Geology of the Taipei Basin

Taiwan results from active convergence between the Chinese Continental Margin on the Eurasian Plate and the Luzon Arc on the Philippine Sea Plate since around 5 Ma (Suppe, 1981; Teng, 1990; Wu et al., 1997) with a rapid convergent rate of about 82 mm/yr in the NW direction (Seno, 1977; Yu et al., 1997; Fig. 4-1A). This oblique

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convergence leads to the southward-propagation of the Taiwan orogeny (Suppe, 1981).

While central and southern Taiwan are presently in full and mature collision (Angelier et al., 1986; Yu et al., 1997; Shyu et al., 2005), the northern part of the mountain belt, including the Taipei region, is now in a state of post-collision with an extensional or transtensional tectonic setting rather than a compressional one (Teng, 1996; Hu et al., 2002), as evidenced by the presence of Quaternary extensional structures (Lee and Wang, 1988; Lee, 1989; Lu et al., 1995), extensional earthquake focal mechanisms (Yeh et al., 1991; Kao et al., 1998), and GPS displacement fields (Yu et al., 1997; Rau et al., 2008; Lin et al., 2010). The Taipei half-graben of Quaternary deposits is therefore formed in a close association with down-dip slips on the Shanchiao Fault, which is considered as the major neotectonic structures responsible for the negative tectonic inversion from compression to extension across the Taipei region (Teng et al., 2001; Fig. 4-1).

The flat low-lying Taipei Basin, a triangular-shaped half-graben filled with late-Quaternary fluvial deposits since c. 0.4 Ma (Wei et al., 1998; Teng et al., 2001), developed on top of the folded Oligo-Mio-Pliocene shallow marine sedimentary rocks, as a fold-and-thrust belt during the earlier stage of mountain building. The late Quaternary terrestrial deposits in the Taipei basin form an asymmetric sedimentary wedge: reaching a maximum depth of about 700 m near the western margin and rather drastically becoming thinner toward the east and south (Fig. 4-1C). These unconsolidated deposits are divided into four major lithostratigraphic units (Teng et al., 1999). From bottom to top, they are: (1) the Banchiao Formation: consisting of intercalated fluvial sands, muds and conglomerates, with occasional pyroclastics and topped by thick laminated mud, with maximum thickness of 380 m with ages ranging from 250 to 400 ka; (2) the Wuku Formation: consisting of fluvial sands and conglomerates, with minor mud and lateritic conglomerates, reaching a maximum thickness of c. 160 m with ages ranging from 80 to 250 ka; (3) the Jingmei Formation:

composed of lateritic fluvial (alluvial-fan) conglomerates with an utmost thickness of 50 m. These particular conglomerates layers are interpreted as the products when the Tahan River was captured into the Taipei basin at 25 to 23 ka (Teng et al., 2004a); (4) the Sungshan Formation, composed of estuarine interbedded sand-mud deposits with a thickness of 50-100 m, was deposited from 23 ka till present (Teng et al., 2000b;

Chen et al., 2010). The basin deposits exhibit prominent lateral facies changes with frequent pinch-outs. However, the widespread lateritic gravels of the Jingmei Formation and the laminated muds in the upper Banchiao Formation (Teng et al., 2004b) usually serve as basin-wide marker beds (Teng et al., 1999).

Geological evolution of the Taipei Basin was proposed by Teng et al. (2001), based

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on interpretation of regional geology. While the Plio-Pleistocene orogeny of mountain building reached its climax in northern Taiwan in Quaternary, the Paleo-Tanshui River, the major river in the Taipei Basin, provided sediments to produce the Linkou fan-delta around the ancient mountain front (Chen and Teng, 1990). Accompanying the waning of compression in the northernmost Taiwan during the middle to late Quaternary (Lee and Wang, 1988) is the vigorous eruptions of the Tatun volcanoes to the north of the Taipei Basin (Wang and Chen, 1990; Song et al., 2000), closely related to the onset of regional extension. Subsidence along western margin of the Taipei basin, as evidenced by several hundred meters thick fluvial deposits, is interpreted to result from the repeated normal faulting of the Shanchiao Fault as inverted from the Hsinchuang fault, an ancient frontal thrust in northern Taiwan (Chiu, 1968; Hsieh et al., 1992). The extensional tectonics turned the Taipei area from rugged mountains gradually into a sediment-receiving basin, and accumulation of fluvial and lacustrine sediments started at about 0.4 Ma (Wei et al., 1998; Teng et al., 2001). Since then the Taipei Basin has kept expanding due to erosion and continual asymmetric subsidence along the Shanchiao Fault in the western edge of the basin.

