Yu-g Chan

Late Cenozoic metamorphic evolution and exhumation of Taiwan

The Taiwan mountain belt is composed of a Cenozoic slate belt (Hsuehshan Range units, HR, and Backbone Slates, BS) and of accreted polymetamorphic basement rocks (Tananao Complex, TC). Ongoing crustal shortening has resulted from the collision between the Chinese continental margin and the Luzon volcanic arc, which initiated ~6.5 Ma ago. The grade and age of metamorphism and exhumation are a key record of the development of the orogenic wedge. Because the Taiwan mountain belt is mostly composed by accreted sediments lacking metamorphic index minerals, quantitative constraints on metamorphism are sparse. By contrast, these rocks are rich in carbonaceaous material (CM) and are therefore particularly appropriate for RSCM (Raman Spectroscopy of CM) thermometry. We apply this technique in addition to (U-Th)/He thermochronology on detrital zircons to assess peak metamorphic temperatures (T) and the late exhumational history respectively, along different transects in central and southern Taiwan. In the case of the HR units, we find evidence for high metamorphic T of at least 340°–350°C and locally up to 475°C, and for relative rapid exhumation with zircon (U-Th)/He ages in the range of 1.5–2 Ma. Farther east, the BS were only slightly metamorphosed (T < 330 °C), and zircons are not reset for (U-Th)/He. From the eastern BS to the inner TC schists, T gradually increases from ~350°C up to ~500°C following an inverted metamorphic gradient. Available geochronological constraints and the continuous thermal gradient from the BS to the basement rocks of the TC suggest that the high RSCM T of the TC were most probably acquired during the last orogeny, and were not inherited from a previous thermal event. Zircons yield (U-Th)/He ages of ~0.5–1.2 Ma. Peak metamorphic T and the timing of exhumation do not show along-strike variations over the TC in the studied area. In contrast, exhumation is laterally diachronous and decreases southward in the case of the HR units. In particular, our data imply that the HR units have been exhumed by a minimum of 15 km over the last few Ma. In the case of the BS, they show far less cumulated exhumation and much slower cooling rates. We propose that most of the deformation and exhumation of the Taiwan mountain belt is sustained through two underplating windows located beneath the Hsuehshan Range and the TC. Our data show significant departures from the predictions of the prevailing model in Taiwan, which assumes a homogeneous critical wedge with dominant frontal accretion. Our study sheds new light on how the mountain belt has grown as a possible result of underplating mostly.

Synorogenic extension and exhumation of the Taiwan hinterland

Structural data from two transects across the active Taiwan arc-continent collision indicate that normal faulting is an important mechanism in the exhumation of the hinterland. The normal faulting extends to at least the brittle-ductile transition, records extension at a high angle to the topographic grain of the orogenic belt, and is responsible for the regional-scale rotation of ductile fabrics. These results are supported by a recent GPS survey of southern Taiwan that reveals extension in the hinterland contemporaneous with shortening in the foreland and collided arc.

Effects of Realistic Surface Topography on Seismic Ground Motion in the Yangminshan Region of Taiwan Based Upon the Spectral-Element Method and LiDAR DTM

We combine light detection and ranging (LiDAR) digital terrain model (DTM) data and an improved mesh implementation to investigate the effects of high- resolution surface topography on seismic ground motion based upon the spectral- element method. In general, topography increases the amplitude of shaking at mountain tops and ridges, whereas valleys usually have reduced ground motion, as has been observed in both records from past earthquakes and numerical simulations. However, the effects of realistic topography on ground motion have not often been clearly characterized in numerical simulations, especially the seismic response of the true ground surface. Here, we use LiDAR DTM data, which provide two-meter reso- lution at the free surface, and a spectral-element method to simulate three-dimensional (3D) seismic-wave propagation in the Yangminshan region in Taiwan, incorporating the effects of realistic topography. A smoothed topographic map is employed beneath the model surface in order to decrease mesh distortions due to steep ground surfaces. Numerical simulations show that seismic shaking in mountainous areas is strongly affected by topography and source frequency content. The amplification of ground motion mainly occurs at the tops of hills and ridges whilst the valleys and flat-topped hills experience lower levels of ground shaking. Interaction between small-scale to- pographic features and high-frequency surface waves can produce unusually strong shaking. We demonstrate that topographic variations can change peak ground accel- eration (PGA) values by 50% in mountainous areas, and the relative change in PGA between a valley and a ridge can be as high as a factor of 2 compared to a flat surface response. This suggests that high-resolution, realistic topographic features should be taken into account in seismic hazard analysis, especially for densely populated moun- tainous areas.

