Active Deformation Front Delineated by a Blind Thrust
Underneath the Coastal Plain, Southwestern Taiwan
Chih-Cheng Barry Yang; Wen-Shan Chen (Department of Geosciences, National
Taiwan University); Leh-Chyun Wu (Department of Geology, Chinese Culture
University); Chii-Wen Lin (Central Geological Survey, MOEA)
We discovered a blind thrust underneath the Chianan coastal plain, which may
indicate the deformation front of the Taiwan fold-and-thrust belt, the tectonic entity created
by arc-continent collision between Eurasian continent and Philippine Sea plate. Based on
strata investigation, drainage analysis and vertical rate calculation, it is obvious that part of
the original Chianan coastal plain has been involved into the Taiwan orogeny belt. The
Kukeng fault and Hsinhua fault bound Chaiyi block in the north and south as a neotectoic
block in the foreland of the fold-and-thrust belt in southwestern Taiwan.
The study area can be divided into three distinct NNE-SSW geomorphic domains:
coastal
plain,
tilted
tableland,
and
low
hill
from
shoreline
mountainward.
Geomorphologically the coastal plain is characterized by meandering rivers, where the
surface slope (Ss) and stream gradient (Sg) are relatively low, but river sinuosity(S) is high
[Ss:0.7m/km, Sg:0.2m/km, S:1.3]. The intermediate tableland domain shows a terrace
surface slightly titled to the west with meanders on the top [Ss:11.8m/km, Sg:2.6m/km,
S:1.4]. Anomaly high river sinuosity in this domain implies the influence given by active
structure. To the east, the domain of low hill, higher and deformed geomorphic surfaces
associated with dissected landscape are dominant. Stream gradient increases but sinuosity
deceases [Ss:86.4m/km, Sg:31.2m/km, S:1.2], indicating it has been uplifted for a while
and is probably undergoing. Independently, we calculated the absolute vertical rates of the
crustal movement by identifying the depositional environments of the corresponding
core-depth versus the radiocarbon ages. Our results reveal that the hanging-wall of the
blind thrust is uplifting(0.4~3.3mm/yr), and the footwall is subsiding(-0.2~-4.7 mm/yr).