SLOW-COOLING OR REHEATING: CAN SW USA THERMOCHRONOLOGICAL DATA BE RECONCILED?

In theory, combining multiple system radiogenic daughter concentration distributions and their associated kinetic data can provide a unique solution of a thermal history. In reality, concentration distributions are rarely accurately known, diffusion parameters are uncertain, and transport mechanisms can be questioned. In practice, using nominal closure temperatures and associated apparent ages inadequately describes complex thermal histories. Thermochronological data from Southwestern USA Proterozoic rocks can be explained using either slow cooling or multiple episodic thermal events. For instance, published thermochronological data from the Arizona transition zone reveal systematic trends with decreasing apparent ages correlating to decreasing calculated closure temperatures (e.g. Hodges et al., 1994; Chamberlain and Bowring, 2000). These, and other studies, argue for overall slow-cooling and long-lived lateral thermal heterogeneities to explain the age distributions. A consequence of the slow-cooling model is that geothermal gradients ranging from 45 to 65°C/km must exist for 10’s to 100’s of millions of years in order to accommodate the thermochronology and the metamorphic petrology that places the rocks at depths corresponding to 2-4 kbar. The high geothermal gradients, which are more indicative of orogenic episodes, seem to be at odds with a stable, slowly cooling craton. The same thermochronological data can be explained using episodic loss events where high temperatures at ca. 1.4 Ga cause partial to complete resetting of isotopic systems. This scenario requires transient high temperature events with or without local 1.4 Ga plutons as well as discrete, low magnitude structural offsets. Both end-member models have limitations and drawbacks, however 1.4 Ga tectonism and thermal events probably play the key role in the apparent age distributions measured in some Paleoproterozoic mid-crustal rocks.