The Taiwan Orogen and Post-Collisional Tectonics in Northern

Taiwan is the product of active convergence between the Chinese Continental Margin and the Luzon Arc since around 5 Ma (Ho, 1986; Suppe, 1981; Teng, 1990;

Lu and Hsu, 1992; Wu et al., 1997) with a rapid convergent rate of about 82 mm/yr currently in the NW direction (Seno, 1977; Yu et al., 1997; Fig. 5-1A). Prior to the Neogene orogeny, the Chinese Continental Margin south and east of the Chinese coast was highly rifted and drifted since Eocene until Miocene due to the opening of the South China Sea (Briais et al., 1993; Lee and Lawver, 1994; Lo et al., 2000) leaving numerous rift-related normal faults and (half-) grabens whose magnitude of extension and subsidence increased eastward (Lin et al., 2003; Teng and Lin, 2004). On land the deformed and metamorphosed Chinese Continental Margin basement crops out along the eastern part of the Central Range against the accreted Luzon Arc of the Coastal Range. To the west sediments on the rifted continental margin were deformed forming the slate belt and the foothills fold-thrust belt with sequence of west-vergent thrust faults which may root into some detachments (e.g. Suppe, 1980; Yue et al., 2005;

Chen et al., 2011). These thrusts may have been affected or partially re-slipped the rift normal faults (e.g. Mouthereau and Lacombe, 2006).

The oblique nature of the convergence leads to the southward-propagation of the Taiwan orogeny (Suppe, 1981), while in the northern part of the island the developed orogen architecture is dissected by extensional structures due to the westward advance of the Ryukyu subduction system and the post-collisional orogen collapse (Suppe, 1984; Teng, 1996; Wang et al., 1999). Central and southern Taiwan are presently in full and mature collision (Angelier et al., 1986; Yu et al., 1997; Shyu et al., 2005), in contrast the northern part of the mountain belt, including the Taipei Metropolis region, is now in an extensional or transtensional tectonic setting (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 Basin where the Taipei Metropolis resides is a half-graben formed by repeated slips of the Shanchiao Fault, an active normal fault considered as the major neotectonic structures responsible for the negative tectonic inversion from compression to extension across the Taipei region, and is filled with late-Quaternary unconsolidated sediments since 0.4 Ma (Teng et al., 2001; Wei et al., 1998). The basin basement is composed of folded Oligo-Mio-Pliocene shallow marine sedimentary rocks as a fold-and-thrust belt during the earlier stage of mountain building, and also outcrops to the east and south of the basin. The late Quaternary terrestrial deposits in the Taipei basin as growth sediments of the Shanchiao Fault

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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. 5-1C). Regional geological synthesis (Teng et al. 2001) illustrates the effects reflecting the tectonic regime change. 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. 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 in the western edge of the basin along the Shanchiao Fault, which is evidenced by several hundred meters thick fluvial growth deposits with its repeated normal faulting, and the fault is interpreted to be inverted from the Hsinchuang fault, the frontal thrust in northern Taiwan during the compressional phase (Chiu, 1968; Hsieh et al., 1992). 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 (Fig. 5-2).

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Fig. 5-2. Lithostratigraphic column of the Taipei Basin deposits (After Teng et al., 1999).

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5.2.2 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; Chapter 4) close to 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). Preliminary geomorphology analysis (Chen et al., 2006; section 4.3) also reveals a series of scarps closely related to the development of the Shanchiao Fault. GPS surveys of the Taipei area showed WNW-ESE extension with a slow rate of 0.08 µstrain/yr across the fault (Yu et al., 1999a). Asymmetric tectonic subsidence related to the Shanchiao Fault across the basin was illuminated through 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 and Luzhou areas demonstrated that the fault has been constantly active in the last 23000 years and the shallow fault zone possesses half-tulip structure (Chen et al., 2010; Chapter 4). The Shanchiao Fault is therefore considered currently active (Chang et al., 1998; Lin et al., 2000), and it’s seismogenic based on the Holocene paleoseismic events proposed by Huang et al. (2007) and a possible historic earthquake in 1694 which induced severe subsidence and inundation in the Taipei Basin forming the Kanshi Taipei Lake (Hsu, 1983).

Despite the knowledge gathered mostly about the current deformation and shallow underground properties of the fault, the geometry of the Shanchiao Fault within the brittle crust has not been constrained. The unavailability of deep seismic profiling

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data and the scarcity of upper crust microseismicity across northern Taiwan (Wang et al., 2006; Wang, 2008) except in the Tatun Volcanoes north of the Taipei Basin where volcano-related seismic signals are identified (e.g. Lin et al., 2005), hamper efforts to delineate the fault plane shape at depth. From analysis of growth faulting in the fault zone a key horizon widespread in the Taipei Basin was identified which encompasses vertical tectonic offset accumulated over at least ten earthquake cycles in 23 kyr; the distribution of tectonic offset revealed by this key marker should reflect the fault geometry beneath that drives hanging wall deformation. In the following the key horizon, the Jingmei Formation top horizon, and the long-term tectonic subsidence it reveals are described. The displacement distribution across the fault is then modeled with simple elastic half-space boundary element method to figure out the best fit fault geometry by try-and-error process. The resultant upper crust fault geometry is discussed for its relations with pre-existing tectonic fabrics including syn-convergence thrusts and pre-orogen normal faults, and for the seismic hazard the geometry may imply.

5.3 Reconstruction of late-Quaternary post-Last Glacial Maximum vertical