CRUSTAL DEFORMATION. Chapter 10

Transcription

1 CRUSTAL DEFORMATION and dgeologic Structures t Chapter 10

2 Deformation Df Deformation involves: Stress the amount of force applied to a given area. Types of Stress: Confining Stress stress applied equally in all directions. Differential Stress stress applied unequally in different directions.

3 Deformational Stress Types of Differential Stress: (1)Compressional lstress shortens and thickens a rock body (associated with convergent plate boundaries). (2)Tensional Stress tends to elongate and thin or pull apart a rock unit (associated with divergent plate boundaries). (3)Shear Stress produces a motion similar to slippage that occurs between individual playing cards when the top of the stack is moved relative to the bottom (associated with transform plate boundaries).

4 Deformation of the Earth s Crust Caused by Tectonic Forces and Associated Differential Stresses

5 Df Deformation Differential stress applied to rocks during tectonic activity causes rocks to respond via df deformation. Strain changes in the shape or size of a rock body caused dby stress. Strained rock bodies do not retain their original configuration during deformation.

6 How Do Rocks Deform? Rocks subjected to stresses greater than their own strength begin to deform usually by folding, flowing, or fracturing. General Characteristics of Rock Deformation: Elastic deformation the rock returns to nearly its original size and shape when the stress is removed. Once the elastic limit (strength) of a rock is surpassed, it either flows (ductile deformation) or fractures (brittle deformation).

7 How Do Rocks Deform? Factors that t influence the strength th of a rock and how it will deform: Depth Temperature Confining Pressure Rock Type Availability of Fluids Time

8 How Do Rocks Deform? Rocks near the surface, where confining pressures and temperatures are low, will behave as a brittle solid and fracture once their strength is exceeded. Rocks at depth, where confining pressures and temperatures are high, will exhibit ductile behavior or solid-state flow, in which changes occur without fracturing.

9 Mapping Geologic Structures When conducting a study of a region, a geologist identifies and describes the dominant rock structures. Describing and mapping the orientation or attitude of a rock layer or fault/ fracture surface involves determining the features. Strike (Trend) The compass direction of the line produced by the intersection of an inclined rock layer or fault with a horizontal plane. Generally expressed an an angle relative to north (for example, N10ºE).

10 Mapping Geologic Structures Dip (Inclination) The angle of inclination of the surface of a rock unit or fault measured from a horizontal plane. Includes both an inclination and a direction toward which h the rock is inclined (for example, 30ºSE). By knowing the strike and dip, geologists can predict the nature of rock structures hidden beneath the surface.

11 Strike and Dip of a Rock Layer

12 A Geologic Map Showing Strike and Dip of Structures

13 Geologist Measuring the Dip of Strata in a Roadcut

14 Crustal Structures Folds During crustal deformation rocks are often bent into a series of wave-like undulations. Anticlines and dsynclines Domes and Basins Monoclines Characteristics of Folds: Most tfolds result ltfrom compressional stresses which shorten and thicken the crust. Most of them occur in a series.

15 Anatomy of a Fold Limbs Refers to the two sides of a fold. Axis (or Hinge) A line drawn down the points of maximum curvature of each layer. Axial Plane An imaginary surface that divides a fold symmetrically. Plunge In complex folding, the axis is often inclined at an angle called plunge.

16 Horizontal (A) and Plunging (B) Anticlines

17 Common Types of Folds Anticline tc upfolded or arched rock layers. Syncline downfolds or troughs of rock layers. Depending on their orientation, anticlines and synclines can be described as Symmetrical, asymmetrical, overturned, recumbent (a type of overturned fold lying on its side ), or plunging. Animations

18 Anticlines and Synclines

19 Insert Animation #30: Folds Formation of Folds

20 Syncline (left) and Anticline (right)

21 Plunging Anticlines and Synclines (Note: the outcrop pattern of an anticline points in the direction it is plunging, whereas the opposite is true for a syncline)

22 Sheep Mountain, A Plunging Anticline

23 Insert Animation #30: Plunging Folds Formation of Folds

24 Monoclines Other Types of Folds Large, step-like folds in otherwise horizontal sedimentary strata. Closely associated with faulting.

