Evolution of Sedimentation vs. Growth Faulting

Combining information on stratigraphic architecture and knowledge on sea level changes since the LGM, the fault zone evolution is depicted in five scenes shown in Fig. 3-7.

(1) Shortly before 25 ka (Fig. 3-7i)

Global and East Asian eustatic sea level plunged to more than 100 m below the present level since 30 ka, as the LGM began (Yokoyama et al., 2007). The sea level around Taiwan stabilized at approximately -140 m (level a) from about 28 ka compared to the present day. The 25-ka paleo-topography as to of the Wuku formation can be estimated by adding the present day -140.8 m deep with 75.6 m of total vertical subsidence due to faulting with our back-stripping method, as described above. We obtain a depth roughly at -65 m for the top of Wuku formation along the Wuku profile (Fig. 3-7i). It means that the Wuku formation was about 75 m high above the sea level at that time. As a result, sediments of the Wuku formation were subject to strong erosion by drainage system in the Taipei basin.

The relatively high local topography 75 m above sea level indicates that no major channels, neither the Tanshui River nor the creeks originated from the Linkou Tableland in the west, existed in the investigated profile, which would form gorge-like incised valleys. The shallow basement at SCF-1 was exposed without cover of late-Quaternary deposits and thus was subject to be weathered (Fig. 3-7i).

Surface scarps resulted from slips on the Shanchiao Fault were likely to be rapidly erased or retreated westward on topography by intense erosion, obscuring attempts to estimate fault activity during this period.

(2) 25 ka to 23 ka (Fig. 3-7ii)

The Jingmei sediments were interpreted to be quickly deposited when the Tahan River was captured into the Taipei Basin during 25-23 ka (Teng et al., 2004a). These alluvial fan conglomerates accumulated within a time span of 2000 year and attained

~30 m thick in the Wuku region. Top of the Jingmei conglomerates should be rather flat by the time deposition ceased in the Wuku Profile because sediment transport was normal to the profile towards the north and no major channels were present. The gravel bed of the Jingmei Formation is 10 m thick onlapping onto the weathered Mio-Pliocene basement at SCF-1, implying an ancient topographic scarp located between SCF-1 and SCF-2. However, we interpret that the Shanchiao Fault was inactive during this 2000-yr-long period because the Jingmei Formation is of uniform thicknesses (i.e. 30 – 33 m) across the main fault between SCF-2 and WK-1.

On the other hand, during 25 ka to 23 ka, the sea level did not change significantly,

52

thus remained about -140 m compared to the present level. So that the top of the Jingmei formation lies about -35 m compared to the present, that is, about 105 m high above the sea level.

(3) 23 ka to ~12 ka (Fig. 3-7iii)

We anticipate that the Taipei basin started to subside along the Shanchiao Fault after completion of the Jingmei conglomerate with probably both the hanging-wall block and the extensional fault block displaced downward. However, braid plain to floodplain sediments of 23-12 ka (lower part of C3 Unit of the Sungshan Formation) exist only at WK-1. It implies that during this 10000-year-long period the hanging wall had subsided enough to accumulate braid plain and flood plain fluvial growth sediments of while the Jingmei gravel would be exposed in the footwall and the extensional fault block region. In the meantime, sea level rose rapidly from -140 m (level a) to -55 m (level b) from ~18 ka to 12 ka (Fig. 3-5) during this period. So that the elevation of the basin floor decreased drastically from 105 m to 10-20 m high above the sea level from 23 ka to 12 ka.

(4) ~12 ka to 10 ka (Fig. 3-7iv)

The situation of next 2000 years in 12-10 ka was similar to the previous period as the sea level rose rapidly, except that the growth strata of fluvial deposits (middle part of Unit C3 of the Sungshan Formation) started to lap on to the area of the extensional fault block between the main and branch faults. This suggests that the subsidence in the extensional fault block region allowed fluvial sediments to accumulate. Sea level rose from -55 m (level b) to about -37 m (level c) during this period, while it still kept slightly lower than the basin ground.

(5) 10 ka to 8.4 ka (Fig. 3-7v)

The start of this period marked the onset of major marine incursions in the basin (Teng et al., 2000b) as sea level gradually rose from -37 m (level c) to -16 m (level e) from 10 to 8.4 ka. Sediments began to pile up across the entire section, and the accommodation space was filled contemporaneously with the rising sea level. Since then the regional topography should be very similar to the flat one today, with less than 5 meters of difference in elevation between the borehole sites. The top horizons of C3 and C2 units, the estuarine deposits during that time, are therefore assumed to be similar in elevation along the Wuku profile in the three boreholes at around 9 ka and 8.4 ka, respectively. Accordingly, while the sea level continued to rise from level d to level e from 9 to 8.4 ka (Fig. 3-7v), we estimate that an extraordinarily rapid sedimentation of Unit C2 with 6.8, 10.1, and 17.4 m thickness at SCF-1, SCF-2, and WK-1, respectively. In the mean time, the Shanchiao Fault slipped vertically 3.3 m on the branch fault and 7.4 m on the main fault during the 600-year interval when C2

53

rapidly deposited in a relative deep, lower estuarine environment.

(6) 8.4 ka to present (Fig. 3-7vi)

From 8.4 ka till present, as the sea level continued to rise from level e (-16 m) to present level and coupled with tectonic fault movement, the estuarine deposits of the C1 unit piled up and completed the uppermost sediments of the Taipei basin in the Wuku profile. The extensional fault block has been observed to subside (relative to the footwall) 7.8 meters and additional 11.3 meters for the hanging wall of the main fault.

54

Fig. 3-7. Interpreted sedimentation and growth faulting and post-LGM development history of the Shanchiao Fault zone in the Wuku profile. Several geological events with confident age controlled were adopted to divided the period into six stages: (i) shortly before the deposition of the Jingmei formation (25ka), (ii) deposition of the Jingmei conglomerates (25 – 23 ka), (iii) shortly before the sea level rising to reach the basin ground level (23 – 12 ka), (iv) the sea level beginning to surpass the basin ground level (12 – 10 ka), (v) basin-wide sedimentation containing a 600-year time span with possible earthquake events coupled with rapid sedimentation (9 – 8.4 ka), and (vi) the latest stage of the sedimentation and growth faulting with gradual sea level rise and stabilized (8.4 – 0 ka). Levels a to e are denoted in Fig. 3-5.

55