Enigma of microcontinent formation

Minakov A.N. ¹, Podladchikov Yu.Yu. ², Faleide J.I. ³

1University of Bergen, Norway; 2University of Lausanne, Switzeland; 3University of Oslo, Norway

alexander.minakov@uib.no

The microcontinentinent formation is enigmatic because the processes related to focusing of deformation just landward the continental fragment are unclear. In addition, the isolated continental fragments are sometimes characterized by uncommon subsidence pattern. These two issues are under the scope in this contribution.

A widely accepted model of microcontinent formation suggests that magmatism plays a major role during splitting-off continental fragments from rifted margins. At the same time, recent geological and geophysical observations imply that a number of microcontinents (such as the Lomonosov Ridge, Jan Mayen, Laxmi Ridge) detach from continental margins as a result of magma-poor breakup despite the proximity to large hotspots. The lack of breakup-related magmatism and magrma-starved early seafloor spreading at these localities imply that another weakening mechanism, sufficient to ensure the detachment of continental fragments, is required. As such mechanism, we propose a short-lived episode of strike-slip deformation just before or during breakup. We perform 2D thermomechanical experiments using a finite element method to simulate reactivation of a rifted margin by oblique rifting followed by isolation of a continental fragment. The modeling results suggest that solely mechanical weakening (cohesion loss) within the shear zone does not generate a weakness large enough to ensure fast and focused necking. Instead, we assume that the localization of deformation is induced by thermomechanical feedback between the temperature-dependent mantle viscosity and strain rates. The efficiency of rift localization initiated by shear heating depends mainly on the initial geotherm and boundary velocities. The narrow region of abrupt lithopsheric thinning (~100 km) is observed for a typical continental geotherm and boundary velocities of about 1 cm/yr. A younger thermal age and/or lower strain rates imply that deformation becomes more distributed.

The sedimentary sections on the Lomonosov Ridge and the Jan Mayen microcontinents is characterized by pronounced erosional unconformities, attributed to postrift period. According to conventional thermal kinematic models, these microcontinents should have subsided to >1 km depth owing to the lithospheric stretching and cooling. We propose an alternative subsidence model for these microcontinents incorporating a simple P-T relation for mantle density. Using this model, we can explain the hiatus by postrift uplift and erosion. The spreading geometry imply the seafloor speading off the cosidered continental fragments was compensated by non-volcanic thinning of the mantle lithosphere at an early stage. In response to a rise in temperature, the mantle mineral composition may have changed through breakdown of spinel peridotite and formation of less dense plagioclase peridotite. The consequence of lithosphere heating and the metamorphic reaction would be postrift uplift followed by rapid subsidence of these structures to the deep-water environment.