Metamorphism and Deformation

Transcription

1 Metamorphism and Deformation

2 Metamorphism takes place at temperatures and pressures that fall between those in which sediments are lithified and those in which rocks begin to melt and form magmas.

3 Types of Metamorphism Regional metamorphism: rocks are buried beneath thick accumulations of sediment and/or undergo tectonic stresses. Rocks can occur over hundreds of thousands of square miles. Contact metamorphism: rocks form when intruding magma contacts the surrounding rock and causes changes due to the heat and release of fluids from the magma.

4 Types of Metamorphism

5 Changes that take place during metamorphism include: Recrystallization of existing minerals, especially into larger crystals. Recrystallization and growth of new minerals: Chemical breakdown of original minerals that become unstable in the metamorphic environment, and formation of new minerals that are stable, such as garnets in the schist rock (shown on the right). Recrystallized quartz minerals in quartzite Garnet Schist Foliation: Deformation and reorientation of existing mineral crystals and the growth of new ones with distinctive orientations. Foliated Schist

6 What factors control the changes that take place during metamorphism? Mineralogy of the original rock (parent rock or protolith ) For example, limestone, comprised of the mineral calcite (CaCO 3 ) is metamorphosed to form marble. Limestone The depth (pressure) and temperature at which the alteration occurred. Deformation and reorientation of existing mineral crystals and the growth of new ones with distinctive orientations. Marble The combined effects of these processes generally produce metamorphic rocks that differ from their protoliths by being coarser-grained and/or foliated.

7 Regional Metamorphism How to tell if a rock has undergone Regional Metamorphism: Rock is widespread Rock is messed up (folded, foliated) Rock contains large metamorphic minerals (often colorful)

8 Contact Metamorphism How to tell if a rock has undergone Contact Metamorphism: Rock is adjacent to an igneous intrusion or (more rarely) volcanic flow Rock is oxidized (often red in color) Formation of clay minerals is common Usually fairly limited in extent compared to regional metamorphism

9 Metamorphic grade refers to pressure/temperature conditions in regional metamorphic rocks. High grade metamorphic rocks were subjected to high temperatures ( o C) and high pressure conditions (equivalent depth of km). Low grade metamorphic rocks were subjected to relatively low temperatures ( o C) and pressures (equivalent depth of 6-12 km). Generally, the higher the grade of metamorphism, the coarser a rock s grain size will be.

10 Idealized schematic of increasing metamorphic grade with depth

11 While Metamorphic rocks contain many of the same minerals found in igneous and sedimentary rocks, certain minerals occur almost exclusively in metamorphic rocks, such as garnet, kyanite or sillimanite. Each mineral found in a metamorphic rock has a specific stability range of pressure and temperature

12 Mineralogical changes during metamorphism of the sedimentary rock shale. Note that the elemental composition will not change, but the mineral composition will reflect the pressure/temperature condition (minerals recrystallize into new minerals).

13 Identification flow chart

14 Non-foliated metamorphic rocks with granular texture. Quartzite Hornfels quartzite Marble (reacts with HCl) hornfels marble

15 slate phyllite schist gneiss Foliated metamorphic rocks are listed with increasing metamorphic grade (and overall grain size): slate, phyllite, schist and gneiss.

16 To differentiate clastic sedimentary rocks from their metamorphosed equivalents look for fracture patterns within the rock. Fractures will occur between sediment grains in sedimentary sandstone or conglomerate. 16 Fractures will occur within mineral crystals in quartzite or lithic clasts, such as pebbles in a metaconglomerate.

17 Elastic (nonpermanent) deformation occurs in rock when weak stresses are applied. In such cases stress and strain are proportional. When the stress is reduced or removed the rock will return to its original form. Ductile (permanent) deformation of rock will occur when the applied stress exceeds the strength of the rock. Rupture occurs when the strain on the rock exceeds its ability to deform.

18 Faulting Folding These rocks had ductile deformation up to a point, but then ruptured. Permanent deformation occurs in rock when shear stress exceeds shear strength. Permanent deformation in rock may occur as brittle fracture (faulting) or ductile (folding) deformation. Folds and faults in marble.

19 Folding and faulting within the above bedrock are examples of ductile (permanent deformation.

20 The lithosphere is in isostatic equilibrium with the asthenosphere (upper mantle). The lithosphere floats on the asthenosphere. When a load, such as an ice sheet, is placed on the lithosphere, it will isostatically depress the lithosphere relative to the asthenosphere. When the ice melts the lithosphere isostatically rebounds. What type of deformation would this be? Elastic

21 The lithosphere can isostatically rise and depress with the filling and emptying of a reservoir. Thinking within the context of isostasy why would you expect the Puget Sound to remain a tectonic basin?

