# Lecture 7 Joints Form perpendicular to weakest stress, often tensile s3 Mostly Chapter 7.

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Lecture 7 Joints Form perpendicular to weakest stress, often tensile s3 Mostly Chapter 7

Joints and veins A fault offsets layers of sediment SS and clay layers
Joint: a fracture without measurable shear displacement (cracks or tensile fractures) Fault: a fracture with measurable displacement Vein: a fracture filled with minerals precipitated from solution A fault offsets layers of sediment SS and clay layers Calcite veins fill joints

Surface morphology Plumose structure: wavy structure on joint

MODES Divergent Shear (Transform) Dip-slip & rotation See figure 6.11

Surface morphology Why does the plumose structure form?
Mode 1 loading: should yield smooth fractures perpendicular to s3. BUT real joints are not perfectly smooth. Rocks are not homogeneous. these imperfections distort the local stress field The stress field at the tip of the propagating crack changes As the crack grows, the stress intensity at the tip of the crack also grows. Also the velocity of the crack tip propagation is proportional to the stress intensity Since the crack is really small at the origin, the stress magnitude may exceed a critical value for cracking. Flaws at the origin

Joint spacing and bed thickness: Closely spaced in thin bedded rx
Joint Spacing in sedimentary rocks Joints are mostly evenly spaced Widely or closely spaced, partially depending on length of time tensile stress applied Model of the sequence of development of joints. Time 1, time before joint forms Time 7, present day Joints form in random sequence, but with regular spacing Minimum distance (dm) joint relieves tensile stress for a critical distance – either side has a decrease in tensile stress. Joint spacing and bed thickness: Closely spaced in thin bedded rx Wide spaced in thick bedded rx

Cutting one string causes only a few remaining strings to relax. Model of the sequence of development of joints. Time 1, time before joint forms Time 7, present day Joints form in random sequence, but with regular spacing Minimum distance (dm) joint relieves tensile stress for a critical distance – either side has a decrease in tensile stress. Greater length of joint has a wider stress shadow Cutting many strings in a row causes a wide band of strings to relax – larger area affected

Joint spacing and Lithology:
Stiffness = Elastic value E, Youngs’ modulus Hookes law where e is the elongation strain Stiff dolomite fractures a few times before the sandstone fractured the first time. Model of the sequence of development of joints. Time 1, time before joint forms Time 7, present day Joints form in random sequence, but with regular spacing Minimum distance (dm) joint relieves tensile stress for a critical distance – either side has a decrease in tensile stress. Dolomite stiffness >> Sandstone Stretch a block Stress in each bed controlled by Hookes law (magnitude of stress depends on E) Beds with large E (stiffness) develop a greater stress and fracture first.

Rocks with low tensile strength develop more closely space joints
AND More tensile strain (stretching) yields more joints Model of the sequence of development of joints. Time 1, time before joint forms Time 7, present day Joints form in random sequence, but with regular spacing Minimum distance (dm) joint relieves tensile stress for a critical distance – either side has a decrease in tensile stress.

Joint arrays Systematic vs nonsystematic joints Systematic joints:
Planar joints Joints are parallel or subparallel. Same average spacing Solids are atoms/ions are connected by chemical bonds (bonds = tiny springs) Nonsystematic joints: Irregular spatial distribution Not parallel to one another. Different average spacing

Joints in the field Why study joints Tectonics (paleostress)
Geomorphology (drainage patterns) Questions to answer in the field Systematic or nonsystematic joints Orientation of joints strike and dip Cross-cutting relations Steno Relative Dating Surface morphology planar plumose Dimensions of joints Joints and lithology which rocks thicknesses have closer Relations of joints and faults and folds. DEMO FOAM Pyroxene Model of the sequence of development of joints. Time 1, time before joint forms Time 7, present day Joints form in random sequence, but with regular spacing Minimum distance (dm) joint relieves tensile stress for a critical distance – either side has a decrease in tensile stress. DEMO

Joints in the field Methods Inventory Sample fracture density
Sample joint orientation (strike and dip of joints) Relate to tectonics Joint trajectory map Frequency diagram Rose diagram Each portrays different data sets

Categories of Brittle Deformation
Frictional Sliding on preexisting fractures Cataclastic flow due grain scale fracturing Shear rupture at acute angle to max. principle stress Tensile cracking perpendicular to dir of min. stess

Cataclastic rocks A Cataclastic rock is a type of metamorphic rock that has been wholly or partly formed by progressive fracturing. Rock fragments are reduced in size by crushing and grinding of existing rock, a process known as cataclasis. The process and is mainly found associated with fault zones. Cataclasite is a type of cataclastic rock that is formed during faulting, consisting of angular clasts in a finer-grained matrix. Cataclasite seen in thin section. Scale is 200 mm

Stress Concentration and Griffith Cracks
A stress concentration is a location in an object where stress is concentrated. An object is strongest when force is evenly distributed over its area, so a reduction in area, e.g. caused by a crack, results in a localized increase in stress. Griffith cracks are preexisting microfractures and flaws in the rock, weakening it. Reason rock failure less than theory

