《构造地质学》（英文版）Chapter 24 The Axial Plane

October- 24 2001 The Axial Plane Folds -Layers are bent, but not broken Fig.10.1

1 GLGY 201 Chapter 10 Folds, Faults and Rock Deformation October 24, 2001 October 24, 2001 The Axial Plane Folds - Layers are bent, but not broken Fig. 10.1

Faults- Layers are broken Fault has moved this much These layers were originally together Strike and dip The orientation of structures in the field DIP Go to"courses" page and look at"Strike and Dip Review

2 Faults - Layers are broken These layers were originally together Fault has moved this much Strike and Dip - The orientation of structures in the Field STRIKE DIP Go to ìcoursesî page and look at ìStrike and Dip Reviewî

STRIKE- The direction of the line formed by the intersection of a horizontal plane with a bedding or fault plane Fig. 10 DIP-The angle formed by the intersection of a bedding or fault plane and the horizontal plane. The angle is measured in a vertical plane perpendicular to the strike. The symbol used to display strike and dip on a map is the following 5 Dip angle The strike and dipl symbols on this map can be used to construct this Cross section Fig.10.5

3 STRIKE - The direction of the line formed by the intersection of a horizontal plane with a bedding or fault plane. DIP - The angle formed by the intersection of a bedding or fault plane and the horizontal plane. The angle is measured in a vertical plane perpendicular to the strike. Fig. 10.4 Fig. 10.5 The symbol used to display strike and dip on a map is the following: S 45 trike Dip angle The strike and dip symbols on this map can be used to construct this Cross Section

Chapter 10: Rock Deformation or what you always wanted to know about folds, faults, and joints Stress= the force applied to a plane divided by the area of the plane lithostatic compressive tensile shear forces directed forces directed stress that stress applied toward one away from acts parallel equally another one another to a plane Rock Deformation The change in volume or shape of an object that results from stress(Force/Area)is called strain The response of rocks to stress can be divided into elastic response: rock returns to original shape ductile or plastic response: permanent deformation without fracture occurs above the elastic limit brittle response: fracturing of a rock with little deformation prior to its rupture

4 Chapter 10: Rock Deformation or what you always wanted to know about folds, faults, and joints Stress = the force applied to a plane divided by the area of the plane. stress applied equally lithostatic forces directed toward one another compressive forces directed away from one another tensile stress that acts parallel to a plane shear Rock Deformation The change in volume or shape of an object that results from stress (Force/Area) is called strain. The response of rocks to stress can be divided into - elastic response: rock returns to original shape - ductile or plastic response: permanent deformation without fracture; occurs above the elastic limit - brittle response: fracturing of a rock with little deformation prior to its rupture

STRAIN This bar, length L Is shortened by compressive stress(o,) To length La The strain(change in shape) is Strain (G =(L1-L2 ·100% Deformed at Deformed at Undeformed Shallow Deep Sample Crustal Crustal Conditions Conditions Fig.10.7

5 STRAIN L1 Is shortened by compressive stress (σ1 ): This bar, length L1: L2 σ1 σ1 To length L2 The strain (change in shape) is: Strain (σ1 ) = (L1 ) (L1-L2) ï100% Undeformed Sample Fig. 10.7 Deformed at Shallow Crustal Conditions Deformed at Deep Crustal Conditions

The fracture response of a rock body is related to the orientation of stress directions o= intermediate stress o,= minimum stress Stresses resolved into three mutually perpendicular vectors of 01=maximum stress unequal magnitude Stress-strain and elastic, plastic and brittle failure Elastic stress proportional to strain body regains shape if stress released Ductile past the elastic limit body only recovers some shape if stress released. Stress is not <T proportional to strain Ductile Brittle Failure Brittle body ruptures, no recovery of shape Elastic Limit Increasing Strain(%)

6 The Fracture response of a rock body is related to the orientation of stress directions. σ3 = minimum stress σ1 = maximum stress σ2 = intermediate stress Stresses resolved into three mutually perpendicular vectors of unequal magnitude Increasing Strain (%) Increasing Stress (F/A) Elastic Limit Stress - strain and elastic, plastic and brittle failure Elastic Elastic = stress proportional to strain, body regains shape if stress released Ductile Ductile = past the elastic limit body only recovers some shape if stress released. Stress is not proportional to strain. Brittle X Failure Brittle = body ruptures, no recovery of shape

RELATIONSHIP OF STRAIN BEHAVIOUR TO STRUCTURE DUCTILE STRAIN FOLDING BRITTLE FAILURE FAULTS FRACTURES JOINTS Rock Deformation Propagation of seismic waves through rocks is an elastic response, rocks return to original shape Ductile(plastic)response of rock layers results in folds= permanent wavelike deformations in layered rocks Brittle response to stress results in faults =a fracture in bedrock along which rocks on one side have moved relative to the other side Rock response to stress is influenced by: type of stress, type of rock, temperature, pressure, fluids, length and magnitude of stress applied

7 RELATIONSHIP OF STRAIN BEHAVIOUR TO STRUCTURE DUCTILE STRAIN FOLDING BRITTLE FAILURE FAULTS FRACTURES JOINTS Rock Deformation Propagation of seismic waves through rocks is an elastic response, rocks return to original shape. Ductile (plastic) response of rock layers results in folds = permanent wavelike deformations in layered rocks. Brittle response to stress results in faults = a fracture in bedrock along which rocks on one side have moved relative to the other side. Rock response to stress is influenced by: type of stress, type of rock, temperature, pressure, fluids, length and magnitude of stress applied

Rock Deformation- Folds Anticline fold with the convex side upward (therefore oldest layers in the middle) Fig.10.9 Syncline fold with the concave side upward ( therefore youngest layers in the middle Usually anticlines and synclines alternate in the field LIMBS BISECTED by an imaginary surface called the axial plane The line formed by the intersection of the axial plane and the surface of a rock layer is the fold axis A fold consists of two limbs Fig.10.10(a)

8 Rock Deformation - Folds Anticline fold with the convex side upward (therefore oldest layers in the middle) Syncline fold with the concave side upward (therefore youngest layers in the middle) Fig. 10.9 Usually anticlines and synclines alternate in the field. A fold consists of two limbs Ö LIMBS BISECTED by an imaginary surface called the axial plane The line formed by the intersection of the axial plane and the surface of a rock layer is the fold axis. Fig. 10.10(a)

Folds with a fold axis that is not horizontal are said to“ plunge The amount of plunge is the angle between the t horizontal and the fold axis-45°for FOLD AXIS this example Note- the axia plane is vertical Fig.10.10(b) in this example Symmetrical Folds Limbs dip symmetrically away from axial plane Axial plane is vertical Fjg.10.12(a)

9 Folds with a fold axis that is not horizontal are said to ìplungeî The amount of plunge is the angle between the horizontal and the fold axis - 45° for this example. Fig. 10.10 (b) Note - the axial plane is vertical in this example. FOLD AXIS Symmetrical Folds Limbs dip symmetrically away from axial plane. Axial plane is vertical. Fig. 10.12 (a)

Asymmetrical Folds Beds of one limbs dip more steeply than beds on other limb Fig.10.12(b) Overturned Folds Beds of both limbs dip in the same direction One limb has been tilted past vertical Fjg.10.12(c)

10 Asymmetrical Folds Beds of one limbs dip more steeply than beds on other limb. Fig. 10.12 (b) Overturned Folds Beds of both limbs dip in the same direction. Fig. 10.12 (c) One limb has been tilted past vertical