A Vertical Exposure of the 1999 Surface Rupture of the Chelungpu Fault at Wufeng, Western Taiwan: Structural and Paleoseismic Implications for an Active Thrust Fault

We mapped and analyzed two vertical exposures—exposed on the walls of a 3- to 5-m-deep, 70-m-long excavation and a smaller 3-m-deep, 10-m-long excavation—across the 1999 rupture of the Chelungpu fault. The primary exposure revealed a broad anticlinal fold with a 2.5-m-high west-facing principal thrust scarp contained in fluvial cobbly gravel beds and overlying fine-grained overbank deposits. Sequential restoration of the principal rupture requires initial failure on the basal, east-dipping thrust plane, followed by wedge thrusting and pop-up of an overlying symmetrical anticline between two opposing secondary thrust faults. Net vertical offset is about 2.2 m across the principal fault zone. From line-length changes, we estimate about 3.3 m of horizontal shortening normal to fault strike. The ratio of these values yields a total slip of 4.0 m and an estimate of about 34° for the dip of the fault plane below the excavation. This value is nearly the same as the 35° average dip of the fault plane from the surface to the hypocenter. Restoration of the exposed gravelly strata and adjacent overbank sediments deposited prior to the 1999 event around the principal rupture suggests the possible existence of a prior event. A buried 30-m-wide anticlinal warp beneath the uplifted crest of the 1999 event is associated with three buried reverse faults that we interpret as evidence for an earlier episode of folding and faulting in the site. The prior event is also recorded in the smaller excavation, which is located 40 m south and is oriented parallel to the larger excavation. Radiocarbon dating of samples within the exposed section did not place tight constraints on the date of the previous event. Available data are interpreted as indicating that the previous event occurred before the deposition of the less than 200 ^(14)C yr B.P. overbank sands and after the deposition of the much older fluvial gravels. We interpret the previous event as the penultimate event relative to the 1999 Chi-Chi earthquake. We estimated the long-term slip rate of the Chelungpu fault to be 10-15 mm/yr during the last 1 Ma, based on previously published retrodeformable cross sections. This rate is, however, significantly higher than geodetic rates of shortening across the Chelungpu thrust where two pairs of permanent Global Positioning System stations suggest 7-10 mm/yr of shortening across the fault. Given the 4 m of average slip, the long-term slip rate yields an interseismic interval of between 267 and 400 yr for the Chelungpu fault.

