VARIABLE SEAFLOOR SPREADING RATES AND GLOBAL SEA-LEVEL: INTRODUCING A RIDGE VOLUME INVERSION TECHNIQUE (SLAB PULL, MANTLE DISCONTINUITY).

摘要

A conflict of viewpoint on short term (10('6)-10('7) yr) plate mechanics may be derived from current geologic literature: the standard view is embodied by the 'constant spreading assumption' and was adopted so as to achieve higher resolution in anomaly timescales than the number of firmly dated points. A second view is more dynamic but less quantifiable and derives from Benioff zone seismicity and plate force studies. In force studies slab pull emerges convincingly as a dominant factor in plate speed. From stress studies one concludes that slabs undergo major interactions in the mantle and have highly changeable geometries; in particular the 670-km discontinuity and indications that slabs do not penetrate it. Taken together this strongly suggests that variable plate speeds might also occur over 10('7) year timescales. Although not apparent from anomaly records, short term variable spreading rates, if widespread enough, may become manifest in sea level changes (through ridge volume dynamics) and in the record of volcanism. Ridge volume dynamics cannot explain eustatic rate changes greater than 1-2 cm/1000 yrs but the possibility that ridge volume contributed to some less rapid inferred oscillations (Vail, 1977) is not inconceivable especially as no other eustatic mechanism would be continually operative throughout the Phanerozoic. Within these theme two quantitative ideas are presented: (1) a ridge topography inversion procedure and (2) a ridge volume inversion procedure using first order sea level as a proxy measure of ridge volume. The first estimates local spreading rate from topography, the second estimates global accretion rate (plate creation rate) from first order long term sea level change. The second has been applied to Phanerozoic data as determined from stratigraphy and geohistory analysis. Although a number of simplifying assumptions were made the estimates of accretion rate thus obtained would be the first prior to about 180 mya: subduction has consumed all older ocean floor making anomaly methods impossible. Accretion rate along with sea-level is the crucial data for 'BLAG-type' early earth climate models.