1. Continuous Thermal Histories from MDD Modeling of 40 Ar/ 39 Ar K-feldspar Analyses and Applications to Extensional Tectonics Martin Wong 1, Phillip Gans 2, Peter Zeitler 3, Bruce Idleman 3 T-t paths Synthetic age spectra 1 Colgate University, 2 UC Santa Barbara, 3 Lehigh University Support from NSF EAR-0948536

2. Tectonics and thermochronology Ehlers (2005) Most tectonic processes impart thermal signatures Thermochronology can be used to address wide range of tectonic questions Inception Duration Magnitude Rate Spatial variation Offers unique insight that is often inaccessible by other methods

3. Thermochronology overview 40 Ar 40 K Decay of parent to daughter product Daughter can diffuse out of the host mineral Volume diffusion is a thermally activated process Closure temperature (T c ) reflects temp below which daughter is retained Isotopic age reflects time since cooling through T c

4. How do we measure T c of minerals in the lab? Diffusion parameters E a and D 0 Where: D = diffusivity (m 2 s -1 ) T = temperature (K) R = Gas constant E a = Activation energy (J mol -1 ) D 0 = frequency factor (m 2 s -1 ) Modified from Reiners and Brandon (2006 ) Diffusion scales exponentially with temperature Volume diffusion in minerals follows an Arrhenius relationship Links reaction rate to temperature R is a constant Diffusion kinetics controlled by E a and D 0

5. Using diffusion experiments to measure T c Laboratory He diffusion from zircon (Reiners et al., 2004) Arrhenius plot note inverse T on x-axis! Linear trend shows a single Arrhenius relationship E a is α to slope D 0 = y–intercept Suggests one diffusion domain with a given T c E a and D 0 used to calculate T c via Dodson’s equation Increasing T

6. From Pollard et al. (2002) with data from P. Fitzgerald, S. Baldwin, G. Gehrels, P. Reiners, and M. Ducea Thermochronometers with wide range of T c are available Most common methods: 40 Ar/ 39 Ar fission track (U-Th)/He analyses Apply multiple thermochronometers to reconstruct T-t path from >500°C to near surface

7. Problems with this approach: What are we missing between point constraints? ? ? Most thermochron data yields point constraints on T–t path Uncertainty in how to connect points What are we missing in between?

8. Problems with this approach: How well do we know T c of our samples? Typically use kinetic data from standards and assume they apply (e.g. Ar-Ar biotite T c = 325 ± 25°C) But compositional, grain size, other effects on T c Don’t typically measure the kinetic data, composition, etc. for each sample Creates uncertainties in T c and thus in the T-t path

9. To track from 350-120°C: Ar-Ar musc Ar-Ar bio FT zircon U-Th/He titanite FT apatite etc Separation time, analytical time, cost, etc. Do the rocks have all of these minerals? Some are trace/rare Multiply by spatial coverage across a field area… Problems with this approach: That’s a lot of work!

10. What would our dream thermochronometer be? Record a continuous thermal history rather than point constraints on T-t path T c range applicable to many geologic/tectonic processes T ½ applicable to wide range of ages Present in many rock types Abundant in sample (ease of separation)

11. 40 Ar/ 39 Ar K-feldspar analyses offer best opportunity to extract continuous thermal histories K–feldspar KAlSi 3 O 8 Based on 40 K  40 Ar (T ½ = 1.25 Ga) Common in many basement rocks Major mineral constituent easy to sample and separate! Appears to record continuous cooling from ~350-150°C Applicable to wide range of tectonic processes from middle to upper crust

12. Why do we think 40 Ar/ 39 Ar K-feldspar analyses yield continuous thermal histories? Apparent age (Ma)Cumulative 39 Ar Single step heating experiment Increasing T-steps 500°C 1100°C Beginning with Lovera et al. (1989), Richter et al. (1991), Harrison et al. (1993) and many others See recent review by Harrison et al. (2005) 40 Ar/ 39 Ar age spectrum - step heat mineral and analyze age at each step Basement K-feldspar step ages get progressively older at higher–T steps Inconsistent with presence of single diffusion domain and T c

13. K-spar diffusion not consistent with a single T c Typical K-feldspar Arrhenius plot Inverse temperature Diffusivity Increasing T Initially follows Arrhenius relationship at low-T Falls off this line at higher–T steps No single E and D 0 Inconsistent with a single diffusion domain with one T c

14. Multiple diffusion domain theory Typical K-feldspar Arrhenius plot Inverse temperature Diffusivity Increasing T Schematic model of MDD theory High T c domain Moderate T c domain Least retentive domain (Low T c ) Ar Low T c domain degas Single E and D 0 Degassing of higher T c domain Consistent with presence of multiple diffusion domains, each with unique T c and volume fraction

15. Likely from microstructures such as exsolution lamellae MDD behavior not present in high–T K-feldspar such as sanidine that lack these structures Creates Ar reservoirs of varying size + diffusivity Each K-feldspar has unique domain structure Ar Source of multiple diffusion domains in K-feldspar?

