Test 1 Flashcards
Primary Structures
Develop during formation of the rock body
example of primary structures
cross bedding, ripples, mud cracks
Secondary Structures
Form as a result of deformation after rocks formed
What are joints?
Tensile fractures with little displacement
How do joints form?
Joints arise from brittle fracture of a rock or layer due to tensile stress.
trend?
the orientation of a linear feature
plunge?
the angle from horizontal of a linear feature
rake?
the angle to a linear featured measured from the strike line of a plane
vein?
fractures filled with minerals that crystallized/precipitated from fluids flowing through rock
fault?
shear fractures that have accommodated notable displacement
folds?
systematically curved layers
Cleavage?
closely spaced surfaces that give
a ‘woody’ appearance
foliation?
Layering produced by ductile deformation
lineations
elongate linear features
shear zones
deformation distributed over a thickness of
rock (meters to kilometers)
most common joint arrays/sets
orthogonal (90 degree angles)
plumose structure?
similar to a feather, shows joint propagation direction
hackle lines?
accentuate plumose structure, like individual fibers of feather
arrest lines?
look like ovals in plumose structure
griffith crack theory
pre-existing microcracks in a rock act as stress “concentrators”, largest properly oriented Griffith Cracks
(i.e., perpendicular to tensile direction)
propagate to form a through-going crack
what can produce cracks?
thermal contraction, outer arc extension, removal of overburden (unloading), exfoliation joints (pluton)
why are joints in uppermost crust (few km)?
Stresses become more compressive with depth to the
point where rocks can’t “pull-apart”
rock strength, depth and temp relationship
strength increases with depth until temps raise to the point where strength begins to decrease.
shear fracture?
Fractures with a component of “sliding” motion
due to compression, most common
shear fracture angle
roughly 30 from sigma 1
normal fault dip angle
around 60
thrust fault dip angle
around 30
Anderson Classification of Tectonic Stress Regimes?
On average, shear fractures form 30°
from s1 and often form as conjugate sets
hanging wall
the block above the fault
hang your lantern on it
foot wall
the block below the fault
stand on it
stratigraphy?
the age of the rocks
The Keys to Describing Slip Along a Fault
direction, sense, magnitude
Slickenlines/Striations:
scratches (lineations) on a surface (slicken surface
Mullions?
Linear deformation structures formed in the interface between a competent and an incompetent rock
(significant viscosity contrast)
Chatter Marks?
Step-like features oriented perpendicular to striations;
give information about the sense of shear
drag folds?
can be used to determine sense of a fault
normal/extensional faults
Dip slip fault on which the hanging-wall
has moved down relative to the
footwall. younger on older
scarps?
triangular faceted spurs due to valleys cutting through a fault scarp. wasatch front
horst
high section of normal fault
graben
low section of normal fault
listric normal fault
bowl looking
domino normal fault
domino sections tilt as extension increases
graben vs half graben
graben surrounded by 2 horsts, half is on one side (listric)
sedimentary growth strata
if it can be dated, great for getting at timing & rates of extension. thickens towards fault. drag folds
rift
forms where the crust is pulled apart by tectonic forces.
3 rift phases
Early extension, Stretching phase, Post rift subsidence and
sedimentation
reverse faults accommodate what?
shortening
what are orogenic systems/wedges like?
snow plow
Two types of structural styles in thrust belts
thin skinned and thick skinned
flat
fault remains parallel to bedding
ramp
fault crosses bedding at an angle
imbricate fans
blind thrust faults, new faults rotate old ones in the snowplow like effect
duplexes
blind thrust faults, includes horses, 3 types.
2 types of strike slip step overs
bends and oversteps
2 directions of strike slip
right and left lateral
what do strike slip fans and duplexes resemble in map view?
horse tails
2 types of duplexes
extensional (neg flower) and contractional (pos flower)
shear fracture
sigma one is at an angle to the fault
echelon veins
tip points away from movement direction
riedel shears
sigma one at an angle to fault, end up with extension in sigma 3
