rocks
o Spacing of thrusts depends on thickness of the thrust sheet (thicker = wider spacing)

Recognition of thrust faults

o Fundamental characteristic: older beds end up above younger rocks
o Stratigraphic sections are generally duplicated
o Igneous rocks or high-grade metamorphic rocks may overlie lower grade or unmetamorphosed rocks.
o Highly folded rocks could be thrust over relatively undeformed rocks.
o May even transport rocks that are still hot over lower grade rocks, creating an inverted
metamorphic gradient
o Because thursts can transport rocks a great distance, rocks of the same age but different sedimentary facies may overlie each other
o All low-angle faults are not thrusts! Normal faults can be rotated to lower angle, and in some cases normal faults form at low angles (see discussion of metamorphic core complexes)
 
Thrust faults in outcrop

o Reverse faults are steeper than 45°
o Thrust faults are shallower than 45° and more common.
o Even the largest thrust faults appear as knife-sharp surfaces that separate rocks that were
far apart before movement on the fault
o Often you can put your finger on the fault
o Evidence of rigid-body translation or rotation following brittle fracture
o Ductile deformation also occurs, but the displacements are much smaller
o Faults have to die out somewhere, and at the end of the fault, along strike, you can
see continuous, unfaulted rocks. So the displacement decreases along strike.
o Strain is generally heterogeneous on the scale of a thrust sheet, and even on outcrop scale.

Mechanical Model for Thrust Fault Systems

o We know that thrust sheets have a large surface area relative to their thickness
-dimensions on the order of 5-15 km thick, 75-150 km in length
o How did they move?
o If you take a thin sheet of rock with those approximate dimensions, the stress applied to the rear of the block must exceed the frictional resistance at the base of the sheet.
o Problem: the stress required exceeds the rock strength by a factor of 10 !
o The problems is analogous to trying to shove a flat slab of Jell-O across the kitchen table without it crumpling.
o This has been called "The mechanical paradox of large overthrusts"
Possible solutions
- Thrust sheets are actually sliding downhill, by gravity (doesn't work: faults dip the wrong way)
- Rocks are deforming ductilely, rather than sliding over a surface (not usually the case)
- Fluid pressure offsets the normal stress at the detachment, thus reducing friction
- The assumption in this paradox is that the entire thrust sheet moves at the same time. This assumption is wrong.
o Actually only a small part of the fault surface is moving at any one time like a wrinkle in a rug being pushed forward or a dislocation moving through a crystal.
 
Bulldozer model
o Think of a thrust system as a WEDGE, such as would form in front of a bulldozer that is pushing snow
- When the wedge reaches a "critical taper" (angle alpha) it begins to slide
- The wedge will grow with time, but the taper will stay the same.
- If the material is weak, or if there is less friction, the taper will be low
- This model matches the shape of most thrust belts
- Probably oversimplified, but it's a good way to think about the process
o Faults usually get younger toward the foreland
o The whole system grows in the direction the thrusts are moving
o Erosion tends to decrease the slope of the wedge. That can lead to fault reactivation in the hinterland to build the taper back up. There is a feedback between surface processes and tectonics.

Active Thrust faults and Earthquakes
o Large thrust faults associated with subduction produce some of the most powerful earthquakes
o One of the best studied earthquake is the 1964 Alaska Earthquake
- Displacement was at least 20 m
- Thrust was shallowly dipping, only 5 to 10°, dipping to the north
- Hypocenter was between 20 and 50 km depth
- Uplifts were locally as much as 10 m, and other areas dropped 1-2 m
- The entire sheet didn't move all at once, but it propagated during the course
of the earthquake, which lasted almost a minute
References:
Price, R. A., The mechanical paradox of large overthrusts, GSA Bull. v. 100, p. 1898-1908, 1988.

4) Normal Faults

-Tectonic setting
Divergent plate boundaries
zones of gravity-driven extension
passive margins
orogenic collapse
-Types of normal fault systems to accommodate extension
-Horst and graben
-Conjugate normal faults
-Listric faults and hanging wall roll-over anticlines
-Growth faults

5) Strike-Slip fault systems

-Tectonic setting

-Development of a strike slip fault
-Strain within a strike slip shear zone –en echelon structures
-Releasing bends and restraining bends
 
6) Quakes
· Faults and Earthquakes
· Strength profile of the crust
· Elastic Rebound theory
· P and S waves
· Quadrants of compressional and dilatational first arrivals
· Focal mechanisms (beach ball diagrams)