16. How do we extract a thermal history from K-feldspar? Directly measuring the kinetics of each sample! Only goal is to adequately reproduce laboratory 39 Ar release with model, not reproduce nature exactly Domain software – Peter Zeitler (Lehigh) measured model E and D o of smallest domain Melting at ~1100°C Ar Linear portion defines E and D 0 of smallest domain Model # of domains, relative size (T c ) and volume fraction to fit measured data Model domain structure

17. Inverse modeling for T-t path – Arvert software (P. Zeitler) Generate random T-t paths impose constraints if desired Produce synthetic age spectrum for each T-t path Via the modeled domain structure What fraction of Ar does each domain hold and when did it cool through its T c ? Compare synthetic to measured age spectrum Take best fit T-t paths and iterate, learning algorithm Initial Monte Carlo T-t paths Synthetic age spectra Measured spectrum

18. Final model results GR-8 Converge on class of best-fit cooling paths Best fit T-t paths reproduce measured age spectrum Rapid cooling = flat age spec. Rapid closure of many domains Slow cooing = age gradient Alter parameters to test model sensitivity (not just one model) T-t paths Synthetic age spectra Measured age spectrum

19. Key assumptions of the MDD approach Ar loss in K-feldspar occurs by volume diffusion Laboratory Ar release adequately mimics diffusion mechanisms and kinetics of sample in nature K-feldspar is anhydrous so no dehydration reactions in vacuo No modification of diffusion domains below the T c of that domain Some issues of secondary importance Somewhat debated, see Parsons et al., 1999; Lovera et al., 2002 Harrison et al. (2005) These are key assumptions for many thermochronometers…

20. Identifying problematic behavior From Harrison et al. (2005) Climbing age gradients at higher T steps required for volume diffusion Excess 40 Ar can produce anomalously old ages in some samples Intermediate age maxima also problematic – source? How to interpret? Generally low % of samples affected We can identify and screen for problematic behavior!

21. So that’s the theory… but does it work? Two case studies applied to extension in the Basin and Range Gold Butte normal fault block, NV Grayback normal fault block, AZ

22. Thermochronology at tilted normal fault blocks Old ages near surface Ages young as depth/temperature increases Zero age below the T c isotherm Age-depth inflection shows position of T c isotherm Prior to extension:

23. Exhumation and cooling of footwall Rapid cooling “quenches” thermochron patterns Samples above T c paleo– isotherm keep same age structure Samples below T c at start of extension cool rapidly, record start of extension After extension and tilting

24. Some info we can derive from thermochron (1) Inception of extension Inflection in age- paleodepth relationship Inception of extension (2) Exhumation rate For samples below T c paleo- isotherm (open at extension) Slope of paleodepth vs. age Exhumation rate (3) Geotherm prior to extension Paleodepth of fossil T c isotherm Must know paleodepth robustly – tilt, paleosurface, intact block Paleo- geotherm TcTc Paleo-surface

25. The Grayback fault block, AZ Proterozoic and Laramide (ca. 75 Ma) granite >100% extension on now gently W-dipping normal faults 90° of eastward block tilt during Tertiary extension (domino–style block rotation) Preserved Oligocene paleo– surface (paleodepth known) Wong et al. (accepted) after Howard and Foster (1996)

26. Prior thermochronology Apatite and Zircon FT (Howard and Foster, 1996) ( U-Th)/He apatite (Wong et al., accepted) Extension began at ca. 25 ± 3 Ma 25 ± 3 Ma inflection Tectonic exhumation rate of ~1 mm/yr ~1 mm/yr exhumation rate Deepest part of block at 220 ± 20 °C at 25 Ma Zr. FT PAZ at 25 Ma 220 ± 20°C

27. Prior thermochronology Apatite and Zircon FT (Howard and Foster, 1996) ( U-Th)/He apatite (Wong et al., accepted) Extension began at ca. 25 ± 3 Ma Tectonic exhumation rate of ~1 mm/yr Deepest part of block at 220 ± 20 °C at 25 Ma Paleo-geothermal gradient of 14–17°C/km at start of extension – Fossil T c isotherms for AHe, Ap. And Zr. FT AHe 14°C/km Ap. FT 17°C/km Zr. FT 17°C/km An excellent place to evaluate K-spar MDD continuous thermal histories!

