Geological Society of America

T153 Thermochronology: Techniques, Applications, and Interpretations II

Tuesday, 18 October 2005
8:00 AM - 12:00 PM

HIGH TO MODERATE TEMPERATURE THERMOCHRONOLOGY AND THE MATURATION OF CONTINENTAL LITHOSPHERE

Paper number: T153-2
Program: Technical Sessions
Time: 8:10 AM – 8:25 AM

Authors

Samuel A. Bowring Presenting Author
Massachusetts Institute of Technology, Cambridge, MA
James L. Crowley
Massachusetts Institute of Technology, Cambridge, MA
Rebecca M. Flowers
Massachusetts Institute of Technology, Cambridge, MA
Daniel MacPhee
Massachusetts Institute of Technology, Cambridge, MA
Blair Schoene
Massachusetts Institute of Technology, Cambridge, MA

Abstract

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.