The Arc–Continent Collision in Taiwan

We present a new compilation of magnetic, geologic, GPS and seismic data and propose that the geometry and kinematics of the Taiwan arc–continent collision are dominated by the partial subduction of a continental margin promontory and associated fracture zone. A prominent magnetic high in the pre-collision zone southwest of Taiwan serves as proxy for the edge of the continental crust of normal thickness (i.e., ~30 km). The high ends abruptly in central Taiwan, suggesting truncation by a NW-striking fault zone interpreted as a rift-related transfer zone. The NW-striking fault zone correlates with a steeply dipping, crustal-scale cluster of earthquakes recognized as the Sanyi-Puli seismic zone, indicating reactivation of the transfer zone. The truncated anomaly and transfer zone define a triangular-shaped continental margin promontory partially subducted beneath Taiwan. Island-wide GPS data show: (1) divergent flow around the promontory; (2) significant shortening between the promontory and the arc in the central part of Taiwan; and (3) significant lateral extrusion of the orogen south of the promontory–arc collision. Collision of the relatively rigid promontory resulted in a tripartite division of the fold and thrust belt as it developed from about 2 Ma to present and the development of a recess or syntaxis in the central part of the Central Range. The syntaxis correlates with the highest topography, thickest crust and highest values of P-wave attenuation, suggesting that it may be an area of high rates of rock uplift and exhumation, consistent with other orogenic syntaxes recognized around the world. This interpretation is also consistent with geomorphic parameters from the central part of the Central Range that indicate high rates of uplift and with the absence of seismicity in this area. Although detailed thermochronologic and kinematic data are limited in central Taiwan, the available data suggest a recent increase in rates of exhumation consistent with the interpretation that the promontory collided relatively recently (~2 Ma).

Kinematic analysis of the Pakuashan fault tip fold, west central Taiwan: Shortening rate and age of folding inception

The Pakuashan anticline is an active fault-tip fold that constitutes the frontalmost zone of deformation along the western piedmont of the Taiwan Range. Assessing seismic hazards associated with this fold and its contribution to crustal shortening across central Taiwan requires some understanding of the fold structure and growth rate. To address this, we surveyed the geometry of several deformed strata and geomorphic surfaces, which recorded different cumulative amounts of shortening. These units were dated to ages 2 ranging from ~ 19 ka to ~ 340 ka using Optical Stimulated Luminescence (OSL). We collected shallow seismic profiles and used previously published seismic profiles to constrain the deep structure of the fold. These data show that the anticline has formed as a result of pure shear with subsequent limb rotation. The cumulative shortening along the direction of tectonic transport (N118E) is estimated to be 1010 +/- 160 m. An analytical fold model derived from a sandbox experiment [Bernard, et al., in press] is used to model growth strata. This yields a shortening rate of 16.3 +/- 4.1 mm/yr and constrains the time of initiation of deformation to 62.2 +/- 9.6 ka. In addition, the kinematic model of Pakuashan is used to assess how tectonics, sedimentation and erosion have sculpted the present-day fold topography and morphology. The fold model, applied here for the first time on a natural example, appears promising in determining the kinematics of fault-tip folds in similar contexts and therefore in assessing seismic hazards on blind thrust faults.

Constraints from rocks in the Taiwan orogen on crustal stress levels and rheology

interval. We estimate a maximum strain rate of 7.0 � 10 � 14 s � 1 by distributing the geodetic convergence rate throughout a region homogeneously deformed under horizontal compression. These stress, strain rate and temperature estimates are consistent with the predictions of widely applied dislocation creep flow laws for quartzite. The samples record stress levels at the brittle-plastic transition, indicating a coefficient of friction (m) of 0.37 in the upper crust consistent with results based on critical taper. Integrated crustal strength of the Hsuehshan range amounts to 1.7 � 10 12 N/m based on our analysis, consistent with potential energy constraints based on topography. Other strength profiles are considered, however high crustal stresses (>300 MPa) conflict with our analysis. The study supports the use of the recrystallized grain size piezometer in quartz as a quick and inexpensive method for resolving stress histories in greenschist facies rocks. For consistency with the independent constraints presented here, we find it accurate to within +20%/� 40%, significantly better than previously recognized.

