Presentation on theme: "Lecture 7 Joints Mostly Chapter 7 Form perpendicular to weakest stress, often tensile 3."— Presentation transcript:
Lecture 7 Joints Mostly Chapter 7 Form perpendicular to weakest stress, often tensile 3
Joints and veins 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 Calcite veins fill joints A fault offsets layers of sediment SS and clay layers
Surface morphology Plumose structure: wavy structure on joint Spreads outward from joint origin
Surface morphology Why does the plumose structure form? Mode 1 loading: should yield smooth fractures perpendicular to 3. 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
Joint Spacing in sedimentary rocks Joints are mostly evenly spaced Widely or closely spaced, partially depending on length of time tensile stress applied Joint spacing and bed thickness: Closely spaced in thin bedded rx Wide spaced in thick bedded rx
Stress shadows Cutting many strings in a row causes a wide band of strings to relax – larger area affected Rigid grid Cutting one string causes only a few remaining strings to relax. Greater length of joint has a wider stress shadow
Dolomite stiffness >> Sandstone 1) Stretch a block 2) Stress in each bed controlled by Hookes law (magnitude of stress depends on E) 3) Beds with large E (stiffness) develop a greater stress and fracture first. Joint spacing and Lithology: 1) Stiffness = Elastic value E, Youngs’ modulus 2) Hookes law where e is the elongation strain Stiff dolomite fractures a few times before the sandstone fractured the first time.
Rocks with low tensile strength develop more closely space joints More tensile strain (stretching) yields more joints AND
Joint arrays Systematic vs nonsystematic joints Systematic joints: Planar joints Joints are parallel or subparallel. Same average spacing Nonsystematic joints: Irregular spatial distribution Not parallel to one another. Different average spacing
Joints in the field Why study joints 1)Tectonics (paleostress) 2) Geomorphology (drainage patterns) Questions to answer in the field 1) Systematic or nonsystematic joints 2) Orientation of joints strike and dip 3) Cross-cutting relations Steno Relative Dating 4) Surface morphology planar plumose 5) Dimensions of joints 6) Joints and lithology which rocks thicknesses have closer 7) Relations of joints and faults and folds. DEMO FOAM Pyroxene DEMO
Joints in the field Methods 1) Inventory a)Sample fracture density b)Sample joint orientation (strike and dip of joints) 2)Relate to tectonics
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 m
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 Sheeting joints form in a location where 1 is horizontal while 3 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 A cooling pluton contracts more than country rock. Here, t (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 t (tensile stress) that allow crack tip to propagate.
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 3 weakest is horizontal, joints form perpendicular to 3
Veins and vein arrays a)Vein array b)Stockwork array of veins (rock shattered and filled by mineral precipitation) 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.
Veins and vein arrays En echelon vein array Fill en echelon joints Develop within a fault zone
Veins and vein arrays 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 En echelon vein array Fill en echelon joints Develop within a fault zone. a)en echelon vein array
Calcite-filled wing crack with tip splay Calcite-filled wing crack with tip splay
Veins and vein arrays Vein fill: block and fibrous veins a) Blocky – vein fill equant - open fracture when mineral precipitated. b) Fibrous – vein crystals long relative to width.
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.
Summary Common questions you should ask when mapping Should I pay attention to joints and veins? It depends. It depends. What is the purpose of the map? You should pay attention to them IF the map purpose is, for example: 1) to locate faults 2) to define variations in permeability 3) to define joint intensity for oil and gas exploration (maybe with drill core data) 4) to define and explore orientation of veins for ore deposits 5) 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.