Origin and Interpretation of joints
Sheeting joints – uplift and exhumation Rocks cool and contract with decrease of burial depth Rocks shrink in the vertical direction (free surface – Earth Surface) Since the rock can not shrink elastically in the horizontal-direction (its still confined) Tensional stresses develop As overburden decreases, the rock expands vertically – the Poisson effect. It contracts in the horizontal direction So the layer of rock stretches like a membrane If the horizontal tensional stresses overcomes compressive stress, it will crack and form joints. Sheeting joints form in a location where s1 is horizontal while s3 is vertical near the ground surface. Joints become more closely placed near the ground surface

Origin and Interpretation of joints
1) Sheeting joints – uplift and exhumation Rocks cool and contract with decrease of burial depth Rocks shrink in the vertical direction (free surface – Earth Surface) Since the rock can not shrink elastically in the horizontal-direction (its still confined) Tensional stresses develop As overburden decreases, the rock expands vertically – the Poisson effect. It contracts in the horizontal direction So the layer of rock stretches like a membrane If the horizontal tensional stresses overcomes compressive stress, it will crack and form joints. A cooling pluton contracts more than country rock. Here, st (tensile stress) is oriented perpendicular to the intrusive contact. After exhumation, joints form parallel to intrusive contact and creates an exfoliation dome.

Mechanical Exfoliation in Granite Yosemite National Park
Source: Phil Degginger/Earth Scenes

Origin and Interpretation of joints
2) Natural hydraulic fracturing Stresses in the Earth’s crust are mostly compressive. How do joints form in such a tectonic environment? Effect of pore pressure on fracture. Increase of pore pressure in a pre-existing crack pushes outward Increase of st (tensile stress) that allow crack tip to propagate. a) Stresses near the crack with high fluid pressure that exceed the magnitude of s3. As a result, tensile stress (st) occurs along crack. b) Crack enlargement. Opening stress >> Closing stress.

Origin and Interpretation of joints
3) Regional divergence High pore pressures in blocks subject to divergence, weakens confining pressure Formation of joints in hanging-wall block with normal faults Tensile stress s3 weakest is horizontal, joints form perpendicular to s3 a) Stresses near the crack with high fluid pressure that exceed the magnitude of s3. As a result, tensile stress (st) occurs along crack. b) Crack enlargement. Opening stress >> Closing stress.

Veins and vein arrays Terminology (see table 7.2)
Vein: A fracture filled with mineral crystals precipitated from fluids. Quartz or calcite are common vein fill Ore minerals occur as vein fill Vein Array: Groups of veins. Vein array Stockwork array of veins (rock shattered and filled by mineral precipitation) a) Stresses near the crack with high fluid pressure that exceed the magnitude of s3. As a result, tensile stress (st) occurs along crack. b) Crack enlargement. Opening stress >> Closing stress.

Veins and vein arrays En echelon vein array Fill en echelon joints
Develop within a fault zone a) Stresses near the crack with high fluid pressure that exceed the magnitude of s3. As a result, tensile stress (st) occurs along crack. b) Crack enlargement. Opening stress >> Closing stress.

Veins and vein arrays En echelon vein array Fill en echelon joints
Develop within a fault zone. en echelon vein array Fractures initiate parallel to s1, at an angle of about 45° to the borders of the shear zone. Fractures open as displacement across the shear zone as it develops and fill with vein material. Once formed, the veins will rotate. If a new increment of vein growth occurs, the new veins will initiate 45° to shear surface. b) Sigmodial en echelon veins due to rotation of older, central part of veins and growth of vein material at ~45° to the shear surface

Calcite-filled wing crack with tip splay

Veins and vein arrays Vein fill: block and fibrous veins
Blocky – vein fill equant - open fracture when mineral precipitated. b) Fibrous – vein crystals long relative to width. Repeated cracking and filling, then sealing of vein. If pressure is high enough, vein cracks and slight openings develop. The crack fills with fluid and minerals precipitate. The process repeats itself, hundreds of times.

Lineament: A linear feature recognized on aerial photos, topographic maps or remotely sensed images. Defined only on a regional scale. Aligned topography, changes in vegetation Represent faults, joints, folds, dikes, or contacts. Lineaments are not always confirmed with ground truth. Repeated cracking and filling, then sealing of vein. If pressure is high enough, vein cracks and slight opening (microns) develop. The crack immediately fill with fluid. The process repeats itself, hundreds of times.

Summary Common questions you should ask when mapping
Should I pay attention to joints and veins? It depends. What is the purpose of the map? You should pay attention to them IF the map purpose is, for example: to locate faults to define variations in permeability to define joint intensity for oil and gas exploration (maybe with drill core data) to define and explore orientation of veins for ore deposits to predict groundwater transport But IF the purpose is, for example, (1) understanding stratigraphy, or (2) the history of folding in high-grade metamorphic rocks then joint analysis will not help.

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