General 1

Question 1

3 fundamental structures

Answer 1

contacts these are boundaries separating rock bodies. These include normal depositional contacts, unconformities, intrusions, faulting, shaving…

Primary Structures These are outcrop scale features that develop as a function of the rock body formation and reflect the local formational conditions. This includes stuff like bedding, granularity, and textures.

Secondary Structures These are our principle focus and is are the rock features that occur after deposition or lithification

Question 2

Accomodation zones in normal fault systems

Answer 2

Generally within continental graben/half-graben systems, the hanging wall’s subsidence is filled with growth strata and become a spot for lakes and other sedimentary accumulation processes.

Question 3

Anderson’s theory of faulting dictates what fault angles

Answer 3

This means that normal faults (where sigma 1 = radial) that the fault plane should dip at 60 degrees (90-30)

Thrust faults should dip at 30 (sigma 1 is lateral and sigma 3 radial so 30 degrees from horizontal)

Strike slip faults should have an inter-fault angle of 60 degrees (sigma 1 and sigma three parallel to surface and the acute block is in the direction of sigma 1)

Question 4

Andersons Theory of Faulting

Answer 4

This says that Earth’s surface has no shear stress along it (there is no shear parallel to the spherical surface). Using Mohr’s circle (there is zero shear in the orientation of sigma 1 and sigma 3) this means that the two perpindicular direction parallel to Earth’s surface MUST be a principal stress directions.

That means that the other principal stress must be radial to earths surface because it is 90 degrees off.

Question 5

Andersons theory of faulting

Answer 5

This says that rocks in the crust are generally in compression and that earths surface has no significant shear stress tangent to it.

There is one orientation of rocks that does not have shear. The principal angles. This means that rocks are generally in a state where the pinciple stresses are radial to the surface and orthogonal to that.

Then given that faults generally form at 30 degrees from sigma one we can derive the orientation of most faults.

Question 6

Anticline

Answer 6

This is when the youngest beds are on the outermost hinge.

This is commonly where the youngest layer makes an upside down u shape. It is convex

Question 7

Antiformal

Answer 7

This is when there is a convex up syncline (upside down U shape but youngest on innermost hinge) It also refers to igneous or meta where age is indeterminate but the shape is convex up

Question 8

apophyses

Answer 8

These are irregular sills/injections from a dyke into the surround country rock

Question 9

Assign S1 and S3 and stress

Answer 9

The marker beds show that s3 is horizontal (the middle moved up based on fold dragging) s1 is 90^o off from s3 hence these results. The drag folding also indicates that the s3 direction is the direction of maximum stress and that the minimum stress is where there is greatest stretch.

Question 10

assign stretch and stress

Answer 10

There is normal faulting that is occurring indicating thinning. This means there will be negative dilation vertically (S3) and the greatest positive dilation laterally (S1). Sigma 1 will be perpindicular to the most negative dilation (compression)

Question 11

Assign stretch and stress

Answer 11

This is left lateral shearing where you can break down the shear into compressional and extensional components. the long axis corresponds to the elongated part of the fold and the compressional component is relating to the thinning in the limbs.

Question 12

Asthenosphere

Answer 12

This is the mechanically different plastic part of earth. At the upper levels it acts like a fluid so it deforms infinitely with shear. This is also where earthquakes end.

Question 13

Asthenosphere-lithosphere boundary depths

Answer 13

~75 km under the ocean, ~225 km under the continent, and the deepest at 700 km

Question 14

Asymmetry in rifted cont. margins.

Answer 14

This occurs most commonly near transition faults along the MOR where the change in slip rates causes a “pileup” of material. It also occurs in rift zones where one side becomes hyper-extended.

Question 15

Attitude

Answer 15

This is the orientation of the fold (N, S, E, W)

Question 16

Attrition + crush breccias

Answer 16

These are breccias with clasts that have been ground and rolled due to friction

Crush breccias are intensely fractured but not displaced. They are crushed in high pressure environments.

Question 17

Axial surface

Answer 17

This is the surface that passes through the hinge lines through layers of the fold.

It is used to document fold orientation.

Question 18

Basal Conglomerate

Answer 18

This refers to the lowest part of the younger layer of an uncomformity often being a conglomerate with clasts from the older layers below.

Question 19

Bedding symbols

Answer 19

regular bedding = perpindicular sign

vertical bedding = strike like with oblique cross line

Overturned bedding = an s with the dip

Horizontal bedding= circle with a +

Question 20

Beta Diagram

Answer 20

This is a diagram used for determining the hinge line of a fold by plotting two great circles representing the two limbs of the fold. The intersection of these great circles is a point, (beta) where the hinge line is.

Question 21

Boudins

Answer 21

This is the flattening and stretching of strong layers that separate and are filled with weaker, ductile layers. They are indicators of shear based on how they “tail” If they are symmetric then it is pure shear, if they are assymetric use drags to indicate the sense of shear.

Question 22

Buckling

Answer 22

This is folding due to end loading.

Question 23

Bulk Modulus

Answer 23

K = Δσ/ΔV where sigma is the hydrostatic stress and the bulk modulus is a measure of compressibility.

Question 24

Byerlees law

Answer 24

This is for joints and it says that sigma = sigma n*tan(phi)

The only difference between this and Coulomb failure is that there is no cohesion, so it is only friction that causes any type of resistance to stress. The amount of friction along a fault is given by phi. The larger the angle of phi the rougher the surface (remember tilted table test).

If a sample is in a stress state beyond that of the line it will fail along the pre-existing fracture. If a sample has a fault plane that is not within the part of mohrs circle that extends past byerlees law then it will not fail.

Question 25

cataclasite breccias

Answer 25

These are breccias that are cohesive and strongly indurated because they tend to form at higher pressures with reactive clasts that effectively produce a cement

Cataclasites .1mm < D < 10mm

Ultraclasite D

Question 26

Cataclastic Breccia

Answer 26

These are breccias that form from the comminution of material due to frictional sliding and associated cataclasis. They generally have very unsorted, angular clasts within a fine ground matrix. It only occurs within brittle rocks and is usually incohesive.

Question 27

Cause of NA orogenic collapse

Answer 27

In the late Creataceous and early Eocene there was active subduction of the Fallaron plate which created Andean style crustal thickening throughout the Western part of the US. It also is what emplaced many of our copper porphyries!

Question 28

Chevron Folds

Answer 28

These are folds that are really angular and kinky. They are generally parallel folds with very discrete hinges

Question 29

Chill zone

Answer 29

This is the fine grained material that forms near the wall of an intrusion. It is usually near contact metamorphosis, auriole, and skarns (reserved for LS)

Question 30

Cleavage

Answer 30

This is a structure that forms in the lower part of the brittle regime (~10 km) common with passive folding and transposition. It is indicated by a staple symbol when mapping.

Geometry: They are closely spaced, planar, woody surfaces associated with folds, and may be oblique to bedding. Generally, it has a “domainal” nature where parts are deformed more than others, creating a very finite “banding” of platy minerals like mica.

Stress/Strain: max shortening perpendicular to sigma 1 and through pressure solution and recrystallization there is a decrease in wavelength and limb thickness

Most common is axial planar cleavage which is perpendicular to sigma 1 and parallel to axial planes.

Question 31

Cleavage

Answer 31

These are closely spaced, regular fractures of distinctly weaker planes.

Question 32

Coloumb failure criterion

Answer 32

This is a line given by sigma = C_o + tan(30) sigma n

If any stress state is beyond the failure criterion then it fails.

Question 33

Concentric/Parallel folds

Answer 33

These are folds that show an even thickness across the beds within the fold.

Question 34

Conical/Plunging Folds

Answer 34

This describes folds that plunge into a flat plane. These are described by the centerline axis (defined as the vector at the tip of the cone) and the opical angle which is the angle between the inflection point and horizontal

Question 35

Conical folds on a stereograph

Answer 35

These will have a great circle created from planes to poles of the bedding transverse to the hinge and the hinge will plot 90^o off in the “center” of the poles.

Question 36

Conjugate fault systems

Answer 36

These are parallel for thrust and normal faults (like wedges that squeeze the material) and are 60/30 for strike slip.

Question 37

Core complex cross-sectional geometry

Answer 37

Metamorphic core complexes tend to be flanked on one side my a low angle detachement fault overlayed by high angle normal faults and interfaced with a mylonitic gneiss (think about this side of the Catalinas). At their center is a granite core related to decrompression melting. The other side is characterized by a listric breakaway fault and a basin with domino style faulting.

