The Cenozoic sedimentary record revealed by the Integrated Ocean
Drilling Program's Arctic Coring Expedition (ACEX) to the Lomonosov
Ridge microcontinent in 2004 is characterized by an unconformity
attributed to the period 44-18 Ma. According to conventional thermal
kinematic models, the microcontinent should have subsided to >1 km depth
owing to rifting and subsequent separation from the Barents-Kara Sea
margin at 56 Ma. We propose an alternative model incorporating a simple
pressure-temperature (P-T) relation for mantle density. Using this
model, we can explain the missing stratigraphic section by post-breakup
uplift and erosion. The pattern of linear magnetic anomalies and the
spreading geometry imply that the generation of oceanic crust in the
central Eurasia Basin could have been restricted and confined by
non-volcanic thinning of the mantle lithosphere at an early stage (ca.
56-40 Ma). In response to a rise in temperature, the mantle mineral
composition may have changed through breakdown of spinet peridotite and
formation of less dense plagioclase peridotite. The consequence of
lithosphere heating and related mineral phase transitions would be
post-breakup uplift followed by rapid subsidence to the deep-water
environment observed on the Lomonosov Ridge today.