Regional restoration across the Kwanza Basin,Angola:Salt tectonics triggered by repeated uplift of a metastable passive margin

摘要

Restoration of a 375-krn (230-mi)-long section across the Kwanza Basin,Angola,shows three stages of deformation detaching on Apt-ian salt,each caused by basement tectonics.First,tilting related to postrift thermal subsidence initiated early Albian deformation,shortly after salt deposition ended.Deformation waned in the late Albian,probably because of thinning of salt lubricant beneath the extensional province.The second phase of deformation was triggered by hitherto unrecognized crustal uplift beneath the continental rise around 75 Ma (Campanian).Uplift led to salt extrusion and seaward advance of the Angola salt nappe over the abyssal plain.Exposure of the nappe toe removed the buttress provided by abyssal-plain cover,which rejuvenated seaward translation.Third,Miocene basement uplift below the shelf steepened the bathymet-ric slope and greatly accelerated downslope translation.This deformation is now slowing because accelerated sedimentation on the abyssal plain reduced the relief of the system and blocked salt-nappe advance.Minor changes in basin configuration led to profound changes in detached deformation.Miocene uplift was only a few hundred meters on the shelf,but this was sufficient to destabilize the system and increase the translation rate from 300 to 3200 m/m.y.(980 to 10,500 ft/m.y.) Deposition of 600 m (2000 ft) of sediment on the abyssal plain in the upper Miocene shifted contractional deformation 150 km (95 mi) landward.We conclude that driving and resisting forces have been precariously balanced for much of the Kwanza Basin's history.