Abstract
The division of the earth’s surface into continents and oceans is a consequence of plate tectonics but a geological paradox exists at continent-ocean boundaries. Continental plate is thicker and lighter than oceanic plate, floating higher on the mantle asthenosphere, but it can rift apart by thinning and heating to form new oceans. In theory, continental plate subsides in proportion to the amount it is thinned and subsequently by the rate it cools down. However, seismic and borehole data from continental margins like the Atlantic show that the upper surface of many plates remains close to sea-level during rifting, inconsistent with its thickness, and subsides after breakup more rapidly than cooling predicts. Here we use numerical models to investigate the origin and nature of this puzzling behaviour with data from the Kwanza Basin, offshore Angola. We explore an idea where the continental plate is made increasingly buoyant during rifting by melt produced and trapped in the asthenosphere. Using finite element simulation, we demonstrate that partially molten asthenosphere combined with other mantle processes can counteract the subsidence effect of thinning plate, keeping it elevated by 2-3 km until breakup. Rapid subsidence occurs after breakup when melt is lost to the embryonic ocean ridge.
Highlights
The aim of this paper is to test the geological causes of problematic subsidence patterns at the margins of ocean basins
A reasonable case can be made for early syn-rift strata being removed before the rift subsides below sea-level, but the question shifts to what causes rifts to be elevated in the first place
Models with low retention assume the asthenosphere is predominantly olivine and in textural equilibrium[54] and that melt can migrate to shallow levels relatively unimpeded[26]
Summary
The aim of this paper is to test the geological causes of problematic subsidence patterns at the margins of ocean basins. We do not know of any models designed to solve the issue of rapid sag after breakup, explanations have been proposed such as continued extension below the crust[7,13,14] We address this shortfall here, by investigating mechanisms which might keep the plate buoyant as it thins and subsequently cause it to subside quickly. There is typically a 3–5 km difference in the height between continental rifts and oceanic rifts (spreading centres) related to a difference in their buoyancy (isostatic equilibrium) It becomes thinner as it stretches, the plate in continental rifts is composed of granitic crust underlain by thick mantle lithosphere. We do not discuss rifted margins associated with very slow separation, where melting is suppressed and mantle can be exhumed[10]
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