of years. This longevity has consequences on their internal
structure, especially in terms of continental growth by tectonic accretion of oceanic terranes, or by arc magmatism,
but also sometimes in terms of continental consumption
by tectonic erosion.
Morphology
A continental margin generally extends from the coast
down to the abyssal plain (see Figure 1 and entry “Morphology Across Convergent Plate Boundaries”). It typically includes a continental platform gently dipping
seaward and a talus with a steeper slope down to the
trench. In detail, active continental margins offer a wide
spectrum of morphologies from narrow and steep ones
(e.g., Porto Rico) to wide and flat ones (e.g., Mediterranean “Ridge” which is a huge evaporitic accretionary
wedge). In general, the continental shelves bordering
active margins are narrower than those along passive margins. The trench marks the deepest seaward termination of
the active continental margin. It coincides with the surface
trace of the subduction fault marking the boundary
between the converging plates. Its depth generally ranges
between 2 and 7 km depending on the age of the
subducting plate (young plate like Juan de Fuca – shallow
trench) and the amount of trench fill sediment (thick
trench fill like Makran – shallow trench).
Birth, life, and death
The discovery of microdiamonds in two billion-year-old
rocks in Canada attests to ultrahigh-pressure metamorphism compatible with subduction in the early Proterozoic
(Cartigny et al., 2004). The ongoing processes responsible
for the sinking of the lithosphere at that time probably differed from those at present since the Earth was hotter than
today. For modern subduction zones, Seiya Uyeda, in
1984, has proposed two modes of subduction mechanisms: a forced one (Chilean type) in contrast with
a spontaneous one (Mariana type), further used by many
authors like Stern and Bloomer (1992). These notions
should now be replaced by compressional versus extensional subduction, based on the observation of the dominant strain within the overriding plate and not on the
cause of the subduction itself (Heuret and Lallemand,
2005). Indeed, the observation of nascent or young (less
than 10 million years old) subduction zones in the western
Pacific has shown that all of them result from ongoing collisions and plate boundaries reorganization (Lallemand
et al., 2005). None of them can be simply explained by
the spontaneous sinking of an old and dense oceanic lithosphere under its own weight. The main driving force
for subduction, after it initiates and develops down to
a depth of about 200 km, is the slab pull exerted by the
excess mass of the slab with respect to the surrounding
mantle (Turcotte and Schubert, 1982; Hassani et al., 1997).
Active Continental Margins, Figure 1 Schematic view of an active continental margin after Lallemand et al. (2005).
10
ACTIVE CONTINENTAL MARGINS
structure, especially in terms of continental growth by tectonic accretion of oceanic terranes, or by arc magmatism,
but also sometimes in terms of continental consumption
by tectonic erosion.
Morphology
A continental margin generally extends from the coast
down to the abyssal plain (see Figure 1 and entry “Morphology Across Convergent Plate Boundaries”). It typically includes a continental platform gently dipping
seaward and a talus with a steeper slope down to the
trench. In detail, active continental margins offer a wide
spectrum of morphologies from narrow and steep ones
(e.g., Porto Rico) to wide and flat ones (e.g., Mediterranean “Ridge” which is a huge evaporitic accretionary
wedge). In general, the continental shelves bordering
active margins are narrower than those along passive margins. The trench marks the deepest seaward termination of
the active continental margin. It coincides with the surface
trace of the subduction fault marking the boundary
between the converging plates. Its depth generally ranges
between 2 and 7 km depending on the age of the
subducting plate (young plate like Juan de Fuca – shallow
trench) and the amount of trench fill sediment (thick
trench fill like Makran – shallow trench).
Birth, life, and death
The discovery of microdiamonds in two billion-year-old
rocks in Canada attests to ultrahigh-pressure metamorphism compatible with subduction in the early Proterozoic
(Cartigny et al., 2004). The ongoing processes responsible
for the sinking of the lithosphere at that time probably differed from those at present since the Earth was hotter than
today. For modern subduction zones, Seiya Uyeda, in
1984, has proposed two modes of subduction mechanisms: a forced one (Chilean type) in contrast with
a spontaneous one (Mariana type), further used by many
authors like Stern and Bloomer (1992). These notions
should now be replaced by compressional versus extensional subduction, based on the observation of the dominant strain within the overriding plate and not on the
cause of the subduction itself (Heuret and Lallemand,
2005). Indeed, the observation of nascent or young (less
than 10 million years old) subduction zones in the western
Pacific has shown that all of them result from ongoing collisions and plate boundaries reorganization (Lallemand
et al., 2005). None of them can be simply explained by
the spontaneous sinking of an old and dense oceanic lithosphere under its own weight. The main driving force
for subduction, after it initiates and develops down to
a depth of about 200 km, is the slab pull exerted by the
excess mass of the slab with respect to the surrounding
mantle (Turcotte and Schubert, 1982; Hassani et al., 1997).
Active Continental Margins, Figure 1 Schematic view of an active continental margin after Lallemand et al. (2005).
10
ACTIVE CONTINENTAL MARGINS