The Midterm. Marks are online Worth 20%, avg. 67% (36-89)

Transcription

1 The Midterm Marks are online Worth 20%, avg. 67% (36-89)

2 Deformation (Ch 8!) The theory of plate tectonics tells us that the Earth s plates are in constant motion They move apart, collide and slide past one another As well as generating earthquakes and volcanoes these processes also deform the rocks of the crust Compressional folding, Domeyko Cordillera, Northern Chile

3 Deformation Many rocks have been deformed by dynamic forces that cause fracturing, folding, and faulting of rocks. Processes that cause deformation are associated with plate boundaries and mountain building processes. Wicander and Monroe (2002)

4 How Does Rock Deform? Deformation is simply a change in shape and/or volume of rocks. We can measure these changes both in the field and the lab in order to try and understand how rocks deform Murck and Skinner (1999)

5 Strike and Dip ration/isu/unit1/u1lesson2.html

6 Stress and strain Stress is the force applied to a given area of rock. It has the same definition as pressure (force acting on a surface per unit area) However, pressure is generally considered to be acting equally in all directions So geologists use the term stress Differential stress Uniform stress Uniform stress can also be thought of as confining pressure or confining stress Murck and Skinner (1999)

7 Stress Stress comes in three flavours: Compression Tension Shear Murck and Skinner (1999)

8 Compression In compression, rocks are squeezed by external forces acting toward one another. Compression causes rock to shorten along the direction of compression. Compression causes folding as well as reverse and thrust faulting. Wicander and Monroe (2002)

9 Tension Tension results from force acting in opposite directions, which tends to lengthen rocks and pull them apart. Tension causes normal faults. Wicander and Monroe (2002)

10 Shear In shear stress, forces act opposite one another in a horizontal plane. Shear stress causes rocks to slip past one another in a horizontal plane as with strike-slip faults. Wicander and Monroe (2002)

11 Strain Strain also comes in three flavours Elastic strain Plastic strain Brittle strain Murck and Skinner (1999)

12 Elastic deformation This is a non permanent change in volume or shape A good example of this is a metal spring Elastic limit Chernicoff and Whitney (2002) Murck and Skinner (1999)

13 Ductile deformation Once the elastic limit is exceeded rocks will deform Ductile deformation is permanent Chernicoff and Whitney (2002) Murck and Skinner (1999)

14 Brittle deformation This occurs when rocks fracture rather than bend Chernicoff and Whitney (2002) Murck and Skinner (1999)

15 Deformation in rocks Murck and Skinner (1999) Plummer et al. (2001)

16 Brittle or ductile? The response to deformation is controlled by the nature of the stress, pressure, temperature, time and rock type Any rock under the great pressures and high temperatures of Earth s interior is more ductile that it would be at Earth s surface.

17 Temperature Solids become more ductile and less brittle at high temperatures Glass can be blown when hot but will shatter when cold This is why the asthenosphere flows

18 Confining pressure High confining pressure reduces brittleness by hindering fracture formation High confining pressure Low confining pressure Murck and Skinner (1999)

19 Rate of deformation The rate at which stress is applied determines if the failure will be brittle or ductile Think of silly putty - it bounces if dropped but shatters if hit with a hammer Strain rate - this is the rate at which a rock is deformed Low strain rate results in ductile behaviour

20 Summary Low temperatures, low confining pressures and high strain rates result in brittle behaviour High temperatures, high confining pressures and low strain rates result in ductile behaviour Brittle-ductile transition - the point at which behaviour changes (~10-15 km)

21 Deformation & Geologic Structures Any feature resulting from rock deformation is a geologic structure, especially fractures, folds, and faults. Folding occurs in response to ductile deformation Faulting occurs in response to brittle deformation The study of these features is structural geology

22 Brittle structures Brittle failure results in fractures in a rock. These basically take two forms Joints are fractures in a rock along which no displacement has occurred. Faults are fractures along which blocks have moved parallel with the fracture surface Joints are generally small but pervasive Faults can vary from micro fractures to continent scale (San Andreas fault or East African Rift)

