Brittle fracture, joints, veins Flashcards
Brittle deformation Mechanisms
- Frictional grain boundary sliding
- Grain rotation
- Fracturing: intragranular and
intergranular
Granular and cataclastic flow
Cataclasis:
fracture and crushing of grains, coupled with frictional sliding along grain contacts and grain rotation
Granular or particulate flow:
achieved by grain translation and rotation
Cataclastic flow:
due to cataclasis
Types of fractures
Fractures are planar or subplanar discontinuities that are very narrow zones/surfaces formed due to stress.
Shear fractures: relative movement parallel to the fracture surfaces
Extension fractures: extension perpendicular to the fracture surfaces, including joints (narrow) and
fissures (wide, filled with air/fluids).
-Veins: fractures filled with minerals.
-Dykes/sills: fractures filled with magmatic rocks
Contractional planar fractures: Stylolites
Modes of fractures
Mode I – Opening mode
(a tensile stress normal to the plane of the crack)
Mode II – Sliding mode
(a shear stress acting parallel to the plane of the crack and perpendicular to the crack front)
Mode III – Tearing mode
(a shear stress acting parallel to the plane of the crack and parallel to the crack front).
Mode I – Opening mode
(a tensile stress normal
to the plane of the crack),
Mode II – Sliding mode
(a shear stress acting
parallel to the plane of
the crack and
perpendicular to the
crack front), and
Mode III – Tearing mode
(a shear stress acting parallel to the plane of the crack and parallel to
the crack front).
Extension fractures form
perpendicular to σ3. If σ3 is tensile
(negative), they are called tensile
fractures.
joints
-approximately planar fractures
-form when the tensile strength of stressed rocks is exceeded
no measurable shear displacement
form parallel to σ1 and σ2,
perpendicular to σ3
Systematic vs nonsystemic joints
joint set vs joint system
Systematic joints: planar, parallel and evenly spaced joints, obvious spatial
relationship
Nonsystematic joints: irregular in form, spacing and orientation, no obvious
spatial relationship
Joint set: A group of systematic joints of common origin form a joint set.
They are often approximately parallel to one another, and equally
spaced.
Joint system: Two or more sets of joints appearing together and usually
intersecting form a joint system.
Joint systems –Orthogonal and conjugate
Orthogonal system:
two sets of joints that are perpendicular to each other
Conjugate system:
two sets of joints with a dihedral angle of significantly less than 90º (e.g., about 30º-60º)
Joint surface: Plumose or plume structure
feathering feature
propagate out from origin
what are the 4 origin of joints
- Differential volume change
- Uplift/unroofing
- Hydraulic fracturing
- Regional Deformation
Differential volume change
Columnar joints: joints breaking rocks into hexagonal columns due to cooling and contraction, usually in lava
flows, but also due to dewatering as seen in mud cracks
strained in all different directions
cools first on the top and cracks, then propagates down the column
Uplift/unroofing
Joints form due to erosional unloading in isotropic rocks (σ3 vertical)
eg., sets of flat lying joints in granitic rocks referred to as sheeting joints or
sheet structure
Exfoliation (or sheeting) joints
pluton is exposed on the surface due to erosion, pluton has vertical expansion which forms cracks
Hydraulic fracturing
Hydraulic fracturing is the generation and propagation of fractures in a rock by a pressurized fluid
Pf > σ3 + T
Pf (fluid pressure) is greater than the sum of σ3 (remote minimum stress) and T (tensile strength)
Hydraulic fracturing, or fracking, is a drilling method used to extract oil or gas from deep in the Earth. In the fracking process, cracks in and below
the Earth’s surface are opened and widened by injecting water, chemicals, and sand at high pressure
Regional Deformation
Joints form due to tectonic deformation:
many joints, particularly those that cut through rocks of different lithologies, appear to be related directly to regional deformation.
Joints associated with faults:
pinnate fractures occurring in the vicinity of a fault plane and intersect the fault in an acute angle pointing in the direction of relative movement of the block containing the pinnate fractures
(where the fault ends there is extension which causes fractures)
Veins
A vein consists of minerals, most commonly quartz and calcite, that precipitated from solution and filled a
fracture
fluids go into fracture (lowest pressure route), pressure decreases and minerals precipitate out of the fluid
planar systemic veins vs stockwork veins
Planar systematic veins:
-planar, mutually parallel, regularly spaced, spatially controlled by a systematic joint set
Stockwork veins:
-irregular
-occur in a pervasively fractured rock
-up to 40 to 50% of the rock is vein material which may completely surround blocks of host material in shattered/fractured rocks
Dilational vs nondilational veins and how you can tell
dilation - open
volume increases
vein appears in the opening
nondilational - replacement
-there was no opening
-in some areas the vein might be thicker than others or not replaced at all (ie one part of the rock was more easily replaced), in some cases may see the trace of the original rock in the vein
Progressive evolution of veins
pure shear - straight veins
- becomes longer and wider
- the ones that formed earlier will be smaller and later will be larger (can see both in the same structure - both can be preserved)
simple shear - sigmoidal veins
- rotates as it propagates
- older ones will be rotated further and be larger
-at some point new ones start in the original orientation and intersect older ones
En echelon
can be straight or sigmoidal
- becom slanted as a result of a fault
Types of vein fill
Blocky veins: crystals in the vein
are roughly equant, growing in open cavity
Fibrous veins: crystals in the vein
are very long relative to their width; crack-seal mechanism
describe the crack-seal mechanism for formation of fibrous veins
A rock contains pore fluids that contain dissolved minerals
A crack in the rock develops and is filled with the fluids.
The fluid pressure within the crack is less than the pores in the surrounding rock. Minerals precipitate out of the solution and mineral fibers form, sealing the crack.
Repetitive processes.
fibrous veins - Syntaxial vs Antitaxial
Syntaxial veins: the vein fill has the same composition as the wall rock (e.g., Quartz veins in a sandstone) and the vein fibers nucleate on the surface of grains in the wall rock and grow inwards to meet at a median line. Further cracking occurs along median line and further growth occurs here.
Antitaxial veins: the vein fill and the wall rock has different composition and the increments of cracking occur at the contacts between the vein and the wall rock (e.g., calcite in sandstone)