28. K-spar results K-spar spectra are systematic – young towards deeper paleodepths No problematic Ar behavior Deeper paleodepths Pre-extensional (~25 Ma) isotherms

29. Top of Laramide pluton (75-80 Ma) K–spar yields flat age spectrum ~ 58-65 Ma Moderate paleodepth (central block – GR-1) GR-1 MDD models suggest rapid cooling (10- 15°C/m.y.) from 300–150°C Suggests post-emplacement cooling of pluton in shallow crust MDD models suggest <150°C by 25 Ma, matches paleo-geotherm prediction 40 Ar/ 39 Ar age spectra MDD thermal model

30. Deeper paleodepths (west central block GR-2) GR-2 Climbing age gradient from 28–55 Ma MDD models suggest slow cooling (2–4°C/m.y.) during early Tertiary Deeper than GR–1 so K–spar did not record rapid post-emplacement cooling Consistent with tectonic quiescence inferred from geologic studies 40 Ar/ 39 Ar age spectrum MDD thermal model

31. Deepest paleodepths (western part of block – GR-8) GR-8 Age flat at low–T steps, 24–27 Ma Climbing age gradient 28–40 Ma MDD models suggest slow cooling 27–40 Ma Rapid cooling (15-17°C/m.y.) after 27 Ma documents start of extension Deeper than GR–2 so catches part of slow cooling but low T c domains stayed open into Oligocene, record rapid cooling 40 Ar/ 39 Ar age spectra MDD thermal model

32. MDD thermal models replicate all key aspects of prior thermochron (1) Extension began at ca. 25 ± 3 Ma (2) Tectonic exhumation rate of ~1 mm/yr (cooling rate 15- 17°C/m.y.) (3) Oligocene paleo-geothermal gradient of 14–17°C/km – GR-8 MDD model at 240°C at ~12 km paleodepth Rapid cooling at 27 Ma Prior thermochron K-spar MDD models Rapid cooling at 25 ± 3 Ma ~ 1 mm/yr 15- 17°C/m.y.(1 mm/yr) GR-8 at ~240°C at 27 Ma Deep footwall at 220°C at 27 Ma (Zr. FT) A single K-feldspar sample (GR- 8) yielded the same info as a large array of AHe and FT analyses!

33. Key points from the Grayback case study Prior thermochron K-spar MDD models 40 Ar/ 39 Ar K-spar MDD thermal models are geologically meaningful Well calibrated to other thermochron- ometers (AHe, FT) Yields the same (more?) info with fewer samples

34. A quick look at the Gold Butte fault block, NV GBT-10GBT-9 Proterozoic units and minor Laramide (ca. 75 Ma) granite Extension on now gently W-dipping normal faults 60° of eastward block tilt during Tertiary extension Miocene paleodepths up to 18 km? Fryxell et al. (1992), Brady et al. (2000)

35. Prior thermochronology Apatite FT ( Fitzgerald et al. 1991, 2009) ( U-Th)/He ( Reiners et al. 2000) apatite Titanite zircon Zircon FT (Bernet, 2002) 40 Ar/ 39 Ar (Karlstrom et al., 2010) Extension began at ca. 17Ma Deepest part of block at 300– 350 °C at 17 Ma, assuming an intact block

36. GBT-10GBT-9 Climbing age gradient from 50– 700 Ma MDD models suggest very slow cooling (<<1°C/m.y.) throughout Paleozoic–Mesozoic Lacks any Miocene record (makes sense given its position) Central part of block GBT-9

37. GBT-10GBT-9 Some problematic behavior 40Ar E and Int. Age Max. Not suitable for MDD modeling But shares climbing age gradient 50–300 Ma as GBT–9 ~20% gas yields Miocene ages at low–T steps Low T c domains recording Miocene rapid cooling Further west: GBT-10 GBT-10

38. GBT-9 Flat age spectrum at 17 Ma 30-40% of 39 Ar released Climbing age gradient 17-70 Ma Further west: GBT-11 GBT-11 Note Y- axis scale change!

