Brittle Deformation

Cracks are useful

Brittle Deformation Permanent change in rocks by fracture or sliding on fractures Fracture: A discontinuity across which cohesion is lost The term fracture includes three basic types of discontinuities: Joint Shear fracture Hybrid shear fracture Vein: fracture filled by secondary minerals

Joint Spacing & Bed Thickness

Faults and joints

Joint sets Devils tower

Systems of joints and shear fractures: Structural Domains are regions with similar joint styles and orientations. They are usually designated on the basis of geographic boundaries, lithologies, etc., and suggest a common origin

En-echelon Tension Gashes

Definitions and Distinctions - General Nature Veins are often associated with joints, when minerals are preciptitated within their openings. Precipitation occurs when P & T conditions are right, or when 2 different solutions mix and subsequently precipitate out minerals. Precipitation of minerals with a fracture can effectively seal the fracture.

Vein

Antiaxial vs. syntaxial vein

Fracture A planar or curviplanar discontinuity Forms as a result of brittle rock failure Under relatively low pressure and temperature conditions in the earth crust Rock fractures range in size from: Microcracks – Intragranular to intergranular (fraction of a mm) Faults - Extend for hundreds of kilometers

Tensile strength and jointing

Joints Elastic regime Are a type of fracture which form due to tension They form parallel to the minimum tensile stress Perpendicular to maximum tensile stress Shearing is zero along joints when they form Also called cracks or tensile fractures Joints form under: Shallow depth, low confining pressure Elastic regime Low temperature, T High pore fluid pressures, Pf

Joints Joints opens up perpendicular to the maximum principal tensile stress i.e, along a principal plane of stress Therefore joints dominantly show separation or opening of the walls of the fractures with no appreciable shear displacement parallel to the plane of the fracture Joints form through the Mode I crack surface displacement Joints are commonly characterized by two matching, rough, discontinuous, and curved surfaces, although they are approximated to be smooth, continuous, and planar

Modes of crack surface displacement Individual cracks, when loaded, propagate, infinitesimally, in three different modes: Mode I – Tensile (Opening) Mode Tensile cracks form normal to the 3 (parallel to the 12 plane) Crack opens infinitesimally perpendicular to the crack plane Crack grows in its own plane; no bending/changing orientation Mode II – Sliding Mode One block moves parallel to the crack normal to the fracture front Mode III – Tearing Mode One block moves parallel to the crack parallel to the fracture front

Modes of crack surface displacement

Modes of fracture

Mode II and III Both are shear mode Do not grow in their own plane. As they start growing, they immediately either: Curve Become mode I cracks Spawn new tensile, wing cracks However, shear fractures and faults are not large mode II or mode III cracks

Joints The planar approximation is justified given that scale of geometric irregularities (e.g. joint surface morphologies, curvature amplitude) is commonly very small compared to the size of the fracture surface Termination of the two opposing surfaces at their distal edge or periphery (fracture front), i.e., a finite extent of the two walls Displacement is zero at the fracture fronts Involve small relative displacement of the originally contiguous points compared to the in-plane dimensions of the fracture walls

Fracture Terminations

Shear Fractures Fractures along which there has been shearing or displacement Shear fractures are small, with small displacement They occur in intact rock during brittle deformation If the amount of displacement is significant and measurable, the shear fracture is called fault

Shear Fractures Shear fractures, form at an angle to the maximum compressive stress and show offset because of the shear traction along the fractures

Terminology Fracture front: The line separating the fractured region from the un-fractured part of the rock Fracture trace: Intersection of the fracture with any surface Fracture tip: The termination of the fracture along the trace of the fracture

Joint Surface Morphology Surface morphology of joints show evidence for initiation, propagation, and arrest Theoretically, mode I loading in an isotropic, homogeneous material should lead to a smooth propagation with a mirror smooth surface Joints however are not smooth, because rocks are commonly neither homogeneous nor isotropic

Out-of-plane Propagation The orientation of the maximum tensile stress in front of a single crack tip may not be parallel to the normal to the parent crack Cracks propagate so that “new’ portions of the crack remain normal to the local maximum tensile stress This requires a crack to leave the plane of the parent crack in order to maintain its orientation relative to the local stress field This out-of-plane propagation is so common in microscopic scale that leads to the formation of rough, non-smooth fractures surfaces

