8.4 Subsidence Related to Tectonic Loading, Subduction, etc.
413
the site investigated. Proximal sites close to the former oeean or remnant basin experienee higher subsidenee than distal, eratonward areas (Fig. 8.l8b,
Venetian basin). With the approaehing overthrust
belt (syn-molasse stage, cf. Fig. 1.3), subsidence aeeelerates and may operate at very high rates in the
proximal part of the foredeep (total subsidence ;:::300
mlMa) for a short time interval.
As soon as plate eollision is eomplete and eonvergenee comes to an end, the downgoing plate is not
further subjeet to inereasing loads. Consequently,
subsidenee slows down quickly. In the northem foreland basin of the Alps, rapid subsidenee is followed,
after a very short time interval, by uplift (Fig. 8.19b).
This may be explained by crustal rebound due to
isostatie adjustment of the thiekened erust, by removal of the negative thermal anomaly built up in the
lithosphere under overthrusting (Kominz and Bond
1986), as weIl as by erosional unloading of the
emerged, high-relief frontal overthrust and fold belt.
8.4.2 Subduetion-Related Are-Treneh Systems
As in foreland basins, fIexure of the lithosphere in
eonjunetion with subduetion is the main proeess in
generating deep-sea trenches (cf. Fig. 1.2). In this
ease, however, the erust is oeeanie and mueh thinner
than eontinental erust. The subdueting oeeanie plate
is underthrust beneath an aeeretionary prism and an
island are or eontinental margin (cf. Seet. 12.5). The
vertieal load of these overriding bodies varies with
the dip angle of subduetion and the land ward inereasing thiekness of the aeeretionary prism and magmatie
are. In addition, lateral forees exerted by eonvergenee and the subduetion rate play some role, at least
in parts of the are-treneh system.
The seaward and eentral portions of an aeeretionary prism, as weIl as the magmatic are, tend to
gain in elevation with time, while the foreare and
backare regions subside. The depth of the treneh
fIoored by the subducting oeeanie plate may be eomparatively stable through time due to the establishment of steady-state eonditions, i.e., a kind of balance between ineoming and aeereting sediment (Seet.
12.5.2).
In the foreare region, seaward parts of the basin
fill may rest on deformed sediments of the aeeretionary wedge (eomposite foreare basins, cf. Seet.
12.5.3). The subsidenee of backarc regions varies in
relation to the lack or oeeurrenee of backare spreading and overthrusting of the magmatie are. Consequently, both the total amount and rate of subsidenee
are very variable in foreare and backare basins, but
they may reaeh high values during their evolution.
During a late stage in ocean closure, the subdueting
oeeanie erust may be replaeed by a thinned eontinental (intermediate ) erust supporting the tendeney to
uplift.
It is evident from these points that the subsidenee
histories of these settings are variable and problematie to simulate.
8.4.3 Pull-Apart Basins
Strike-slip movements eommonly operate at high
rates, but pers ist only for limited time periods. Partieularly in transtensional systems, they eause significant crustal extension or the opening of erustal
"gaps" whieh ean be filied by upwelling magma. The
subsidenee history of these basins therefore resembles to some extent that of rift basins. The early stage
of basin formation is aeeompanied by rapid subsidenee whieh, due to lateral heat loss, may be even
higher than that of rift basins. In this case, subsidenee ean be modeled in a way similar to that deseribed in Seet. 8.3 .1 (cf. Sect. 12.8).
However, many puIl-apart basins show a eomplex
subsidenee history with more than one pulse of rapid
subsidence. In addition, subsidenee may be followed
by aperiod of uplift.
These complications have been described from the onshore
Tertiary pull-apart basins in California, such as the Los
Angeles, Sacramento, Salton Trough, Ventura, and other
basins (Dickinson et al. 1987; Mayer 1987). Changes in
subsidence trends are probably caused by the transition
from transtensional to transpressional defonnation (cf.
Sect. 12.8), but other processes such as volcanism associated with wrench faulting rnay also playapart.
8.4.4
Summary (Subsidenee Caused by
Teetonie Loads, ete.)
Orogenie basins subjeeted to tectonic loading
(renmant and foreland basins, many backare
basins) are eharaeterized by eonvex-up seetions in their subsidenee eurves. This is in eontrast to rift basins, puIl-apart basins, passive
eontinental margin and oeeanie basins whieh
show a eoncave-up shape of their subsidenee
eurves indieating exponential deeay.
- In the later stages of their evolution, however,
the subsidence of orogenie basins slows and is
frequently followed by uplift.
- Episodie thrusting andlor strike-slip, voleanie
events, and variations in the fIexural behavior
of the erust involved ean lead to irregular,
poorly predietable subsidenee eurves.
413
the site investigated. Proximal sites close to the former oeean or remnant basin experienee higher subsidenee than distal, eratonward areas (Fig. 8.l8b,
Venetian basin). With the approaehing overthrust
belt (syn-molasse stage, cf. Fig. 1.3), subsidence aeeelerates and may operate at very high rates in the
proximal part of the foredeep (total subsidence ;:::300
mlMa) for a short time interval.
As soon as plate eollision is eomplete and eonvergenee comes to an end, the downgoing plate is not
further subjeet to inereasing loads. Consequently,
subsidenee slows down quickly. In the northem foreland basin of the Alps, rapid subsidenee is followed,
after a very short time interval, by uplift (Fig. 8.19b).
This may be explained by crustal rebound due to
isostatie adjustment of the thiekened erust, by removal of the negative thermal anomaly built up in the
lithosphere under overthrusting (Kominz and Bond
1986), as weIl as by erosional unloading of the
emerged, high-relief frontal overthrust and fold belt.
8.4.2 Subduetion-Related Are-Treneh Systems
As in foreland basins, fIexure of the lithosphere in
eonjunetion with subduetion is the main proeess in
generating deep-sea trenches (cf. Fig. 1.2). In this
ease, however, the erust is oeeanie and mueh thinner
than eontinental erust. The subdueting oeeanie plate
is underthrust beneath an aeeretionary prism and an
island are or eontinental margin (cf. Seet. 12.5). The
vertieal load of these overriding bodies varies with
the dip angle of subduetion and the land ward inereasing thiekness of the aeeretionary prism and magmatie
are. In addition, lateral forees exerted by eonvergenee and the subduetion rate play some role, at least
in parts of the are-treneh system.
The seaward and eentral portions of an aeeretionary prism, as weIl as the magmatic are, tend to
gain in elevation with time, while the foreare and
backare regions subside. The depth of the treneh
fIoored by the subducting oeeanie plate may be eomparatively stable through time due to the establishment of steady-state eonditions, i.e., a kind of balance between ineoming and aeereting sediment (Seet.
12.5.2).
In the foreare region, seaward parts of the basin
fill may rest on deformed sediments of the aeeretionary wedge (eomposite foreare basins, cf. Seet.
12.5.3). The subsidenee of backarc regions varies in
relation to the lack or oeeurrenee of backare spreading and overthrusting of the magmatie are. Consequently, both the total amount and rate of subsidenee
are very variable in foreare and backare basins, but
they may reaeh high values during their evolution.
During a late stage in ocean closure, the subdueting
oeeanie erust may be replaeed by a thinned eontinental (intermediate ) erust supporting the tendeney to
uplift.
It is evident from these points that the subsidenee
histories of these settings are variable and problematie to simulate.
8.4.3 Pull-Apart Basins
Strike-slip movements eommonly operate at high
rates, but pers ist only for limited time periods. Partieularly in transtensional systems, they eause significant crustal extension or the opening of erustal
"gaps" whieh ean be filied by upwelling magma. The
subsidenee history of these basins therefore resembles to some extent that of rift basins. The early stage
of basin formation is aeeompanied by rapid subsidenee whieh, due to lateral heat loss, may be even
higher than that of rift basins. In this case, subsidenee ean be modeled in a way similar to that deseribed in Seet. 8.3 .1 (cf. Sect. 12.8).
However, many puIl-apart basins show a eomplex
subsidenee history with more than one pulse of rapid
subsidence. In addition, subsidenee may be followed
by aperiod of uplift.
These complications have been described from the onshore
Tertiary pull-apart basins in California, such as the Los
Angeles, Sacramento, Salton Trough, Ventura, and other
basins (Dickinson et al. 1987; Mayer 1987). Changes in
subsidence trends are probably caused by the transition
from transtensional to transpressional defonnation (cf.
Sect. 12.8), but other processes such as volcanism associated with wrench faulting rnay also playapart.
8.4.4
Summary (Subsidenee Caused by
Teetonie Loads, ete.)
Orogenie basins subjeeted to tectonic loading
(renmant and foreland basins, many backare
basins) are eharaeterized by eonvex-up seetions in their subsidenee eurves. This is in eontrast to rift basins, puIl-apart basins, passive
eontinental margin and oeeanie basins whieh
show a eoncave-up shape of their subsidenee
eurves indieating exponential deeay.
- In the later stages of their evolution, however,
the subsidence of orogenie basins slows and is
frequently followed by uplift.
- Episodie thrusting andlor strike-slip, voleanie
events, and variations in the fIexural behavior
of the erust involved ean lead to irregular,
poorly predietable subsidenee eurves.