Salt flows downslope, irrespective of overburden. In salt basins on passive margins, the salt will tilt and flow towards the ocean immediately after continental rifting has ended due to thermal subsidence. Using real examples, as well as physical and numerical models, tilting is shown to be relatively rapid, enhanced by isostatic rebound updip and loading downdip where salt pools and inflates behind an outer high. In the Santos, Campos and Kwanza basins, this outer high is represented by an embryonic mid-Atlantic ridge, amplified in height by the differential weight of the inflating salt. Widespread extension and translation of overburden, utilizing both seaward- and landward-dipping normal faults, characterizes the early evolution of the inboard region. Inflation and contraction occur outboard, the effects of which tend to expand in a landward direction over time. Rapid accumulation of salt implies wholesale dewatering of pre-salt sediments, the water possibly permeating the salt once it has reached a burial depth of c. 3 km. The process of thermal subsidence, salt drainage and isostatic amplification is an efficient mechanism for moving sediment on passive margins tens of kilometres seaward during a relatively short period and helps explain why great thicknesses of salt can accumulate there in the first place.
The Quetico alkaline province of the western Quetico metasedimentary belt comprises at least 20 small intrusions with distinct alkaline affinity that were emplaced during a short period of time in the late Archean. Bulk compositions vary from ultramafic, through
mafic, intermediate and felsic rocks, which display mutual intrusive relationships. Carbonatite dykes form a minor component of the intrusions. The alkaline affinity of the magma(s) parental to the intrusive bodies is suggested by the presence of phlogopitic biotite inclusions in high Na2O augite,
together with the occurrence of secondary, blue-alkali amphibole along fractures, and evidence for early crystallization of apatite.
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