Ideal Fracture Displacement Modes

Rock deformation experiments of rocks loaded to failure under triaxial compression demonstrate symmetrical orientation of fractures with respect to the three effective principal stresses: σ₁’ > σ₂’ > σ₃’ where compressive stress is positive. The type of fracture that will develop is dependent upon the value of minimum effective principal stress (σ₃’), the difference between the maximum and minimum effective principal stresses (σ₁’- σ₃’), and the tensile strength of the rock.

There are three ideal displacement modes of fractures based on the angle with respect to σ₁’: mode I, mode II, and mode III.

 

Mode I displacement is referred to as opening or tensile mode and is purely extensional. These fractures develop at an orientation perpendicular to σ₃’ and within the σ₁’ stress plane.

 

Mode II displacement is referred to as forward shear mode, where the fracture surfaces slide over one another in a direction perpendicular to the fracture tip.

 

Mode III displacement is referred to as transverse shear mode, where the fracture surfaces move relative to one another in a direction parallel to with the fracture tip. Both mode II and III fracture planes are oriented parallel to σ₂’ and at an angle less than 45° to σ₁’.

Source: Agust Gudmundsson (2011) Rock Fractures in Geological Processes

Trishear Fault-Propagation Folding

Fault-propagation folds are produced by deformation that takes place just in front of the propagating fault. The fault tip propagates upsection, and the fold develops above the ramp with uniform fold angles.

 Figure: A) Kink-band model, B) Fold above thrust fault,

C) Fold above reverse fault, D) Fold above normal fault.

(Erslev 1991)

 

Fault-propagation fold hinges tighten and converge downward, forming a triangular zone of deformation that is concentrated on the tip of the propagating fault. This downward convergence of deformation is modeled as triangular shear zones. This lends the name to trishear fault-propagation folds.

Figure: Models of homogeneous and heterogeneous

fault-propagation trishear folds. A) Thrust faults,

B) Reverse faults, C) Normal faults. (Erslev 1991)

Understanding the geometry of fault-propagation folding is useful in creating balanced models of fold and thrust belts. Fault-propagation trishear folds are common in the Laramide structures of the Bighorn Basin in Wyoming.

All information from: Eric A. Erslev (1991) Trishear fault-propagation folding

Rigid Body Deformation & Shear Strain

Deformation is a change in form or shape. Rock masses can be translated or rotated as rigid units during deformation, without any internal change in shape. Fault blocks moving during deformation with no internal distortion.

A displacement field shows the change in position points before and after deformation in a group of displacement vectors. The displacement field does not, however, show how the particles moved during deformation history, but links the undeformed and deformed states. Particle paths show the motion of those points during deformation.

Rotation indicates rigid rotation of the entire deformed rock body. It involves uniform rotation of the rock volume relative to an external coordinate system. Large-scale rotations occur in thrust nappes or tectonic plates, usually around vertical axes. Fault blocks may rotate around horizontal axes in extensional settings.

Translation is where every particle in the rock body moves in the same direction, over the same distance. Displacement fields consist of parallel vectors of equal length. Translation of nappes can occur over 10s or 100s km.

Shear strain describes the strain due to rotation about an axis. It is deformation which involves change in internal shape.

Simple shear is a special type of constant-volume plane strain deformation. No stretching or shortening of lines or movement in the third direction of particles occurs. It is non-coaxial deformation, meaning that lines parallel to the principal strain axes have rotated away from their initial positions.

Subsimple shear is a spectrum of planar deformations between pure shear and simple shear. Internal rotation is less than for simple shear.

Pure shear is a perfect coaxial deformation. Particles parallel to the principal axes do not rotate from their initial positions. Pure shear is a plane strain with no volume change associated.

Rigid body deformation (rotation & translation),

and shear strain (simple, subsimple, & pure shear).

From Haakon Fossen's Structural Geology (2010).

ImageJ - Thin Section Porosity

I'm not very patient, and I will spend more time trying to fiddle with a program to get it to do what I want rather than read through documentation. I will also try Google. So, I was very happy when I found a very simple and thorough video about exactly what I wanted: how to measure porosity in a thin section using ImageJ.

Thin Section Porosity ImageJ uploaded by Chris Liner on YouTube gives simple, step-by-step instructions on calculating porosity in a thin section.

I hope this helps other people looking for the same thing!

