**3. Strike-slip basins**

Strike-slip faults can accommodate localized compression or extension at continental margins, in island arcs, and also within continents. Sedimentary basins commonly develop where the fault kinematics are divergent with respect to the plate vector along strike-slip faults. Since the 1980s, various classifications of strike-slip basins have been formulated [4, 11, 36–40]. Common characteristics of strike-slip basins [4, 39] include: (1) elongated geometry, (2) asymmetry of both sediment thickness and facies pattern, (3) dominance of axial infilling, (4) coarser-grained marginal facies along the active master fault, (5) finer-grained main facies, (6) depocenter migration opposite to the direction of axial sediment transport, (7) very thick strata relative to the burial depth, (8) high sedimentation rate, (9) abrupt lateral and vertical facies changes and unconformities, (10) compositional changes that reflect horizontal movement of the prove‐ nance, (11) abundant syn-sedimentary slumping and deformation, and (12) rapid subsidence in the initial stage of basin formation.

**Figure 5.** Strike-slip basins at plate convergent margins. Red triangles, trench-linked; black squares, indent-linked; pur‐

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**Figure 6.** Geometrical models of (A) a spindle-shaped fault-bend basin and (B) a rhomb-shaped stepover strike-slip basin. Colored areas indicate subsiding basins. (C) Multistage evolution of stepover basins. As a result of step-wise propagation of one of the master faults, a new basin (3) is created, but the pre-existing basins 1 and 2 become inac‐ tive, resulting in a long (high *l*/*w* ratio) strike-slip basin with progressive depocenter migration. Diagrams are modified

from [42]

ple circles, plate-boundary transform faults. Numbers correspond to those in Table 1

There are many strike-slip basins along plate convergent margins (Figure 5 and Table 1). Here I classify strike-slip basins into four types, discussed in turn below.


**Table 1.** Modern and ancient (\*) examples of strike-slip basins according to the types of strike-slip faults. Numbers correspond to those in Figure 5

**2.3. Plate-boundary transform zones**

modern examples of this type (Figure 4).

in the initial stage of basin formation.

Fault-bend basin Vienna Basin1

Fault-termination basin Yinggehai Basin15

correspond to those in Figure 5

**3. Strike-slip basins**

Plate-boundary transform faults develop between two plates rotating around the poles that define the relative motion between them [9]. The San Andreas Fault, Dead Sea Fault, Alpine Fault (New Zealand), and the northern and southern margins of the Caribbean Plate are

32 Mechanism of Sedimentary Basin Formation - Multidisciplinary Approach on Active Plate Margins

Strike-slip faults can accommodate localized compression or extension at continental margins, in island arcs, and also within continents. Sedimentary basins commonly develop where the fault kinematics are divergent with respect to the plate vector along strike-slip faults. Since the 1980s, various classifications of strike-slip basins have been formulated [4, 11, 36–40]. Common characteristics of strike-slip basins [4, 39] include: (1) elongated geometry, (2) asymmetry of both sediment thickness and facies pattern, (3) dominance of axial infilling, (4) coarser-grained marginal facies along the active master fault, (5) finer-grained main facies, (6) depocenter migration opposite to the direction of axial sediment transport, (7) very thick strata relative to the burial depth, (8) high sedimentation rate, (9) abrupt lateral and vertical facies changes and unconformities, (10) compositional changes that reflect horizontal movement of the prove‐ nance, (11) abundant syn-sedimentary slumping and deformation, and (12) rapid subsidence

There are many strike-slip basins along plate convergent margins (Figure 5 and Table 1). Here

Izumi Group\*3 St. George Basin\*4 Suwa Lake5

Salan Grande Basin10

Tokushima Plain20

**Table 1.** Modern and ancient (\*) examples of strike-slip basins according to the types of strike-slip faults. Numbers

**Trench-linked strike-slip faults Plate boundary transform**

Beppu Bay17 Gulf of California18

**faults**

Ridge Basin\*6 Death Valley7

Dead Sea Basin11 Cayman Trough12 Cariaco Basin13 Salton Trough14

I classify strike-slip basins into four types, discussed in turn below.

**Indent-linked strike-slip**

**faults**

Marmara Sea2

Stepover basin Thai Basin8 Matsuyama Plain9

Malay Basin16

Transpressional basin Aceh Basin19

**Figure 5.** Strike-slip basins at plate convergent margins. Red triangles, trench-linked; black squares, indent-linked; pur‐ ple circles, plate-boundary transform faults. Numbers correspond to those in Table 1

**Figure 6.** Geometrical models of (A) a spindle-shaped fault-bend basin and (B) a rhomb-shaped stepover strike-slip basin. Colored areas indicate subsiding basins. (C) Multistage evolution of stepover basins. As a result of step-wise propagation of one of the master faults, a new basin (3) is created, but the pre-existing basins 1 and 2 become inac‐ tive, resulting in a long (high *l*/*w* ratio) strike-slip basin with progressive depocenter migration. Diagrams are modified from [42]

#### **3.1. Fault-bend basins**

Fault-bend basins result from vertical displacement of normal faults in front of releasing bends corresponding to gentle transverse **R** (synthetic Riedel) faults connected to stepped master **Y** (principal displacement) faults (Figures 1 and 6A). The basin geometry is generally spindleshaped or lazy-Z-shaped in plan view [38]. This type is considered to represent an early stage of the evolution of a pull-apart basin [12].

