HR: 15:25h
AN: V42F-07    [PDF]
TI: Modeling Lithospheric Isostatic and Thermal Evolution: Application to the Proterozoic Orogen of the Southwestern United States
AU: * Flowers, R M
EM: rflowers@mit.edu
AF: Massachusetts Institute of Technology, Dept. of Earth, Atmospheric and Planetary Sciences, 77 Massachusetts Ave., Cambridge, MA 02139 United States
AU: Royden, L H
EM: lhroyden@mit.edu
AF: Massachusetts Institute of Technology, Dept. of Earth, Atmospheric and Planetary Sciences, 77 Massachusetts Ave., Cambridge, MA 02139 United States
AU: Bowring, S A
EM: sbowring@mit.edu
AF: Massachusetts Institute of Technology, Dept. of Earth, Atmospheric and Planetary Sciences, 77 Massachusetts Ave., Cambridge, MA 02139 United States
AB: The integration of high-resolution cooling histories with exhumation constraints and thermobarometric, heat production and heat flow data can be used to understand long-term lithospheric thermal and isostatic evolution. Cratonic lithosphere retains a record of the net temperature dependent lithospheric density change, as net lithospheric heating causes density decrease, uplift and erosion, while net cooling causes density increase, subsidence and sedimentation. We use this to construct isostatically consistent primary lithospheric geotherms, and use a finite-difference model to compute the time-dependent change of the geotherms following assembly to steady-state. The Proterozoic orogen of the southwestern U.S., characterized by disparate crustal domains with distinct cooling records, levels of exposure, and peak metamorphic conditions, is an ideal location to evaluate the controls on lithospheric thermal and isostatic evolution. In West-central AZ, apparent protracted cooling (1$\deg$C/m.y.) in 3-4 kbar exposed rocks of the Hualapai block contrasts with rapid cooling (25-100$\deg$C/m.y.) in 1-2 kbar exposed rocks of the Ash Creek block. Our thermal analysis indicates that observed differences in radiogenic heat production (3.1-5.2 microW/m$^{3}$ for Hualapai vs. 0.7-1.4 microW/m$^{3}$ for Ash Creek), but similar initial lithospheric mantle temperatures (900-1000$\deg$C), can explain the distinct thermal and isostatic records of the two blocks. Thus, heat production variation not only significantly influences crustal cooling histories, as has been previously documented, but also can induce diverse isostatic responses during geotherm evolution. The approach applied in this study can place important constraints on feasible, isostatically consistent models for cratonic lithosphere, and provides a framework with which better understand its assembly, stabilization and preservation.
DE: 8130 Heat generation and transport
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting