Lithospheric Strength and Rift Migration controls on Syn-rift Stratigraphy and Breakup Unconformities at Rifted Margins

Abstract:

Syn-rift stratigraphy and distribution of erosional unconformities related to breakup vary vastly among margins and cannot be explained by classical rifting models. Here we use numerical modelling to predict their spatio-temporal distribution. We show that syn-rift stratigraphy mimics rift architecture which is controlled by lithospheric strength. Basinward rift migration during extension produces 1) oceanward younging, syn- and post-tectonic sequences, 2) Rift Migration Unconformities, RMUs, pre-dating break-up, and 3) a Break-up Unconformity, BU, only extending over the most outer margins, since breakup does not represent a sudden, large release of stress. For small syn-rift sedimentation, the RMUs and BU appear to laterally merge forming a margin-wide single unconformity. In symmetric, wide conjugates, arising for weak lithospheres as the South China sea, a long phase of distributed deformation with little subsidence, results in early syn-rift sediment over most of the margins. RMUs merge into a single event marking the subsequent focusing of deformation into a narrow break-up area, which experiences short-lived intense thinning and subsidence. In asymmetric conjugates, lateral rift migration transports shallowly deposited, early syn-rift sediments from the narrow to the wide, hyperextended margin, resulting in a condensed syn-tectonic sequence and a single BU in the narrow margin, and a series of RMUs in the wide one. For very weak lower crusts, lateral rift migration generates large syn-rift sag basins in the wide margin, as in Angola and Congo margins. Our models resemble the observed margins tectonic diversity, and may be used as templates for interpretation of their distal, unexplored areas.