台灣地震地研究-台灣西南部活斷層研究─台灣西南部活斷層的大地構造分析

The continental margin to the east and south of China comprises an active
marginin the East China Sea, a collision mountain belt in Taiwan, and a passive
margin in the South China Sea. These three segments were generally regarded as
separate tectonic entities and their interrelations have long been the subject of debate.
Here we synthesize available information to outline the tectonic and geological
background of the China margin, examine the link between Taiwan and the
neighbouring China margins, and thereby establish a Cenozoic evolutionary model.
The China margin is floored with a pre-Cenozoic continental basement covered
with an up to 10-km-thick pile of Cenozoic sedimentary strata. The continental
basement has been invariably stretched and molded into a series of northeast-trending
horsts and grabens. Except in the Okinawa Trough of the East China Sea, the
Cenozoic sedimentary cover typically exhibits a two-tier tectonostratigraphic structure,
with narrow Palaeogene rift basins draped by a blanket-like Neogene-Quaternary
sequence. The two-tier structure prevails in the entire inner part of the China margin,
including the Taiwan Strait off western Taiwan. In the outer China margin, however,
the two-tier structure persists only in the South China Sea, and is in stark contrast with
the collisional orogen of Taiwan and the Ryukyu arc of the East China Sea.
By untangling the contractional deformation of the northern Taiwan mountain
belt, it has been possible to reconstruct a pre collisiona tectonostratigraphic section
with a distinctive two-tier structure shown by a Palaeogene half-graben covered with
a Miocene drape sequence. When put together with Palaeogene rift basins of the
Taiwan Strait, it becomes clear that the precollisional continental margin of Taiwan
resembles that of the South China Sea, characterized by two lines of Palaeogene rift
basins. Hence before the collision started in Late Miocene times, Taiwan was part of
the passive South China margin that extended northward to the southern Ryukyu area.
Ever since the end of the Cretaceous, the China continental margin has been
dominated by extensional tectonics, regardless of the presence or absence of
subduction zones. In the Early Cenozoic, extensive crustal attenuation resulted in
region-wide subsidence and formation of rift basins. Extension in the South China Sea
culminated in Late Oligocene times, when part of the outer margin was drifted away
by the opening ocean basin. In the East China Sea, the margin remained intact and
became separated from the South China Sea margin by a trans-form fault. From the
Miocene onward s, the South China Sea margin has been passively subsiding, sporadically punctuated with basaltic volcanism. In the East China Sea margin, the
Okinawa Trough has opened and the Ryukyu volcanic arc thrived. The NE edge of
the South China Sea margin was deformed as the Taiwan orogen.