Article ID | Journal | Published Year | Pages | File Type |
---|---|---|---|---|
8912883 | Earth-Science Reviews | 2018 | 27 Pages |
Abstract
The plate tectonics theory established ~ 50â¯years ago has formed a solid framework for understanding how the earth works on all scales. In this theory, movement of the tectonic plates relative to the subjacent asthenosphere is one of the fundamental tenets. However, the nature of the boundary between the lithosphere and asthenosphere (LAB) beneath ocean basins remains under debate. The current consensus is that the oceanic lithosphere thickens with age by accreting asthenosphere material from below, and reaches its full thickness (L) of ~ 90â¯km at the age (t) of ~ 70â¯Ma. This lithospheric thickening fits the relation L â t1/2, consistent with conductive cooling to the seafloor. A puzzling observation is that although conductive cooling continues, the oceanic lithosphere ceases to grow any thicker than ~ 90â¯km when tâ¯>â¯70â¯Ma. Small scale convection close beneath the LAB has been generally invoked to explain this puzzle, but why such convection does not occur until L ~ 90â¯km at tâ¯>â¯70â¯Ma has been a matter of conjecture. In this paper, we summarize the results of many years of experimental petrology and petrological studies of oceanic basalts, which indicate consistently that the LAB is a petrological phase boundary marking the intersection of the geotherm with the solidus of amphibole (pargasite)-bearing peridotite. That is, petrologically, the LAB is an isotherm of ~ 1100â¯Â°C with L â t1/2 for tâ¯<â¯70â¯Ma and an isobar of ~ 3â¯GPa (~ 90â¯km) for tâ¯>â¯70â¯Ma. This unifying concept explains why the LAB depth increases with age for tâ¯<â¯70â¯Ma and maintains constant (~ 90â¯km) for tâ¯>â¯70â¯Ma. The LAB, that is intrinsically determined by petrological phase equilibria, does not require small-scale convection. However, because the mantle above the LAB is the conductive lithosphere (pargasite-bearing peridotite) and below the LAB is the viscosity-reduced convective asthenosphere (peridotite + incipient melt), the small-scale convection in the asthenosphere close beneath the LAB (~90â¯km) under older (tâ¯>â¯70â¯Ma) seafloors becomes possible, whose convective heat supply balances the conductive heat loss, maintaining the constant heat flow, seafloor depth and lithosphere thickness.
Related Topics
Physical Sciences and Engineering
Earth and Planetary Sciences
Geology
Authors
Yaoling Niu, David H. Green,