EXAM 2 Flashcards
thrust faults place…
older rocks on top of younger
thrust systems propagate towards…
foreland
how are new thrust faults created
when stress required to move fault > stress required to break rock a thrust falut is created
the geometry of fold and thrust systems depends on…
- friction coefficient on fault surfaces
- failure strength of the rocks
thrust fault example: foreland rocks become harder to break
reactivation of older fault was easier than fracturing more competent lithology in foreland
where is friction on faults higher in thrust faults
new faults form closer to the ramp of existing faults
thrust fault example: high friction faults and strong material in foreland
it may be more energetically favorable to produce a “back thrust”
thrust fault example: low friction faults and strong material in foreland
reactivation of multiple older faults because faulting is easier than breaking
thrust fault example: very weak layer between hard basement and soft overlying sediment
low friction surface makes it easiest to continue faulting at contact before breaking overlying layers
types of thrust systems
- fault bend fold
- in-sequence thrust system
- out-sequence thrust system
- duplex
- fault-propagation fold
fault bend fold
thrust cuts up section at footwall ramp
in-sequence thrust system
older to younger
out-sequence thrust system
younger to older
types of extensional fault systems
- rift
- basin and range
- normal fault systems
- large displacement normal faults
- domino fault model
draw a rift system
yay
what are grabens
the downthrown blocks in extensional systems (hanging wall)
what are horts
the upthrown blocks (footwall)
draw a basin and range system
yay
what are ranges
ranges are the exhumed footwall blocks
what is exhumation
the transport of a rock to the surface
what is need denudation
the mechanical and chemical of erosion, weathering, and mass wasting “removal of material”
what is tectonic denudation
removal of material by faulting
results of exhumation
- normal faulting exposes rock
- weathering breaks down the rock
- erosion or mass wasting transports the sediment to the basins
normal fault system
parallel closely spaced faults
large displacement/magnitude extension
- during continued extension, it becomes more efficient to slip on a lower angle fault; some stretching is accommodated by rotation
domino fault model
- faults shallow as fault blocks rotate
- faults become too shallow and new higher angle faults will develop
space problem
rigid block rotation along planar faults creates gaps in the crust, which is impossible
how to solve space problem
- roll over
- antithetic faults
- synthetic faults
types of folds
- chevron fold
- concentric fold
- box fold
fold opening
gentle: 180-90 degrees
open: 120-70 degrees
tight: 70-30 degrees
isoclinal: 30-0 degree
dip of axial surface
- upright
- plunging upright
- horizontal inclined
- recumbent
- vertical
- plunging inclined
- reclined
tools to describe a fold
- tightness
- dip of axial plane
- plunge of hinge line
what is fabric built of
minerals and mineral aggregates with a preferred orientation that penetrate the rock at the microscopic to centimeter spacing scale
foliation
planar structure formed by tectonic processes, and includes cleavage, schistosity, and mylonitic foliations
cleavage
a low temperature version of foliation best developed in rocks with abundant platy minerals
draw scale and type of foliation
yay
cleavage formation
- slaty cleavage
- phyllitic cleavage
what is slaty cleavage
- pressure solution cleavage
slaty cleavage with clay minerals
- pressure at grain boundary causes dissolution of quartz in the presence of a fluid
- clay minerals will dissolve and regrow with the long axis perpendicular to sigma 1
phyllitic cleavage
- slaty cleavage becomes more well defined as mica minerals continue to grow
- rock begins to look shiny but individual mica grains cannot be seen with the eye or hand lense
schistosity
as metamorphic grade increases, mica grain grow larger. The foliation becomes less planar as it wraps around stronger minerals. Layers of mica are continuous through the rock and individual grains are visible in hand sample (other metamorphic minerals start to grow)
gneissic foliation
foliation defined by compositional banding caused by transposition of compositional domains or metamorphic processes
transposition
flattening and rotation into parallelism
metamorphic processes
melting causes segregation of melt (quartz and feldspar) from residuum (biotite, garnet, amphibole)
lineation
a fabric element in which one dimension is considerably larger than the other two
examples of lineation (elongation)
mineral aggregates or stretched pebbles
example of lineation (lines of intersection)
line along which two planar elements intersect
example of lineation (surface lineation)
slickenline, striation..
stretching lineations
lineation formed by rotation, recrystallization or dissolution/precipitation of a mineral
intersection lineations
a lineation commonly formed when two foliations cross-cut
draw L-tectonite
yay
draw S-tectonite
yay
shear zone
commonly considered the ductile-component of fault zones that cut the crust
ramsay-type shear zone
- foliation initially develops perpendicular to sigma 1
- as shear strain increases, foliation rotates to near-parallel with shear zone boundary
how to find ramsay-type shear zone
can use angle between foliation and shear zone boundary to calculate shear strain
draw ramsay-type shear zone diagram
yay
kinematics
in a shear zone, deflection of foliation records the sense of shear (top-right or top-left)
s-c fabrics
- s (schistosity) = foliation
- c (cisaillement) = shear band
porphyro clast
relatively large grains that form before deformation but change simple or orientation during deformation
types of kinematic indicators
- sigma clast
- delta clast
- phi-clast
diagram of each kinematic indicators
yay
ductile deformation mechanisms
during foliation development, crystallization is the primary deformation mechanism
recrystallization
is a process whereby strained grains are replaced by unstrained grains
dislocation glide
-dislocations move along the lattice plane
- dislocation pile up when they cannot continue moving, creating a wall (undulose extinction)
dislocation climb
-dislocation can climb from one lattice plane to another to avoid dislocation pile ups and continue moving (recovery mechanism)
dislocation creep
when dislocation glide and climb happen together (most common mechanism)
dynamic recrystallization with dislocation creep
during dislocation creep, dislocations move toward grain boundaries
dynamic recrystallization with dislocation creep
at higher temperatures, dislocation climb (recovery) becomes more efficient, so subgrain formation is more rapid and the subgrains become new grains
grain boundary migration
at higher temperatures, grain boundary migration becomes rapid and dislocations move to grain boundaries, which grow outwards
