The 40Ar/39Ar release spectra from the 13 K-feldspar samples are shown individually in Appendix VIII, with all surface and all subsurface samples combined on two plots for visual comparison. The 36Ar/40Ar and 39Ar/40Ar ratios for gas fractions from the plateau portion of the release spectra are plotted so that the 40Ar/36Ar initial ratio and the best estimated age for the sample can be determined (i.e. the y-intercept gives the 40Ar/36Ar initial ratio and the X-intercept gives the age). Isochron ages are given for samples which yielded reliable isochrons in Appendix VIII. The other samples have complicated release spectra which do not form isochrons. These release spectra can be interpreted by examining the plateau portion of the plot and determining a mean age for those temperature steps which fall within the plateau as shown in the age data tables shown in Appendix VIII.

All of the microclines sampled showed relatively similar Arrhenius relations (Appendix VIII) and thus have similar diffusion parameters and closure temperatures. The calculated closure temperature for these K-feldspars is calculated to be 110 +/- 30 degrees C, based upon a cooling rate of 5 degrees C/Ma. Thus the isochron ages discussed below represent the time that the samples cooled below approximately 110 degrees centigrade. The closure temperatures calculated are shown in Appendix VIII.

All of the samples show complication by the presence of high excess 40Ar. This contamination can be seen by the saddle-shaped release spectra and negative slope on many of the plots. The excess argon results in ages which are artificially older than the actual radiometric age and create complications in interpreting the data. Both the Marlin 1-4 and Eagle Springs #1 wells show older cooling ages at depth than shallower in the well. This inverting of the radiometric age is the result of more excess 40Ar at greater depths in the basin. This excess 40Ar is probably the result of reaching temperatures capable of diffusing argon, approximately 100 degrees C. Attributing the excess 40Ar to any one geological source is difficult.

Although all of the release spectra are complicated by excess 40Ar, with the exception of the deeper Marlin and Eagle Springs samples, they show no signs of Recent 39Ar loss. The calculated ages therefore tell us the last thermal event greater than approximately 110 C which outgassed the samples.

A second complication observable in the release spectra is the apparent presence of multiple activation energy sites in the samples. This appears to be the case for sample M-5-C which shows a strong stepping in the release spectra pattern. These patterns depend upon the activation energy of the mineral and it appears that low and high activation sites exist in the same grains, probably as a result of inhomogeneities such as exsolution lamellae. The details of variability in activation energy are still poorly understood for feldspars.

A possible difficulty arises if multiple provenance sources are responsible for a microcline population. For instance, if Precambrian and Paleozoic microclines are present in a sample that experiences a thermal event which only partially outgasses the microcline, then a mixture of ages can be present in a sample or through a well profile. In this case however, the diffusion parameters suggest that the well samples have been totally reset or outgassed. This results in a homogenization of ages reflecting the timing of the thermal event. Age variance with depth in the Marlin and Eagle Springs Wells appears to be the result of variation in the amount of excess argon. There may be greater amounts of excess argon with increasing depth and temperature in a given valley or basin. Diffusion of excess 40Ar can occur at temperatures near 80-100 degrees C which would be expected in most valleys based upon the modern thermal gradients as indicated by bottomhole temperature data.

K-feldspars analyzed from surface samples M-1-Va, M-3-C, M-5-C, and M-9-C show feldspars derived from a very old source terrane which in the case of M-9-C may be approximately 1400 Ma. The release spectra for the surface samples show age gradients which suggest a normal slow cooling and diffusion history, complicated by excess 40Ar and apparent multiple activation energy sites within the samples. The surface samples show no signs of being thermally heated or outgassed since approximately 150 Ma which is the youngest age as shown by M-5-C.

Sample M-1-Va is located southwest of Cortez in the hills between the southern Cortez Mountains and northern Toiyabe Range. The release spectra show a long plateau which forms an isochron with an age of about 359.9 +/- 6.8 Ma. This Late Devonian/Early Mississippian age is coincident with the initiation of Roberts Mountains thrusting as a result of the Antler orogeny. The Valmy Formation here has not seen the effect of post-Mississippian thermal events.

Sample M-3-DP is located in the central Pinon Range east of Pine Valley. It shows a complicated release spectra which indicates excess 40Ar and appears to suggest a cooling age of about 803.6 +/- 4.9 Ma. This suggests a Precambrian provenance for the feldspar in the Diamond Peak Formation, which has a bearing on tectonic reconstructions. The Diamond Peak in the central Pinon Range does not show the effects of any thermal events since the Proterozoic.

Sample M-5-C is located northeast of Elko in the Peko Hills area. The sample shows a relatively typical release spectra with initial young ages which are followed by older ages. This sample contains the youngest ages observed in any of the surface samples analyzed for this evaluation. The strong stepping of the spectra is probably the result of multiple activation energy sites in the feldspar analyzed. The release pattern suggests that this Chainman Formation sample was last re-heated in the Jurassic, about 150.8 +/- 0.9 Ma, during a thermal event which did not totally outgas and reset the initial sample age.