Requirements: ImageJ, thin section photographs

Isostasy - Airy & Pratt

Isostasy

• describes the state of equilibrium of a lithospheric plate floating on the asthenosphere
• the weight of columns of rock, at some depth called the depth of compensation, is everywhere equal

 Airy Isostasy

mountains have a crustal root that compensates for the additional relief (variation in thickness)

Pratt Isostasy

density varies laterally, so that mountains have a lower density than higher density, thinner portions of crust

Possible Mechanisms of Crustal Subsidence

Mechanisms that can generate sufficient subsidence to create basins:

Crustal thinning    extensional stretching, erosion during uplift, and magmatic withdrawal
Mantle-lithospheric thickening    cooling of lithosphere following either cessation of stretching or heating due to adiabatic melting or rise of asthenospheric melts
Sedimentary and volcanic loading    local isostatic compensation of crust and regional lithospheric flexure, dependent on flexural rigidity of lithosphere, during sedimentation and volcanism
Tectonic loading    local isostatic compensation of crust and regional lithospheric flexure, dependent on flexural rigidity of underlying lithosphere, during overthrusting and/or underpulling
Subcrustal loading    lithospheric flexure during underthrusting of dense lithosphere
Asthenospheric flow    dynamic effects of asthenospheric flow, commonly due to descent or delamination of subducted lithosphere
Crustal densification    increased density of crust due to changing pressure/temperature conditions and/or emplacement of higher-density melts into lower-density crust



Principles of Sedimentology and Stratigraphy (5th ed.), Sam Boggs, Jr.

Seismic Waves

Body Waves

• penetrate the body of the Earth
• travel faster in more elastic rocks
• body wave velocities increase with depth in the interior of the Earth
• subject to refraction and reflection
• increased rock temperature = decreased velocity
• increased confining pressure = increased velocity

Figure 1. A) P-wave motion. B) S-wave motion.

P-Waves

• primary waves, compressional waves
• fastest seismic waves
• wave motion: energy moves as a succession of compressions and expansions in the direction of wave travel - an accordion-like push-pull movement
• each square in the figure changes from square to rectangle to square again as the waves move through the rock
• travel through solids, liquids, & gases

S-Waves

• secondary waves, shear waves
• slower than P-waves
• wave motion: rock segments vibrate perpendicularly (at right angles, up-and-down or side-to-side) to the direction of wave travel - this more complex motion causes S-waves to travel more slowly
• travel through solids only

Surface Waves

• large-motion waves that travel through the outer crust of the Earth
• wave pattern resembles ripples caused when a pebble is dropped in a pond
• slowest seismic waves
• cause of destruction during an earthquake since they are channeled through the thin crust and their energy is less rapidly dissipated than body waves

Figure 2. a) Rayleigh wave motion. b) Love wave motion. 

c) Surface expression of wave motion of Rayleigh & Love waves

Rayleigh Waves

• wave motion is similar to waves in an ocean (see figure)

Love Waves

• wave motion is a shear which moves the surface from side to side

Metamorphic Index Minerals

Metamorphic index minerals form under specific temperature and pressure conditions.

This diagram shows shale being metamorphosed.

Chlorite and muscovite form at relatively low temperatures. Garnet forms at higher temperatures and pressures. Sillimanite indicates the highest level of temperatures and pressures.

Shale is metamorphosed to slate and then to phyllite. Schist is next to form, then gneiss when high-grade metamorphism is reached. Beyond 800°C, the rock may completely deform by melting.

Another version showing kyanite.

Bowen's Reaction Series

Bowen's Reaction Series is the order of mineral crystallization as a magma slowly cools.

The right branch of the chart is the continuous series of crystallization because the plagioclase minerals maintain the same basic crystal structure but change continuously in calcium and sodium content away from calcium-rich plagioclase towards a sodium-rich variety.

The left branch of the chart is the discontinuous series of crystallization because the reactions result in minerals of distinctly different structure.

Potassium feldspar, muscovite mica, and quartz do not react with the melt. By the time they crystallize, there is little liquid left.

Bowen's Reaction Series allows geologists to recognize why mineral variations exist in igneous rocks. A volcanic rock from an early eruption may be rich in iron, magnesium, and calcium and thus produce basalt. Therefore, later eruptions might be depleted in iron, magnesium, and calcium but enriched in potassium, sodium, and silica. These rocks would be less basaltic and more andesitic in composition.

Source: The Earth Through Time by Harold Levin

Principle of Superposition

The principle of superposition states that in a sequence of undisturbed sedimentary layers, the oldest layers are on the bottom and the youngest layers are on the top.

This principle was formulated by Nicolas Steno in the 17th century.

Thrust faults can alter the understanding of the principle of superposition because they occur parallel to bedding and can be hard to detect. They can create situations where older layers overly younger layers. Scientists of Nicolas Steno's time were unaware of thrust faults.

Anderson's Theory of Faulting

Assuming that there is no shear stress at the Earth's surface (shear stress cannot occur in fluids), one of the principal stress components must be vertical and thus the other two must be horizontal.