Across the whole complex there is the same sense of shear creating the mylonitic fabric

Question 38

Crystal Fiber Veins

Answer 38

These are oriented growths of calcite and quartz which originate from saturated fluids causing joints to form. When the pressure of the fluid is relieved the crystals precipitate. The change in direction of the crystals indicates how the principle stresses changed over time aka the dilation history

Question 39

Cut-offs

Answer 39

The footwall and hanging wall cut-off are the points where the marker beds intersect the fault surface and terminate

Question 40

Decollment zone

Answer 40

This is when there is detachment from lower layers and the formation of a cupsate. It is common in concentric folds.

Question 41

Deformation

Answer 41

This is the complete kinematic evolution of the rock body of interest.

It includes: traslation (rigid), rotation (rigid), strain: dilation/change in volume, and distortion

Question 42

dip of a plane

Answer 42

This increases from out to in. Dip is greatest at the north south line

Question 43

Dip of plane to pole

Answer 43

The dip of a plane as a pole on a stereonet is measured from 0 being the inside and 90 being the outside. Alternatively it is 90- angle from outside

Question 44

Domino style faulting

Answer 44

This is similar to listric normal faulting but both the footwall and hanging wall rotate so that way the hanging wall dips towards the fault and the footwall rotates so bedding is near perpindicular to the fault surface.

Question 45

Drag folds

Answer 45

These are folds that occur along the interface of a fault and appear to resemble frictional resistance to sliding along the fault. They are actually caused by the propagation of the fault surface from depth. For example, in a normal fault (which represents extension) before the shear plane physically divides strata in a normal fault, the hanging wall will be extended and the materials near the fault “collapse” deforming those on the surface upwards. You are basically removing the support below the layers causing slumpling.

Question 46

Dynamic Analysis

Answer 46

This is the attempt to understand deformation as a function of stress, shear, strain, and rheology…

Question 47

Effective stress

Answer 47

This is the confining pressure-fluid pressure. Fluid pressure has a very significant impact on the stress state of a rock at depth.

Question 48

En Echelon Segment

Answer 48

This is where a fringe meets the exposed surface. It is usually at an angle to the main face.

Question 49

En echelon tension gashes

Answer 49

These are opening that form in simple shear zones. They have s shapes and open in the direction of minimum stress.

Question 50

Extension (e)

Answer 50

This is the ratio of the change in length to the original length.

e = (L_f - L_o)/L_o and can be expressed as a percent

Question 51

Fault rocks as a function of depth

Answer 51

0-4 km of depth: Breccia/gouge which has angular clasts, can be incohesive, and comminuted material. Gouge is very fine breccia.

4-10 km: cohesive cataclasite (cement) which is angular clasts surrounding by hard muddy material. It is cohesive and usually fluid related. (can become breccia during exhumation)

10+: mylonites This is when quartz begins to ductily deform and you have extremely strained fabrics.

Question 52

Fault Scarp

Answer 52

This is a stair-step like surface which coincides with fault zones where the total displacement is given by the summation of the step heights

Question 53

Fault Surfaces

Answer 53

These are discrete (usually elliptical) surfaces where slip occurs due to faulting. The maximum displacement occurs at the center (r=0) and decreases outward reaching 0 at the tip line loop

Question 54

Fault symbology

Answer 54

These are always with thick lines

Normal faults = line with a dot and dip

Thrust fault = line with teeth and dip

Strike slip = arrows

approximate location = dashes

concealed = vertical dashes

uncertain = dashes with question marks

Question 55

Faults

Answer 55

Discrete fracture surfaces/zones where slip has occurred and is recorded. These are the result of compressional stress compensation and are formed from the preferred alignment of microcracks forming a shear fracture. These occur at phi of ~30^o from the principal stress (sigma 1)

Question 56

Faults on maps

Answer 56

If it is strike slip then it is double arrows

If it is not vertical then there is an arrow pointing in the dip direction with the dip.

If it is normal then sticks with dots are normal an on the hanging wall. If it is thrust then there are teeth on the HW.

Generally we map the fault trace which is the planar feature interfacing the topographic surface.

Question 57

Finding a axial trace steoreograph

Answer 57

1. plot poles to the bedding planes on both sides of fold
2. Align the dots so they lie along a great circle. Draw the great circle.
3. ) Count 90 along the horizontal and plot a point. This is beta
4. The plunge of beta is found by rotating beta to north and counting from the inside out
5. ) Align beta so that it aligns to the great circle representing the plunge. Draw
6. Connect the outer edges of the plane.

This represents the plane of the axial surface and beta is the trend and plunge of the fold axis.

Question 58

Finding strike and dip from two apparent dip/dipdirection

Answer 58

1. plot the dip/dip direction as points
2. rotate the stereonet so that the dots align to a great circle.
3. This is the true plane

Question 59

Finding true strike and dip from plunge, strike, and dip

Answer 59

To find the true strike and dip of a plane that intersects two surfaces first find the strike and dip of each of the faces. Then find the rake from the plane to the intersection of the bed of interest. Make sure to note the rake and direction.

Plot the surfaces as planes with lines at the rake. Align to be on a great circle and plot. This plane is your true plane.

Question 60

Flexural folding and slip

Answer 60

This is the idea of folds where less competent layers assume the bulk of strain/deformation. This means that during folding there is shear along the boundaries of layers and that the more competent layers are more likely to maintain their original mid-point length.

Flexural slip is when weaker beds (shales, mudstones…) accommodate the slip of harder, less elastic rocks nearby This creates shear through folding. The individual displacements are small but rapidly accumulate.

Question 61

Fluid Pressure ratio

Answer 61

P_f/P_r = ρ_f g h_f/ρ_r g h_r This represents the fluid pressure from a column divided by the lithostatic pressure

Question 62

Fold description terms

Answer 62

Wavelength, width, height, interlimb angle, symmetry, and vergence is common

Question 63

Fold orientation labels

Answer 63

Find the strike and dip of the axial surface and the trend/plunge of the hinge line.

Question 64

Fold symmetry terms

Answer 64

If folds are assymetric terms like “top-to-east” or “top-to-right” to give a sense of shear. This would correlate to an east-vergent fold.

Question 65

Fold Tightness

Answer 65

This is given by the interlimb angle which can be found via direct measurement or by taking strike/dip of the inflection points of the limbs. Plotting these as poles and align them to a great circle to find the angle between them.

Question 66

Fold Trains

Answer 66

These describe regional periodicity in waveforms of folding.

Question 67

Folds

Answer 67

These are when the rock behaves like a ductile material when compensating for stress/strain which results in curved, bent, or crumpled strata.

Question 68

Folds: mapping and arrows

Answer 68

1. map marker beds and define hinge zones
2. connect the hinge zones and draw an axial trace
3. define anticline vs syncline and use an arrow to show plunge direction.

Synclines will have arrows pointing inwards and dips pointing inwards (oldest beds at center)

anticlines will have dips and lines pointing outwards (oldest beds out)

Overturned beds will be in the same direction but use the original dip direction to ID.

Question 69

Foliation

Answer 69

This is penetrative planar fabric that forms because of mineral recrystallization and plastic deformation at temperatures exceeding 300 celsius

Question 70

Foliation

Answer 70

Any penetrative planar elements

Within metamorphic rocks foliation is a f(plasticity) and a secondary structure

Within igneous rocks foliation is a f(primary structure)

It includes layering produced by ductile deformation

Question 71

Footwall

Answer 71

This is the bottom rock. It has the resting surface for the rock above.

Question 72

Force

Answer 72

This is something that changes the motion of a body via acceleration

F = ma in Newtons

Question 73

Forced folds

Answer 73

These are folds that are related to faulting and the fault geometry

Question 74

Fracture Spacing Ratio (FSR)

Answer 74

This is defined as the median spacing/bed thickness. Usually it increases with bed thickness

Question 75

General shear

Answer 75

Any mix of pure and simple shearing. It is also in plane. This will result in a mixture of smooshing and translation.

Question 76

Gneissic Structure

Answer 76

Penetrative plane layering with compositional banding, mineral laminae, and quartz eigens

Question 77

Griffith Crack Theory

Answer 77

This is a form of kinematic analysis that says that cracks that are perpendicular to σ₃ form joints to relieve tension. It also speculates that the largest microcrack perpendicular to tension will be the origin for a joint and other joints that are not orthogonal close when the joint forms.

Question 78

Hanging Wall

Answer 78

This is the wall of a fault that lies on the top. The fault undercuts the rocks.