Crack Propagation Paths Out of plane propagation is characterized by a combination of two end member crack propagation paths: Twist: leads to segmentation of the crack into several smaller crack planes. Represents a rotation of the local maximum tensile stress in the initial yz plane. Rotation is about an axis in the crack plane || to propagation direction Tilt: Causes the crack tip to rotate without segmentation. Rotation is about an axis in the crack plane _|_ propagation direction

Mesoscopic Joint Surface Although microscopic out-of-plane propagation is common, joints appear smooth on the mesoscopic scale. This is due to the homogeneous nature of the remote stress field Even when the crack leaves its plane, the general propagation path remains normal to the remote maximum tensile stress The microscopic out-of-plane propagation leads to the development of joint surface morphology

Plumose Surface Morphology Helps to interpret rupture nucleation, propagation, and arrest Develops largely due to local twists and tilts during propagation of a fracture which would otherwise be planar. Barbs: surface irregularities In homogeneous rocks, barbs trace to the point of origin (an original crack) In inhomogeneous rocks (sandstone, shale), barbs radiate from either a bedding plane or an inclusion in the bed (e.g., fossil, concretion, clast) The point of origin may vary from bed to bed in shale or siltstone

Joint Surface Ornamentation An ideal joint consists of a planar, smooth main joint face that is often surrounded by rough fringes.

Fringes project outward from the main joint face, and are the outermost extent of the joint. They are areas where the energy required to propagate the joint is dissipated. Fringes are characterized by smaller swarms of en-echelon joints that are miss-aligned with respect to the main face.

Hackles Many joint faces have ornamentation that indicate the location that the fracture originated, and the direction that the fracture propagated. These features are: Origins - the site of initial porpagation, analogous to the hypocenter of an individual earthquake. Likely the site of some sort of initial flaw in the material. Hackles - are linear to curved lines that originate from but converge to the origin. Ribs - are present on most surfaces that display hackles, and record the position of the fracture front at any given instant of fracture growth, and are generally sub-perpendicular to the orientation of hackles. Ribs

Hackles Hackles often display a feather-like geometry called plumose structure, where the hackle ‘feathers’ radiate from one or more axis.

Plumose Structure

Plumose Structure The simplest type of plumose structure are marked by straight plume axes, with hackles that radiate symmetrically from the axis.

Plumose Structure Curving plumes are more complicated, where new plumes may ‘blossom’ off an earlier hackle. FYI - These fractures propagate at about 300 mph!

Conchoidal fracture showing ribs but no hackles Well developed ribs and hackles in a well cemented SS.

Plumose Structures

Well developed ribs & hackles in plexiglass The presence of ornamentation on joint faces indicates that fractures do not form as a smooth, continuous process. Ornamentation reveals that the process, although occuring at great velocities, is a complex process where the fracture is slowed down, arrested, sped up and made to branch during fracture growth.

Plumose Structure There are 3 modes of fracture - they are effectively end members, that can be mixed - resuting in mixed-mode fractures. They are: Mode I - opening mode fractures - that open in a direction perpendicular to the fracture plane. Mode II - sliding mode fractures - where motion is parallel to the plane of the fracture. Mode III - scissoring mode fractures - where motion is about a pole to the fracture face.

In a given idealized fracture, we see all 3 modes active simultaniously. There are 3 parts to this idealized fracture: 1) The main joint face (Mode I & II), which is ringed by; 2) Tilts (Mode II),, which are at the ends of the fracture that are in the slip direction, and are at some angle (20° - 30°) to the main face, and; 3) Twists (Mode III), which are found on the remaining sides of the main face. The angle of the twists and tilts is related to the direction of s1

Mechanical stratigraphy - alternating weak and strong layers can produce complex fracture propagation patterns - here, the layers of shale arrest the fractures, which then continue with subsequent increseases in stress.

Patterns of Intersection and Termination of Joints: Depending on the degree of exposure, the pattern we might see will vary…

Patterns of Intersection and Termination of Joints: