540
subduction zones is about one fourth of the global
modem river load. In times not affected by human
activities (global sediment yield about 3 to 4
km 3 /year) about one half of the river load may have
been absorbed by subduction zones and about one
fifth was probably removed by frontal accretion and
subcrustal underplating. Long-tenri (pre-30 Ma) values for the total amount of subducted sediment appear to have been lower (around 1.0 km 3 /year) than
the post-30 Ma values. In any case, sediment
subduction explains why only small proportions of
old (>50 to 100 Ma) deep-sea sediments are preserved.
Sediment accreted by underplating and absorbed
by the mantle may have contributed to the slow
growth of the continents (in the order of 1.5 to 2.0
km 3 /year) as weIl as to the generation ofnew magma
and juvenile igneous rocks (about 1.65 km 3 /year).
11.7.4 Summary (Global Sediment Recycling)
A major part of the preserved sediment masses
of medium to young ages consists of recycled
older sediments.
- The present-day existing fraction from the initial total mass of older sedimentary rocks, deposited in a certain time unit, is defined as the
survival rate. This rate generally decreases with
the age of the initial sediment.
- The residence time or half-life of the sediments
of particular basins increases from active via
passive margin basins to intracontinental basins.
For the post-Devonian time, the average half-life
of all types of sediment was -130 Ma. After this
Chapter 11 Sediment Supply, Subsidence, and Basin Fill
The magma of the volcanic arcs seerns to be fed only
to a very small part by subducting sediment. Continents can also shrink in areas where subduction erosion is significant (global long-term average about
1.0 km 3 /year).
All these data are subjected to modifications in line with
the progress in our knowledge. Nevertheless, the presently
available data are useful for an understanding and preliminary evaluation of the processes of sediment recycling.
This processes exert a profound influence on the (changing) composition of sediments with time and they are important in studies on geochemical cycles. The latter, such
as the areal and temporal distribution of certain elements
and element ratios in turn can support, refine, or modify
our present-day knowledge about long-term sediment recycling. Basin filling and basin destruction play, in combination with tectonic processes and isostasy, a great part in the
evolution of the continents and the entire globe.
time, half of the original sediment mass was
eroded.
Easily soluble evaporites and carbonates are
destroyed faster than shales, sandstones, and
volcanogenic rocks.
The most resistant rocks (e.g. quartzitic sandstones, chert, and ironstones ) are relatively enriched in very old sequences.
A significant part of the oceanic (deep-marine)
sediments is consumed along subduction zones.
It ultimately contributes to the growth of the
continents and/or it is absorbed by the mantle.
subduction zones is about one fourth of the global
modem river load. In times not affected by human
activities (global sediment yield about 3 to 4
km 3 /year) about one half of the river load may have
been absorbed by subduction zones and about one
fifth was probably removed by frontal accretion and
subcrustal underplating. Long-tenri (pre-30 Ma) values for the total amount of subducted sediment appear to have been lower (around 1.0 km 3 /year) than
the post-30 Ma values. In any case, sediment
subduction explains why only small proportions of
old (>50 to 100 Ma) deep-sea sediments are preserved.
Sediment accreted by underplating and absorbed
by the mantle may have contributed to the slow
growth of the continents (in the order of 1.5 to 2.0
km 3 /year) as weIl as to the generation ofnew magma
and juvenile igneous rocks (about 1.65 km 3 /year).
11.7.4 Summary (Global Sediment Recycling)
A major part of the preserved sediment masses
of medium to young ages consists of recycled
older sediments.
- The present-day existing fraction from the initial total mass of older sedimentary rocks, deposited in a certain time unit, is defined as the
survival rate. This rate generally decreases with
the age of the initial sediment.
- The residence time or half-life of the sediments
of particular basins increases from active via
passive margin basins to intracontinental basins.
For the post-Devonian time, the average half-life
of all types of sediment was -130 Ma. After this
Chapter 11 Sediment Supply, Subsidence, and Basin Fill
The magma of the volcanic arcs seerns to be fed only
to a very small part by subducting sediment. Continents can also shrink in areas where subduction erosion is significant (global long-term average about
1.0 km 3 /year).
All these data are subjected to modifications in line with
the progress in our knowledge. Nevertheless, the presently
available data are useful for an understanding and preliminary evaluation of the processes of sediment recycling.
This processes exert a profound influence on the (changing) composition of sediments with time and they are important in studies on geochemical cycles. The latter, such
as the areal and temporal distribution of certain elements
and element ratios in turn can support, refine, or modify
our present-day knowledge about long-term sediment recycling. Basin filling and basin destruction play, in combination with tectonic processes and isostasy, a great part in the
evolution of the continents and the entire globe.
time, half of the original sediment mass was
eroded.
Easily soluble evaporites and carbonates are
destroyed faster than shales, sandstones, and
volcanogenic rocks.
The most resistant rocks (e.g. quartzitic sandstones, chert, and ironstones ) are relatively enriched in very old sequences.
A significant part of the oceanic (deep-marine)
sediments is consumed along subduction zones.
It ultimately contributes to the growth of the
continents and/or it is absorbed by the mantle.