12 Basin Evolution and Sediments
12.8 Pull-Apart Basins
12.8.1 General Characteristics
12.8.2 Sediments ofPull-Apart Basins
12.8.3 Examples ofPull-Apart Basins
12.8.4 The Guaymas Basin Model
12.8.5 Summary (Pull-Apart Basins)
12.8.1 General Characteristics
Many of the basin types discussed above are affected
more or less by strike-slip motions, particularly those
described in Sect. 12.7. In this section we deal with
some special features typical of pull-apart basins
which originate predominantly from this mechanism.
Pull-apart basins occur on both continental and oceanic crust in different tectonic settings:
- Along strike-slip boundary zones (transform
faults) between rigid, continental plates.
- In zones of oblique subduction along volcanic
arcs.
- In zones of "continental escape" in response to
collision (cf. Sect. 12.7.3).
Pull-apart basins are generated at the releasing bend
of transfarm faults and display a certain evolution
from a small initial stage (e.g., spindie shape) via a
"lazy-S" and rhomboidal basin to a more mature,
elongate trough stage (Fig. l2.42a). Furthermore,
there are more complicated strike-slip systems with
various basin morphologies, such as basins associated with the bifurcation of faults. Pull-apart basins
are typicaHy narrow (less than 50 km wide). Along a
widely extended strike-slip zone, several puH-apart
basins may be created simultaneously. SmaH basins
are commonly underlain by thinned continental crust,
while greater strike-slip motions lead to the formation of new oceanic crust in one or several special
troughs within the larger basins, as for example in
the Gulf of Califomia (see below). Extremely long
pull-apart basins (e.g. the Cayman trough in the Caribbean Sea) may develop a short spreading ridge
comparable to mid-oceanic ridges.
The displacement along strike-slip faults may be
accompanied either by extension (transtensian) at a
releasing bend or compression (transpressian) at a
restraining bend along the fault line in map view
(Fig. 12.42b, cf. Fig. 1.4). Transpression leads to
reverse fault separation, vertical uplift, thrusting, and
folding (Fig. 12.42b,c) and is therefore ineffective in
creating sedimentary basins of some size. Transtension, however, and simple, "sharp" puH-apart motion
form important basins with substantial sediment fills
(Fig. 12.43). The size of these basins is mainly controlled by fault separation and overlap of the master
faults. Large puH-apart basins commonly result from
simultaneous strike-slip and trans form-normal extension. Further prominent features of strike-slip faults
are "en echelon" faults and folds which may occur
within or adjacent to the principal displacement zone.
In profile, strike-slip zones show characteristics as
shown in Fig. 12.42c. The sediments may be segmented in wedge-shaped bodies with lateraHy variable facies and thickness. Both normal and reverse
separation faults may be present in the same depth
range and along the same fault at different levels.
Due to high slip rates (commonly 1-10 cm/a), subsidence of pull-apart basins is fast. The subsidence of
narrow puH-apart basins is more rapid than that of
rift basins of infinite length. In the former case, the
heat provided by the upweHing magma under the
stretched crust is laterally lost and thus subsidence
accelerated (cf. Sect. 8.3). Later, subsidence IS
mainly driven by the increasing sediment load.
A model calculation using the local Airy model for a
crustal block of 10 km width subjected to a stretching rate
of 3 cmla and a continuous sediment 10ading of 1 mlka
(Fig. 12.42d) shows that subsidence during aperiod of 0.2
Ma can reach 10 mmJa and create a 2 km deep basin
(Pitman and Andrews 1985; see also Sawyer et al. 1987).
In this initial phase the basin is sediment-starved in spite of
the high sedimentation rate assumed. Later, sedimentation
exceeds subsidence which is now only driven by sediment
loading. When the basin is filled up, the sediments have
reached a thickness of 4 to 5 km.
The Los Angeles basin deve10ped since the middle Miocene (11 Ma B.P.) and accumulated in its center a sequence
of 9-10 km in thickness (average rate of total subsidence
-1 mmJa). About 4 km result from pure1y tectonic subsidence. A pull-apart basin assumed be10w the northeastem
Nile delta subsided at a rate of -5 mmJa during the last
7000 years (Stanley 1988b). The Dead Sea basin may have
subsided since the late Pliocene at a rate of 2-3 mmJa (cf.
Sect. 11.3.6).
