Continental margins that are attached to
the adjacent oceanic crust and do not have a plate boundary
along the margin are known as passive or trailing margins.
Most parts of the east coasts of North and South America,
the west coasts of Europe and Africa, and most of the coastlines
of India, Antarctica, and Australia are passive margins,
many of which are characterized by thick accumulations of
marine carbonates, shales, and sandstones. Conditions for the
formation, accumulation, and preservation of hydrocarbons
are met along many passive margins, so contemporaneous
and ancient passive margin sequences are the focus of intense
petroleum exploration.
Trailing or passive margins typically develop from a continental
rift and first form an immature passive margin, with
the Red Sea being the main example extant on the planet at
this time. Rifting along the Red Sea began in earnest by 30
million years ago, separating Arabia from Africa. The Red
Sea is characterized by uplifted rift shoulders that slope generally
away from the interior of the sea but have narrow
down-dropped coastal plains where steep mountain fronts
are drained by wadis (dry streambeds) with alluvial fans that
form a typically narrow coastal plain. These have formed
over stretched continental crust, forming many rotated fault
blocks and grabens, intruded by mafic dike swarms that in
some places feed extensive young volcanic fields, especially in
Saudi Arabia. Since much of the Red Sea is located in tropical
to subtropical latitudes, it has developed thick carbonate
platforms along the stretched continental crust. As rifting and
associated stretching of the continental crust proceeded, areas
that were once above sea level subsided below sea level, but
different parts of the Red Sea basin reached this point at different
times. Together with global rises and falls of sea level,
this led to episodic spilling of salty seawater into restricted
basins which would then evaporate, leaving thick deposits of
salt behind. As rifting continued these salts became buried
beneath the carbonates and shales and sandstones, but when
salt gets buried deeply it rises buoyantly, forming salt domes
that pierce overlying sediments and form exceptionally good
petroleum traps. Parts of the passive margins along the Red
Sea are several kilometers thick and currently exhibit some of
the world’s best coral reefs. The center of the Red Sea is
marked by steep slopes off the carbonate platforms, leading
to the embryonic spreading center that is only present in
southern parts of the sea. Abundant volcanism, hot black
smoker vents, and active metalliferous and brine mineralization
on the seafloor characterize this spreading center.
As spreading continues on passive margins, the embryonic
or Red Sea stage gradually evolves into a young oceanic
or mature passive margin stage where the topographic relief
on the margins decreases and the ocean to passive margin
transition becomes very flat, forming wide coastal plains such
as those along the east coast of North America. This transition
is an important point in the evolution of passive margins,
as it marks the change from rifting and heating of 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 nine or even 12.5 miles (15–20 km), making passive margin
deposits among the thickest found on Earth.
See also CONTINENTAL MARGIN; DIAPIR; DIVERGENT OR
EXTENSIONAL BOUNDARIES; HYDROCARBON; PLATE TECTONICS.
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