Header. Volcanic matrix from three basalts and one dacite sample from the Black Mountains and Black Hills, respectively, were separated to perform

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1 DR000 Data Repository Item Geochronology Sample Preparation and Analysis Volcanic matrix from three basalts and one dacite sample from the Black Mountains and Black Hills, respectively, were separated to perform 0 Ar/ 9 Ar geochronology (Fig. A and Table A). Matrix separates (0-80 µm size fraction) were produced using magnetic separation, heavy liquids and hand picking techniques to a purity of >99%. The separates were then washed in acetone, alcohol, and deionized water in an ultrasonic cleaner to remove dust and then re-sieved by hand using a 80-µm sieve. Sample aliquots of approximately 000 mg (x0 mg) were packaged in copper capsules and sealed under vacuum in quartz tubes. The samples were irradiated for 0 hours (irradiation package KD7) in the central thimble facility at the TRIGA reactor (GSTR) at the U.S. Geological Survey, Denver, Colorado. The monitor mineral used in the package was Fish Canyon Tuff sanidine (FCT-) with an age of 7.79 Ma (Kunk et al. 98; Cebula et al. 986) relative to MMhb- with an age of 9. ±. Ma (Alexander, et al. 978; Dalrymple et al. 98). The type of container and the geometry of samples and standards are similar to that described by (Snee et al. 988). Samples were analyzed at the U.S. Geological Survey Thermochronology Laboratory in Denver, Colorado on a VG Isotopes Ltd., Model 00 B Mass Spectrometer fitted with an electron multiplier using the 0 Ar/ 9 Ar step heating method of dating. For additional information on the analytical procedure see Kunk et al. (00). The argon isotopic data was reduced using an updated version of the computer program ArAr* (Haugerud and Kunk, 988). We used the decay constants recommended by (Steiger and Jäeger, 977). Table A shows 0 Ar/ 9 Ar step-heating data for the basalts and dacite and includes the identification of individual steps, existence of plateau, and total gas ages. Total gas ages represent the age calculated from the addition of all of the measured argon peaks for all steps in a single sample. The total gas ages are roughly equivalent to conventional K/Ar ages. No analytical precision is calculated for total gas - -

2 ages. Plateau ages (not present) are identified when three or more contiguous steps in the age spectrum agree in age, within the limits of analytical precision, and contain more than 0% of the 9 Ar released from the sample. References Alexander, E. C., Jr., Mickelson, G. M., and Lanphere, M. A., 978, Mmhb-: a new 0 Ar/ 9 Ar dating standard, in Zartman, R. E., ed., Short papers of the fourth international conference, geochronology, cosmochronology, and isotope geology: U.S. Geological Survey Open-File Report 78-70, p Cebula, G. T., Kunk, M. J., Mehnert, H. H., Naeser, C. W., Obradovich, J. D., and Sutter, J. F., 986, The Fish Canyon Tuff: A potential standard for the 0 Ar/ 9 Ar and fission track dating methods: Terra Cognita, v. 6, n., p. 0. Dalrymple, G. B., Alexander, E. C., Lanphere, M. A., and Kraker, G. P., 98, Irradiation of samples for 0 Ar/ 9 Ar dating using the Geological Survey TRIGA reactor: U.S. Geological Survey Professional Paper 76, p. Kunk, M. J., Sutter, J. F., and Naeser, C. W., 98, High-precision 0 Ar/ 9 Ar Ages of Sanidine, Biotite, Hornblende, and Plagioclase from the Fish Canyon Tuff, San Juan Volcanic Field, South-central Colorado: Geological Society of America Abstracts with Programs, v. 7, p. 66. Kunk, M. J., Winick, J. A., Stanley, J. O., 00, 0 Ar/ 9 Ar Age-Spectrum and Laser Fusion Data for Volcanic Rocks in West Central Colorado: U.S. Geological Survey, Open-File Report 0-7, p. 9. Haugerud, R. A., and Kunk, M. J., 988, ArAr*, a computer program for reduction of 0 Ar- 9 Ar data: U.S. Geological Survey, Open-File Report 88-6, 68 p. Snee, L. W., Sutter, J. F., and Kelly, W. C., 988, Thermochronology of economic mineral deposits: Dating the stages of mineralization at Panasqueira, Portugal, by high precision 0 Ar/ 9 Ar age spectrum techniques on muscovite: Economic Geology, v. 8, p. -. Steiger, R. H., and Jäeger, E., 977, Subcommission on geochronology: Convention on the use of decay constants in geo- and cosmo-chronology: Earth and Planetary Science Letters, v. 6, p Figure Caption Figure A Age spectra and inverse isochron ages from 0 Ar/ 9 Ar geochronology of glass matrix separates from basalt and dacite lava flows offset by the Blackwater fault. All ages in the plots are -sigma uncertainty. Vertical extent of boxes represents age uncertainty for individual heat steps. Black mountains basalt field shows stratigraphically consistent - -

