**5. Discussion and conclusion**

#### **5.1. Age of U1 and U2**

In our seismic data, the seismic boundaries U1 and U2 are remarkable unconformities, which show similarity in the truncation relationship with underlying strata. The ages of the unconformities U1 and U2 formation are thus significant, revealing more information on regional tectonic events. Based on drilling results [8], the strata overlying the unconformity U1 were dated back to approximately Pleistocene and the strata beneath the same unconformity were dated back to Late Miocene [8]. Accordingly, the unconformity U1 bears a hiatus of Late Miocene-Quaternary age, while there is still no available drilling information reporting the existence of the unconformity U2 up-to-date. In this way, the age of the unconformities U2 may be alternatively proposed by additional local Late Miocene-present tectonic events.

The Quaternary magmatism is also a significant feature in post-collisional environment in northern Taiwan, resulting in the NTVZ [30]. The ages of the NTVZ have been dated, mainly distributing from 2.8 to 0.2 Ma. During this period, three age groups of before 2.6 Ma, 2~1 Ma, and after 1 Ma have been identified [30]. We note that the first age group may represent the earliest signals of the post-collisional state [30]. Among the three groups, the second age group is mainly composed of the volcanic edifice formed at the Pengjia Islet (2.1 Ma), the Tatun Volcanic Group (1.5 Ma), and the Keelung Volcanic Group (1.4 Ma). We consider that, the formation of the unconformity U2 and deposition of the SU II are likely simultaneous to the second age group of the NTVZ [8]. Another point we note is that most of the NTVZ events may cease no later than 0.2 Ma [30], suggesting that 0.2 Ma may be a critical timing of tectonic environment change in the NTVZ area. Collectively, we suggest the unconformities U1 and U2 may represent the age of 1.5 Ma and 0.2 Ma, respectively.

#### **5.2. Late Neogene basin of north offshore Taiwan**

**Figure 7.** Selected seismic sections showing the termination feature of the U1 and U2. See **Figure 5** for locations.

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

The formations of the unconformities U1 and U2 shall be related to the tectonic development of the Late Miocene-present sedimentary basin of northern offshore Taiwan, which has been considered as post-rift basin [1, 2], back-arc basin [3–5], post-collisional basin [6–8], and strikeslip basin [10]. Among the four competing models, the strike-slip motion has been proposed for long time, as it may be caused by the formation of the East China Sea Shelf Basin or the opening southern Okinawa Trough [9, 10]. However, the shape of the Late Neogene basin does not present as a rhombic shape reflecting strike-slip pull apart basins. In addition, the strike-slip structures have not been fully supported by mapping result from marine reflection seismic survey. The post-rift basin model is proposed as northern offshore Taiwan being considered as an extension of the East China Sea Shelf Basin. While in our study area, the post-rift shall have been ceased since the activities of both Taiwan orogenic belt to the south and the back-arc extension of the Okinawa Trough to the southeast are more immediate and are more likely to dominate. On the basis of the geographical connection of our study area with the Okinawa Trough and the Taiwan mountain range, the rest back-arc basin and post-collisional basin models are better candidates for the Late Neogene basin formation.

Our reflection seismic data show that U1 is an unconformity that truncated the underlying fault blocks and the tilting strata. We consider that those tilting strata may indicate a drastic uplift event prior to the formation of the unconformity U1. As a result, the back-arc basin model may be less likely to ascribe for strong tectonic uplift, once the back-arc basin is more dominated by crustal extension. In this way, we tend to agree with the post-collisional basin model. We suggest that the unconformity U1 represents an age mark of the post-collisional stage that started to receive sediments; the SU I were the strata deposited before the mountain collapse, and the SU II were deposited probably after the post-collisional basin had formed. The thickness variation of the SU II depends on the distribution of faults, indicating that SU II may be formed in association with fault-controlled subsidence [36, 37, 38]. The unconformity U2 shall be a regional tectonic event later in the Quaternary. Based on its erosional and relatively flattening feature upon tilting fault blocks, we propose that the unconformity U2 may be related to the cessation of the fault blocks rotation and change of regional subsidence rate. The thickness of the SU III does not vary greatly, indicating that the fault-controlled subsidence was followed by a relatively stable, probably a thermal-controlled subsidence [36, 37, 38].

#### **5.3. Tectonic evolution and implication of post-collisional basin**

**Figure 8** is a sketch cartoon showing the development of the unconformity U1 and U2 and of the related seismic units in the post-collisional tectonic setting in northern offshore Taiwan. After reaching the culmination of orogenic activities in northern offshore Taiwan (~2.6 Ma; **Figure 8A**), the post-collisional magmatism was also about to begin. Afterward, the mountain range began to collapse, leaving the unconformity U1 at 1.6 Ma and SU II (**Figure 8B**). The initial subsidence was fault-controlled and was likely to be dominated by the rotation of the fault blocks, reflecting a mechanical stretching typical of early stages of rifting basin development [36, 37, 38]. The fault-controlled subsidence and volcanic activities may cease at late Quaternary (~0.2 Mal **Figure 8C**), resulting in a change in regional subsidence rate, a horizontally distributed unconformity U2, and a relatively even-thick SU III. In this model, the lowermost seismic unit SU I is a pre-Late Miocene sequence, representing the main body of collapsed imbricated thrusts. The middle seismic unit SU II caused by probably more rapid, fault-controlled subsidence indicates the depositional sequence of early stage mountain collapse. The uppermost seismic unit SU III, on the other hand, resulted from slower, probably thermal-control subsidence and indicates the depositional sequence of late stage of mountain collapse.

Seismic Stratigraphic Features of the Late Miocene-Present Unconformities and Related Seismic Units… http://dx.doi.org/10.5772/intechopen.70819 73

**Figure 8.** A schematic cartoon showing the development of the U1 and U2, with the tectonic scenario of post-collisional in northern Taiwan. Blue rectangle indicates the sites referring to our study area. We interpret the U1 and U2 indicate the onset and cessation of fault-controlled subsidence at 1.5 Ma and 0.2 Ma, respectively.

Also in this model, the volcanic magmatism of the NTVZ may be reducing since unconformity U2 appears to indicate the cessation of active extension. However, previous geophysical observations have shown that in addition to siliciclastic sediments, andesitic intrusions were also found at several places offshore northern Taiwan [39]. Recent observations about volcanic activities suggest that there are even younger, Holocene volcanic events in the NTVZ area, and hydrothermal activities in the Tatun Volcanic Group area are still very common at present [40, 41]. New seismological evidence clearly indicates that a deep magma reservoir is beneath Taipei [42]. Also, increasing active submarine volcano, gas plumes, and topography lineaments have been identified in the offshore area [43–45]. To sum up, investigation on whether the volcanic activities are still potential threatening geohazards, along with the geophysical and geochemical monitoring on present-day activities, remains important. We wish that the better understanding of the unconformities formation and relevant stratigraphic architectures will provide a good insight into Late Pleistocene-Holocene volcano-tectonic evolution of the NTVZ.