23 Faults Klamath Falls, Oregon, Wicander and Monroe (2002) Not all faults penetrate to the surface, but those that do often show a fault scarp, a cliff or bluff formed by vertical movement along the fault plane. Magnitude 6.1, Takhar Province, Afghanistan, February 4, 1998, Chernicoff et al. (2002)

24 Faults There is some terminology we must know to understand faults: Fault plane (fault scarp) Hanging wall Footwall Wicander and Monroe (2002)

25 Hanging wall & footwall The footwall block lies beneath the fault plane and the hanging wall block lies above Chernicoff and Whitney (2002)

26 Fault types We can only think of the movement on faults in terms of relative movement Normal or dip-slip faults Reverse faults Strike slip faults Oblique slip faults some combination Chernicoff and Whitney (2002)

27 Normal dip-slip faults All movement on a dip-slip fault is parallel to the dip of the fault plane, that is, movement is up or down the fault plane. In a normal fault, the hanging wall moves down the fault plane. Normal faults result from tensional stress. footwall hanging wall Wicander and Monroe (2002)

28 Horst & Graben structures Horst - uplifted block with faults dipping away Graben - downthrown block with faults dipping towards each other Murck and Skinner (1999)

29 Reverse dip-slip faults Reverse faults are dip-slip faults where the hanging wall has moved up the inclined fault plane. In reverse faults, the dip of the fault plane is >45. Formed by compressional stress footwall hanging wall Wicander and Monroe (2002)

30 Thrust faults Reverse faults with fault plane dips of <45 are called thrust faults Murck and Skinner (1999)

31 Strike-Slip Faults A Strike-slip faults are caused by shearing forces, which cause blocks on either side of the fault plane to slide laterally past one another. B Wicander and Monroe (2002) San Andreas Fault, Calif.

32 Strike-slip faults As the observer looks across the fault plane to the opposite side, the offset feature will lie to the left for a leftlateral (sinistral) fault strike-slip fault and to the right for a rightlateral (dextral) strikeslip fault. Sinistral Dextral Murck and Skinner (1999)

33 San Andreas fault. The Pacific plate appears to be moving NW relative to the North American plate Chernicoff and Whitney (2002)

34 Oblique slip. Chernicoff and Whitney (2002)

35 Joints These are formed in a variety of ways but are characterised by the fact that no movement has occurred along the fracture Some form as a result of regional stress, others because of cooling or pressure release Marquette, Michigan, Wicander and Monroe (2002)

36 Joints Joints can form in the upper parts of anticlines which undergo tension as the rocks flex around the fold axis Wicander and Monroe (2002) B

37 Joints Joints form when rocks are stretched by simple tension or shear stress. Plummer et al. (2001)

38 Brittle or ductile? The response to deformation is controlled by the nature of the stress, pressure, temperature, time and rock type Any rock under the great pressures and high temperatures of Earth s interior is more ductile that it would be at Earth s surface.

39 Folds When rocks deform in a ductile manner they bend and flow to form folds They range in scale from mm to km There are three basic fold types Monoclines Synclines Anticlines Rockies, Thompson and Turk (1998)

40 Some terminology Fold limb Murck and Skinner (1999)

41 Monoclines Monoclines are simple bends or flexures in otherwise horizontal or uniformly dipping layers. Monoclines often drape deep fractures in the rock along which vertical movement has occurred. Wicander and Monroe (2002) Big Horn Mtns., Wyo.

