Chapter 4: Fault Zone Characteristics and Basin-wide Distribution of Post-Last
4.5 Distribution of Tectonic Subsidence since the Last Glacial Maximum
4.5.1 Jingmei Formation top horizon as a key marker
From growth faulting analysis performed in the Wuku (Chen et al., 2010; section 4.4.1) and the Luzhou profiles (section 4.4.2) in the central portion of the Shanchiao Fault, it’s noted that the top of the Jingmei Formation as the original Jingmei alluvial fan surface serves to be a good marker documenting tectonic subsidence succeeding its completion at about 23 ka. The Jingmei Formation top horizon is therefore considered a precious key horizon with the following advantages: 1. the Jingmei Formation was formed in a snapshot in geological sense and is widespread across the basin except in the northwestern corner; 2. the Jingmei Formation top horizon is expected to possess a regular, flat to conical geometry as the top surface of a broad alluvial fan should be; 3. it’s a distinct and easy-to-recognize lithological boundary in boreholes as the Jingmei Formation is the last conglomerate to be deposited in the basin except in the northern corner (pyroclastic deposits sourced from the Tatun volcanoes) and the southeastern part (later fan deposits from the Xindian River).
What’s more, many boreholes drilled for engineering or construction purposes in the Taipei Basin encountered the conglomerate before finishing, making the depth information of this particular horizon widely available. Taking the possible earthquake recurrence interval as 1000 years from paleoseismic investigations by Huang et al.
(2007), the Jingmei Formation top horizon has documented fault displacement over ten earthquake cycles and is thus representative of late-Quaternary fault behavior.
This work integrates the available borehole records throughout the Taipei Basin to investigate spatial variation of the Jingmei Formation top horizon and the inferred vertical deformation caused by the Shanchiao Fault.
4.5.2 Data and result
The geological drilling data incorporated in this study is acquired from the online open-source borehole data bank maintained by the Central Geological Survey, available on
http://210.69.81.70/geo/frame/gsb88.cfm. In total 520 borehole records
in the Taipei Basin reached the target horizon with clear geological information are assembled in the compilation. While some of them belong to the deep drillings carried out by the Central Geological Survey for research purpose, most of the data are coming from engineering consultation companies for construction purposes. The compilation is presented in Fig. 4-13. The values shown are meters below the (present-day) sea level corrected for the ground elevation of the drilling sites. The Jingmei Formation top horizon exhibits a sharp dive from about 30 m to over 100 m deep in about 1 km at the western border of the basin as documented in Wuku and Luzhou areas (section 4.4.1, 2; Chen et al., 2010). East of the trough the horizon92
rapidly rises to 60 m deep in 4 km in the central part of the basin, followed by a much slower ascend back to around 40 m deep at the eastern margin of the basin. The horizon is deeper or non-existent near and north of the lower reach of the Tanshui River channel. The horizon depth is also greater in the central part than in the southern part of the basin, while a slight cone-shape bulge exists in the southeast.
Despite the easy-recognition of the Jingmei Formation top horizon, some minor conglomerate layers deposited later on is known to exist along the borders of the basin, many are colluvial in origin and are thus very limited in spatial and temporal scale, while some may be originated from tributaries of the Tanshui River. When the identification is in doubt, records of nearby boreholes are cross-checked to discriminate local conglomerates from the Jingmei Formation. Nonetheless some data compiled in the eastern part of the basin is marked ‘less certain’ as a caution that the true Jingmei Formation top horizon in the area is ambiguous (Fig. 4-13).
Fig. 4-13. Basin-wide compilation of depth distribution of the Jingmei Formation top horizon. A prominent trough near the western boundary of the basin is observed and is a product of fault offset.
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4.5.3 Factors affecting the depth distribution of the Jingmei Formation top horizon
Contrary to the spatial extent and thickness distribution of the Jingmei Formation (Teng et al., 2004), the present configuration of the Jingmei Formation top horizon does not resemble its alluvial fan origin. The first order control is hold by slips of the Shanchiao Fault which produced the offset in the western basin margin and the gradual return in depth across the basin. The resultant tectonic subsidence is therefore largest and almost equal in Luzhou and Wuku areas, and then decreases along strike towards south. The deeper horizon depth found near and north of the lower reach of the Tanshui River is probably resulted from river erosion, leading to speculation that the Tanshui River has not changed its course significantly since the LGM as the associated depression is quite localized along its present channel; another possible reason is that the area is located on the outer boundary of the original alluvial fan, which might help explaining a similar northward plunging of Jingmei Formation top horizon depths south of the Keelung River course in the eastern part of the basin.Another depression in the northeast boundary of the basin near the channel of the Keelung River is also likely the result of fluvial erosion of lesser extent. The conical hump in the southeast is the sub-alluvial fan drained by the Xindian River, which was formed together with the main fan produced by the Tahan River and continued to be active beyond 23 ka till more recently (Teng et al., 2004).
The Jingmei Formation top horizon depth variation in the western to central Taipei Basin is mainly the product of tectonic deformation by activities of the Shanchiao Fault. Taking the horizon depth 29.1 m in SCF-1 in Wuku Profile as the undeformed reference on the footwall, tectonic subsidence since LGM after the deposition of the Jingmei paleo-alluvial fan around 23 ka can be inferred and calculated. The central part of the Shanchiao Fault zone is composed of at least two long, narrow extensional fault blocks with offsets about 25 and 50 m before reaching the exact hanging wall with offset more than 70 m in the Wuku-Luzhou region. Moving further away from the fault in the fault-normal Wuku-Sanchung-Taipei Profile (Fig. 4-14) the offset quickly decreases to about 30 m in the center of the basin in the Sanchong area, and is then gradually reduced to 10 m in the Xinyi-Nankang region at the eastern extreme of the basin, while most of the central Taipei City has a tectonic subsidence between 20 to 30 m. The along-strike variation of offset from the central portion to the southern portion of the fault (Fig. 4-15) is also significant and rather systematic, which drops from about 60 m to 30 – 40 m north of the Tahan River in Hsinchuang and 20 – 0 m south of the Tahan River in Banchiao. Such southward fault-parallel changes in the horizon depth may be reflecting the fault deformation but contribution from the original fan geometry, which should be higher upstream in the south with unresolved gradient, is of unknown proportion and difficult to be excluded.
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Fig. 4-14. Variations of the Jingmei Formation top horizon depth (A), and the deduced tectonic subsidence amount (B) and rate (C) since ~ 23 ka in the Wuku- Sanchung-Taipei profile normal to the fault trace across the fault.
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Fig. 4-15. Variations of the Jingmei Formation top horizon depth (A), and the deduced tectonic subsidence rate (B) since ~ 23 ka in the Wuku-Hsinchuang-Banchiao profile sub-parallel to the fault trace across the fault.
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