3 series of Pliocene ages, ca..6 to.8 Ma. Dacite of the Black Hills is not younger than Late Miocene. - -

4 Oskin and Iriondo Figure A Apparent Age (Ma) 8 A B C Basalt Flow : Sample BWF-0-7 B C A D Isochron Age =.6 ± 0.08 Ma [ 0 Ar/ 6 Ar] i = 0 ± 8, MSWD =.9 Steps B through D, 6.6% of 9 Ar K Basalt Flow : Sample BWF-0-6 D E F G H I Isochron Age =.7 ± 0.0 Ma [ 0 Ar/ 6 Ar] i = 97 ±, MSWD =. Steps E through I, 7.9% of 9 Ar K Basalt Flow : Sample BWF-0-06 A B C D E F G H Isochron Age =.77 ± 0. Ma [ 0 Ar/ 6 Ar] i = 86 ±, MSWD = 0.0 Steps D through H, 8.8% of 9 Ar K E F J I J 7 6 A BC D E F G H Dacite of the Black Hills: Sample BWF-0-6 Isochron Age = 7. ±.07 Ma [ 0 Ar/ 6 Ar] i = 6 ± 7, MSWD = 0.69 Steps D through F, 60.% of 9 Ar K Cumulative % 9 Ar K Released

5 Table A. 0 Ar/ 9 Ar step-heating data Step Temp. % 9 Ar Radiogenic 9 Ar 0 k Ar* Apparent Apparent Apparent Error C of total Yield (%) (Moles x 0 - ) 9 Ar k K/Ca K/Cl Age (Ma) (Ma) BWF-0-7 basalt matrix J = 0.00 ± 0.0% wt = 7. mg #, & KD7 A ± 0.06 B ± 0.0 C ± 0.0 D ± 0.0 E ± 0.0 F ± BWF-0-6 basalt matrix J = ± 0.0% wt = 999. mg #9, 0, & KD7 A ± 0.7 B ± 0.06 C ± 0.0 D ± 0.0 E ± 0.0 F ± 0.0 G ± 0.0 H ± 0.0 I ± 0.0 J ± BWF-0-6 basalt matrix J = 0.00 ± 0.0% wt = 00.7 mg #, 6, 7 & 8KD7 A ± 0. B ± 0.09 C ± 0. D ± 0.0 E ± 0.0 F ± 0.06 G ± 0.0 H ± 0.0 I ± 0.08 J ± BWF-0-6 dacite matrix J = ± 0.0% wt = mg #,, & 6KD7 A ± 0.08 B ± 0.0 C ± 0.0 D ± 0.0 E ± 0.0 F ± 0.0 G ± 0.0 H ± Ages calculated assuming an initial 0 Ar/ 6 Ar = 9.± 0. All precision estimates are at the one sigma level of precision. Ages of individual steps do not include error in the irradiation parameter J. No error is calculated for the total gas age.

Sample and analytical information:

Sample and analytical information: Timing, magnitude, and style of Miocene deformation, west-central Walker Lane belt, Nevada R. Ernest Anderson, U.S. Geological Survey, Kernville, CA 93238 Byron R. Berger, U.S. Geological Survey, MS 964,