ALL map symbols go onto the hanging wall

Question 79

Heave and throw

Answer 79

Heave is the horizontal component of the displacement along a fault. Throw is the vertical component.

Question 80

Heterogeneous deformation

Answer 80

This is irregular and non-uniform deformation which is best broken into vectors of homogenous deformation.

Question 81

Hinge line

Answer 81

This is the 1d vector that describes the trend and plunge of the hinge line.

It is also called the fold axis.

Question 82

Homogeneous deformation

Answer 82

This is the systematic and uniform deformation of a body. In this form of deformation straight lines stay straight (although rotated and changed) and circles become ellipses.

Question 83

Horst and Graben structures

Answer 83

These are characterized by high areas flanked by normal faulting on both sides. They are consistent with andersons theory of faulting (highs have normal lows have thrust) often underlined by a detachment fault defined as a low-angle normal fault most commonly found at depth. This fault determines if individual structures are synthetic (dip in the same direction) or antithetic (dip in the opposite direction)

Question 84

How are stress trajectories oriented on a buckled layer/topography?

Answer 84

It is similar to a bending moment in a beam where at the uppermost crest sigma 1 is vertical (normal faulting) and at lower areas it is horizontal (thrust faulting)

Question 85

How do columnar joint sets form?

Answer 85

Because the middle of the basalt is usually the hottest part, the joints start at both the top and the bottom to propagate and meet in the center

Question 86

How do real rocks behave in response to stress?

Answer 86

Real rocks have an initial elastic deformation which is followed by strain hardening (an increase in E) or strain softening (a decrease in E)

Question 87

How does cleavage form?

Answer 87

1. The rotation of minerals (small contribution): grains orient perpendicular to sigma 1 so that their smallest width orientation creates maximum compaction
2. Directional recrystallization. This is when things like mica recrystalize due to high pressures and form “sheets” or laths of crystals perpindicular to sigma 1.
3. pressure solution (primary factor): Quartz dissolves, micas do not. In addition, pressure solution is going to be greatest in the direction perpinduicular to the greatest principle stress which means that grains will “elongate” perpindicular to that direction.

Question 88

How does depth impact faulting/folding

Answer 88

Below 12 km mylonites form

from 12-10 km is the crystal plastic regime where breccias become quasi mylonites because T=250-350 C

Above 10 km is the frictional regime where rocks are rather brittle. This is the region that produces breccias

Question 89

How does elasticity impact folding?

Answer 89

IF there is a large difference in elasticity between a stronger layer in a weaker host the folds will become ptygmatic whereas is there contrast is small then buckling occurs and it creates cupsate lobate folds/

Question 90

How does fluid pressure influence failure

Answer 90

Fluid pressure lowers both principal stresses moving Mohr’s circle to the left without changing the differential stress. This occurs until it intersects tensile failure or coloumb failure.

Question 91

How does rock strength change in each rheologic regime

Answer 91

In the brittle regime strength is depth dependent. In the ductile regime strength decreases exponentially with temperature and then in the viscous regime it is strain rate dependent.

Question 92

How does strength change with increasing strain rate, temperature, and depth?

Answer 92

With strain rate strength increases

With temperature strength decreases

With depth strength increases

Question 93

How does strength vary with depth and lithology?

Answer 93

Rocks that are weak on the surface (mafic rocks) are strong at depth and rocks that are strong in the brittle regime (quartzite) are weak at depth.

The rheology of quartzite controls rock deformation.

Question 94

How does strength vary with temperature?

Answer 94

As temperature increases strain rate increases exponentially and strength decreases exponentially.

Question 95

How to find strike on a topo map

Answer 95

Connect the topo lines of equal elevation where the upper/lower bound of the structure intersects said topo line.

This is because strike is defined as the direction where elevation does not change.

Question 96

How to plot a plane

Answer 96

1 mark the strike on the circumference of the plot and rotate it to north

2 count from the center out to the dip circle and draw

Question 97

How to plot planes to poles

Answer 97

1. plot the great circle representing the plane
2. rotate the great circle to N-S and count 90 degrees along the horizontal line
3. plot the pole

Question 98

How to plot rake on a plane

Answer 98

1. ) using strike and dip of the plane find the great circle (rotate CC to strike and count out from center to dip
2. ) With the great circle oriented N-S count along the circle to the rake angle. Plot a dot.
3. ) Find apparent dip by rotating the plot until the dot is on the N-S line and count in.
4. ) find trend by returning the great circle to N-S and drawing a line from the center of the plot to the radius. Where this intersects the circumference of the circle is the trend.

Question 99

Identify: The rock, fabric, SC surfaces, and sense of shear

Answer 99

The C-surfaces are the linear planes where the mica exists, S surfaces are elongation surfaces that are 45 degrees off. The sense of shear is top to the right as noted by how the S surfaces merge into the C surfaces by curving right.

Question 100

Implosion Breccias

Answer 100

These are effectively rock bursts that form from holes in the fault. They are usually cemented by hydrothermal cements and clays.

Question 101

Incohesive Breccia series

Answer 101

This only refers to brittle fault rocks with angular clasts in a fine matrix and represents positive dilation.

megabreccias D> .5m

breccias 1mm

microbreccias.1mm

gouge D

Question 102

Inflection Points

Answer 102

These are where the second derivative of the line that describes the strike of the fold switches sign. It is where convexity switches sign.

Question 103

Into and out of the page symbols

Answer 103

A circle with an x is into the page and a circle with a dot is coming out of the page. It is like an arrow from a bow.

Question 104

Intrusive contacts

Answer 104

This is the contact of igneous rock and country rock

Question 105

Joint Intersection Types

Answer 105

Joint intersections help to indicate the pathway of formation.

Y-interceptions are characteristic of thermal shrinking and the development of hexagonal columns

T-intersections are common in unloading and result in orthogonal joint sets where the terminating joint is young than the throughgoing

X-intersections occur when joints meet at acute angles but both are continuous

Question 106

Joint Ribs

Answer 106

These are concentric bands around the origin that represent where the joint was arrested during propogation (slowed or ceased) They form slight lumps or decreased relief of the plumes intersecting them.

Question 107

Joint Saturation

Answer 107

This is the concept that there is some maximum joints/volume of joints that will not be exceeded by the rock. Stress transition model states that when the joint saturation is exceeded the positive dilation switches the tensile stress to compressive. Stress-transfer model says that if undersaturated the next joint will form at the midpoint of the prior two

Question 108

Joint significance (4 econ and geomorph)

Answer 108

The economic value of joints is that they are conduits for hot fluids to precipitate metals and other valuable things. This also makes them good for geothermal and hydrocarbon storage. Joints also enable quarrying.

Joints are also potent weathering agents and can become enlarged or contracted via deformation. They also act as conduits for groundwater and potential contamination

Question 109

joint surfaces

Answer 109

These are rounded fans that form from a localized stress perturbation. They record the propagation of the joint fracture.

Question 110

Joint System

Answer 110

This describes the presence of 2+joint sets in a rock body

Question 111

Joints

Answer 111

These are smooth, planar features that cut through rock bodies and layers with ~0 displacement across the joint. These occur perpendicular to tension and are brittle failure. They tend to be continuous and through-going and occur at intervals of cm-km. In 3d they are elliptical and follow the path of maximum tension but are in a plane perpendicular to the primary tensile stress.

They primarily occur in the uppermost few kilometers of the crust where tension can occur and occur in sets which are families of parallel, evenly spaced joints

During the fracturing of the rock the propagation path minimizes shear at the joint tip and maximizes tensile stress.

Question 112

Joints vs. Shear fractures

Answer 112

Formationally joints are only from tension whereas shear fractures are from shearing.

Shear fractures intersect at 60^o joints do not

Shear fractures will have slickenlines

Question 113

Joints: Dynamic analysis

Answer 113

This is the idea that joints form due to tensile stresses which are most commonly from cooling or unloading.

Question 114

Kinematic Analysis

Answer 114

This is understanding the motion of materials during displacement not only their positions at t=0 and t>0 but s(t). Thus, interpreting structures as a function of x,y,z,t with the goal to identify the displacement rate and path

Question 115

Kinematics of joint formation

Answer 115

The origin of the joint is always the nucleas for the plumose structure and the main face is perpendicular to the tensile stress. They are formed in some form of mode I jointing.

Question 116

Kink folding

Answer 116

This is similar to buckling where there is one spot that shows a kink. There are z-folds (dextral) and s-folds (sinistral). They are defined by high anisotropic stress states post buckling in layers that have large amounts of cohesion.

