Description:

The Great Unconformity was defined by Powell in the late 19th century and generally represents at time gap of over a billion years. Accompanying this time gap is a thermochronological gap. Micas and amphiboles from basement rocks below the Great Unconformity often yield apparent ages >1400 Ma and fission track ages are generally less then 100 Ma. Thus we are left without a rock record or a temperature record for a very large time interval. K-feldspar 40Ar/39Ar thermochronology can fill this thermochronological void, as it generally records the time-temperature history between about 150 to 300¡C. We have just received an NSF grant to work on the thermal history below the Great Unconformity and will soon provide a first-hand record for this important time interval. Preliminary results show that large regions of the southwest USA were exhumed at different times and acted as independent blocks. Working with graduate students and a post-doc over the next few years we hope to define the thermal histories of these independent blocks. Structural studies will focus on what features are accommodating the thermally distinct boundaries.

Not only does the Great Unconformity represent a billion years of missing rock record, the basement below has a billion years of missing thermochronological record. Because K-feldspar has an argon retentivity range of ~150 to 300°C, K-feldspar thermochronology will provide the critical information to fill the thermochronological void.