The empirical relationship between crustal stretching
and maximum sediment thicknesses of puH-apart bas-
12.8 Pull-Apart Basins
12.8.1 General Characteristics
12.8.2 Sediments ofPull-Apart Basins
12.8.3 Examples ofPull-Apart Basins
12.8.4 The Guaymas Basin Model
12.8.5 Summary (Pull-Apart Basins)
12.8.1 General Characteristics
Many of the basin types discussed above are affected
more or less by strike-slip motions, particularly those
described in Sect. 12.7. In this section we deal with
some special features typical of pull-apart basins
which originate predominantly from this mechanism.
Pull-apart basins occur on both continental and oceanic crust in different tectonic settings:
- Along strike-slip boundary zones (transform
faults) between rigid, continental plates.
- In zones of oblique subduction along volcanic
arcs.
- In zones of "continental escape" in response to
collision (cf. Sect. 12.7.3).
Pull-apart basins are generated at the releasing bend
of transfarm faults and display a certain evolution
from a small initial stage (e.g., spindie shape) via a
"lazy-S" and rhomboidal basin to a more mature,
elongate trough stage (Fig. l2.42a). Furthermore,
there are more complicated strike-slip systems with
various basin morphologies, such as basins associated with the bifurcation of faults. Pull-apart basins
are typicaHy narrow (less than 50 km wide). Along a
widely extended strike-slip zone, several puH-apart
basins may be created simultaneously. SmaH basins
are commonly underlain by thinned continental crust,
while greater strike-slip motions lead to the formation of new oceanic crust in one or several special
troughs within the larger basins, as for example in
the Gulf of Califomia (see below). Extremely long
pull-apart basins (e.g. the Cayman trough in the Caribbean Sea) may develop a short spreading ridge
comparable to mid-oceanic ridges.
The displacement along strike-slip faults may be
accompanied either by extension (transtensian) at a
releasing bend or compression (transpressian) at a
restraining bend along the fault line in map view
(Fig. 12.42b, cf. Fig. 1.4). Transpression leads to
reverse fault separation, vertical uplift, thrusting, and
folding (Fig. 12.42b,c) and is therefore ineffective in
creating sedimentary basins of some size. Transtension, however, and simple, "sharp" puH-apart motion
form important basins with substantial sediment fills
(Fig. 12.43). The size of these basins is mainly controlled by fault separation and overlap of the master
faults. Large puH-apart basins commonly result from
simultaneous strike-slip and trans form-normal extension. Further prominent features of strike-slip faults
are "en echelon" faults and folds which may occur
within or adjacent to the principal displacement zone.
In profile, strike-slip zones show characteristics as
shown in Fig. 12.42c. The sediments may be segmented in wedge-shaped bodies with lateraHy variable facies and thickness. Both normal and reverse
separation faults may be present in the same depth
range and along the same fault at different levels.
Due to high slip rates (commonly 1-10 cm/a), subsidence of pull-apart basins is fast. The subsidence of
narrow puH-apart basins is more rapid than that of
rift basins of infinite length. In the former case, the
heat provided by the upweHing magma under the
stretched crust is laterally lost and thus subsidence
accelerated (cf. Sect. 8.3). Later, subsidence IS
mainly driven by the increasing sediment load.
A model calculation using the local Airy model for a
crustal block of 10 km width subjected to a stretching rate
of 3 cmla and a continuous sediment 10ading of 1 mlka
(Fig. 12.42d) shows that subsidence during aperiod of 0.2
Ma can reach 10 mmJa and create a 2 km deep basin
(Pitman and Andrews 1985; see also Sawyer et al. 1987).
In this initial phase the basin is sediment-starved in spite of
the high sedimentation rate assumed. Later, sedimentation
exceeds subsidence which is now only driven by sediment
loading. When the basin is filled up, the sediments have
reached a thickness of 4 to 5 km.
The Los Angeles basin deve10ped since the middle Miocene (11 Ma B.P.) and accumulated in its center a sequence
of 9-10 km in thickness (average rate of total subsidence
-1 mmJa). About 4 km result from pure1y tectonic subsidence. A pull-apart basin assumed be10w the northeastem
Nile delta subsided at a rate of -5 mmJa during the last
7000 years (Stanley 1988b). The Dead Sea basin may have
subsided since the late Pliocene at a rate of 2-3 mmJa (cf.
Sect. 11.3.6).
The empirical relationship between crustal stretching
and maximum sediment thicknesses of puH-apart bas-