Structure and Growth Faulting at the Luzhou Profile in the

In the Luzhou area immediately south of the Tanshui River, several well-documented boreholes are available east of the Guanyinkeng Fan in the basin plain (Fig. 4-4, 4-9A). The SCF-5, 6, and LC-1 are boreholes drilled by the Central Geological Survey (CGS) aimed at studying the basin deposits and the Shanchiao Fault (Lin et al., 1999; Huang et al., 2007), and the BH-1 to 9 are boreholes drilled for construction purposes (available from the CGS engineering geology data bank). The Shanchiao Fault was considered to be located between the SCF-5 and 6 boreholes (Lin, 2005) and two paleoseismic events at 11 ka and 9 ka were proposed (Huang et al., 2007). All of the boreholes are close to each other, within a one-km-long profile normal to the trend of the Shanchiao Fault (Fig. 4-9).

Situated in a geological environment similar to that at the Wuku area, analysis of basin sediments recovered in the SCF-5, 6 and LC-1 boreholes reveals analogue growth strata architecture. The basement is 125 m deep at SCF-5, 221 m at SCF-6 and

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deeper than 300 m in LC-1, meanwhile members of basin deposits including the Banchiao, Wuku and Sungshan formations are significantly thickened and down-shifted in the east (Fig. 4-10). As shown in the Wuku Profile, the Jingmei Formation and the Sungshan Formation were accumulated during the latest sea level rise and therefore better preserve the interactions between faulting and sedimentation (Chen et al., 2010). The Jingmei Formation is thicker towards the east (21.5 m, 26.5 m, 31 m at SCF-5, 6 and LC-1, respectively), probably reflecting the difference in height of the eroded basin ground composed of the Wuku Formation before the formation of the Jingmei alluvial fan; while the top horizon of the Jingmei Formation should be rather flat and regular as the surface of a broad alluvial fan should be, but now it’s much deeper in the east (112 m in LC-1 compared to 72.5 m in SCF-6 and 62.1 m in SCF-5). The Sungshan Formation on top again forms an eastward-thickening wedge and exhibits a significant westward-increasing hiatus with the underlying Jingmei Formation (radiocarbon age data summarized in Table 4-1).

The three units in the shallowest Sungshan Formation defined in the Wuku area can also be applied in the SCF-5, 6 and LC-1 boreholes due to the similar sedimentary characteristics and identical horizons, while a 4-unit scheme previously defined in SCF-5 and 6 (Huang et al., 2007) for the Sungshan Formation is difficult to be applied to the LC-1 borehole. The lower C3 unit is found between 62 and 50.4 m deep and started deposition at ~ 11 ka in SCF-5, 72.5 to 39.2 m deep starting at ~ 14 ka in SCF-6, and 111.7 to 50 m deep starting at ~ 23 ka as continuous with Jingmei deposition in LC-1 (Teng et al., 2000b). The C2 unit is present at 30.4 to 23.7 m in SCF-5, 39.2 to 29.4 m in SCF-6, and 50 to 33.5 m in LC-1; both the C2 and C1 units thicken and deepen towards the east. Mollusks are abundant in the C1 and C2 units signifying the marine presence in the Taipei Basin since 9 ka. The depths of the key horizons including the tops of the Jingmei Formation, C3 and C2 units in LC-1 are quite similar to those found in WK-1, as well as transgression records inferred from foraminifer assemblages (Huang, 2006).

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Fig. 4-9. (A) Map of the Luzhou area indicated in Fig. 4-1B, showing locations of the boreholes used in this study, the interpreted Shanchiao Fault trace (dashed lines), and geomorphic features. (B) Interpreted Luzhou geological profile consisted of three westerly branch faults and a main fault to the east, resembles a half-tulip structure.

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Fig. 4-10. Stratigraphic correlation between boreholes (SCF-5, SCF-6, and LC-1) of the Luzhou Profile. Location of the boreholes is indicated in Fig. 4-9A.

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Table 4-1: Radiocarbon age data incorporated in the Luzhou Profile.

Borehole Data

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In addition to SCF-5, 6, and LC-1, nine boreholes (BH-1 to 9) for construction use aligned NW-SE were drilled in this area. These boreholes recorded the depth variation of the Jingmei Formation top horizon, as the Jingmei Formation is the last conglomerate to be deposited in most of the Taipei Basin especially the western part.