They can result of bending moments, dilation, in-kink rotation (torque), or shear.

Question 117

Kinks

Answer 117

These occur when the joint is arrested and the tensile stress is reoriented. This occurs near the surface which is how en echelon traces and fringes form.

Question 118

Labels on folds

Answer 118

These have arrows pointing from the oldest to youngest beds, they indicate the facing direction

Question 119

Lengthening

Answer 119

This is the concept in faulting changing the net surface distance of an area. In normal faulting the crust is lengthened and thinned. It thrust faulting it is shortened and thickened. To determine the %stretch you find the ratio of the original map view length to the new length.

Question 120

Linear Elastic Fracture Mechanics (LEFM)

Answer 120

This is the study of how heterogeneity in the structure of a material changes the stress state. It posits that brittle materials have a fracture toughness given by σ_m and when the local stress intensity > σ_m then a flaw is created to lower the localized stress to a subcritical state

Question 121

linear feature mapping symbology

Answer 121

Foliation = shaded triangle with dip

Vertical foliation = shaded diamond

Cleavage = a [ symbol with the dip inside in the direction of strike

Vertical cleavage = I

lines = dot with an area with the plunge angle

Question 122

Linear features

Answer 122

These are features that are 1D and are described as a vector with trend and plunge. Always be sure to indicate the direction of plunge (Ex: 18^o NW would be a line that is pointing NW in the downward direction)

Question 123

Lineations

Answer 123

This is ANY linear feature on a rock

These are any preferred alignment of materials due to shear flow.

Question 124

Lines on stereonets

Answer 124

Lines plot as points on the stereonet. a plunge of 90 will be at the center.

Question 125

Listric

Answer 125

This is a fault that has a curved surface which approaches horizontal with depth.

Question 126

Listric normal fault geometry

Answer 126

These are faults that shallow with depth. They are defined by a rollover monocline where thinning and rotation has deformed the beds, ‘drag folds’ on the upper/lower surfaces, and growth strata (alluvium) thickening towards the fault surface. In comparison to domino faulting, the footwall remains stationary and “blocking” does not occur.

Question 127

Lithosphere

Answer 127

The outermost part of the Earth that create the “Solid” plates. It is composed of 4-9 km thick oceanic crust and 25-70 km continental crust.

Oceanic crust is mafic and forms from the decompression melting of the asthenosphere. It commonly has igneous intrustions (gabbros and dikes) along with extrusive pillow basalts. It is younger than ~200 mya.

Contintental Crust is very heterogeneous and the majority of it has been constantly deformed since the Precambrian. The oldest continental crust is >4Ga

Question 128

Lock Up Angle

Answer 128

This is the maximum angle from sigma 1 where a plane will slip. It is determined by finding the intersect of mohr’s circle and coloumbs failure. At this point the upper intercept of the circle with byerlees law shows the maximum angle from sigma 1 where a plane will slip. Anything beyond that and the rock will form a new fracture.

Question 129

Main Joint Faces

Answer 129

These are planar/elliptical faces with a large plumose and rib structure. The fringe is defined by smaller plumose structures on the outer edges of the main face that are slightly hackly.

Question 130

mapping: line thickness meaning

Answer 130

thin lines are reserved for depositional and intrusion based contacts

thick lines are used for faults

Question 131

Marker beds

Answer 131

These are any traceable bed. They are critical to mapping and you can use dots or line-dot-line sequences to draw them for mapping.

Question 132

Median Trace

Answer 132

This is the line/surface that defines the connection of the inflection points of the fold.

Question 133

Microcrack types

Answer 133

There are intragranular (within a grain), transgranular (accross 1+ grains), intergranular (along the grain edges.

Question 134

Modes of Loading

Answer 134

These are the three kinds of loads that propogate at the joint tip and along the tipline.

Mode I: Opening which is pure extension perpindicular to the fracture plane

Mode II: Sliding which is pure shear parallel to the plane and in the plane

Mode III: Scissoring this is shear within the fracture plane and motion parallel to the tipline but perpendicular to the upper and lower bed faces.

There is also mixed modes of loading.

Question 135

Mohovic Dsicontinuity

Answer 135

This is the lower bound of the lithosphere. It represents a compositional boundary within the lithosphere.

Question 136

Monoclines

Answer 136

These are “regional tables/platforms” Where there is one life or decline. It looks like play-doh was layed over a cliff

Question 137

MPa to kbar conversion

Answer 137

1 kbar = .1 GPa

Question 138

Mylonitic foliation

Answer 138

This is a foliation that occurs at ~12+ km in high shear zones where there is a brittle deformation of feldspar and garnet porphyroclasts (augens) and plastic deformation of quartz ribbons.

Question 139

Normal fault Symbol

Answer 139

This has a line with a dot

Question 140

Normal faults

Answer 140

These enable extension and relieve tension. They are caused by σ₁ = σ_v On the surface this is usually the weight of the rock.

Normal faults are identified by the appearance of the hanging wall rocks being younger than the footwall indicating that the hanging wall moved down.

It omits stratigraphic layers in the sequences when looking on the fault surface.

Question 141

oblique slip nomenclature

Answer 141

These are named by adding handedness to the normal name for the fault. Ex: Normal right handed fault

Question 142

Orthographic Projection

Answer 142

This is the concept of using 2d representations of 3d surfaces/objects.

Question 143

Overturned folds

Answer 143

This refers to any fold where a limb’s stratigraphic up is not pointing towards the surface. If the fold is inclined the back limb can be overturned but the front limb can be non-overturned based on how you define back and front

Question 144

Parallel Folds

Answer 144

These are folds where the bed thickness remains constant throughout the fold. It indicates lower temperature and shallower folding.

Question 145

Particle paths

Answer 145

These are drawings showing how material moves during deformation. If it is pure shear then the particle paths are hyperbolic around the origin showing how the material moves down then is smooshed outwards. If it is simple shear the particles move in a more “translated” sense of motion where their paths are not precisely curved.

Question 146

Passive folds

Answer 146

These are folds where the hinges are thick and the limbs are thin but consistent throughout the rock. This is common in high grade metamorphic rocks where there is high average ductility and low ductility contrast. This can be aided by “flattening” which is the dissolution of parts of the rock.

Question 147

Physical and Geometric elements

Answer 147

Physical elements are elements of structures we can directly measure (bed thickness, bedding…)

Geometric elements are intangible elements of structures that we identify through analyzing physical elements.

Question 148

Pi Diagram

Answer 148

If we plot limbs of a fold as planes to poles these will connect to create a great circle known as the pi circle which represents a plane perfectly perpindicular to the hinge line which will plot 90⁰ off the pi circle center

Question 149

Planar Features

Answer 149

These are features that can be described as a 2d plane in space. They are described by a strike and dip

Question 150

Planes on stereonets

Answer 150

Planes with a dip of 90 degrees plot along the N-S line horizontal planes are on the primitive circle. The strike of a plane is the line connecting the two outer edges of the great circle

Question 151

Plotting a plane from apparent dip

Answer 151

Given strike, apparent dip, apparent dip direction.

1. Make a tick at the strike and at apparent dip. Rotate the plot CC so the strike tick is at north. Draw the strike line (plane with dip=90)
2. Rotate CC so that the dip tick is in the E mark
3. From the inside out count out the apparent dip and draw the great circle.
4. true dip is found when strike is at N

Question 152

Plotting Mohr’s circle

Answer 152

the center is at the average principle stress. r = sigma 1 - center

Any point on the circle as a plane within a rock is measured 2*theta clockwise from sigma three (on the plot) where theta is the angle from sigma one to the plane in the rock. This is the same as 90-normal theta (from horizontal) or is the angle from the pole of the shear plane to sigma 1.

Question 153

Plumose structure

Answer 153

This occurs on the faces of joints and are feather like patterns propagating from the origin of the joint outward

Question 154

Plunging folds

Answer 154

These are folds that have a plunging hinge line.

Question 155

Poles on stereonets

Answer 155

Lines are projecting from the center to the outer edge so vertical lines are in the center of the plot and horizontal lines (vertical planes if planes to poles) are on the outer edge

Question 156

Porphyroclast Systems

Answer 156

There are two primary structures that form when porphyroclasts are in high shear environments.

Sigma structure: This is where there is simple shear but the porphyroclast does NOT rotate. It looks like a tension gash.

Delta Structure: this is where there is rotation and simple shear of the porphyroclast so the “tails” of the partial melt are non-linear.

