Dynamics of long-term sea-level change and vertical motion of continents

Abstract

We formulate and apply dynamic models to better understand mantle processes and evolution, the vertical motion of continents, and regional and global sea-level change since 100 Ma. We show that evolving mid-to-upper mantle upwellings explain observed anomalously shallow bathymetry, the negative geoid, and the low seismic shear velocity anomalies in the Ross Sea region of Antarctica. These upwellings create a long-lived dynamic topography high, and the Campbell plateau of New Zealand experienced excess subsidence as it moved away from this upwelling. We then use instantaneous models globally to demonstrate that upper-to-mid mantle upwellings, located in the Indian Ocean, Ross Sea, northeast Pacific, and west Atlantic, are the primary cause of high-amplitude geoid minima that are localized within the longer wavelength geoid trough created by Mesozoic slabs. We propose that these upwellings constitute an unrecognized mode of mantle upwellings, potentially developed in response to the ancient subduction zones. In an alternative approach, we apply inverse models to North America (NAM), and find that the vertical motion and relative sea level were controlled by Farallon slab subduction. The Farallon slab was flat-to-shallow lying in the Late Cretaceous and in turn controlled the marine inundation of the western NAM. During the Cenozoic, the Farallon slab sank into the lower mantle, while NAM moved westward in a mantle reference frame, resulting in the dynamic uplift of the western half and dynamic subsidence of the eastern half of NAM. We then use dynamic models and hypsometric analysis to show that the proposed dynamic subsidence potentially explains discrepancies between low-amplitude of sea-level fall inferred from subsidence analysis of New Jersey boreholes compared to sea-level curves based on global data sets. Finally, we formulate dynamic models based on a hybrid approach, accounting for long-term sea-level change factors self-consistently. We infer the relative importance of dynamic topography versus other factors in controlling regional sea level and relative large-scale vertical motions, and calculate a global sea-level curve. We find that the eustatic sea-level fall since the Late Cretaceous is driven by changes in the age of the ocean floor, but is partially offset by dynamic topography.