HR: 08:00h
AN: S11A-01 INVITED     [PDF]
TI: Speculations on Some Problems of Continental Evolution
AU: * Jordan, T H
EM: tjordan@usc.edu
AF: Department of Earth Sciences, University of Southern California, Los Angeles, CA 90089-0740
AB: Don Anderson is a true student of T. C. Chamberlin, who in his famous 1890 paper (probably as an unstated response to Kelvin's unrealistic bound on the age of the Earth) cautioned how ``a working hypothesis may with the utmost ease degenerate into a ruling theory.'' Questioning the ruling theory has been a specialty of Don's---a skill he has recently demonstrated in his vigorous critique of the hypothesis-cum-ruling-theory that deep mantle plumes are a dominant mechanism for anomalous volcanism in the continents and oceans. Though he was my teacher, I have rarely had the occasion to agree with an Andersonian view of the world, even in the multiplicity of its working hypotheses, so I appreciate this opportunity to support his notion that the instabilities responsible for certain types of anomalous volcanism---in particular continental flood basalts---arise in the upper, not lower, mantle. The subject of my presentation will be the evolution of the continental tectosphere. For some time my thinking has been based on the idea that the chemistry and temperature of the deep (sublithospheric) tectosphere are mutually regulated by the dynamic requirement of isopycnic balance. Current data on the substructure of the cratons seem to support this hypothesis, at least as a first approximation, but it places severe constraints on continental formation that have not yet been reconciled with salient aspects of geologic history. The transition sometime in the early to mid Proterozoic from thick to relatively thin tectosphere can plausibly be explained by the exhaustion of mantle peridotitites with magnesium numbers in excess of 92 that were depleted prior to 4 Ga, but the long history of intracratonic volcanism, as evidenced by pervasive dyke swarms and episodes of basaltic flooding, remains a puzzle. I will therefore explore the possibility that the \begin{it}magmatic\end{it} stabilization of the cratons proceeded during an extended interval following their \begin{it}tectonic\end{it} stabilization by supercontinent collisions. According to this line of thinking, remnants of oceanic (i.e., formerly convecting) upper mantle trapped within the accreting cratons conductively cooled, subsided to form sedimentary basins, and eventually became unstable in basin inversion events that produced large volumes of basaltic magma by decompression melting. The resulting depletion led to the stabilization of these tectospheric ``flaws'', although their existence remains evident as regions of lower Mg numbers and higher temperatures (and thus lower seismic velocities) in near-isopycnic balance with the surrounding tectosphere. Data from southern Africa supporting this speculative model will be discussed.
DE: 1025 Composition of the mantle
DE: 7218 Lithosphere and upper mantle
DE: 8110 Continental tectonics--general (0905)
DE: 8125 Evolution of the Earth
DE: 8414 Eruption mechanisms
SC: Seismology [S]
MN: 2003 Fall Meeting