Transcript Sedimentary Basins related to Volcanic Arcs

Sedimentary Basins
related to Volcanic
Arcs
M08353 Basin Analysis
Reading - start with:
• Reading, H.G.: Sedimentary Environments
– 2nd edition. Tectonics & Sedimentation chapter
by Mitchell & Reading
– 3rd edition. Volcaniclastics chapter by Orton, p.
549-
Volcanic arcs may develop...
• within oceanic lithosphere, where ocean
floor subducts beneath ocean floor, and
an island arc results, e.g. Lesser
Antilles arc
• or at the edge of a continent, where
oceanic lithosphere subducts beneath
continental lithosphere, and a
continental margin magmatic arc
forms, e.g. Andes
Basins related to volcanic arcs
• fore-arc
• back-arc
• intra-arc
• All may be either submarine or
subaerial, or may have marine &
subaerial parts
• Much sediment is supplied from active
arc.
Fore-arc basins
• Lie in the arc-trench gap, between
volcanic arc and submarine trench
• range from small basins on trench slope
to large basins (50 to 100 km wide, and
> 500 km long) with thick fills (several
km)
• Basins tend to become wider and
shallower with time, partly because of
accretion at trenches
fore-arc basins
Sediment sources:
• volcanic arc
• outer arc
• longitudinally from
a continent
Tectonic style varies:
• compressional
• extensional
• strike-slip
Back-arc basins
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•
•
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lie behind the magmatic arc
often the site of extension & thinning of crust
may overlie either ocean or continental crust
oceanic back-arc basins are eventually
subducted and destroyed, or preserved in
thrust complexes related to ocean closure.
• back-arc basins on continental crust - more
varied facies, because of terrigenous input;
higher preservation potential.
Intra-arc basins
• Sedimentary basins within magmatic arcs,
between volcanoes, or between older and
younger belts of the arc
• Some are fault-bounded and subside rapidly.
Faulting due to extension within arc, or
flexure of lithosphere due to weight of
volcano.
• With time, position of the arc migrates, and
basins may change between intra-arc, backarc and fore-arc.
Sediment supply and
transport
• Sediment supply varies according to volcano
behaviour, governed by magma viscosity and
gas content.
• In deep water, explosive activity is
suppressed by hydrostatic pressure.
• More silicic magmas in more evolved arcs therefore greater explosive activity, more
supply of pyroclastic sediment.
Sediment transport and
deposition is controlled by:
• topography - both subaerial and
submarine
• volcanic processes, especially eruption
column height, direction of pyroclastic
flows
• sediment transport systems - e.g. rivers,
prevailing winds