#### **3.2. Stepover basins**

As the master faults continue to propagate, they overlap and pull the crustal blocks farther apart, with lengthening geometries that gradually change from lazy-Z-shaped fault-bend basins to rhomboid-shaped stepover basins (Figure 6B). The basins subside by extension along strike-slip fault systems where the sense of *en échelon* segment stepping coincides with the sense of the slip (i.e., right-stepping faults have dextral displacement). The term 'pull-apart basin' was originally introduced to explain a depression in the Death Valley whose sides were pulled apart along releasing bends or oversteps of faults [41]. According to the pull-apart mechanism, two sides of the basin are bounded by faults with primarily horizontal displace‐ ment, and the other two sides are bounded by listric or transverse faults.

Stepover basins generally maintain their length/width ratio [2], as expressed by the following relationship between the length (*l*) and width (*w*) of a pull-apart basin based on the dimensions of natural pull-apart basins (Figure 6):

$$
\log l = c\_1 \log w + \log c \tag{1}
$$

propagation of the master fault, accompanied by the creation of a new stepover basin, has

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Very high *l*/*w* ratios can also result from continuous transtension leading to extreme thinning of the crust and rupture. This process induces magmatic activity, high heat flow, and then the generation of new oceanic crust that may be younger than the overlying sedimentary succes‐

Fault-termination basins are developed in transtensional stress domains at the ends of strikeslip faults where normal or oblique slip faults diffuse or splay off to terminate the deformation field [44]. If a part of a crustal block undergoes translation within the block, it results in shortening/uplift at one end and extension/subsidence at the other (Figure 7). Basins formed by such subsidence are referred to as fault-termination basins or transtensional fault-termi‐

Modern examples include the Yinggehai Basin (Song Hong Basin) along the Red River Fault zone [45], the Malay and Pattani basins in the Gulf of Thailand [46], several segmented basins in the Gulf of California [44, 47], the northern Aegean Sea [48, 49], and Beppu Bay along the

Transpressional basins tend to develop along oblique convergent margins whose subsidence results from flexural loading of the hanging-wall crust, similar to foreland basins adjacent to uplifted blocks [52–54]. Such basins are usually long, narrow structural depressions that lie

The Sumatra forearc basins are modern examples of this type. Uplift of outer arc highs bounded by trench-linked strike-slip faults may cause flexural subsidence on the forearc side and

The wide variability of strike-slip faults makes it difficult to develop a simple model of the formation of strike-slip basins and their sedimentary facies. Although the geometries of such basins depend on the amount of fault displacement, the angle and distance between overstep‐ ped faults, and the depth of detachment of the faults, the basins are generally elongate, narrow, and deep. Several representative examples of the strike-slip basins described in this section

resulted in the basins high *l*/*w* ratio (Figure 6C) [42].

sion (e.g., the Cayman Trough or the Gulf of California).

Median Tectonic Line (Figure 5 and Table 1) [50].

generate elongate wedge-shaped sedimentary basins.

show a range of basin evolutionary paths and filling processes.

**4. Examples of strike-slip basins**

**3.3. Fault-termination basins**

nation basins [44].

**3.4. Transpressional basins**

parallel to the master faults.

The best fitting constants have been found to be c1 = 1.0 and c2 = 3.2, which yield *l*/*w* ≈ 3.2 with a 95% confidence interval about the ratio of 2.4 < *l* / *w* < 4.3.

In sandbox experiments [42], a spindle-shaped basin appears in the first stage of basin evolution and is bounded by master **Y** faults and their synthetic Riedel (**R**) faults. Subsequently, antithetic Riedel (**R'**) faults replace **R** faults, leading to a rhomb-shaped basin. The *l*/*w* ratio depends on the angles α and β (Figure 6):

$$1/w = 1/\tan\alpha + 1/\tan\beta\tag{2}$$

The mean angle between **R** and **Y** faults in the experiments is *a* =*β* =30<sup>o</sup> ; that is, *l*/*w*=3.5. This value is consistent with those of natural basins.

As overlapped offsets of the master strike-slip faults propagate, basins elongate and finally become long pull-apart basins. The Dead Sea Basin, with a length of 132 km and a width of 18 km (*l*/*w*=7.2), is considered to have been formed by the coalescence of three successive and adjacent sedimentary basins whose depocenters migrated northward with time [43]. Although each sub-basin has a *l*/*w* ratio typical of a pull-apart basin (2.4, 3.3, and 2.6 from south to north), propagation of the master fault, accompanied by the creation of a new stepover basin, has resulted in the basins high *l*/*w* ratio (Figure 6C) [42].

Very high *l*/*w* ratios can also result from continuous transtension leading to extreme thinning of the crust and rupture. This process induces magmatic activity, high heat flow, and then the generation of new oceanic crust that may be younger than the overlying sedimentary succes‐ sion (e.g., the Cayman Trough or the Gulf of California).