25 Dome Other Types of Folds Upwarped displacement of rocks. Circular or slightly elongated structure. Oldest rocks in center,,younger rocks on the flanks.

26 Basin Other Types of Folds Circular or slightly elongated structure. Downwarped displacement of rocks. Youngest rocks are found near the center, oldest rocks on the flanks.

27 Crustal Structures Faults Fractures in rocks along which appreciable displacement has taken place. Fault Zone Displacements along multiple interconnected faults. Sudden movements along faults are the cause of most earthquakes. Classified by their relative movement which h can be Horizontal, Vertical, or Oblique

28 Types of Faults

29 Summary of Fault Types Dip-Slip Faults: Normal (gravity) associated with divergent plate boundaries. Reverse and Thrust associated with convergent plate boundaries. Strike-Slip Faults: Lateral (right and left) associated with transform plate lt boundaries.

30 Dip-Slip Faults Movement is mainly parallel to the dip of the fault surface. Parts of a dip-slip fault include the hanging wall (rock surface above the fault) and the footwall (rock surface below the fault).

31 Dip-Slip Faults Normal Fault (gravity) Dip-Slip Faults Hanging wall block moves down relative to the footwall block. Tensional stress Accommodate lengthening or extension and thinning of the crust. Associated with divergent plate boundaries. Most are small with displacements of a meter or so. Larger scale normal faults are associated with structures called fault-block mountains (Teton Range in Wyoming, Basin and Range Province in Nevada).

32 Normal Faulting Animations:

33 Formation Insert Animation #29: Faults Normal of Normal Faults

34 Normal Faulting Fault Block Mountains Fault-Block Mountains Basin and Range Province in Nevada topography generated by a system of roughly north to south trending normal faults. Movements along these faults have produced alternating uplifted blocks called horsts (form elevated ranges) and down-dropped blocks called grabens (form basins). Half-grabens a tilted fault block in which the higher side forms mountainous topography and the lower side forms a basin that fills with sediment. Detachment Fault nearly horizontal fault extending up to hundreds of kilometers into the subsurface. Smaller faults are connected to this larger fault. Boundary between ductile and brittle deformation.

35 Dip-Slip Faults Reverse and Thrust Dip-Slip p Faults Hanging wall block moves up relative to the footwall block. Reverse faults have dips greater than 45 o and thrust faults have dips less than 45 o. Strong compressional stress. Accommodate shortening and thickening of the crust. Associated with convergent plate boundaries.

36 Reverse Fault Animations:

37 Formation Insert Animation #29: Faults Reverse of Reverse Faults

38 Idealized Development of Lewis Overthrust Fault near Glacier National Park

39 Fault-Associated Folding

40 Strike-Slip Faults Dominant displacement is horizontal and parallel to the strike of the fault. May produce broad zones of roughly parallel fractures up too several kilometers in width. Shear stress. Associated with transform plate boundaries.

41 Formation Insert Animation #29: Faults Strike-Slip of Strike-Slip Faults

42 Types of Strike-Slip Faults Right-Lateral as you face the fault, the opposite side of the fault moves to the right. Left-Lateral as you face the fault, the opposite side of the fault moves to the left. Transform Fault Large strike-slip fault that cuts through accommodates motion between two large crustal plates (San Andreas Fault).

43 A Right-Lateral Strike-Slip Fault Animations:

44 The San Andreas Fault System

45 Name the Type of Fault Below

46 Name the Type of Fault Below

47 Name the Type of Fault Below Insert Animation #28: Exposing Metamorphic Rock

48 Name the Type of Fault Below

49 Name the Type of Fault Below

50 Name the Structuret

51 Name the Structuret