HR: 11:50h
AN: T22B-07    [Abstracts]
TI: Birch's Crustal Heat Production-Heat Flow Law: Key to Quantifying Mantle Heat Flow as a function of time
AU: * Blackwell, D D
EM: blackwel@smu.edu
AF: Southern Methodist University, Dept. of Geological Sciences, Dallas, TX 75275-0395, United States
AU: Thakur, M
EM: mathakur@smu.edu
AF: Southern Methodist University, Dept. of Geological Sciences, Dallas, TX 75275-0395, United States
AB: Birch (1968) first showed the linear correlation of surface heat flow and radioactive heat production (Q_s = Q_o + bA_s ) in granites in New England, USA and discussed implications to the vertical scale of radioactive heat generation in the crust. Subsequently similar relationships have been found worldwide and numerous papers written describing more details and expanding the implications of Birch's Law. The results are a powerful contribution from heat flow research to the understanding of the lithosphere and its evolution. Models are both well constrained experimentally and simple in implications. However, there still exist thermal models of the crust and lithosphere that do not have the same firm foundation and involve unnecessary ad hoc assumptions. A main point of confusion has been that the several of the original relationships were so low in error as to be considered by some to be "fortuitous". Interestingly a "similar" relationship has been proposed based on regional scale averaging of Q_s -A_s data. A second point of confusion is that one admissible crustal radioactivity distribution model (the constant heat generation to depth b) has been criticized as unrealistic for a number of reasons, including the effect of erosion. However, it is appropriate to refer to the Q_s -A_s relationship as a law because in fact the relationship holds as long as the vertical distribution is "geologically realistic." as will be demonstrated in this paper. All geologic and geophysical models of the continental crust imply decreasing heat production as a function of depth (i.e. the seismic layering for example) except in very special cases. This general decrease with depth is the only condition required for the existence of a "linear" Q_s -A_s relationship. A comparison of all the Q_s -A_s relationships proposed for terrains not affected by thermal events over the last 150 to 200 Ma shows a remarkably uniformity in slope (10 ± 3 km) and intercept value (30 ± 5 mWm^-2 ). Therefore these parameters of Birch's Law equation represent the starting place for discussions of lithospheric thermal regime and evolution. The stability of the values of intercept Q_o for areas with thermal ages of Paleozoic and older prove that the lithosphere heat flow does not vary significantly with age as is demonstrated in the companion paper. The minimum mantle heat flow for preMesozoic thermal terrains is 20 - 25 mWm^-2. This value is consistent with the lack of indication from xenolith data that lithosphere thickness changes with age and with theoretical models of mantle convection.
DE: 1020 Composition of the continental crust
DE: 1060 Planetary geochemistry (5405, 5410, 5704, 5709, 6005, 6008)
DE: 5400 PLANETARY SCIENCES: SOLID SURFACE PLANETS
DE: 5418 Heat flow
DE: 8125 Evolution of the Earth (0325)
SC: Tectonophysics [T]
MN: 2007 Fall Meeting