**2. Geological backgrounds**

Connecting Japan to the north and Taiwan to the south, the East China Sea is a marginal sea over a broad continental shelf (**Figure 1a**). The basin of the East China Sea shelf, also known as East China Sea Shelf Basin, is the largest Cenozoic sedimentary basin of the East Asia continental margin [2]. There are several sub-basins at the southern end of the East China Sea Shelf Basin (**Figure 1b**) [11]. These sub-basins were formed in Paleogene, filled with the syn-rift sedimentary sequence and covered by Neogene post-rift sedimentary sequence [1, 12].

Seismic Stratigraphic Features of the Late Miocene-Present Unconformities and Related Seismic Units… http://dx.doi.org/10.5772/intechopen.70819 63

basins. In terms of regional stratigraphy, they provide not only the thickness and the distribution of the sedimentary sequences but also the contact relationships between the sedimentary sequences, representing stratigraphic lap-out and geological unconformities. In addition to sea level change, tectonic events are generally accepted as a general cause for these seismic unconformities, indicating the basin formation mechanisms directly. In this study, we aim at analyzing the dominant tectonic events and the present-day tectonic setting of northern offshore Taiwan based on newly collected marine reflection seismic data, which reveal the

62 Seismic and Sequence Stratigraphy and Integrated Stratigraphy - New Insights and Contributions

The northern offshore Taiwan is located at the junction among the southernmost part of the East China Sea, the south-western extension of Okinawa Trough and the northern tip of the Taiwan orogenic belt (**Figure 1**). The northern offshore area of Taiwan is surrounded by different geological units highlighting that several basins have influenced its tectono-sedimentary evolution. It could be the part of the post-rift stage of the Paleogene rift basin in the East China Sea [1, 2]. It may also be proposed that it is dominated by relict back-arc basins, which were controlled by a progressive eastward migrating subduction of the Pacific plate and ended up in the present Okinawa Trough [3–5]. The progressive southward migration of the Taiwan orogenic belt could be a practical mechanism for basin formation as well [6–9]. Recently, strike-slip motion along the East Asia continental margin is considered to play a role on the basin evolution of the East China Sea [10]. A re-appraisal of which basin formation mechanism is more dominant for the Neogene basin development in the northern offshore Taiwan

This chapter provides new seismic stratigraphic information on the Neogene tectonic setting in the northern offshore Taiwan through seismic interpretation. The stratigraphic architecture of the Neogene sedimentary sequences in the northern offshore Taiwan has been reconstructed through the geological interpretation of high-resolution seismic profiles. Here, we examine two high-resolution reflection seismic profiles of different orientations northern offshore Taiwan (**Figure 2**). One of the profiles runs in NE-SW direction, showing a northward dipping sequence. The other one, on the other hand, runs in NW-SE direction, showing a series of tilted fault blocks. Most significantly, both profiles feature obvious, angular unconformities. The geological and tectonic significance of these unconformities is discussed in this

Connecting Japan to the north and Taiwan to the south, the East China Sea is a marginal sea over a broad continental shelf (**Figure 1a**). The basin of the East China Sea shelf, also known as East China Sea Shelf Basin, is the largest Cenozoic sedimentary basin of the East Asia continental margin [2]. There are several sub-basins at the southern end of the East China Sea Shelf Basin (**Figure 1b**) [11]. These sub-basins were formed in Paleogene, filled with the syn-rift sedimentary sequence and covered by Neogene post-rift sedimentary

subsurface stratigraphic features.

is thus required.

study.

sequence [1, 12].

**2. Geological backgrounds**

**Figure 1.** Geological settings and basin location in the study area. (a) Regional setting map showing the approximate location of the East China Sea, of the Okinawa Trough (thin dashed line) and of the Taiwan-Sinzi Folded Zone (thick dashed line). (b) Bathymetric map showing the location of the Paleogene rift basins identified by seismic data (backslash area) and back-arc basin identified by magnetic data (slash area), southern Taiwan-Sinzi Folded Zone (vertical bar line), and Northern Taiwan Volcanic Zone (red area and triangles. (1) Pengjia Islet, (2) Mianhua Islet, (3) Keelung Volcanic Group, (4) and (5) Kuanyinshan, (6) Tatun Volcanic Group, and (7) Tsaolingshan). The black thin lines indicate 200 m contour, and the Red thick line indicates the location of the schematic profile shown in **Figure 1c**. (c) Sketch profile across the Taiwan orogenic belt at its culminating stage The green and yellow area indicate the deformed Tertiary-present sequence, respectively.

**Figure 2.** Bathymetric map with location of seismic lines. Red lines indicate the locations of the selected seismic profiles. White circles along the red lines indicate the locations of the shot points. Blue square A indicates the location of the exploration wells.

The Okinawa Trough is a long, N-S trending that connects Japan and Taiwan (**Figure 1**). It is a back-arc basin formed by extension within the continental lithosphere landwards of the Ryukyu trench-arc system [13, 14]. The initial opening of the northern and middle part of the current Okinawa Trough occurred during the Miocene, while it delayed until the Pleistocene in the southern part of the Trough [15, 16]. It is also believed that this back-arc basin may be suited even westwards before Miocene, controlling the emplacement and the tectonic setting of the East China Sea Shelf Basins [3, 5].

The Taiwan is located between the large marginal seas of the East China Sea and the South China Sea and includes a young orogenic belt, formed by Late Miocene collisional events in the SE Eurasian continent (**Figure 1b** and **c**) [17, 18]. In addition to metamorphic basement rocks, the body of the Taiwan orogenic belt is mainly composed of metamorphic basement rocks and deformed Tertiary rift basins, including Paleogene syn-rift and Neogene post-rift sedimentary sequences [19]. Since the Taiwan orogenic belt is moving southward, different evolutionary stages of the orogenic belt features along the trending of Taiwan have been distinguished [20, 21]. The central part of the Taiwan, which is characterized by high mountain peaks of Taiwan mountain ranges, is bearing the culmination of the collisional activity (**Figure 1b**) [22, 23]. Accordingly, the northward descending topography of the mountain range represents the northward decline of the collision [6].

The Taiwan-Sinzi Fold Zone is another significant geological unit in this area (thick dashed line in **Figure 1a**; purple area in **Figure 1b**) [8, 9, 24–28]. It runs approximately along the shelf break, trending parallel to the Okinawa Trough and representing a structural high separating the East China Sea Basin to the west and the Okinawa Trough to the east. In the northern and middle part, the Taiwan-Sinzi Folded zone is characterized by structural highs of the acoustic basement, while it shows folded structures and tilted blocks in the southern part, interpreted as the earliest Taiwan orogenic belt [7, 8, 29].

Northern Taiwan area is also characterized by the Quaternary-present magmatic activities, which is also known as Northern Taiwan Volcanic Zone (NTVZ). The NTVZ is composed of several groups of the volcanic fields, including the earliest (before 2.6 Ma) Tatun Volcanic Group, Mianhua Islet, and Sekibisho; later (2~1 Ma) Tatun Volcanic Group, Pengjia Islet and Keelung Volcanic Group; and the latest (after 1Ma) Tsaolingshan, Kuanyinshan, and Kobisho (**Figure 1b**). They occur probably in association with the westernmost part of the Ryukyu Arc or in response to the extensional magmatism of the northern Taiwan mountain range [6, 30, 31].
