Appendix A

Analytical Procedures

40Ar/39Ar laser fusion analyses were conducted at the Clair argon laboratory, MIT using procedures similar to those described in Hodges and Bowring (1995). 100µm-thick, polished sample sections were cut into 0.5cm-square pieces and packaged in aluminum foil for irradiation. Samples were irradiated in the C5 position of the McMaster University nuclear reactor for 10 hours with a total power of 20MW. The conversion efficiency of 39K to 39Ar was monitored using Taylor Creek rhyolite with an assumed age of 28.34 Ma (Renne et al., 1998). K2SO4 and CaF2 synthetic salts were used to enable corrections for interfering nuclear reactions. Samples were fused with a New Wave Research/Mechantek Nd-YAG (ArF) excimer (uv) laser at a current intensity of 28-30kV for 8-12 minutes. Each pseudotachylyte sample site was probed with a 42µm beam, rastered over a 300µm-square area that was chosen to minimize the incorporation of cataclastic fragments. Ultramylonite samples were probed with a 30µm beam along 3-4 discrete lines per analysis. Each line was chosen to track regions of biotite-rich matrix and avoid very fine-grained (~5-15µm) plagioclase and occasional quartz. Released gasses were purified for 10-15 minutes with Al-Zr and Fe-Zr-V getters and then admitted into a MAP 215-50 mass spectrometer for Ar isotopic analysis using an electron multiplier. System blanks were measured before and after each sample, and all data were corrected for blanks and mass-fractionation effects. Final data reduction was conducted with the program ArArCalc (Koppers, 2002). The integrated 40Ar/39Ar age for each sample was calculated as a (% 39Ar) weighted mean of 13-19 individual analyses, and sample errors were calculated as standard errors of the weighted mean. Precision limits represent propagated measurement uncertainties and are reported at the ee level.

AP1

40Ar/39Ar analytical data for fine-grained biotite from ultramylonite samples. 40Ar* is radiogenic 40Ar. Weighted age is the mean age of individual analyses weighted on 39Ar (%). Individual analyses numbers preceded by a '*' were not used when calculating the weighted age.

Table AP1. 

ID 39Ar/40Ar 36Ar/40Ar 40Ar* (%) 39Ar (%) Age (Ma ± 2σ)
  weighted age (n=15)     340.60 ± 5.40
  004 Tres Arboles ultramylonite, J=0.0151
1 0.0739 0.000453 88.20 2.81 335.60 ± 46.06
2 0.0748 0.000740 82.06 5.33 331.67 ± 29.54
3 0.0734 0.000365 90.26 4.90 337.58 ± 12.60
4 0.0735 0.000349 90.65 7.03 337.17 ± 12.50
5 0.0715 0.000385 89.79 9.79 345.83 ± 15.28
6 0.0720 0.000417 89.03 8.70 343.31 ± 6.16
7 0.0724 0.000746 81.93 6.24 341.96 ± 11.22
8 0.0722 0.000273 92.53 6.55 342.76 ± 12.98
9 0.0728 0.000411 89.16 6.12 339.90 ± 13.18
10 0.0737 0.000489 87.37 6.65 336.23 ± 11.04
11 0.0741 0.000506 86.98 5.06 334.42 ± 17.46
12 0.0762 0.000737 82.11 2.56 326.34 ± 29.50
13 0.0719 0.001123 75.08 3.19 344.09 ± 22.66
14 0.0715 0.000205 94.28 13.40 345.71 ± 5.16
15 0.0722 0.000578 85.41 11.66 342.72 ± 16.30
*16 0.0921 0.001480 69.70 2.56 273.97 ± 41.54
*17 0.1023 0.001860 64.63 3.77 248.48 ± 56.12
*18 0.0992 0.001781 65.49 5.98 255.52 ± 29.76
ID 39Ar/40Ar 36Ar/40Ar 40Ar* (%) 39Ar (%) Age (Ma ± 2σ)
  weighted age (n=19)     342.53 ± 5.26
  006 Tres Arboles ultramylonite, J=0.0152
1 0.0693 0.000314 91.49 4.33 357.96 ± 34.48
2 0.0729 0.000519 86.71 5.12 341.50 ± 15.66
3 0.0726 0.000475 87.69 5.60 342.78 ± 9.74
4 0.0722 0.000278 92.41 5.87 344.80 ± 12.06
5 0.0738 0.000701 82.83 4.43 337.92 ± 7.88
6 0.0729 0.000496 87.21 5.50 341.45 ± 36.84
7 0.0746 0.000615 84.62 5.87 334.64 ± 30.12
8 0.0723 0.000591 85.12 3.74 344.39 ± 25.10
9 0.0740 0.000484 87.49 5.07 336.97 ± 23.24
10 0.0725 0.000773 81.39 2.28 343.20 ± 28.68
11 0.0733 0.000376 90.01 4.99 340.05 ± 8.48
12 0.0700 0.000338 90.91 6.69 354.67 ± 29.92
13 0.0729 0.000422 88.90 5.66 341.80 ± 28.26
14 0.0708 0.000331 91.08 5.68 350.76 ± 26.08
15 0.0734 0.000537 86.31 6.07 339.64 ± 15.18
16 0.0742 0.000411 89.16 5.38 336.19 ± 27.32
17 0.0755 0.000485 87.46 7.68 331.01 ± 24.84
18 0.0723 0.000542 86.19 5.09 344.45 ± 11.34
19 0.0706 0.000684 83.18 3.78 351.87 ± 10.08
*20 0.0915 0.001575 67.74 1.02 277.42 ± 123.14
*21 0.0903 0.001719 66.47 2.28 280.70 ± 62.06

