Paper No. 5
Presentation Time: 2:30 PM
PLATE RECONSTRUCTIONS REQUIRE HIGH CRETACEOUS SPREADING RATES AND RIDGE VOLUMES
High Cretaceous seafloor spreading rates have been implicated in the production of CO2 and thus, the greenhouse climate as well as in the generation of high eustatic sea level. The sources of data on which spreading rates are based are spreading plate models that are based, in turn, on magnetic anomalies and the time scales used to date them. Both have been refined over the past 20 years since the last ridge volume calculations were made and both retain some uncertainty. Subduction of seafloor necessarily reduces the accuracy of estimates of past ocean volumes at increasingly older times. However, the Atlantic and Indian Oceans (AIO) provide a relatively complete record of spreading rates for the past 170 Ma, because subduction is minor in these basins. The volume of the AIO ridges grew through time to about 50 Ma as plates systematically fragmented Gondwana. The subsequent contribution of oceanic area from AIO oceans remained generally high with variations that depend on the time scale applied to magnetic anomalies. The area of the Pacific Ocean is slightly larger than the Atlantic and Indian Oceans combined and, thus, was probably nearly twice its current size in the Early Cretaceous. Here seafloor from the N, E and SE sides of the late Cretaceous Pacific Plate have been subducted along with all or most of the Farallon-Phoenix and the Kula-Phoenix ridges. The spreading rates of these ridges can be inferred from the remaining triple junctions on the western half of the Pacific Plate. The lengths of these ridges depend on the plate reconstruction chosen. A reconstruction at 90 Ma allows examination of spreading rates and the age distribution at this time. Spreading rates were very high and in the absence of slow spreading AIO ridges, require significantly higher spreading rates compared to those of the last 50 Ma. This is reflected in the area-age distribution of sea floor, which reflects the younger nature of ocean floor compared to the area-age distribution observed today.