Building and Destroying Continental Mantle

Abstract

Continents, especially their Archean cores, are underlain by thick thermal boundary layers that have been largely isolated from the convecting mantle over billion-year timescales, far exceeding the life span of oceanic thermal boundary layers. This longevity is promoted by the fact that continents are underlain by highly melt-depleted peridotites, which result in a chemically distinct boundary layer that is intrinsically buoyant and strong (owing to dehydration). This chemical boundary layer counteracts the destabilizing effect of the cold thermal state of continents. The compositions of cratonic peridotites require formation at shallower depths than they currently reside, suggesting that the building blocks of continents formed in oceanic or arc environments and became “continental” after significant thickening or underthrusting. Continents are difficult to destroy, but refertilization and rehydration of continental mantle by the passage of melts can nullify the unique stabilizing composition of continents.

Constraints on shear velocity in the cratonic upper mantle from Rayleigh wave phase velocity

“…The younger platforms are thought to be hotter due to higher heat production (Nyblade and Pollack, 1993). Temperature within cratons has been estimated using shear velocity (Fowler, 2005), mantle xenoliths (Lee et al, 2011), and surface heat flow measurements (Rudnick et al, 1998). Currently, temperature estimates for cratons at depth do not correlate with observed velocities (Dalton and Faul, 2010).…”

The Ocean – Continent Transition Zones Along the Appalachian – Caledonian Margin of Laurentia: Examples of Large-Scale Hyperextension During the Opening of the Iapetus Ocean

“…The Alpine Tethyan OCT ophiolites contain only minor amounts of mafic igneous rocks (basaltic lavas, sheeted dyke complexes and gabbros) and are instead characterized by blocks of ancient subcontinental mantle exhumed by top-basement detachment faults and overlain by extensional being re-enriched in basaltic melt components) and therefore continental mantle is the product of at least two major processes: melt depletion followed by refertilization or other major metasomatic enrichment processes such as Si enrichment (Lee et al 2011). Sub-continental lithospheric mantle exhumed at OCTs on presentday passive margins is dominated by moderate (50%) to highly (95-100%) serpentinized peridotites (Kodolányi et al 2012).…”

Coexisting Early Cretaceous High-Mg Andesites and Adakitic Rocks in the North China Craton: the Role of Water in Intraplate Magmatism and Cratonic Destruction

“…Hydration of the lithosphere may not only significantly reduce the viscosity contrast between the lithospheric mantle and the underlying asthenosphere (Niu, 2005;Lee et al, 2011;Xia et al, 2013), but also promote refertilization-driven destabilization of the craton (Kusky et al, 2014;Zheng et al, 2015;Xiong et al, 2015). Therefore, we propose that destruction of the NCC was caused by erosion, replacement, modification or partial melting associated with the hydration of the lithosphere.…”

“…Ancient cratons, the most stable parts of the continents, are underlain by thick thermal and mechanical boundary layers that have been largely isolated from the convective asthenospheric mantle over billion year timescales (Kelly et al, 2003;Carlson et al, 2005;Lee et al, 2011). However, several lines of evidence suggest that the eastern part of the NCC has been destroyed since the Phanerozoic (Menzies et al, 1993;Griffin et al, 1998;Xu, 2001;Gao et al, 2002;Wu et al, 2003;Zheng et al, 2007).…”