the lithosphere to drifting and cooling of the lithosphere. The cooling of the lithosphere beneath the passive margin leads to gradual subsidence, typically without the dramatic faulting that characterized the rifting and Red Sea stages of the margin's evolution. Volcanism wanes, and sedimentation on the margins evolves to exclude evaporites, favoring carbonates, mudstones, sandstones, and deltaic deposits. The overall thickness of passive margin sedimentary sequences can grow to 9 or even 12.5 miles (15-20 km), making passive margin deposits among the thickest found on Earth.
Most ancient passive margins have gone through stages of evolution similar to those described for the Red sea, so they form a distinctive assemblage of rocks in the geological record. In general ancient passive margins can be recognized first, in that they are located on the flanks of cratons, continents, or microcontinents. The rocks of the passive margin typically overlie older continental crusts although some overlie other rock sequences such as rift deposits that record a geologic history of the margin prior to its development as a passive margin. The passive margin sequence has a geometry where initial rift phase deposits are overlain by a seaward thickening and seaward deepening wedge of sedimentary rocks, typically grading from a sandy shore facies, to an offshore muddy facies, and in cases where the climate permitted, to a carbonate platform. The passive margin sequence is flanked by deep water facies rocks that, during collisional orogenesis, may be thrust on top of the shallow water sediments of the passive margin. In some cases these thrust sheets contain slivers or large thrust sheets of oceanic crust and lithosphere known as ophiolites.
The oldest known well-preserved passive margin is the Steep Rock Lake belt, which formed between 3.0 and 2.7 billion years ago in the Superior Province of Canada, although many smaller and disrupted candidates exist elsewhere in the world including a 2.9-2.7 billion-year-old passive margin in the North China craton, and a 2.7 billion-year-old margin in Zimbabwe. Passive margins show a cyclic or episodic distribution of peak abundances through geological time, with the most abundant extant passive margins found at 1950, 550, and 0 million years ago, corresponding to times of supercontinent dispersal. The opposite is also true—times where there were relatively few passive margins, such as 1750 and 300 million years ago, correspond to times of supercontinent formation.
The present day total length of passive margins on the Earth is 64,500 miles (104,000 km), and these have an average age of 104 million years and a maximum age of 180 million years. Studies of ancient passive margins by U.S. Geological Survey geologist
Dwight Bradley show that they have a mean life span of 187 million years and a maximum life span of 550 million years. Divided by age, the mean life span changes from 182 million years for the Archean and Paleoproterozoic, 211 million years for the Neo-proterozoic, 145 million years for the Cambrian to Carboniferous, and 142 million years for the Carboniferous to present. Overall there is a general trend for passive margins to have a longer life span with younger geological ages, suggesting that the rate of plate tectonic processes may have been faster in Pre-cambrian times.
See also African geology; Asian geology; continental margin; divergent plate margin processes; hydrocarbons and fossil fuels; plate tectonics; subsidence.
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