Developing and Testing a Model to Quantify Extension of Passive Margins with Implications for Sea Level Change

Numerous factors influence global sea level fluctuations, yet the effect of passive margin extension has received little attention. Extension of continental crust reduces the total surface area of oceanic crust.  Consequently, sea level must rise to accommodate the loss of basin volume.  The magnitude of oceanic volume change can be calculated by determining the thickness of stretched crust and assuming that stretched crust replaced deep ocean.  Quantifying how much the crust has stretched is a rather elaborate process.  First, total tectonic subsidence is calculated from three distinct datasets: 1) sediment thickness, 2) water depth, and 3) age of the ocean floor.  Then, we use a uniform stretching model and assume reasonable physical properties for the lithosphere, asthenosphere, and sediments to obtain crustal thickness values for every unit cell centered on 5 minute grid points.  The timing of the resulting change in sea level is constrained by the onset of rifting and the breakup age.

Whether or not the modeled results correlate well with direct observations is of paramount importance when considering the robustness of this analysis.  Accordingly, we compared our results with seismically derived crustal thickness, water depth, and sediment thickness data obtained from three, published, seismic studies in coastal North America.  Our data and the seismic data usually pertained to slightly different locations.  Therefore we compare the seismic data to our closest grid point (CGP), and to an average (AVE) including 8 surrounding grid points.  Linear regression was used to determine R² values of .809 (CGP) and .822 (AVE) for crustal thickness, .979 (CGP) and .971 (AVE) for water depth, and .853 (CGP) and .871 (AVE) for sediment thickness.   Our modeled results compare well with the seismic data, and can be used to calculate the change in ocean volume, and sea level, due to extension of the continental crust.