Sample M-9-C was taken from the Chainman Formation along the western flank of the Pancake Range. The release spectra for this sample is very complicated and difficult to interpret as a result of excess argon. The feldspar found in the Chainman has not seen the effects of a thermal event greater than approximately 120 degrees C since the Latest Cambrian or Earliest Ordovician, about 502.6 +/- 1.5 Ma.

The subsurface microcline samples appear to contain microclines which are similar to the surface outcrop samples but have been thermally heated and outgassed due to burial and/or an increase in the geothermal gradient, perhaps as a result of igneous activity. The ages calculated from the well samples are much younger than all of the surface outcrop samples. They are complicated by a high contamination of excess 40Ar, and in some cases by low radiogenic yields. Because of these complications, the isochron age is considered the best estimate age for the sample.

Four of the well samples have Late Eocene to Early Oligocene isochron ages between 34.6 and 47.5 Ma. These are the Eagle Springs #1, Amoco Blackburn #4, Willard Pease #1, and the Gulf Gose Eu Fed. #1. The samples from the oil producing areas of Eagle Springs in Railroad Valley and Blackburn in Pine Valley, are indicating an Eocene thermal event between 41.7 and 47.5 Ma.

Two samples were taken from the Shell Eagle Springs #1 well. The deepest sample was taken from the Chainman Formation at 8740-9200' and the shallower sample is from Tertiary sediments at 3250-3500'. The sample from 3250-3500 feet shows a well defined plateau in the spectra complicated somewhat by excess argon. A well defined Eocene isochron age of 41.7 +/- 2.8 Ma reflects the cooling age of the last thermal event capable of hydrocarbon generation of about 100 degrees C.

The deeper Eagle Springs sample shows a very complicated release spectra with a stronger influence from excess argon than the shallower sample. The excess argon in the deeper sample gives anomalously old ages in respect to the shallower sample. This could be the result of an unusually retentive feldspar which did not loose its argon during the 41.7 Ma event shown in the 3250-3500 sample, but this seems geologically unreasonable. The older sample has probably seen a thermal history similar to the younger sample. Age increments that get as low as approximately 77 Ma are present in this sample and are probably more representative than the older values.

The Amoco Blackburn #4 well sample is from cuttings of the Chainman Formation at 7310-7520'. This sample has a complicated saddle-shaped release spectra and gives a poorly defined isochron, and also shows the highest 40Ar/36Ar initial ratio of all the samples analyzed. The isochron reflects an Eocene thermal event at about 47.5 +/- 4.0 Ma, very similar to the age present in the Tertiary Eagle Springs sample. This suggests that the Blackburn oil field area last saw a thermal event capable of oil generation about 47.5 Ma.

The Pease #1 Federal sample is located in the northern portion of Cave Valley. The sample is from cuttings of the Chainman Formation taken from 4700-5000'. The release spectra is once again complicated by excess 40Ar, but shows a long and well defined plateau region which gives an Early Oligocene isochron age of 34.6 +/- 1.3 Ma.

The Gulf Gose EU Federal #1 well is located in the southern portion of White River Valley. The sample analyzed is from cuttings of the Chainman Formation from 4540-4640'. The release spectra for the Gulf well shows a well developed plateau which gives a latest Eocene isochron age of 38.2 +/- 0.3 Ma for the last thermal event greater than 110 degrees C.

The Suntide 1-A sample is located between northern Railroad and southern Newark valleys. Cuttings form the Chainman Formation from 1270-1500' were analyzed. Unfortunately, the release spectra plot is rendered uninterpretable as a result of very high excess 40Ar. One age increment does show an age of 45.7 +/- 2.2 Ma. which is in line with the four samples which show isochrons between 34.6 and 47.5 Ma. This may be suggesting that the thermal history of the Suntide 1-A is similar to the Pease, Eagle Springs, Gulf, and Blackburn samples.

The Wexpro Jiggs #2 sample is located in Huntington Valley, west of the Ruby Range metamorphic core complex. Cuttings from the Tertiary sediments from 5000-5300' were analyzed. The release spectra for this sample indicates a negatively sloping gradient reflecting excess argon. A fairly well developed plateau is developed and show an isochron age of 15.2 +/- 0.4 Ma. This age is significantly younger than all of the other samples analyzed. This 15.2 Ma Miocene thermal resetting is probably the result of the final emplacement of the adjacent Ruby Range core complex.

Two samples were taken from the Marlin 1-4 well in northern Butte Valley. The results from this well are anomalously old with respect to all of the other wells sampled. The upper interval sampled is from cuttings of Tertiary sediments at 2800-2810'. The release spectra for this sample is complicated, in part by excess argon, and the ages do not form an isochron. The mean of several ages suggest a Middle Triassic age of about 237.1 +/- 0.7 Ma for the maximum time of the last thermal event.

The deeper Marlin sample is from cuttings of the Chainman Formation at 7330-7470'. This release spectra is very complicated and affected by excess argon, and is without a clear isochron age. Like the Eagle Springs samples, the deeper sample is also showing an older age than the shallower sample. Several step ages suggest an Early Mississippian mean maximum age of about 330.2 +/- 2.7 Ma for the last major thermal event. There appears to be more excess argon in the deeper sample suggesting that the 330.2 Ma age is anomalously old.