AP2

40Ar/39Ar analytical data for pseudotachylyte samples. 40Ar* is radiogenic 40Ar. Weighted age is the mean age of individual analyses weighted on 39Ar (%). Individual analyses numbers preceded by a '*' were not used when calculating the weighted age.

Table AP2. 

ID 39Ar/40Ar 36Ar/40Ar 40Ar* (%) 39Ar (%) Age (Ma ± 2σ)
  weighted age (n=16)     344.95 ± 5.08
  9819 Los Tuneles pseudotachylyte, J=0.0153
1 0.0714 0.000480 87.57 4.09 350.31± 23.64
2 0.0710 0.000563 85.74 6.89 352.17 ± 11.38
3 0.0704 0.000625 84.40 6.06 354.70 ± 22.02
4 0.0742 0.000762 81.61 8.37 338.30 ± 15.86
5 0.0749 0.000814 80.61 9.41 335.43 ± 14.18
6 0.0746 0.001091 75.62 8.09 336.73± 35.66
7 0.0704 0.000495 87.24 5.24 354.75 ± 34.28
8 0.0720 0.000717 82.51 9.47 347.77 ± 19.34
9 0.0739 0.000380 89.90 2.76 339.38 ± 25.50
10 0.0723 0.000707 82.72 7.29 346.49 ± 13.92
11 0.0730 0.001160 74.47 5.63 343.44 ± 14.50
12 0.0724 0.000928 78.47 6.19 346.07 ± 34.16
13 0.0736 0.000906 78.87 3.47 340.56 ± 27.54
14 0.0721 0.001009 77.03 8.18 347.37 ± 25.36
15 0.0721 0.000875 79.45 4.81 347.11 ± 46.58
16 0.0730 0.000684 83.17 4.06 343.39 ± 18.84
ID 39Ar/40Ar 36Ar/40Ar 40Ar* (%) 39Ar (%) Age (Ma ± 2σ)
  weighted age (n=18)     348.19 ± 2.22
  9820 Los Tuneles pseudotachylyte, J=0.0155
1 0.0728 0.001043 76.44 2.09 348.02 ± 16.64
2 0.0732 0.000468 87.84 9.03 346.45 ± 18.16
3 0.0720 0.000538 86.28 6.08 351.62 ± 8.96
4 0.0742 0.000287 92.17 6.91 342.27 ± 10.32
5 0.0712 0.000373 90.06 7.21 355.24 ± 9.04
6 0.0730 0.000676 83.34 5.15 347.40 ± 23.04
7 0.0738 0.000763 81.61 4.67 343.83 ± 19.08
8 0.0711 0.000725 82.35 4.69 355.68 ± 16.36
9 0.0742 0.001103 75.41 4.06 342.42 ± 26.98
10 0.0734 0.000433 88.65 5.71 345.80 ± 29.70
11 0.0729 0.000381 89.88 7.87 347.79 ± 21.64
12 0.0724 0.000959 77.92 4.41 349.82 ± 29.28
13 0.0736 0.000810 80.69 5.53 344.78 ± 12.86
14 0.0739 0.000810 80.68 4.14 343.58 ± 18.94
15 0.0715 0.000444 88.39 4.38 354.04 ± 24.36
16 0.0728 0.000468 87.84 6.32 348.16 ± 9.26
17 0.0716 0.000499 87.14 5.97 353.34 ± 13.32