Question 157

Primary deformation structures

Answer 157

These are structures that form during lithification/deposition. These commonly include assymetric intraformational folds (magmatic flow deformation) and soft sediment deformation structures

Question 158

Primary Structures

Answer 158

This includes structures that form because of the internal/inherent nature of creating rocks. This includes bedding, flow layering, intrusions… They are described using contact type, lith/mineralogy, texture, resistance, and color.

Stratigraphic up can be identified with ripple marks, cross beds, mudcracks, and pillow structures which are all primary structures

Question 159

Principal stress regimes based on faulting

Answer 159

Normal faults have sigma 1 radial (like gravity) and sigma 3 laterally. They dip at ~60

Thrust faults have sigma 1 laterial and sigma 3 radial. These dip at ~30

Strike slip has sigma 1 and 3 lateral and sigma 2 radial. The wedge being pushed by sigma 1 has an inner-angle of 60 and dip at ~90.

Question 160

Process zone

Answer 160

This is the concept that when a rock is subjected to far field stresses that create tensile points in the structure this creates a variety of microcracks and the joint eventually forms by propagating along the microcracks perpendicular to tensile sterss.

Question 161

Progressive Strain Significance

Answer 161

The relative path of deformation of any material may differ dramatically. This means that the start/end state do not characteristically define the strain state of the rock.

Question 162

Psuedotachylite

Answer 162

This is false tachylite (lightning rocks). It forms from the frictional melting of rock. Not grinding or fracturing. It is generally glassy to crystalline and fills sills/gaps within the generation surface

Question 163

Pure shear strain

Answer 163

This is a type of plane strain where one direction is extended and the other is compressed. It is co-axial deformation where there is not a rotation in the principal stretching axes. This also means that the compression to extension ratio is relatively 1:1. The change in area is also zero.

Question 164

Rake

Answer 164

This is a measure of the acute angle of a line in a plane measured In the plane.

ALWAYS note the direction that the lineation is moving down towards.

Question 165

Regional folding terms

Answer 165

On a regional level folds can be described as anticlinorium and synclorium. These can be considered first order folds that have localized anti/synclines superimposed onto their structure

Question 166

Relationship between Metamorphic Core Complexes and Orogenic Collapse

Answer 166

Across the globe, similar patterns of continental extension related to orogenic collapse is correlated with the presence of metamorphic core complexes. In NA these occur in the basin and range extensional system from Mexico through Canada. They tend to be domal arches ranging from 25 to 60 km in length, bounded by low angle normal faults, and display a unidirectional sense of shear, this is simple shear

Question 167

Relay Ramps

Answer 167

Within a series of partially connected normal faults the relay ramps refer to the inclines between tip points.

Question 168

Release/Unloading Joints

Answer 168

These are joints that form perpindicular to the principle stresses and represent the dilation of the surface above/below the nuetral axis

Question 169

Rheologic difference in asthenosphere and lithosphere

Answer 169

the viscosity of the lower lithosphere is ~10²³ poise

The viscosity of the upper asthenosphere is ~10²¹ poise

Question 170

Rheology, Elastic, plastic, and viscous behavior

Answer 170

This is the mechanical behavior of rocks. It has three end-members, elastic, plastic, and viscous.

Elastic strain is defined by recoverable deformation through the hysteresis loop where stress is related to strain by hookes law in a linear fashion as defined by E.

Plastic Behavior is defined by materials having an irreversible change in shape. Ductile rocks have undergone plastic deformation and are at a point where changes in strain occur continuously even if stress is constant. Below the critical stress E=infinity so there is no change in strain with stress.

Viscous Behavior is described by “solid-state flow” where small proportions of melt and long time scales enable materials to act quasi-fluid like where differential stress = viscosity x strain rate

Question 171

Ridge in Groove lineations

Answer 171

These are the toole marks used to describe slickenlines. They often accompany crystallographic reorientation

Question 172

Riedel Shears

Answer 172

These are strike, slip mini faults which form +-30^o from sigma 1

Question 173

Right hand rule

Answer 173

The planar feature dips to the right so if you put your right hand down dip your thumb points in the direction of strike

Question 174

Rock strenght

Answer 174

This is the amount of differential stress required to cause failure in the brittle regime or flow within the plastic regime.

Question 175

Rotational planar normal faulting

Answer 175

This is “domino-style” or “bookshelf” faulting defined by fault dip decreasing with increased slip.

Question 176

S-C fabrics

Answer 176

S-surfaces (planes of schistosity/foliation/flattening) this is the plane of S₁ perpindicular to sigma 1 and

C surfaces which are usually linear and form planes of maximal shear. They bisect the intersetion of sigma one and S₁

To identify: look at the relatively repetitive and planar surfaces of micas within the higher grade fabrics these will correspond to the C-surfaces and 45 degrees from that will be the curvy S-surfaces.

Question 177

SCAT plots

Answer 177

These are a type of analytical plots for analyzing azmuth and dip vs depth of a drillhole. They can show faulting or folding via cusps.

Question 178

Schistosic Structure

Answer 178

Penetrative planar layers of mica and lath-shaped minerals

Question 179

Secondary Structures

Answer 179

These are structures that are the result of deformation after the rocks are formed. It is the primary focus of the class. They include things like joints, veins, faults, folds, cleavage, lineations, and microstructural deformation.

Question 180

Shear Strain

Answer 180

This is a measure of rotation due to shear

γ = Δx/y = tan(ψ) where ψ = the angle of the roation of an internal element.

Question 181

Shear zone

Answer 181

This is an area that can be small or large and is described by many sub-parallel interconnected slip surfaces. These form at depth where rock is more plastic than brittle.

Question 182

Shear Zones

Answer 182

This describes offset over a zone that can represent cm-km width. This represents basal faulting where temperature is high enough to cause plastic deformation. They tend to form mylonites.

Question 183

Signifigance of Folds (3 econ)

Answer 183

1. ) structural traps for oil and gas
2. saddle reef deposits which are lode quartz precipitating in the low P of the fold axis
3. Strata-bound deposits: These are commonly Pb-Ag deposits

Question 184

Signs of Surface Fault Expression

Answer 184

Turncation of units, mismatched ages of units, repeated stratigraphy, and/or missing data

Question 185

Similar Folds

Answer 185

These are folds where the limbs thicken/thin evenly between layers. Usually hinges are thick and inflection points are thin. It indicates mid ductility.

Question 186

Simple shear

Answer 186

This is an end member of strain defined by noncoaxial deformation. This is shear in a typical form where the principle stretching axis rotates over deformation. It is where the primary force/s are parallel and separated by some distance, not in line/co-axial. It is characterized by elements being deformed along a line parallel to the direction of shear.

Question 187

Slaty Cleavage

Answer 187

These are fine parallel laminae

Question 188

Slickenlines

Answer 188

These are toole lines that show displacement along faults and record the displacement history of the fault. They are generally straight, fine, delicate. They may cement breaking material from the other wall to create hooks, lumps, or spikes.

Question 189

Slickensides

Answer 189

These are polished surfaces or mineral coating that form from the displacement and associated heat

Question 190

slickolites

Answer 190

Crystals that form during the displacement of material along a fault with the aid of stylolitic pressure dissolution. This generally creates a pock-marked surface of rods and cones.

Question 191

Slip

Answer 191

This is a term that describes the actual relative displacement.

separation is the measure of the apparent relative displacement.

Question 192

Slip Vector

Answer 192

This includes the slip direction, magnitude, and sense. It is the displacement vector.

Question 193

Slip vs new fracture formation via mohrs circle

Answer 193

Using mohrs circle, coloumb failure, and byerlees law we can define the space where slip will occur before a new fracture forms to accommodate failure. If a mohrs circle intersects byerlees law at stresses below coloumb failure then any pre-existing fracture within that orientation will slip before failure occurs. If the plane is oriented such that mohr’s circle intersects the coloumb failure criterion before the plane intersects byerlees law or the coloumb failure then a new fault is formed.