Digital Elevation Model Differencing and Error Estimation from Multiple Sources: A Case Study from the Meiyuan Shan Landslide in Taiwan

In this study, six different periods of digital terrain model (DTM) data obtained from various flight vehicles by using the techniques of aerial photogrammetry, airborne LiDAR (ALS), and unmanned aerial vehicles (UAV) were adopted to discuss the errors and applications of these techniques. Error estimation provides critical information for DTM data users. This study conducted error estimation from the perspective of general users for mountain/forest areas with poor traffic accessibility using limited information, including error reports obtained from the data generation process and comparison errors of terrain elevations. Our results suggested that the precision of the DTM data generated in this work using different aircrafts and generation techniques is suitable for landslide analysis. Especially in mountainous and densely vegetated areas, data generated by ALS can be used as a benchmark to solve the problem of insufficient control points. Based on DEM differencing of multiple periods, this study suggests that sediment delivery rate decreased each year and was affected by heavy rainfall during each period for the Meiyuan Shan landslide area. Multi-period aerial photogrammetry and ALS can be effectively applied after the landslide disaster for monitoring the terrain changes of the downstream river channel and their potential impacts.

Quantitative analysis of movement along an earthquake thrust scarp: a case study of a vertical exposure of the 1999 surface rupture of the Chelungpu fault at Wufeng, Western Taiwan

A vertical exposure across the principal thrust scarp of the 1999 Mw 7.6 earthquake allows quantification offault slip. The exposure is located on the active Chelungpu fault near Wufeng, along the range front of the fold-and-thrust belt in western Taiwan. The 1999 surface ruptures at the Wufeng site are characterized by a west-facing 2 to 3 m high principal thrust scarp and an east-facing lesser backthrust scarp. We mapped a 15 mlong, 5 m-deep exposure across the principal thrust scarp and characterized complex deformation structures, which include a main basal thrust fault, a wedge thrust, and a pop-up anticlinal fold with two secondary opposing thrust faults. The vertical displacement across the principal thrust scarp is measured directly from the offsets of the same sedimentary horizons between the hangingwall and the footwall. The average vertical displacement is 2.2 ^ 0.1 m, and the maximum displacement is 2.5 m, at the crest of the small pop-up fold. Horizontal displacement estimates were determined using line- and area-balancing methods. With line-length methods we estimated a horizontal displacement of 3.3 ^ 0.3 m across the principal scarp for four sedimentary horizons. For area balancing, first we selected three horizontal soil/sand deposits with a total thickness of about 0.5 m. The estimate yields an average horizontal displacement of 4.8 ^ 1.0 m. Using these individual and relatively thin stratigraphic layers yielded significant standard deviations in displacement estimates as a result of thickness variations. Second, we used the 3 mthick overbank soil/sand and the lower part offluvial pebble/cobble to calculate a horizontal displacement of 2.6 ^ 0.2 m with the area-balancing technique. According to the geometry of the dip angle (35‐ 408) of the basal thrust, the line-length measurement and the 3 m-thick package area balancing both providedreasonable results of horizontal displacement. By comparing thedifferent deposits applied to theline- and area-balancing methods, we interpret that decoupling of deformation occurred between the lower fluvial gravels and the upper overbank sand and mud deposits. Due to lesser confining pressure at the surface, additional deformation occurred in the upper 1 ‐ 2 m thick overbank deposits. This additional deformation yielded further vertical uplift of 0.3‐ 0.5 m and horizontal displacement of 0.2 ‐ 0.8 m around the core of the pop-up fold. Our work suggests that determination of slip across surface thrust ruptures varies as a function of the mechanical behavior of young late Quaternary deposits. q 2003 Elsevier Ltd. All rights reserved.

Surface rupture and behavior of thrust faults probed in Taiwan

Taiwans destructive Chi-chi earthquake of September 21, 1999, was a dramatic expression of active tectonic processes at a complex collisional plate boundary. It resulted in more than 2,400 causalities and tens of billions of dollars in property loss. During the earthquake, an 80-km stretch of the countrys mountainous backbone moved upward and westward along the range-bounding Chelungpu thrust fault (Figure la). A team of earthquake geologists from the United States, in collaboration with geoscientists from Academia Sinica, National Taiwan University and the Central Geological Survey of Taiwan, worked together to address questions concerning the recurrence of large-magnitude earthquakes along reverse faults in Taiwan.