**3. Discussion**

The correlation results (Figure 15) show thicker sediments in the western part of the central block below the Twin Tuff bed. The thickness of sediments between the base of the upper formation and Twin Tuff bed consistently increases from section 5 to section 2, even though section 2 is located in the west of the fault separating the western and central blocks. This may show that the fault was inactive before the Twin Tuff deposition, and another fault, which is not indicated on the geologic map and is running west of the study area, was

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

Thickness of the sediments between the Twin and Pum Tuff beds is almost constant in the central block. However, the Red beds tend to be thicker to the east, and the sediments between the local flooding surface (F in Figure 15) and Exo Tuff bed tend to be thicker to the west, suggesting a temporary seesaw subsidence during the deposition between the

The sediments above the Pum Tuff bed (Figure 15) tend to be thicker in the eastern part of the central block. This thickness change and the appearance of the thicker lake depos‐ its in the eastern part clearly indicate that the depocentre was shifted in the eastern part of the basin. The seesaw subsidence seems to have ceased just before the Pum Tuff deposition. The thicker sediments to the east indicate that the fault separating the central and eastern blocks may have been formed in this phase. Note the thickness variation between the Pum and Ma Tuff beds—which tend to be thicker from section 3 to 5, but have thicker sediments in the same horizon in section 1, indicating larger subsidence around section 1 than section 3. This suggests that the fault separating the central and western

Such a seesaw subsidence pattern suggests that the study area was located on the accom‐ modation zone [3, 5] during the deposition of the lower part of the upper Nakali Forma‐ tion. The Case C fault linkage and accommodation zone proposed in [5] (Figure 1) is inferred

Seesaw subsidence was reported from the Santo Domingo Basin in the Rio Grande Rift system [3], which has been long lived from the Oligocene to Pleistocene, because of changes in the shift of the active part of the faults forming the accommodation zone. This study showed a gradual facies shift because of such long-term seesaw subsidence (Figure. 8 in [3]). In case of the Nakali Formation, the movement's scale is much smaller and shorter than the case in [3]. This seesaw subsidence may have been related to the development of the block-bounding faults, which propagated either from the south or north. Such a temporary seesaw subsidence pattern may be the typical subsidence pattern of the Case C accommodation zone (Figure 1) when the zone is incorporated into a larger basin be‐ cause of the merger of smaller basins. This result additionally suggests that the constant thickness sediments within a half-graben fill could be the consequence of the seesaw

active instead.

for this case.

Twin and Pum Tuff beds (Figure 15).

subsidence happening in a short period.

blocks also became active after the Pum Tuff bed deposition.