22 Figure 9-5 Permanent deformation can be compressional, tensile and translational (shearing). The tectonic setting greatly influences the nature of the stress being applied to the crust or rock. Think about which type of tectonic margin would yield compressional stresses versus tensile or shearing.

23 Controlling Factors on Ductile versus Brittle Fracture Deformation 1. Temperature 2. Confining Pressure Near the San Andreas Fault, Palmdale, CA 3. Strain Rate 4. Lithology 5. Time

24 How is temperature related to the ductility of rock? Think about glass blowing.

25 Glacial ice can deform ductily when the confining pressure is high (ice is constrained within the U-shaped valley) and the strain rate is low.

26 Marble is typically brittle but can deform ductily when the strain rate is low.

27 This interlayered sandstone and shale deforms both ductily and by brittle fracture.

28 Vertical faults are classified by offset of the hanging wall relative to the footwall. When the hanging wall is down-dropped relative to the footwall the fault is classified as normal. When the hanging wall is upthrown relative to the footwall the fault is considered reversed or thrust if it is low-angled.

29 How would you classify the above fault. Could you draw the fault line and direction of motion on this image?

30 The basin and range province is characterized by normal faulting. Is the stress field extensional or compressional tectonics?

31 The range front fault along the Sierra Nevada is characterized by normal faulting. Which side of the fault is up-thrown (footwall)? Which side of the fault is downdropped (hanging wall)?

32 Could you draw the fault line and offset arrows for this reverse fault?

33 Overturned folds will often evolve into thrust faults with continued compressive stress. Collisional tectonic mountain ranges are often characterized by fold and thrust fault structures.

34 An overturned fold and thrust fault can place older Precambrian rock over younger Paleozoic rock, such as the case in the Canadian Rocky Mountains (shown above). Can you draw the thrust fault with displacement arrows?

35 Is this where you would draw the thrust fault?

36 Horizontal (strike-slip) faults are classified by relative motion of the fault block as seen by the observer across the fault line. They are classified as either rightlateral or left-lateral strike-slip depending upon the relative motion.

37 What kind of strike-slip fault is the San Andreas? Can you draw in the fault and show the displacement arrows?

38 The San Andreas Fault is a right-lateral strike slip fault. Horizontal motion between two tectonic plates is defined as a transform fault.

39 Ductile deformation of rock results in fold structures. What type of stress results in folding? Folds have two limbs and an axis. The axial plane is an imagery plane that extend through the axis.

40 Folds are classified based on symmetry of their limbs relative to the axial plane. Which two folds do not have limbs that are symmetric relative to their axial planes?

41 Monoclines form when the stress field is unidirectional.

42 Anticlines and synclines are symmetric folds that form from bi-directional compressive stress. The anticline on the right is plunging into the slide (north).

43 As anticlines and synclines erode note the outcrop pattern in the center of the eroding fold. Which fold will expose the oldest rock in the center (near its axis) as it erodes? Note the difference in outcrop pattern of the plunging anticline versus the non-plunging folds on the left of the slide.

44 What kind of fold is shown above? What direction was the stress being applied?

45 What kind of fold is shown above? Note that the river erodes along the axis of the fold. Why do you think this is the case (where would the rock be weakest?)?

46 Note that the fold pattern of a plunging anticline closes in the direction of plunge (into slide).

47 Note that the fold pattern of a plunging syncline opens in the direction of plunge (towards you).

48 What kind of plunging fold is shown above? (Arrow points in plunge direction).

49 Zigzag fold structures form as a series of plunging anticlines and synclines are eroded following tectonic uplift. These types of fold structures are common in uplifted fold and thrust belt mountain ranges, such as the Appalachian Mountains in the eastern U.S.

50 The zig-zag folds seen in the Appalachian Mountains is the result of erosion of plunging folds. The Harrisburg region of Pennsylvania, U.S. is characterized by these structures.

51 Domes and basins are simply special cases of anticlinal and synclinal folds structures. Note that similar to anticlines the oldest rock is exposed in the center of an eroding dome and the youngest rock is exposed in the center of an eroding basin.

52 Igneous intrusive bodies can cause upwarping and the formation of domal structures.

53 Figuring out the tectonic history of an area can be very difficult. Complicated Norwegian Rocks