Question 194

Slip-fiber lineations

Answer 194

These are ridge like growths that form on the fault surface due to asparities during the translation. It acts as a low pressure area for displaced material. The smooth way is the direction of slip. These generally have chatter marks which are the cliffy parts of the lineations

Question 195

Steps to create subsurface contour maps

Answer 195

1. ) plot hole collars
2. ) plot layer intercepts
3. ) make contours with the knowledge that faults displace contours

Question 196

Steps to creating a cross sections

Answer 196

1. ) choose the transect (usually perpendicular to the strike of the main features)
2. ) Create topographic profile by “Draping” the transect line onto the “surface”
3. ) Transfer the station point information and other relevant knowns onto the profile. Usually this starts with the dip of beds near the surface while making sure to account for apparent dip.
4. ) Construct interpretation
5. ) QA/QC by making sure that conservation of mass holds (midpoint L = C1)

Question 197

Steps to finding apparent dip

Answer 197

1. ) draw the dipping plane in map view (x-y)
2. ) Use a cross section perpendicular to strike to find the contour line at some depth d where x = d/tanθ and θ is the dip. x will be real normal distance and will need to be converted to map units
3. ) At whatever angle you are looking to cross the structure the apparent dip is found by measuring the distance from the contour z=0 to the created contour at z =d. This is x₂.
4. ) convert x₂ to real units and θ₂ (apparent dip) = tan^-1 (d/x₂)

Question 198

Steps to finding intersecting planes (orthogonally)

Answer 198

1. ) in map view draw the two intersecting planes
2. ) Draw two subsurface contours at depth = d for each plane with dips; θ₁ θ₂ and distances from the drawn line in map view given by d/tanθ = x_1,2
3. ) draw these contours
4. ) find the map distance from the original intersect to the new intersect. Measure this distance (x₃)
5. ) the plunge of the line of intersection is given by tan^-1(d/x₃) and the trend is determined via measuring

Question 199

Strain

Answer 199

This is a type of deformation defined by a change in the volume of a rock body (dilation) or a change in the shape (distortion). It is a form of non-rigid deformation which results in a change of the spatial arrangement of the materials of a rock.

If we know the size of the body at t=0 then we can quantify strain.

Question 200

Strain Analysis

Answer 200

This a form of structural analysis based in descriptive analysis that only analyzes distortion and dilation

Question 201

Strain analysis

Answer 201

This is the analysis of strain of a rock body that defines how every line passing through the centroid changes in length and angle during deformation.

Question 202

Strain angle equation

Answer 202

tan ϕ_d = tan ϕ (S₃/S₁)

where ϕ_d = the angle between S₁ (max extension) and some line AFTER deformation

and ϕ is the angle between the same line and S₁ before deformation

Where the line is arbitrarily drawn based on field measures and S1 is considered a direction in which the maximum strain occured.

Question 203

Strain Ellipse

Answer 203

This is an ellipse or ellipsoid that shows the finite stretch of the material in every direction. It models what would happen to a sphere in that state of strain.

Question 204

Strain rate

Answer 204

This is the extension/time. In real geologic terms we cannot match the incredibly slow timescales of tectonic which acts as a caveat to lab experiments.

Question 205

Stratigraphic Throw

Answer 205

This is the displacement between two marker beds at a fault surface

Question 206

Strength vs. depth curve/chart

Answer 206

This shows three main parts:

An elastic part given by mohrs failure criterion where stress is linearly related to strain by youngs modulus and brittle deformation dominates. At these depths joints then faults occur. At the lower depths, quartz becomes plastic (T300-350 C)

The yield strength (~10-15 km) indicates the elastic-plastic transition and is where earthquakes nucleate (greatest stress). It is also where von mises failure begins.

After this an exponential decay of strength occurs marked by the brittle-ductile transition. It begins with flexural flow fold then plastic flow (qtz) and viscous flow (strain dependent so stress is constant)

Question 207

Strength vs. depth diagram

Answer 207

This is simply the upper half of mohrs circle plotted vertially so normal stress is considered proportional to depth where P=30 Mpa*h (km)

Question 208

Stress

Answer 208

This is force that tends to deform a body. The difference is that deformation is a function of how force is distributed over an area.

It is used to describe all of the tractions on all surfaces that have the origin at the point of interest. It is the tensor that describes the force/area acting within a volume of rock.

Question 209

Stress/strain in bending layers

Answer 209

The bending of a group of rocks is just like the bending moment of a structure where the upper areas experience stretching (normal faulting) and the lower layers have compression (thrust faulting). In a cross section of a thick folded layer the uppermost layer is likely to have P-solution based boudinage and T-joints. The mid-layers will have X-joints and conjugate normal/thrust faults. The lower layers are likely to have increasingly dense slaty cleavage.

Question 210

Stretch

Answer 210

This is L_f/L_o it is the ratio of the final length to the initial length and denoted by S

S₃ = the minimum finite stretch

S₁ = the maximum finite stretch

Question 211

stretching lineation

Answer 211

These are the plastic elongation of mineral due to shearing or constriction. It occurs very commonly with calcite

Question 212

Structural analysis approach

Answer 212

1. Identify geometry: orientation, shape of the rock units, and nature of contacts (via mapping)
2. ID kinematics: movement during deformation and includes, translation, rotation, distortion, and dilation
3. ID dynamics related to deformation because of stresses/forces generated by tectonic processes.

Question 213

Structural Geology

Answer 213

Study of the architecture of Earth’s Crust from deformation

Question 214

Structure and sense of shear

Answer 214

This is a delta structure because there is clearly rotation. The tails indicate clockwise rotation hence the sense of shear is top to the right.

Question 215

Structure Contour Maps

Answer 215

These are subsurface topographic maps of the geologic planes.

Question 216

Stylolites

Answer 216

These are sharp zigziags in the rock that are the result of pressure dissolution. In pressure dissolution S₁ = S₂ =1 but S₃<0

Question 217

Subsurface contour maps

Answer 217

These describe the geometry and topography of subsurface features.

Question 218

Superposed Folds

Answer 218

These are when folds get folded. Oftentimes instead of having limbs that go straight like a parabola they will have slight bends and wiggles that are synforms(U shaped) and antiforms (Upside down U)

Question 219

Syncline

Answer 219

This is when the youngest layer in on the innermost hinge of the fold. Alternatively, it is a fold that is convex in the direction of the oldest beds.

Question 220

Synformal

Answer 220

This is a way of describing an anticline where the youngest layers are on the outermost hinge but at the lowest elevation. It is usually convex up but the youngest layer is at the base.

Question 221

Tectonic Joints

Answer 221

These are “cross fold” joints that form parallel to the principal stresses for a tectonic regime on a regional scale. They generally form because pore pressure is perpindicular to the principal stresses and thus propagate parallel to the principal stresses

Question 222

Tectonites

Answer 222

These are rocks that are foliated and/or lineated due to having flowed in a solid state. They are shear zone rocks.

S tectonites have schistosity (foliation) only due to flattening (pure shear). It is likely smashing play-doh

L: Lineation only where there is shearing and/or constriction. Like a pasta maker

LS: Foliation and lineation from general shear. The most common tectonite.

Question 223

Tensile buildup model

Answer 223

This postulates that the rock starts in some stress state in the compressive regime but P_f slowly builds and pushes sigma 3 into the tensile regime and a joint forms when it intersects the Coloumb Envelope of Failure.

This repeats and creates several plumose structures

Question 224

Things that form from pressure solutions

Answer 224

Stylolites: Pressure solution of limestone

“dimples”: pressure solution “drilling” of one cobble into another.

Passive folds: folds that have been “chopped” parallel to the main fold axis (large cleavage)

Cleavage

Question 225

Three Point Problem

Answer 225

This is finding the strike and dip of a plane based on three elevations.

1. ) Draw the triangle in map view that represents the three points where z1>z2>z3
2. ) Using map scale find the distance from z1 to z3 in map scale. This is x1-3
3. ) θ1-3 = sin^-1 (z1-z3)/(x1-3)
4. ) Contruct a simple line given by z = z1-tanθ x
5. ) z2 must lie on the line between z1 and z3. so z2 = z1-tanθ x. Solve for x. This is the hypotenuse of a triangle from z1 at θ. Use xsinθ = horizontal distance.
6. ) This horizontal distance can be plotted on map view between z1 and z3. The line connecting z2 and this point is the strike of the plane.
7. ) Use this line to find the strike at z1 then measure the perpindicular map distance between the contours.
8. ) θ = tan^-1(z₁-z₂)/x = true dip

Question 226

Thrust fault Mapping

Answer 226

These are shown with little teeth on the hanging wall

Question 227

Thrust faults

Answer 227

These are faults that relieve contraction/compression. The hanging wall moves up relative to the footwall which is shown by having older rocks on younger rocks or having higher grade on lower grade. It also repeats stratigraphic sequences.

