Sea-Level Change (1990) / Chapter Skim
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8 Long-Term Eustasy and Epeirogeny in Continents
8 Long-Term Eustasy and Epeirogeny in Continents
Pages 141-158
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From page 141... ...
Many other possibilities exist for changing sea level by tens to hundreds of meters. For instance, a change of the pattern of subduction can cause the ocean basins to become on average younger or older and so produce a sea level change that is in principle caused by a similar effect to that caused by varying the amount of seafloor produced per unit time interval.
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From page 142... ...
Fluctuations in relative sea level could cause a pumping action whereby some of the sediment deposited on the slope during high stands is eroded away during low stands and deposited on the continental slope or rise, or into the deep ocean basins by the mechanism of turbidity currents. These arguments suggest that long-term flooding values should not change drastically, even if the area of the continental crust has changed appreciably or if the volume of ocean has grown with time.
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From page 143... ...
Al, A2, and t are taken from the continental hypsographic curve. The change in depth of the oceans is given by h + d: d represents the isostatic response of the ocean basins to the extra load of the water; the change in freeboard, h, is less than the change in ocean depth.
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From page 144... ...
She investigated the errors produced by inaccurate estimates of spreading rates caused by errors in the time scale of reversals, inaccurate estimates of ridge lengths, the effect of uncertainties in her calculations of variation in areas of oceanic crust, which would be older than 150 m.y.
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From page 145... ...
(1981) also added to this volume the effect of other volcanic activity in the ocean basins.
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From page 146... ...
over the whole area of the ocean basins deeper than 1 km, this translates to an equivalent water volume of 0.825 x 10~6 m3 (see Table 8.1~. This effect is in the opposite direction to that of the ridge-crest volume effect, since the sedimentation on the ocean floor tends to decrease the r~dge-crest topographic effect somewhat.
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From page 147... ...
on area as a function of depth in the ocean basins to calculate the total volume change on allowing the water So far, we have used only continental hypsometry to determine the additional area of the oceans as freeboard is decreased, so that we could calculate what the freeboard change is for a change in volume of the ocean basins or the water in them. But a far more significant use can be made of hypsographic curves to determine directly the change in
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From page 148... ...
The crosses represent mean values from results from six individual continents, and the horizontal bars show the limits of the standard error of this mean value. The open circles show the value calculated from the total amount of flooding and the world hypsographic curve.
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From page 149... ...
eustatic sea-level diagram. EPEIROGENY OF THE CONTINENTS An alternative way of estimating the average change in freeboard is to measure the amount of flooding for individual continents and to use the hypsographic curve for each continent to determine a freeboard change for each continent in turn.
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From page 150... ...
We prefer the unbiased calculation that supposes that the modern hypsographic curves are in general like those of previous eras apart from a coherent change discussed in the next section, and that the best estimate of eustatic sealevel change is obtained by the calculation that assumes this, i.e., the one done in Figure 8.4. The epeirogenic movements suggested by Bond (1979)
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From page 151... ...
Alternatively, since they did not produce any details of how the hypsographic curve was established except to give one with the required average continental elevation, it is possible that other hypsographic curves could be developed that require a smaller freeboard increase but that still satisfy the requirement of average elevation outlined by Southam and Hay (1981~. An alternative possibility is that the continents have thermally contracted since the splitting up of Pangea.
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From page 152... ...
is in some way connected with this large density of hot spots. CHANGING THE AGE DISTRIBUTION OF THE OCEAN BASINS Changes in the rate of seafloor spreading, as measured by the area of oceanic crust produced per unit time interval, can produce significant changes in the average depth of the basins and thus change sea level, as discussed earlier.
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From page 153... ...
. In order to calculate the effect of a consumption rate that is not uniform, we have made a calculation in which we start with an oceanic crust that has a uniformly decreasing area versus age plot, and have the total consumption of crust of all ages equal to the production of new crust.
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From page 154... ...
is, Calculation of average depth of the oceanic crust al \ ~ Am, lows us to determine the change in depth of the inland sea. \ ~ ~Due to the isostatic response of the continental crust by ~\ ~ ~5, loading of an added water depth, the water depth is in ,l~ , , ~ ~ |
From page 155... ...
If the changes in spreading occurred in the Pacific, then they would be matched by changes in subduction rates of the Pacific Ocean basin, and in particular in subduction rates at the western margin of North America. It is possible that during increased rates of subduction to the west of the Western Interior Seaway, the shallow basin could be caused to subside tectonically, whereas during times of reduced subduction rate, uplift of the basin could occur.
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From page 156... ...
Because of these vertical motions, estimates of freeboard change measured at one place may not indicate true eustatic sea-level changes. Sea-level changes necessary to produce the large effects of water depth seen in the Western Interior Seaway of North America are likely to be caused by an amplification of a eustatic change.
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From page 157... ...
(19841. Oxygen isotope record of ice volume history: 100 million years of glacio-eustatic sea-level fluctuation, in Interregional Unconformities and Hydrocarbon Accumulation, J
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From page 158... ...
A HARRISON of flexure and the geological evolution of the Pacific ocean basin, Nature 283, 532-537.
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