THE RIFT TO DRIFT TRANSITION AT NON-VOLCANIC MARGINS: INSIGHTS FROM NUMERICAL MODELLING

Abstract

"Non-volcanic" rifted margins exhibit very little evidence of synrift magmatism, even where the continental crust has been thinned to such an extent that the mantle has been exhumed across a transitional zone (up to ~100 km wide), called the continent-ocean transition (COT). Using dynamical models of rifting, we explore how extension velocity, mantle composition and potential temperature influence the nature and extent of the COT and compare our results to observations at the West Iberia margin (WIM) and the ancient margins of the Liguria-Piemonte Ocean (LP) now exposed in the Alps. We find a first order relationship between extension velocity and the amagmatic exposure of mantle at the COT. For very slow half extension velocities, (<6 mmyr), mantle exhumation begins before melting. at these velocities, by the time melting starts rift centre, area of exhumed has moved sideways creating a cot, width which increases with decreasing velocities. however, 10 yr, velocity probably appropriate for wim and lp,prior to exhumation. in this case, our models show that starts, ~4.5 km column melt been produced, much more than ~2km maximum mean thickness inferred cot margins. even considering 25% produced may be trapped mantle, as slow-spreading mid-ocean ridges, still is from observations. thus, extension alone cannot explain practical absence synrift magmatism lp. we find formation wide, amagmatic requires either was depleted basaltic constituents>10% prior to rifting or that its potential temperature was ~50 °C lower than normal (≤ 1250°C). © 2006 Elsevier B.V. All rights reserved.