σv = σ1 normal fault

σv = σ2 reverse fault

σv = σ3 strike-slip fault

Anderson's Theory of Faulting

Normal, Reverse, and Strike-Slip Faults

Top 10 Signs You Might be a Geologist

10. You've responded "yes" to the question, "What have you got in there, rocks?"
9. You've taken a 15-passenger van over "roads" that were really intended only for cattle.
8. You've found yourself trying to explain to airport security that a rock hammer isn't really a weapon.
7. Your rock garden is located inside your house.
6. You've hung a picture using a Brunton as a level.
5. Your collection of beer cans and/or bottles rivals the size of your rock collection.
4. You consider a "recent event" to be anything that has happened in  the last hundred thousand years.
3. Your photos include people only for scale and you have more pictures of your rock hammer and lens cap than of your family.
2. You've been on a field trip that included scheduled stops at a gravel pit and/or a liquor store.
1. You have uttered the phrase "Have you tried licking it?" with no sexual connotation involved.

Ramsay Fold Classifications

John Ramsay's Fold Classification

John Ramsay proposed a classification scheme for folds that is used to describe folds in profile based upon curvature of the inner and outer lines of a fold, and the behavior of dip isogons.

dip isogon: a line that connects points of equal inclination or dip on the outer and inner bounding surfaces of a folded layer

Class 1 - Dip isogons converge downward towards axial surface, signifying that the curvature of the outer arc is less than that of the inner arc

Class 1A - Limbs thicker than hinges

Class 1B - Layer thickness constant; parallel fold

Class 1C - Limbs thinner than hinges

Class 2 - Dip isogons are parallel, signifying that the curvature of the outer arc exactly matches the curvature of the inner arc; similar fold

Class 3 - Dip isogons diverge downward towards axial surface, signifying that the curvature of the outer arc is greater than that of the inner arc

Class 1B (parallel) and Class 2 (similar) folds are the most common folds seen in the field. Concentric folds are a special case of Class 1B (parallel) folds where the outer and inner bounding surfaces define arcs with a common center of curvature. These types of folds are common in upper crustal tectonic settings, where most deformation occurs by processes that only permit limited ductile flow of rock. Most of the deformation is accommodated by slip on bedding or layer boundaries (flexural slip folding). Class 2 (similar) folds have relative thinning of the limbs and thicking of the hinges. These types of folds are common in metamorphic terranes, where most deformation occurs by processes that permit extensive ductile flow of rock.


Sources:
Folding and Fracturing of Rocks, John G. Ramsay, 1967
Structural Geology of Rocks, 2nd Edition, George H. Davis & Stephen J. Reynolds, 1996
http://ocw.mit.edu/courses/earth-atmospheric-and-planetary-sciences/12-113-structural-geology-fall-2005/lecture-notes/part6_dctl_fldfb.pdf

Near passerine? How can you be near passerine?

Near passerine? How can you be near passerine? Doesn't that just make you non-passerine?

I know very little about bird lineages, and I recently read an article about the replacement of non-passerine birds in Europe with passerine birds. I was trying to determine what passerine versus non-passerine birds were, generally, but when I looked up woodpeckers I found that they, and several other lineages, were known also as "near passerines." My search led me to citations that stated that near passerines are a group that are believed to be related to true passerines due primarily to ecological similarities. It is thus far undetermined whether or not all near passerines are related to true passerines, but new molecular data apparently makes it appear unlikely. So, that is, briefly, a near passerine.

Subduction zones and earthquakes

I am still waiting to hear from a couple friends in Japan, and I hope to hear that they and their families are safe. There are also extensive fears about nuclear radiation, as well as problems with evacuations and shelter for those displaced, throughout the Tohoku region. There are many good resources for information on these factors, so I will not focus on them.

Instead I will focus on the geological effects of this earthquake, since I understand these a bit better.

The 8.9 magnitude earthquake has shifted the Earth of its access by about 16.5 cm. This will cause the planet to rotate faster and shorten the length of the Earth's day by 1.8 millionths of a second or so. It has also moved the coastline of Japan in the Tohoku region by about 4 m to the east in some regions.
The cause of this earthquake is due to the subduction zone which lies to the east of Japan (the thick line to the east of the epicenter on the image below). The Pacific Plate is being subducted, or pulled under, the North American Plate, on which Japan lies. The motion of the scraping of the two plates together builds up strain energy which then must be released at some indeterminable period. The release of the strain results in an earthquake, in this case a very large earthquake.

When the movement of the Earth occurred underwater, it displaced a large bit sea water and thus created the large tsunami which hit the coastal regions of the Tohoku region. The Pacific Plate's maximum westward movement was about 20 m, with the movement along the fault reducing as you go away from the epicenter. This is also why the amount of shaking and damage decreased away from the epicenter and fault.





