Paleo plate motions, the vertical motion of plates, and seismic tomographic images place fundamental constraint on models of mantle convection. We have developed forward and inverse models of mantle convection using these constraints self consistently. Using new paleogeographic concepts and reconstructions, implementations of adjoint and inverse models of convection, and stratigraphic forward models in the plate frame, we apply our method to North America over the last 100 Myr. In order to make a closer link between plate motions and mantle convection, we have developed a new paleogeographic concept ('dynamically closing plate polygons') and developed plate reconstructions 140 Ma to the present. Seismic tomography is used explicitly by using an adjoint of the equations in CitcomS. We have implemented a forward-adjoint looping that solves for the initial conditions while minimizing the difference between predicted and observed present day structure. Since the scaling from seismic anomalies to mantle temperature and mantle viscosity are both uncertain, we apply additional constraints from regional sea level observations on an inverse problem. Finally, prediction of vertical motions (dynamic topography) from the forward models is translated into the plate frame and tested with paleo-shorelines, sediment isopachs, and tectonic subsidence curves. Applied to North America since 100 Ma, we find that as North America moved westward, a long wavelength dynamic topography depression swept eastward over the continent. The Farallon slab, as imaged by seismic topography, is currently below the central Atlantic coast of the US. Sea level fall since the Cretaceous, inferred from boreholes on the Atlantic coast are estimated to be less than 100 m, 100 to 200 meters lower than inferred from either ridge volume or the average flooding of continents. Using this discrepancy between regional and eustatic sea level, as well as the well known anomalous Cretaceous subsidence (and subsequent uplift) of the western interior seaway, we are able to constrain the inverse model of mantle convection. The self-consistent model suggests that the marine flooding of the western interior sea way was not an unusual 'event' in which North America regionally subsided. Rather, North America moved over a more or less fixed downwelling and the "event" was only recorded when eustatic sea level was elevated. The use of the US Atlantic margin as a stable reference frame for sea level is called into question and the unusually small sea level fall may have been caused by a gradual subsidence of the east coast as it moved over the Farallon slab downwelling.
AGU Fall Meeting Abstracts
- Pub Date:
- December 2007
- 3040 Plate tectonics (8150;
- 7270 Tomography (6982;
- 8120 Dynamics of lithosphere and mantle: general (1213);
- 8157 Plate motions: past (3040)