18 0.0732 0.000564 85.72 5.76 346.40 ± 8.92
ID 39Ar/40Ar 36Ar/40Ar 40Ar* (%) 39Ar (%) Age (Ma ± 2σ)
  weighted age (n=13)     429.26 ± 3.54
  98109 La Calera pseudotachylyte, J=0.0154
1 0.0564 0.000456 88.13 12.42 435.47 ± 14.50
2 0.0581 0.000460 88.03 7.31 423.96 ± 15.00
3 0.0578 0.000123 96.49 10.86 425.98 ± 8.44
4 0.0583 0.000094 97.30 8.59 423.10 ± 11.04
5 0.0578 0.000516 86.77 4.34 425.98 ± 16.86
6 0.0564 0.000176 95.06 8.61 435.36 ± 15.96
7 0.0576 0.000641 84.08 3.34 427.34 ± 13.90
8 0.0567 0.000311 91.58 2.89 433.79 ± 18.96
9 0.0569 0.000445 88.37 7.27 432.46 ± 10.80
10 0.0576 0.000575 85.47 8.87 427.13 ± 14.46
11 0.0575 0.000677 83.34 8.12 428.12 ± 7.42
12 0.0565 0.000117 96.65 8.09 434.71 ± 18.16
13 0.0579 0.000352 90.58 9.29 425.60 ± 8.20
*14 0.2782 0.011328 23.41 0.48 97.27 ± 211.38

AP3

Calculated ambient host rock temperature for samples 9819 and 9820 at the time of pseudotachylyte generation. Method follows that of O'Hara (2001) and uses the volume ratio of clast fragments/matrix (W/M) to calculate the temperature (Tlow) with the equation: W/M = (Thigh – Tlow) / Thigh. Melting temperature (Thigh) ~ 927˚C, and 2 sigma errors for Tlow are estimated at ±70˚C (O’Hara, 2001).

Table AP3. 

Sample Number of points Clast points Matrix points Clast/matrix ratio Calculated Temperature
9819 210 77 133 0.579 232 ± 70˚C
9820 203 73 130 0.562 253 ± 70˚C

AP4

Calculated Ar closure temperatures for 10µm and 20µm biotite grains using the equation of Dodson (1973):

with analytical constants from Harrison et al. (1985). Convergence of calculated temperatures (Tc) was easily reached within 4 iterations.

Constants (Ann56 biotite values from Harrison et al., 1985):

E (activation energy) = 47 kcal/mol

D0 (frequency factor) = 0.077

A (geometry = infinite cylinder) = 27

dT/dt (linear cooling rate) = 5˚C/m.y.

Table AP4. 

Radius of grain (µm) Number of iterations Closure temperature
10 4 236.6˚C
20 4 251.6˚C

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