39. GBT-10GBT-9 MDD model suggests slow cooling/isothermal residence at ~300 ± 25°C from early to middle Tertiary Matches well with projection of Miocene paleo–geothermal gradient Captures start of rapid tectonic exhumation at 17 Ma Identical to prior thermochron Further west: GBT-11 GBT-11 17 Ma

40. GBT-10GBT-9 Looks good so far, but… There is a K-spar age repetition between GBT-11 and GBT-7 GBT-11 GBT-7 GBT-11 shown at same age scale as GBT–7 for comparison GBT-7 is further west (deeper paleodepths) but is older… breaks the pattern

41. GBT-10GBT-9 GBT-4/5 Ages drop younger again Remarkably similar form to GBT-11 Kspar Age flat at 17 Ma at low–T steps Climbing age gradient to 35-50 Ma Western footwall: GBT-4 and 5

42. MDD thermal models of GBT–4 and GBT–5 capture the inception and rate of extensional exhumation perfectly… K-spar MDD: 250–275°C at 17 Ma GBT-10GBT-9 GBT-4/5 17 Ma geotherm. projection: 350°C But MDD models suggest they were 75- 100°C colder at 17 Ma than the projected Miocene geotherm. would suggest…

43. GBT-10GBT-9 Two “problems” with Kspar dataset: Age repetition and MDD models of westernmost footwall are too cold How can we resolve this? K–spar ages from GBT-9, 10 and 11 make sense Match expected thermal behavior from prior thermochronology GBT-11 GBT-10 GBT-9 GBT-10GBT-11 Decreasing ages towards deeper hotter footwall

44. GBT-10GBT-9 Two “problems” with Kspar dataset: Age repetition and MDD models of westernmost footwall are too cold How can we resolve this? Data from the deepest footwall repeats this pattern GBT–7 matches GBT-10 GBT–4/5 match GBT–11 GBT-11 GBT-10 GBT-11 GBT-4/5 GBT-7 GBT-10 GBT-4/5

45. GBT-10GBT-9 GBT-11 GBT-7 GBT-10 GBT-4/5 A shallowly west dipping normal fault with ~4-5 km of slip at ~13 km paleo– depth would match these data exactly Possible normal fault Gold Butte block may not be an intact block Possibly cut by at least one major normal fault Max paleodepth at Gold Butte may be ~12 km May reduce the total required slip on the bounding Lakeside Mine fault Would explain lack of mylonites in western footwall, lack of Miocene Ar mica ages

46. K-feldspar as a tool to ID cryptic faults and quantifying slip? Prior thermochronology shows no obvious age repetitions in this part of footwall Why? Fault motion above T c of those thermo- chronometers? Lack of resolution? K-feldspar analyses may offer a high-resolution tool for identifying cryptic faults and constraining slip

47. Conclusions Laboratory Ar diffusion studies of K-feldspar are consistent with the presence of multiple diffusion domains (MDD) within a single mineral The domain structure from each sample can be modeled based on lab diffusion data MDD thermal histories are possible T-t paths when they reproduce the measured 40 Ar/ 39 Ar age spectrum via the modeled domain structure Two case studies from tilted normal fault blocks document the utility of the MDD approach

48. Conclusions The MDD approach can produce highly accurate and geologically meaningful thermal histories MDD results appear well calibrated to other thermochronometers in these two case studies 40 Ar/ 39 Ar K–feldspar data may be more sensitive than other thermochronometers in some cases – potential utility for identifying cryptic faults and constraining slip K–feldspar MDD thermal models are a potentially powerful but underutilized tool for addressing a wide range of tectonic problems

49. Bonus slides

50. Using diffusion kinetics to calculate T c The Dodson equation From Harrison et al. (2005) after Dodson (1973) Where: E = Activation energy D 0 = frequency factor R = Gas constant A = Geometry constant (sphere, slab, etc) r = effective diffusion length scale dT/dt = cooling rate If we know E and D 0 for a given thermochronometer, we can solve for T c Necessary? Or combine with prior slide?

51. GBT-10GBT-9

52. GBT-10GBT-9 GBT-11 GBT-4/5 Sample from the deepest footwall gets younger again (GBT 4 and GBT 5) They look remarkably like GBT-11…