PG HC Generation -- Paleotemperatures.pptx

PG HC Generation -- Paleotemperatures

1. Paleotemperatures Paleosurface temperatures Burial history Paleoheat flow

2. Paleosurface temperatures Based on present-day surface temperatures, modified for Change in latitude (continental drift) Change in elevation above sea level Change in water depth Change in global climate

3. Change in latitude www.scotese.com Reconstruction of continental positions through time

4. Temperature at sea level at any paleolatitude Assume the equation of Barker (2000) can be applied to get us started: T ( o C) = 27.6 - 0.0414•L - 0.00599•L 2 L = latitude in degrees

5. Change in paleoelevation above sea level Apply adiabatic lapse rate concept Temperature decreases by 6.5 o C/km of elevation gain

6. Changes in global climate www.scotese.com

7. Effect of paleowater depth Apply nomograph of Beardsmore & Cull (2001), which was designed for present world

8. Effect of global climate change on sea-floor temperature

9. Example 1 Calculate the surface temperature in central Algeria at 265 Ma, at an elevation of 1800 m in the mountains believed to have formed during the Hercynian orogeny

10. Step 1 Estimate paleolatitude Using Scotese’s set of paleomaps, we find the following reconstruction at 255 Ma

11. www.scotese.com ●

12. Estimated paleolatitude is about 3 o S In today’s world, using Barker’s (2000) equation, the temperature at sea level at that latitude would be about 27.4 o C .

13. Step 2 Correction for elevation Temperature reduction: 1.8 km above sea level * 6.5 o C/km = 11.7 o C

14. Step 3: Correction for global climate change ●

15. 265 Ma is difficult to reconstruct Temperature was increasing rapidly at about this time. Global climate was probably several degrees (10 o C?) warmer than the modern world, which is cool

16. Estimated temperature on the Permian mountain: 27.4 o – 11.7 o + 10 o = 25.7 o C

17. Example 2 Calculate the temperature at the sea floor in 200 m of water along the coast of Gabon at the start of the Turonian

18. Start of Turonian: 94 Ma www.scotese.com ●

19. Estimated paleolatitude = 22 o S Applying the nomograph of Beardsmore & Cull (2001), we find a present-day temperature of 14.3 o C at 200 m water depth at 22 o S latitude

20. ●

21. Scotese shows Turonian as about 12 o C warmer than the present ●

22. How will this warmer surface temperature affect bottom-water temperatures? At 200 m, the warmer surface probably gives warmer sea-floor temperatures – but the difference will not be as great as at the surface

23. Sea-floor temperature change with 1 o C surface temperature change

24. At 200 m water depth the change is about 0.54 o C for every 1 o C change in temperature at the surface

25. Best estimate Temperature at 200 m water depth at 20 o S latitude 94 Ma: 14.3 o + 0.54*12 o = 20.8 o C

26. Burial history Burial history affects thermal history Our main concern here is uncertainty in burial history and its effect on knowledge of thermal history

27. Burial history: uncertainties about paleotemperatures Continuous subsidence: very minor uncertainty Minor unconformities (short duration, minor erosion): minor uncertainty Major unconformities Long duration, little erosion: minor? uncertainty Long duration, much erosion : major uncertainty Short duration, much erosion : major uncertainty

28. Major erosion

29. Pre-erosion burial (before 300 Ma) controls maturation history of Paleozoic rocks Subsequent events are of no importance to those rocks

30. Therefore, correct reconstruction of the Hercynian unconformity is very important

31. Geologic methods of reconstructing events at an unconformity have already been discussed Later we will discuss the use of thermal indicators to test our reconstruction

32. Paleoheat flow Varying the paleoheat flow is widely used to fit calculated maturity to measured data However, although variations in paleoheat flow do occur, they are probably not as common, strong, or rapid as many believe There are significant geological and physical constraints on variations in paleoheat flow

33. Why does basal conductive heat flow change? Change in thickness of lithosphere Rifting Other extension (Pull-apart) Mantle diapirs Hot spots Subduction Obduction

34. Why do changes in thickness of lithosphere affect heat flow from the asthenosphere? D T/ D Z = Q/ l Q = l * D T/ D Z D T is difference in temperature between asthenosphere and surface, and is essentially constant l (average conductivity) doesn’t change much as lithosphere thins or thickens Therefore, heat flow (Q) changes in inverse proportion to thickness of lithosphere ( D Z)

35. Models for heat-flow history of rifts McKenzie model Original model Non-instantaneous stretching Non-uniform stretching Two- or three-layer models Waples modification

