HR: 16:55h
AN: U44A-03    [Abstracts]
TI: Age distribution of passive margins through earth history and tectonic implications
AU: * Bradley, D C
EM: dbradley@usgs.gov
AF: U.S. Geological Survey, 4200 University Drive, Anchorage, AK 99508, United States
AB: The ages and lifespans of all existing passive (Atlantic-type) margins plus 59 ancient ones were compiled. Passive margins have existed on Earth almost continually since at least 2685 Ma. Their abundance has fluctuated dramatically, and most of the fluctuations have clear tectonic causes. For the past 2200 Ma, the compiled age distribution of passive margins appears to be robust and not an artifact of an incomplete rock record or flaws in the compilation. It closely tracks all the first-order highs and lows of the seawater 87Sr/86Sr curve, which has been derived from utterly independent data. The main features of the age distribution are as follows: (1) A present-day maximum in number and aggregate length of passive margins corresponds to a time of continental dispersal following breakup of Pangea. (2) A 250- 350-Ma minimum corresponds to Pangea's greatest extent. (3) A 500-600-Ma maximum represents a time of continental dispersal following staged breakup from 600 to 1000 Ma of one or more larger continents (Rodinia in most models). (4) Passive margins are rare between 1000 and 1650 Ma, and none are known at all between 1650 and 1750 Ma. This 750-m.y. low in the passive margin record coincides with the heyday of massif anorthosites. Whereas the Mesoproterozoic may have seen few modern-style Wilson Cycles involving the opening and closing of Atlantic-type oceans, it nonetheless was a time of plate tectonics involving subduction and collision (e.g., Grenville orogeny). (5) Passive margins were abundant between 1750 to 2250 Ma. The close of this interval at 1750-1800 Ma was marked by the collisional assembly of Laurentia, Baltica, and other cratons, which may have been part of a supercontinent (Columbia in some models). A maximum at 1850-2050 Ma corresponds to a time of dispersed small continents. (6) The record of passive margins before 2250 Ma is patchy. It definitely extends back to 2685 Ma (Kaapvaal craton, W margin), and possibly to about 3000 Ma. For all but a few margins with an unusual tectonic history such as re-rifting, the lifespan was calculated from the time between the rift-drift transition and the passive margin to foredeep transition. The present-day passive margins have a mean age of 104 m.y. and a maximum age of 180 m.y.; these are partial lifespans. Fifty-nine ancient margins have a mean lifespan of 187 m.y. and a maximum lifespan of 550 m.y. Subdivided into natural age groupings, mean lifespans are 182 m.y. for the Archean to Paleoproterozoic, 211 m.y. for the Neoproterozoic, 145 m.y. for the Cambrian to Carboniferous, and 142 m.y for the Permian to present. Of the 59 ancient margins, 20 had lifespans that were longer than that of the oldest modern margins, and all 20 are either wholly or partly Precambrian in age. Several margins in the Mesoproterozoic and Neoproterozoic are very long-lived by modern standards, though in each case, the tectonic interpretation is debatable and (or) age control is poor. The tentative world-record holder, at 550 m.y., is the Mesoproterozoic eastern margin of Siberia. The number of long-lived Precambrian margins is inconsistent with the widely held notion that the tempo of Wilson Cycles was faster in the Precambrian than at present, as has been be predicted from the long-term decline in Earth's radiogenic heat production. Thus, the duration of Wilson Cycles involving passive margins is not a good proxy for rates of plate motion. Greater heat production in the Precambrian might still be linked to faster rates of plate motion if the effect was confined to oceanic (Pacific-type) plates.
DE: 8105 Continental margins: divergent (1212, 8124)
DE: 8125 Evolution of the Earth (0325)
DE: 8157 Plate motions: past (3040)
SC: Union [U]
MN: 2007 Joint Assembly