HR: 09:45h
AN: G41A-08    [Abstracts]
TI: Building on Current Space-Based Geodesy to Infer Long-Term Tectonic Reduction in Continental Area
AU: * Hahn, B C
EM: bhahn@mantle.geo.sunysb.edu
AF: Stony Brook University, Department of Geosciences , Stony Brook, NY 11790 United States
AU: Kreemer, C
EM: kreemer@mailhost.geologie.ens.fr
AF: Laboratoire de Geologie de l'Ens, UMR 8538 24 rue Lhomond, Paris, 75231 France
AU: Holt, W E
EM: wholt@mantle.geo.sunysb.edu
AF: Stony Brook University, Department of Geosciences , Stony Brook, NY 11790 United States
AU: Silver, P G
EM: silver@dtm.ciw.edu
AF: Department of Terrestrial Magnetism, Carnegie Institution of Washington, Washington, D.C., 20015 United States
AU: Haines, A J
EM: haines@esc.cam.ac.uk
AF: Cambridge University, Bullard Laboratories, Cambridge, CB3 0EZ United Kingdom
AB: GPS network data gathered globally over the past decade have been analyzed to calculate plate velocities and a world strain rate model {\it (Kreemer et. al 2003; http://gsrm.unavco.org)}. Use of this global model has allowed us to quantify the current changes in the areal extent of the continental crust and oceanic basins. After applying a correction for the recoverable effects of elastic strain in subduction collision zones, we find that tectonic processes alone produce a reduction in continental area equal to approximately 0.18$\pm$0.02 km$^{2}$yr$^{-1}$ and an oceanic crust creation rate of 2.7$\pm$0.02 km$^{2}$yr$^{-1}$ (primarily at mid-ocean ridges, but also through extension at shallow back-arc basins). This implies that 6% of generated oceanic area is not destroyed through subduction but is taken up through continental deformation. While these estimates represent the current rate of change calculated from GPS, we can infer longer-term rates of similar order through comparisons with geologic data. The global velocities from the global strain rate model used for this study correlate well with velocities determined from 3 Myr averaged plate motion models from sea-floor spreading rates and fault azimuth data {\it (DeMets et. al 1990, 1994)}, implying that the areal change rates estimated here are a reflection of the long-term, 3 Myr average rate. Further, using sediment volume delivered to oceans as a proxy for surface uplift volume produced by specific collision events, we can estimate the collision zone areal reduction rate for the formation of older orogens for the past 450 Myr. Similar estimates can be made for areal creation produced by major rifting events. Without a counterbalance to this tectonic areal reduction, our current rate implies an approximate 10% reduction in total continental area every 100 Myr. The reduction is due to the disparate temporal and geographic prevalence of continental compression versus extension. That is, continental collision and subduction occur throughout the Wilson cycle, while major rifting events are generally singular and relatively short-lived for a particular cycle. If constant or slowly increasing continental area is to be maintained (as suggested in several continental growth models), 7.65$\pm$0.90 km$^{3}$yr$^{-1}$ of new or recycled volume must be added to the continental landmass through one or more of the following methods: sediment accumulation at continental margins producing additional area; introduction of new continental volume through volcanism; incorporation of oceanic crust underlying margin sediment into the continental mass during collision; or some additional processes. Any of these processes will introduce chemical changes to the continental crust and may help explain the structural, compositional, and tomographic disparity between ancient and younger continental regions. We argue that continental areal reduction is a long-term consequence of plate tectonics and likely had a profound influence on the character and evolution of continental crust at least through the Phanerozoic.
DE: 1206 Crustal movements--interplate (8155)
SC: Geodesy [G]
MN: 2004 AGU Fall Meeting