**3.3. Fault-termination basins**

**3.1. Fault-bend basins**

**3.2. Stepover basins**

of the evolution of a pull-apart basin [12].

of natural pull-apart basins (Figure 6):

depends on the angles α and β (Figure 6):

value is consistent with those of natural 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

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‐

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

The best fitting constants have been found to be c1 = 1.0 and c2 = 3.2, which yield *l*/*w* ≈ 3.2 with

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

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

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),

 b

*l w*/ 1 / tan 1 / tan = + a

<sup>1</sup> log log log *lc w c* = + (1)

(2)

ment, and the other two sides are bounded by listric or transverse faults.

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

a 95% confidence interval about the ratio of 2.4 < *l* / *w* < 4.3.

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‐ nation basins [44].

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