Question 228

Tightness terms

Answer 228

120-180 = gentle

70-120 = open

30-70 = tight

<30 = isoclinal

Question 229

Tip and Tipline

Answer 229

The tip of a fracture is the place where the fracture propgates from. The tipline in the axis that connects the tips at the upper and lower bounds of the rock

Question 230

Traction

Answer 230

This is stress in vector form where normal stress is normal to the plane and shear is parallel to the plane

Question 231

Transform faults

Answer 231

These are discrete or broad areas where steeply dipping faults shear past one another. They accommodate for shear.

Oceanic transfer faults are very important because they enable spreading centers to have different rates of spreading.

Question 232

Translation

Answer 232

This is defined by all the point matter within a rock being displaced along equal and parallel displacement vectors. This means that the internal distances between elements remains constant but their absolute location in space changes.

Question 233

Transposition

Answer 233

Transposed rocks are rocks with lenses parallel to the main cleavage planes. It is layering due to deformation. The process begins with folded bedding which become elongated similar folds. These eventually get “chopped up” by pressure solution and the relict hinges become lenses within the rock parallel to cleavage.

Question 234

Transpressive and transtensive deformaiton

Answer 234

Transpressive motion represents oblique compression and transtensive motion represents oblique tension

Question 235

Types of cleavage

Answer 235

Slaty cleavage, crenulation cleavage (cuts older folding),

Question 236

Types of crystal fiber veins

Answer 236

Syntexial veins describes veins where material precipitates from the walls to the center

Antiaxial veins have crustal that precipitate from the center to the walls (the center crystals are oldest.

Question 237

Types of extensional systems?

Answer 237

Previous continental (passive) margins from the breakup of pangea. These are major petroleum systems (like the artic)

Mid-ocean ridges: commonly symmetric mountain chains with graben structures or asymmetric with detachment fault

Continental rifts associated with hot mantle upwelling (East Africa, Rio Grande). They include high angle normal faults that become listric at depth. They are shaped like a shield.

Highly elevated regions of Earth (Tibet) due to gravitational collapse.

Continental extension zones (basin and range) where a mix of back-arc basin extension and transtension from the san andreas create graben structures.

Question 238

Types of foliation (in order of increasing grade)

Answer 238

Phyllitic foliation: sheeths of small micas

Schistose foliation: coarser grained micas without banding

Gneissic foliation: This has clear banding that formed during deformation and metamorphism

Migmatite: This looks like guts. It occurs when partial melting occurs (T>650 C)

Question 239

Types of planes

Answer 239

Bedding planes, fault planes, joints, dike, walls/cuts, veins, foliations.

Question 240

Types of rock bodies based on rheology

Answer 240

There are ductile rock bodies which accommodate deformation without losing cohesion of internal strength by distributing deformation throughout the rock’s volume. An example of this would be the asthenosphere.

Brittle rock bodies accommodate deformation by a near loss of cohesion along discrete fractures (residual frictional cohesion)

Elastic rock bodies have deformation linearily related to the stress load as described by Hooke’s Law

Question 241

Types of slip (4)

Answer 241

Strike-slip: dip is ~90 with motion parallel to the strike

Dip slip: slip along inclined surface with the hanging wall and footwall (normal and thrust faults)

Oblique slip: inclined translation with dip ~30-60.

Rotational Faults: These are faults where one block moves about an axis relative to the other.

Question 242

Types of Strain

Answer 242

Progressive strain is the motion of every point throughout the deformation path

instantaneous strain: a snapshot of strain at some dt

Finite strain: The final result of deformation

Question 243

Types of stress based on Mohr’s circle

Answer 243

Differential stress is the diameter of mohrs circle. It causes distortion

Mean stress is the average of the principle stresses. It causes negative dilation.

Deviatoric stress is the radius of the circle. It causes distortion.

Question 244

Types of structural geology lines

Answer 244

mineral lineations

slickenlines

line of intersection between two planes

Question 245

Unconformity and types

Answer 245

This is any pause in deposition and/or erosion

This includes

nonconformities: young ig or sed on old meta or ig

angular unconformities: sedimentary layers over sed layers at a different orientation

disconformity: Parallel beds of similar composition usually id’ed using dating.

Question 246

Unloading Joints

Answer 246

These are the most common form of joints and oftentimes form sugar cube joints due to Poisson effect and dilation.

This is a type of unloading jointing where the ellipsoid shape of the pluton creates radial and tangential stresses for jointing. An example of this is half dome. If the rock being exhumed is a pluton (oftentimes having an elliptical shape) it will create joints that are radial and tangential to the ellipsoid. these are exfoliation joints

Question 247

vergence

Answer 247

This is common within flexural folding where the upper and lower surfaces of a fold experience shear. It is said to be verging to a direction based on the degree to which the tops/bottoms are sheared.

Question 248

Volume of a sphere and ellipsoid

Answer 248

V(sphere) = (4/3)*pi*r³

V_e = (4/3) pi a*b*c and r = (abc)^1/3

Question 249

Von Mises Failure Envelope

Answer 249

This is a failure envelope describing failure at high confining pressures. At these depths, lithostatic pressure dominates and failure occurs at about 45 degrees therefore 2theta = 90 and the critical stress becomes constant.

Question 250

What creates joints

Answer 250

Heterogeneity in the rock mass creates localized stresses that then create the joints. They are created due to thermal tension, unloading, or exfoliation (unloading for igneous rocks/plutons)

Question 251

What is the geothermal gradient after 15 km of depth?

Answer 251

The geotherm increases to about 25 C/km

Question 252

What is the process of fault reactivation and formation

Answer 252

When a fault first forms (theta = 30 from sigma 1) only the differential stress and not the average stress decreases. It ten slowly increases until it intersects byerlees law and will slip along that fracture. As the differential stress increases the angle between sigma 1 and theta increases (measured by the upper intercept) which corresponds to a lower angle of dip for a normal fault. This continues to increase until mohrs circle intersects coloumbs failure criterion (at 2*theta = 60) and a new fault forms, relieves stress, and the process repeats.

Question 253

What is the shape of normail faults in 3d?

Answer 253

They are shovel shaped. where there is maximum displacement at the center and this approaches 0 with decreasing distance to the tip point/line where the displacement = 0

Question 254

What is theta for a new fault

Answer 254

Theta is always =30^o

Question 255

What structures are associated with normal faults.

Answer 255

The hanging wall is extended as a function of slip along the surface. This means that as displacement increases towards the center of the fault the hanging wall slumps inward to create a syncline which plunges into the fault. In the footwall, the removal of overburden creates a bulge aka a syncline which plunges away from the fault surface.

Question 256

What type of loads are within Earth?

Answer 256

Generally rocks within Earth will experience gravitational loading, thermal loading due to dilation, displacemement based loading, and pore pressure.

Question 257

When do topographic lines not match?

Answer 257

When doing structural topographic maps, faults cutoff topographic lines.

Question 258

When does rotation occur?

Answer 258

Rotation is the translation of material about a central point. It is described by the radians of rotation

It occurs most common within overlying rigid deformation (where rigid faulting underlies a more ductile upper layer that deforms. It also occurs with listric normal faulting, kink folding, and mylonation.

Question 259

Where is theta measured from in the stress equations?

Answer 259

Theta is measured from the plane of interest to the maximum stress.

Question 260

Yield, ultimate, rupture Strength

Answer 260

Yield Strength is the point when strain softening starts to occur. This is not the peak stress but the point slightly below it.

Ultimate Strength is the point of maximum stress

Rupture Strength is the point where slipping occurs (instant drop in stress without change in strain)

Question 261

Detachment Faults and Surfaces

Answer 261

Detachment faults are low angle (<30^o) which form underneath graben systems and at the interface of the brittle crust and the metamorphic core complexes. In the latter case they are marked by chloritic alteration (green color) which forms due to fluid alteration during the process of exhumation.

They can be confused with thrust faults.

Question 262

What lies below the brittle fault rocks in a metamorphic core complex detachment fault?

Answer 262

This is underlayed by 500 m to 2 km of mylonitized orthogneiss/metasedimentary rocks with potentially undulatory foliation. Deeper than this is the injection complex which has a series of leucogranitic dikes cutting the mylonitized gneisses. As they approve the shear zone they become oriented parallel to the detachment surface.

Question 263

Metamorphic Core complex tectonostratigraphy with depth

Answer 263

Starting at the detachment fault (low angle fault separating brittle crust from core complex)

chloritic breccia (green breccia between cm and 10s of m thick)

cataclasite resistant layer (~1 m)\

mylonitic shear zone (gneisses with hundreds to km of m thick)

injection complex (dikes in the gneisses)

undeformed core

Question 264

What causes the doming of meta core complexes?