36. Original McKenzie model All heat comes from asthenosphere Instantaneous uniform stretching Stretching factor (tectonic) b = ratio of original lithospheric thickness to new thickness Increase in basal HF because of thinning of lithosphere Thermal b = ratio of new HF to original HF Thermal b = Tectonic b Exponential relaxation to original basal HF after >200 my

37. McKenzie b values 1.0: no extension 2.0: North Sea (typical of many failed rifts) ~4.2: formation of oceanic crust In original model b is constant across a basin In modified models b is maximum in basin center, decreasing to 1.0 at margins

38. Non-instantaneous stretching If stretching occurs over ~20-30 my or less, it can be considered to be instantaneous Rifting normally takes 10-20 my Thus McKenzie’s original mathematical analysis is valid

39. Uniform stretching? Modern applications: b varies across a basin Different degrees of stretching for different layers of lithosphere Upper crust & Lower crust-mantle (2 layers) Upper crust & Lower crust & Mantle (3 layers) Lower layer(s) stretch more than upper crust An average b can be calculated

40. McKenzie model: b = 2

41. McKenzie model: b = 4

42. Change in basal heat flow during rifting In the original McKenzie model, since all heat is assumed to come from the asthenosphere (no RHP), the basal HF increases by the factor b during rifting. If we assume that some heat comes from RHP in the lithosphere, then the situation is more complex.

43. Waples modification: two opposing effects Increase in heat flow due to thinning of insulating layer (lithosphere) (as in McKenzie model) Decrease in heat flow due to thinning of radiogenic sources in lithosphere

44. Increase and decrease in basal HF are not simultaneous, and not equal in amount

45. Decrease in heat flow from loss of RHP is felt first near the surface, since most of RHP is in shallow upper crust Increase in heat flow from thinning of insulating layer is felt later , since the source of asthenospheric heat is much deeper

46. How fast can basal conductive heat flow increase? Rifting normally takes 10-20 my Hot spot time scale similar Pull-apart basins can evolve more rapidly Effects of deep lithospheric processes are not felt immediately near the surface

47. Delay in arrival of heat pulse at surface for different degrees of stretching and lithospheric thinning

48. Waples modification Assumes shallow radiogenic and deep sources for heat Assumes there is loss of RHP during extension Assumes there is gain of deep-sourced heat Assumes loss occurs earlier than gain

49. b = 2

50. Differences between Waples & McKenzie models Waples predicts initial lowering of HF before it increases McKenzie predicts much higher synrift HF McKenzie predicts much greater decrease in HF during relaxation Waples predicts final HF will be lower than pre-rift HF (lost RHP cannot be recovered; McKenzie predicts they will be the same because he doesn’t consider RHP ) In Waples model thermal b ≠ tectonic b

51. Commercial software generally includes McKenzie model

52. Application of McKenzie model to other types of extensional basins McKenzie model is commonly applied to pull-apart basins There is no direct evidence that it is applicable in those situations It seems likely that convective heat transfer is responsible for most of the heating in such basins

53. Hot spots, mantle plumes Conceptually similar to McKenzie-type extension Lithosphere thins due to upwelling asthenosphere No thinning of radiogenic crust Original McKenzie model may be appropriate, but with reduced value of Beta (no change in radiogenic heat contribution)

54. Emplacement of plutons Plutons are relatively shallow Relaxation time at end of event will be short No thinning of radiogenic crust Modified McKenzie model with rapid relaxation may be appropriate (no change in radiogenic heat contribution)

55. Foreland basins Most-common type of non-extensional basin No major change in thickness of crust or lithosphere No major change in basal heat flow

56. Any tectonic setting can be modeled by making assumptions about the geologic process and its parameters

57. Convective heat transfer 

58. Convective heat transfer Probably important in all extensional settings Often important in backarc settings Extremely important in pull-apart basins Very difficult to model

59. Problems in modeling convective heat transfer Timing is poorly known Events are of very short duration, and duration is not known Intensity is poorly known Multiple events probable Events are local and highly variable Not included in McKenzie or Waples models

60. Thermal relaxation (cooling) Conduction: slow; depends on thickness of section above heating event (e.g., McKenzie model: time scale 100-200 my) Convection: rapid; depends on fluid-flow system (time scale << 1 my) Conductive relaxation models should not be applied to convective systems

61. Summary of paleoheat flow Conduction and convection must be modeled separately We have good (?) models for conduction in rifts No completely appropriate models for conduction in other extensional basins Many difficulties in modeling convection , which is often very important Conductive paleoheat flow varies less through time than many believe Conductive heating is not fast, but is faster than cooling Convective heating and cooling are fast and local in time and space

