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WOU ES 406 - Brittle Deformation and Fractures

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43 Intro. to Brittle Deformation and Fractures I. Introduction A. Fractures = brittle rupture of rock medium in response to stress 1. Most common geologic structure; 2. cracks in rocks/minerals in which cohesion of material is lost. B. Geological importance of fractures; 1. Conduits for fluids (water, gas, oil), a. increase permeability, b. avenues for enhanced weathering, c. influence fluid flow to wells, d. Serve as planes of weakness for construction/mining (1) "hydrofracing" in petroleum/groundwater industry e. Increase risk of slope failure/rock slides C. Terminology 1. joints: cracks in rock in which no appreciable displacement has occurred 2. faults: cracks in rock in which appreciable displcacement has occurred D. Types of fractures 1. Extensional Fractures a. Mode 1=extensional fracture where relative motion is perp. to fracture plane, (1) known as joints, gashes, veins. (2) Fracture planes are parallel to maximum force. 2. Shear Fractures: relative motion parallel to fracture plane (i.e. faults) a. Mode 2=slides perp. to edge of fracture = Strike Slip (1) motion parallel to strike of fracture plane b. Mode 3=slides parallel to edge of fracture = Dip-Slip (1) motion parallel to dip of fracture plane c. Oblique extension fracture or mixed mode fracture (1) Hybrid between Modes 2 and 3 E. Methods of Observation and Analysis44 1. Distribution of geometry of fracture system 2. surface morphology of fractures 3. Cross-cutting relations between fractures 4. Geometric relationship between fractures and other structures II. Joints: Mode I Fractures A. Joints Defined 1. unfilled mesoscopic fractures exhibiting no evidence of shear a. Most common structural element at every outcrop. b. Commonly joints form before faults which are later reactivated in shear. c. In sedimentary rocks most joints are vertical or bed normal, B. Geometry of Natural Mode I Fractures 1. Joint Types: Terminology a. Joint Set: adjacent fractures of similar geometry (1) strike orientation and dip b. Joint System (1) two or more joint sets affecting the same body of rock c. Systematic and nonsystematic joints (1) Systematic joints (a) planar (b) parallel (c) regular spacing (2) Non-systematic joints (a) "curvy cross-joints" (b) curviplanar, irregular in geometry (c) irregular spacing (d) commonly abut against older fractures d. Sheet joints/Exfoliation joints (1) curved extension fractures (2) "release" fractures due to removal of overburden via erosion (3) subparallel to topographic form (4) exfoliation domes: onion-like appearance due to sheeting joints e. Columnar joints (1) vertical fractures that form hexagonal or pentagonal forms (2) form from cooling of igneous bodies/shrinkage45 f. Veins (1) mineral filled fractures (a) calcite, quartz fillings most common g. Gash joints (1) en echelon organization (2) commonly mineral filled (3) may be S or Z shaped (4) commonly form in response to shear motion h. Pinnate Fractures (1) secondary fractures form in response to shear along fault planes (2) form acute angles with fault (3) show relative shear sense of fault motion III. Geometry of Mode I Fracture Systems in Three Dimensions (Geometric Analysis) A. Orientation of Fractures 1. Strike and dip measurements 2. Joint Sets: Fractures of similar orientation in space a. Must. I.D. sets in field 3. Joint orientation vs. Lithology a. mechanical properties of varying lithology can result in deviation of fracture orientation in uniform stress field 4. Problem: curving fractures a. if fracture not planar, must take care in measuring and interpreting orientation of "curvy cross joints". B. Scale and Shape of Fractures 1. Lateral and vertical extent of fracture a. vertical terminations at bed contacts? or through-going? b. lateral extent 2. Nature of fracture termination (shape) a. branching b. en echelon c. curving/overlap46 3. Scale of fracture a. megascopic b. mesoscopic c. microscopic C. Spacing of Fractures 1. Distance between fractures of a given set, measured perpendicular to the fracture surface 2. Known Controls a. systematic vs. nonsystematic (1) systematic = regular spacing (2) nonsystematic = irregular spacing b. Bed Thickness in Layered Rock (1) As Bed Thickness >, Fracture spacing > logrithmically (and vice versa) c. Lithology (1) Rheologic properties of rock will affect fracture spacing (a) e.g. coal vs. shale D. Spatial Pattern and Distribution of Fracture Systems 1. Map plots of fracture orientation (strike and dip) a. Relation to other structure (1) bedding attitude (2) folds, faults 2. Form Line Map a. Plot fracture orientation at individual outcrop locations b. extend lines between data points to created a continuous "form line" (1) allows greater visual recognition of patterns in map view over large areas IV. Features of Mode I Fracture Surfaces A. "Fractography" - analysis of morphology on the surface of fractures 1. Plumose Markings or Hackle Plume a. Hackle- regular pattern of subtle ridges and grooves on surface of fracture (1) commonly shows radiating pattern from a point or central axis (2) Most commonly displayed on fine-grained lithologies (mudstone, chalk) (a) hard to find on coarse sandstone47 b. Rib Marks (1) ripple shaped ridges that form transverse to hackle lines c. Hackle Markings and Fracture Propagation (1) Plumose structure = distinct evidence of mode I extension (2) plumose structure forms in response to rapid fracture propagation upon cracking (3) the direction of divergence of the hackle plume points in the direction of propagation (4) Rib Marks = arrest lines, where fracture propagation temporarily halted. d. Slickenside Lineations or "Slickenlines" (1) Grooved striations that form on fracture surface in response to shear (a) result from abrasive/polishing action of crushed rock caught in shear zone (b) distinct evidence that fractures are either of Mode II or Mode III origin e. Mineralized fracture surfaces (calcite, quartz commonly) (1) Suggest fluids driving force of rock fracture (2) fracture served as fluid conduit V. Abutting Relations / Cross-cutting Relations A. Fracture termination against another fracture 1. first-formed fracture through-going 2. second-formed fracture propagates to first fracture and stops due to break in mechanical properties of the rock B. Fractures


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