HIGH TO MODERATE TEMPERATURE THERMOCHRONOLOGY AND THE MATURATION OF CONTINENTAL LITHOSPHERE

Continental lithosphere during orogenesis has a complicated and transient thermal structure both at the near-surface and at depth. To gain a full understanding of the maturation of continental lithosphere, it is necessary to deduce evolving thermal conditions over geological time-scales based on present day exposures. The application of multiple high to moderate temperature U-Pb thermochronometers allows tracking of transient thermal patterns within the lithosphere as it evolves into a regime characterized by very slow conductive cooling and long-term isostatic exhumation typical of cratons.

Uranium-bearing accessory minerals with reasonably well-known diffusion parameters and bulk closure temperatures include zircon and monazite (T_cb > 1000 °C), titanite (T_cb ca. 650 °C), apatite (T_cb ca. 450 °C) and rutile (T_cb ca. 400 °C). ID-TIMS geochronology can be used to generate high-precision thermochronologic data on the sub-grain scale, permitting us to exploit the dependence of closure temperature on cooling rate and effective diffusion dimension. A further advantage of the U-Pb system is that it allows for the quantitative evaluation of closed-system behavior, useful for evaluating multiple pulses of reheating and slow cooling. These data, in conjunction with Ar-Ar thermochronologic data, numerical modeling and heat production information, can be used to construct precise and accurate thermal histories of rocks over temperature ranges of >600 °C and time periods of billions of years. The accessory minerals used for U-Pb thermochronology can also be dated using (U-Th)/He techniques, thereby extending the temperature range to the last stages of near-surface exhumation. Combining these studies of exposed rocks with data from lower crustal xenoliths provides broad insight into the 4-D post-assembly stabilization of continental lithosphere.

We illustrate the power of this approach with examples from Archean and Proterozoic cratonic regions that experienced both rapid- (>100 °C/Myr) and slow-cooling (<1 °C/Myr) during continental assembly, stabilization and thermal reactivation.