The Jingmei Formation top horizon in the Luzhou Profile displays down-east staircase geometry with three steps (Fig. 4-11A): 7.5 m between BH-1 and 2 where a 10 m offset is also present between SCF-5 and 6, ~ 15 m between BH-4 and 5, and more than 22 m between BH-8 and 9 or up to 33 m between BH-8 and LC-1. Such spaced plunges of Jingmei Formation top depth were not likely to be present when it was deposited considering the form of the original alluvial fan.

Based on stratigraphic correlation of the Sungshan Formation it is revealed that fault offset occurs both between SCF-5 and 6 and Between SCF-6 and LC-1, with the latter containing greater slips. The offset is 4 m between LC-1 and SCF-6 and 5.7 m between SCF-6 and 5 from 8.4 ka till present, and 6.8 m between LC-1 and SCF-6 and 3.1 m between SCF-6 and 5 from 9 ka till 8.4 ka. Taking account the depth changes of Jingmei Formation top horizon in the entire Luzhou Profile, the Shanchiao Fault is made up of a main fault in the east (between BH-8 and 9) and at least two branch faults in the west (one between BH-4 and 5, another between BH-1/SCF-5 and BH-2/SCF-6), whose averaged slip rate since ~ 23 ka is around 1.5, 0.7, and 0.4 mm/yr (Fig. 4-9B). The LC-1 is situated on the hanging wall as the WK-1 in the Wuku area. The branch fault located between SCF-5 and 6 may again rooted in the basin sediment-basement interface as the sheared section at the base of basin deposits recovered in SCF-6. The existence of another sheared section at the base of basin sediment in SCF-5, as well as the deeper depths of key horizons in SCF-5 compared to those in SCF-1 of the upstream Wuku area (Chen et al., 2010), suggests that at least one more branch fault exists west of the SCF-5. As a whole, the Shanchiao Fault in the Luzhou Profile has a form of half tulip structure, containing a main fault extended from the steep basement rise east of borehole SCF-6 and three westerly branch faults developed within the loose basin sediments (Fig. 4-9B). Correlation of lahar layers in the Wuku Formation also agrees with the proposed fault configuration (Song et al., 2007). The entire Shanchiao Fault zone is therefore of several hundred meters wide, and only the two westernmost branches have corresponding fault scarps. Although none of the boreholes of the Luzhou Profile seems to be on the actual footwall of the Shanchiao Fault, exact fault vertical throw and subsidence rate can be inferred by taking the data of SCF-1 on the footwall in the Wuku Profile. The calculated tectonic subsidence rate since ~ 23 ka of the Luzhou Profile is shown in Fig. 4-11D; the hanging wall has been subsiding about 3 mm/yr since the LGM, a rate identical to that obtained in the Wuku Profile in the central portion of the fault (Chen et al., 2010). The

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detailed evolution of post-LGM tectonic subsidence in the fault zone is rather complicated as exhibited in Fig. 4-12. The westernmost branch fault west of SCF-5, probably equivalent to the one between SCF-1 and 2 in the Wuku Profile, has had a nearly constant subsidence rate of 1 mm/yr except for non-activity during the C2 deposition (9-8.4 ka). Slip was variably divided among the branch fault between SCF-5 and 6 (Huang et al., 2007) and the main fault between SCF-6 and LC-1; while the two faults combined yielded the slip rates almost identical to that on the main fault (between SCF-2 and WK-1) in the Wuku Profile. Such pattern of varying slip rate partitioning among main and branch faults suggests that fault displacement was not constantly distributed within the fault zone; one particular branch fault may be highly active in some periods or earthquake events while be dormant in other times.

Fig. 4-11. Depth variations of the Jingmei Formation top horizon and the deduced tectonic subsidence rates since ~ 23 ka in the Luzhou Profile

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Fig. 4-12. (A) Sedimentation rate and tectonic subsidence rate of the C1 and C2 units at the Luzhou boreholes. (B) Accumulative vertical slips for the Shanchiao fault since the LGM documented in the Luzhou Profile.

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4.4.3 Geological interpretation of the Shulin Profile in the southern end of the fault