42 Monoclines Thompson and Turk (1998) Thompson and Turk (1998)

43 Monoclines Murck and Skinner (1999)

44 Anticlines Anticlines are arched or convex-upward folds with the oldest rocks in the core of the fold. Calico Mtns., Calif. anticline Wicander and Monroe (2002)

45 Synclines Synclines are trough-like or concavedownward folds with the youngest rocks in the core of the fold. Calico Mtns., Calif. syncline Wicander and Monroe (2002)

46 Folds Anticlines and synclines do not necessarily correspond to highs and lows on Earth s surface British Columbia, Canada Wicander and Monroe (2002)

47 Folds Anticlines and synclines often occur together in alternating series. Upright folds have vertical axial planes and limbs that dip at the same angle in opposite directions. Wicander and Monroe (2002)

48 Anticlines and synclines. Chernicoff and Whitney (2002)

49 Folds Folds with non-vertical axial planes and limbs that are in opposite directions and at different angles are inclined folds. Wicander and Monroe (2002)

50 Folds Folds with non-vertical axial planes and limbs that dip in the same directions, but at different angles are overturned folds. Wicander and Monroe (2002)

51 Folded Rock Layers Where deformation has been especially intense, folds sometimes have horizontal axial planes. These are recumbent folds. Overturned and recumbent folds are particularly common in mountain ranges formed by compression along convergent plate margins. Wicander and Monroe (2002)

52 Plunging Folds If a fold s axis is not horizontal, the fold is plunging Anticlines have outcrop patterns that point in the direction of plunge. Wicander and Monroe (2002) A B

53 Plunging Folds Synclines have map patterns that open in the direction of plunge. A series of alternating, plunging anticlines and syncline forms a zig-zag, or sawtooth map pattern Wicander and Monroe (2002) A B

54 Plunging folds Chernicoff and Whitney (2002)

55 Plunging folds An aerial view of Sheep Mountain in Wyoming shows that it is a plunging anticline. The plunge direction is out of the viewing plane. Wicander and Monroe (2002)

56 Domes and Basins Domes and basins are circular- to oval-shaped folds. In eroded domes, the oldest rocks lie at the middle of a bull s eye map pattern and all layers dip away from the center. In basins the youngest rocks lie at the middle of a bull s eye map pattern and all layers dip toward the center. Most of Michigan is a large basin. Wicander and Monroe (2002)

57 Domes and basins. Chernicoff and Whitney (2002)

58 Dome Rawlings, Wyoming, Press and Siever (2000)

59 Summary Murck and Skinner (1999)

60 What type of fold? Symmetrical Asymmetrical Chernicoff and Whitney (2002)

61 What type of fold? Overturned Recumbent Chernicoff and Whitney (2002)

62 Orogens a.k.a mountain belts Orogens are elongate belts of intensely deformed and metamorphosed rocks Oros - Greek for mountain Genesis - come into being Active mountain belts such as the Alps, Andes and Himalayas are clearly the result of plate tectonics So if we can identify them in older rocks we have evidence for plate tectonics

63 Ancient orogens The oldest part of the Appalachian is ~600 Ma There is evidence for orogens as old as 1.5 Ga and maybe even older Appalachians Chernicoff and Whitney (2002)

64 Isostasy In order to understand orogens we need to understand isostasy Chernicoff and Whitney (2002)

65 The Principle of Isostasy The principle of isostasy indicates that Earth s crust floats in equilibrium with the denser mantle below, like an iceberg floats in water. If mountain ranges had no roots, a positive gravity anomaly would be expected, but such has not been observed. In contrast, no anomaly would be expected if they have roots. The buoyant force from extension of the low-density roots into the underlying high-density mantle causes mountains to stand high Wicander and Monroe (2002)

66 Isostatic Rebound Thick accumulation of sediment or glacial ice atop continental crust can push it lower into the mantle. Once the load is removed, the crust rises back to a position of equilibrium in a process known as isostatic rebound. Isostatic rebound affects mountain ranges. Due to isostatic rebound as mountains erode, progessively deeper levels are brought to the surface. If rebound continues long enough, only the plutonic and metamorphic rocks of the root will be left to record the existence of the former mountain range. Wicander and Monroe (2002)

67 Isostasy and erosion As the mountain range is eroded the root rises to balance the loss of material Because the surrounding lithosphere is strong it is a slow process for the root to push its way up This process can take 100 s of millions of years

68 Cratons These are areas that have been stable and undeformed for at least 1 Ga Stable means isostatically balanced May or may not have been mountain belts Murck and Skinner (1999)