Answer 264

The combination of ductile flow of the weak, hot lower crust/moho region and tectonic unloading causes the core complex to act like a bubble in oil.

Question 265

Rolling hinge

Answer 265

This is a way of thinking about how core complexes form. It shows the breakaway and detachment of the upper crust as the lower crust upwells and is fed by the moho/upper mantle.

Question 266

Extension-parallel corrugations

Answer 266

These are lobe-like plunging antiformal corrugations that occur on the detachment fault side of metamorphic core complex. They plunge into the direction of the footwall recession. They are basically the result of stretching.

Question 267

What type of extension leads to the formation of core complexes?

Answer 267

There are generally two types of rifts: wide rifts and narrow rifts with indiviual characteristics based upon the moho temperature and the crustal thickness. Core complexes from in wide rifts where moho temperature is high (high ductility) and crustal thickness is high

Question 268

Supradetachment basin vs. half-graben basin

Answer 268

A half graben basin is flanked on both sides by high mountains with high-angle normal faults and has sedimentation/depocenter in the middle where growth strata increase in thickness towards the mountain range.

Supradetachment basins are wider and the depocenter becomes thicker away from the mountains unlike the growth strata of half-grabens. They are flanked by a low angle fault with basins that are far from the mountains.

Question 269

What type of shear fabrics exist?

Answer 269

porphyroblast systems (delta and sima types)

mica fish (lenses of mica that form in the directions of stretching)

SC fabrics (C fabrics are linear and show highest shear)

Fractured grains (like a toppled loaf of bread)

Question 270

What is the hypothesis for the development of Tibetan Rift systems?

Answer 270

They start as half-grabens with a structural low adjacent to the normal fault high. This low causes isostatic rebound at depth that starts to “bulge” the half graben basin into a basin high. At depth this causes the normal fault to flatten. The flattening of the normal fault initiates the onset of breakaway faults in the growth strata. The continual decline of the hanging wall repeats this process, and the basin high continues to enlarge until you reach the point where there is a detachment fault at the surface.

Question 271

Where are thrust systems found?

Answer 271

They are found at convergent margins where topographic highs create normal faulting and the topographic lows are dominated by thrust faulting

Question 272

Ocean-Continent Margins

Answer 272

These Andean/cordilleran-style thrust belts are characterized by a trench, forearc, arc, plateau, retroarc thrust belt, and foreland basin. The plateau is the highest point along the figurative cross-section given and is a point where normal faulting occurs. It has a weak lower crust due to the high heat of the upwelling magma. This means that it is more easily squeezed upwards where it cools and collapses.

Question 273

Continent-Continent margin

Answer 273

Alpine/Himalayan-style convergence is similar to the ocean-continent collision except that a trench does not form and there is a generally thickening of the crust that causes isostatic rebound in the plateau region. There is a similar collapsing a double sided thrust belts though.

Question 274

General architecture of fold-thrust belts

Answer 274

From low to high elevation. The foreland is the inner-continental basinward part of the belt and made of younger, lower grade rocks. It will be in front of an emergent imbricate fan which has newer thrusts at lower elevations. This merges with the plateau aka the hinterland where the duplex roof exists (the first thrust fault) These are underlain by younger thrusts too. This is where the highly sheared, old, and high-grade rocks exist.

All of this sits above the decollment fault which is the “main fault” that the other faults feed into. It is relatively flat but dips towards the hinterland. It represents a decoupling within the crust

There are usually turned over antiformal folds in the hanging wall and synclines in the footwall.

Question 275

Thrust belt propagation

Answer 275

The first ramp thrusts over the flat which then eventually begins to thrust too. This increases the dip of the dip of the earlier thrusts and also makes the youngest faults those that are closest to the foreland.

Like normal faults, thrusts cut up-section (originate at depth). They are connected like stair-steps with ramps and flats that progressively reach the surface. This is because they take advantage of planes of weakness (parallel to bedding)

Question 276

Where are the oldest rocks in a thrust belt?

Answer 276

These are usually in the hinterland which is the oldest fault. This means that it has had the greatest amount of displacement and uplift. This correlates to a greater amount of erosion which exposes the oldest rock.

The elevated hinterland is important to the development of large thrust belts where the weight of the elevated rock helps to push the younger foreland thrusts.

Question 277

Forced folds

Answer 277

These are thrusts that form over relatively stationary and stagnate ramps. They “cut” the rock along the fault and slide along the thrust.

Question 278

Thrust sheets, Klippes, and windows

Answer 278

Thrust sheets generally refer to large interior thrusts. The thrusting layers can be eroded to expose the younger material below, windows, and there can be parts that become seperated from the main sheet. These are klippes

Question 279

Salients

Answer 279

These are foreland features of thrust belts where they can form “lobes” that overlay the interior rocks.

Question 280

Classes of fault-related folds

Answer 280

There are fault-bend folds where layers bend over the thrust. The upper layers will be thinned. It is similar to a rollover monocline.

fault propagation folds These are rock layers that fold ahead of the propagation tip. These create the turned over antiformal folds in the hanging wall. The syncline axis in the footwall is near the propagation tip.

During Continental continental collisions there are also “snake head folds” which occur in relatively weak, less brittle crust where a large lobe basically plops onto the footwall.

Question 281

Duplex

Answer 281

This is the uppermost thrust. It forms the earliest and is rotated to steeper and steeper dips as it overrides the foreland material. These occur when the inactive fault is thrusted above the active ramp to be on the surface of the previously adjecent layers.

Question 282

Duplex architecture

Answer 282

There are generally several thrust sheets that layer onto one another. In the foreland are the youngest thrusts and the hinterland has the oldest thrusts.

The lower layer where “decoupling” occurs is the floor fault and the roof fault is the boundary between the thrusted layers and the overlying rock.

A horse is where the thrusted layer interfaces with the inclined layer below.

Question 283

How are large thrust sheets emplaced?

Answer 283

The integral of the frictional forces needed to forces 100’s of km of thrust sheet onto another rock is beyond the magnitude of conventional tectonics. The rock would shatter before thrusting in this matter. The caveat is water pressure.

Water pressure acts as a negative weight and enables the thrust sheet to more effectively slide onto the material below it. It decreases mean stress without changing differential stress (drives the process)

Additionally, thrust sheets move in piecewise dislocations. movement and strain is not homogeneous.

Question 284

Foreland Basins

Answer 284

These are depressions that form on the frontside of fold-thrust belts (in the foreland) where the topographic load of the fold-thrust belt causes lithospheric flex. Foredeeps seperate the fold-thrust belt from the forebulge.

Question 285

Types of ocean-continent margins

Answer 285

The key concept to understanding convergence in this context is that oceanic plates simply “fall” like a weak noodle.

advancing subduction zone refers to when the rate of falling is less than the horizontal displacement of the plate. This causes for retroarc thrust sheets. See the Andes for an example (the pacific plate has the maximal spreading rates)

retreating subduction zones refer to when the spreading rate is less than that of the falling rate of the plate. This leads to the trench moving oceanward and the development of backarc extension See the West coast (also influenced by transtension)

Question 286

Trench translation rate

Answer 286

This refers to the “subduction rate” of the plate and is related to the rate at which the oceanic plate sinks while horizontal translation occurs.

when convergence rates exceed subduction the trench advances towards the continent and retroarc thrusts form.

When convergence rates are less than subduction rate the trench moves oceanward and the continental crust is extended. This is a retreating trench.

Question 287

What drives tectonics

Answer 287

Gravity. The differing rates of convergence and divergence lead to strike slip faults through.

Question 288

What happens when strike-slip faults are not parallel?

Answer 288

releasing bends are transtensional pull apart basins

restraining bends are high relief transpressive thrusts and folds.

Slip along thrusts and related fault mechanisms due to uneven strike-slip mechanisms leads to transverse ranges and seismeic hazards.

Question 289

Flower Structures

Answer 289

Strike slip systems generally have a curvy architecture that branches from a main fault at depth to a series of roughly parallel surface scarps.

extensional flower structures are those that form in transtensive basins. They are generally a series of listric normal faults very similar to classic horst and graben structures.

contractional flower structures are convex down structures that are like pimples. They form thrusts.

Question 290

What are the influences that cause strike slip faults to be rather significant seismic hazards?

Answer 290

Because they are deep there is a large area of surface displacement and because friction is really the only resisting force the slip rate is high.