Under the combining influences of sea level fluctuations, volcanic activities, drainage system changes, and tectonic processes, the basin was infilled with various types of sediments, including alluvial, lacustrine, marine and pyroclastic deposits, up to 700 m thick as mentioned above.

4.2.2 The active Shanchiao Fault

The Shanchiao Fault was mapped (Chang et al., 1998; Lin et al., 2000; Lin, 2001;

Huang et al., 2007; Chen et al., 2004, 2006) along the topographic boundary between the Linkou Tableland and the Taipei Basin, sub-parallel to the Hsinchuang Fault (Lin, 2001; Teng et al., 2001), with features indicating that the steeper Shanchiao normal fault may merge into the Hsinchuang thrust fault at depth (e.g. Hsieh et al., 1992).

Following the late-Quaternary tectonic inversion, tectonic subsidence from down-dip slips on the Shanchiao Fault led to formation and development of the Taipei Basin.

Left-lateral transcurrent motion together with clockwise block rotation is also present along the Shanchiao Fault, based on studies on regional structural geology, paleomagnetism, and GPS measurements (Lu et al., 1995; Lee et al., 1999; Rau et al., 2008).

Many efforts have been made to characterize this active fault. Shallow reflection seismic profiling across the Shanchiao Fault imaged vertical offsets of Holocene sediments at shallow depth, although the location of the main fault remains questionable (Wang and Sun, 1999; Shih et al., 2004). GPS surveys of the Taipei area showed WNW-ESE extension with a slow rate of 0.08 µstrain/yr across the fault (Yu

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et al., 1999a). Asymmetric tectonic subsidence related to the Shanchiao Fault across the basin was illuminated through recent study on 30-year-long levelling data (Chen et al., 2007) as well as recent InSAR data (Chang et al., 2010). Huang et al. (2007) correlated stratigraphy of three sets of boreholes, and proposed three paleoseismic events during the Holocene (i.e. at 8500, 9200, and 11100 years b.p., respectively).

Radon and helium anomalies in soil-gas along the fault zone were documented (Walia et al., 2005) indicating the presence of possible active faults and deep fracture- advection system. Growth faulting analysis at the central portion of the fault in Wuku area demonstrated that the fault has been constantly active in the last 23000 years and the shallow fault zone seems to possess half-tulip structure (Chen et al., 2010).

Preliminary geomorphology analysis (Chen et al., 2006) also revealed a series of scarps closely related to the development of the Shanchiao Fault. The Shanchiao Fault is therefore considered currently active (Chang et al., 1998; Lin et al., 2000) while our knowledge toward it is still limited.

In the following the detailed topographic analysis in the fault zone is first presented, and then what have been found in the Wuku Profile at the central of the fault are demonstrated to be extended along strike and across the basin to illuminate fault zone structure and tectonic subsidence. The results shed light on basic information including the location of the fault zone and fault displacement pattern, as well as insights on rheological controls on shallow fault zone structure and the surface imprint of active faulting.

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Fig. 4-1. (A) General tectonic framework of Taiwan. A: Coastal Range; B: Backbone Range; C: Hsueshan Range; D: Western Foothills; E: western Coastal Plain. (B) Simplified geology of the Taipei area. Four geological domains are defined in the Taipei area as indicated by different colors shown in the legend. The thick red lines are the Shanchiao Fault traces (Chen et al., 2006). Thin black lines within the Taipei Basin are the basement depth contour of 100 m interval (adapted from Teng et al., 2001). The areas shown in Figs. 4-2, 4, 6, and 9 are marked. (C) Geological cross section of the Taipei Basin (modified from Teng et al., 1999).

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