fractures_intro - Intro. to Brittle Deformation and...

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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 Analysis
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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/shrinkage
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This note was uploaded on 10/02/2011 for the course GEOLOGY 406 taught by Professor Taylor during the Spring '11 term at Western Oregon University.

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fractures_intro - Intro. to Brittle Deformation and...

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