62. Testing of proposed paleotemperature models Compare measured values of thermal indicators with values calculated using the proposed burial and thermal model Correct any misfits by changing the proposed model (optimization)

63. Optimization of paleotemperatures Not always useful or necessary. Examples: Continuous subsidence Minor unconformities Lack of thermal events Most appropriate for major unconformities May be useful for convective heating May be useful for extensional events Must be done in conjunction with geologic concepts

64. Data for optimization of paleotemperatures Thermal indicators Vitrinite reflectance (Ro) (VR) VIRF FAMM Fission track data

65. Vitrinite reflectance Requires vitrinite, which is kerogen formed from the hard, structural parts of terrestrial plants Measures % of light reflected coherently from surface of vitrinite particle Reflectance increases with increasing maturity Reflectance can also be calculated for any proposed thermal history

66. Vitrinite reflectance Measures cumulative thermal history Includes effects of time, temperature Standard technique Numerous problems Values should be used cautiously

67. Vitrinite reflectance problems Caving (cuttings samples) Misidentification of vitrinite Mud or oil contamination Reworking Oxidation Lack of vitrinite Suppression

68. Vitrinite reflectance A particular problem is suppression , which means that measured Ro values are lower than they should be Suppression is caused by presence of hydrogen-rich material within vitrinite VIRF and FAMM were developed to solve this problem

69. VIRF V itrinite & I nertinite R eflectance & F luorescence Offered by Newman Energy Research Christchurch, New Zealand Replaces vitrinite reflectance

70. FAMM F luorescence A lteration of M ultiple M acerals Offered by CSIRO Sydney, Australia Replaces vitrinite reflectance

71. FAMM Two fluorescence parameters measured Ro equivalent is estimated from fluorescence data, which were calibrated with standard coal samples that show no suppression

72. Fission-track data Apatite Zircon Records timing and amount of cooling

73. Fission-track data Not useful unless there has been significant cooling (>20 o C) (known or suspected)

74. Apatite measures cooling around 100 o C Zircon measures cooling around 250-300 o C

75. Fission-track data Useful for verifying and quantifying … Major unconformities (~500 m erosion or more) Hydrothermal events Volcanism

76. Fission-track vs Reflectance Reflectance, VIRF and FAMM record total thermal history, but cannot give detail on specific events FT records timing and amount of specific cooling events, which can then be used to work out specific heating events FT and reflectance supplement each other

77. Applications of thermal indicators: verification of proposed thermal history

78. Malay Basin: confirmation that no recent heating event is required

79. Apatite Fission Track example Eviondo-1 well Equatorial Guinea Rifted South Atlantic Mid-Cretaceous volcanism Modest Paleogene erosion Recent volcanism?

80. Initial model

81. Initial model

82. Eviondo-1: Initial and Final Models

83. Final model

84. Final model

85. Conclusion from FT A small adjustment to erosional model (200 m more erosion) was necessary Additional conclusion from combined Ro and FT: Neogene thermal event (Cameroon volcanic Line) began much later than proposed (3 Ma vs 25.7 Ma)

86. Applications of thermal indicators: estimating amount of erosion Question: How much erosion occurred at the Hercynian (Late Paleozoic-Cretaceous) unconformity in central Algeria?

87. Summary-1 Paleosurface temperatures are estimated from consideration of global climate, paleolatitude, paleoelevation, and paleowater depth Paleosurface temperatures are not further optimized unless an error is discovered

88. Summary-2 Paleoheat flow trends and values are estimated from tectonic history Conductive and convective heat transfer must be considered separately Conductive HF depends on thickness of lithosphere and of crust McKenzie model and Waples modification describe rift basins Application to pull-apart basins speculative

89. Summary-3 Convective heat transfer is important or very important in extensional basins Convective effects are difficult to model because they are local in both time and space Convective heat transfer can decrease much faster than conductive HF Convective effects, where important, must be included somehow in models

90. Summary-4 Burial history also controls thermal history Major erosional events must be reconstructed correctly

91. Summary-5 Thermal indicators help reconstruct thermal history Ro, VIRF, FAMM; Fission track During optimization we can adjust paleoheat flow, amount of erosion, convective heating

92. When we have finished optimizing paleotemperatures, we are finished with building the geologic framework for our model. From this point onward we will not change any of the input parameters we have worked with so far. The only exception is if you discover an error, in which case you will have to repeat the entire optimization process from the point of the error.

93. The next step is to focus on the parts of the model that cannot be optimized: Characteristics of source rocks Calculation of maturity and generation Output