BBC - How the quake has moved Japan

8.9 Magnitude Earthquake Offshore Japan

An 8.9 magnitude earthquake occurred offshore of Honshu, Japan, on Friday, March 11, 2011, at 14:26 local time. This is the largest earthquake the country has ever experienced, and the seventh largest ever recorded. It occurred 382 km northeast of Tokyo.

The towns along a 2,100-km range of eastern coastline of Honshu were affected by the earthquake and its 19 aftershocks, most of which were greater than 6.0 magnitude. A 7-metre tsunami was launched which carried cars, boats, and even planes inland. At least 60 people have been killed.

Tsunami warnings have been issued for the west coast of South America, the west coast of the U.S., Hawaii, New Zealand, and other areas in the Pacific: Pacific Tsunami Warning Center.

More News:
Earthquake: Japan Hit by 8.9 Earthquake/Japan Tsunami Warning (Sydney Morning Herald)

Mediterranean Sharks Resulted from Wrong Turn

A new study involving Mediterranean great white sharks suggests that they are more closely related to sharks from Australia and New Zealand, and less similar to those of the Atlantic Ocean as previously thought.  It is believed that the group of Mediterranean sharks arrived about 450,000 years ago after making a "wrong turn" on their return to the location of their birth.  This period of time was an interglacial period, in which extreme current variations were occurring.  The change in warm and cold currents may have been significant enough to alter the course of the migrating sharks.  If only a few shark pups were born in the Mediterranean waters, the study indicates that this would be all that was needed to begin a new cycle of migration to the same location for future generations.  This is an interesting study which may have implications for other species migrations during interglacial periods.

Quetzalcoatlus

Quetzalcoatlus was a Late Cretaceous pterosaur known from North America, and a member of the family Azhdarchidae - known as advanced, toothless pterosaurs characterised by long, stiffened necks.  Its name comes from Quetzalcoatl, a feathered serpent deity of the peoples of central Mexico, Nicaragua, and Honduras.

Please visit Shiraishi Mineo's Jurassic Gallery, the talented artist who created this image, with a beautiful gallery of images of dinosaurs, pterosaurs, and other prehistoric animals.  This was my favourite image of all the Quetzalcoatlus images I have seen so far.

The feeding style adopted by Quetzalcoatlus is controversial.  Originally it was theorised that Quetzalcoatlus fed on fish, scooping them from the sea as it flew overhead.  It has also been suggested that the pterosaur was a scavenger, because some remains had been found in an area devoid of lakes or rivers, and was far inland.  But the shape of its jaw suggested to others that it could have fed by skimming over the sea and collecting fish in its mouth.  This skimming technique was later disproved because the energy costs for the shape of the pterosaur would be too high, as well as the fact that the remains were found in an inland area rather than a coastal environment.  Another theory suggests that Quetzalcoatlus hunted by terrestrial stalking, hunting small vertebrates on land or in streams.  The modern analogue for this behaviour is in storks.  Quetzalcoatlus would have walked on its hind legs and folded wings.

One important point to remember is that pterosaurs were not dinosaurs!  Dinosaurs are terrestrial animals only!  This includes plesiosaurs and similar marine creatures contemporaneous with dinosaurs, which were actually marine reptiles.

Fossil Discovery of Utah Ceratopsians

Fossils of new species of horned dinos found in Utah (BBC)

Scientists digging in south-central Utah found two new species of ceratopsians in the Grand Staircase-Escalante National Monument, in the Kaiparowits Formation.  They found a specimen with a particularly large horn over the nose and a massive head which they dubbed Utahceratops gettyi.  They also found a very ornately-ornamented specimen with a total of 15 horns on its head which they called Kosmoceratops richardsoni.

Top: Utahceratops gettyi
Bottom: Kosmoceratops richardsoni


Both dinosaurs lived on the ancient landmass of Laramidia, which was formed when a late Cretaceous inland seaway split the North American continent in half into the western landmass of Laramidia and the eastern landmass of Appalachia.  Many fossils have been found in the northern part of the ancient Laramidia landmass, but until recently fossils from the southern portion have been lacking.  The National Monument in southern Utah has produced an abundance of fossil specimens and may begin to make up the disparity between northern and southern fossils.

Utahceratops

Further analysis could provide answers to many questions, such as why there was not a lot of exchange between dinosaurs in the northern and southern parts of Laramidia (a physical barrier?) or why there was such a high diversity of large animals living in a relatively small area (there were about two dozen different species of large dinosaurs living on a landmass a quarter of the size of the African continent).

For the scientific paper: New Horned Dinosaurs from Utah Provide Evidence for Intracontinental Dinosaur Endemism

California Water Crisis

http://www.redorbit.com/news/science/1643127/california_farmers_to_lose_primary_water_supply/