Hsuehshan Range Research Articles

Mountain building process of the Taiwan orogeny.

The Taiwan orogeny, an example of arc-continental collision, exhibits complex geological structures and rapid exhumation. Many models have tried and failed to fully capture the dynamics of these processes. We developed a comprehensive thermomechanical model that considers the transition from brittle to ductile behavior with depth, lithology-dependent erosion and observed decollement and backstop geometries. This model successfully reproduces the intricate structures observed within the Taiwan orogeny, aligns with structural complexities, metamorphic temperature profiles, thermochronological records, strain distributions, and the rates of exhumation and cooling and elucidates the roles of ductile deformation and ramp structures in forming the Hsuehshan Range and the Western fold and thrust belt. The insights from this model offer potential applicability to other orogenic wedges worldwide.

Early to Middle Eocene larger foraminifera from the Ryukyu Tectonic Arc in Amakusa, Kyushu, Japan and Hsuehshan Range, Taiwan

Early to Middle Eocene larger foraminifera from the Ryukyu Tectonic Arc in Amakusa, Kyushu, Japan and Hsuehshan Range, Taiwan

The exhumation history of the middle Hsuehshan Range, Taiwan, as revealed by zircon thermochronological modeling

The Taiwan orogenic belt has high precipitation and high exhumation rates (3–6 km/Myr), making it a natural laboratory for understanding how erosion, tectonics, and climate interact. Understanding how rock exhumation rates have evolved since the late Miocene is key to unraveling climate–tectonic interactions. In this study, we present 8 new zircon fission track (ZFT) and (UTh)/He (ZHe) cooling ages (including 2 unreset ages) from 5 samples from 1700 to over 3300 m elevation around the highly incised Dajia River in the core of the Hsuehshan Range (termed the mid-HR in this study) in central Taiwan. Two of the ZFT and ZHe ages (5.3 Ma and 2.6 Ma) are the oldest ages recorded in this region. These ages suggest that the cooling rate of the mid-HR is lower than that of the Backbone Range and Yushan. With these and previously published cooling ages, we modeled the exhumation and cooling history using a 2-D thermal diffusion equation with time-varying erosion. The ZFT and ZHe ages are best explained by a steadily increasing exhumation rate up to 3.4 + 0.5/−0.3 km/Myr from 7 to 1 Ma. The exhumation rate in the last million years is not as well constrained but appears to have increased at a slower rate. Our findings differ from those of previous studies that propose that the exhumation rate increased rapidly in the last 1–2 Myr. We propose that the steady increase in the exhumation rate in the mid-HR indicates increasing topographic relief, which is likely caused by progressive crustal thickening.

The Exhumation History of the Middle Hsuehshan Range, Taiwan as Revealed by Zircon Thermochronological Modeling

The Exhumation History of the Middle Hsuehshan Range, Taiwan as Revealed by Zircon Thermochronological Modeling

Subduction-collisional processes between the Eurasian and Philippine Sea plates: Constraints from thermal-age paths of the Taiwan Orogen

Records of the thermal-age path of orogenic rocks are crucial in understanding subduction and collisional processes between a continent and an arc. Tectonic events and thermochronological data of the Taiwan Orogen have been extensively investigated. We use the zircon UPb youngest ages as the (initial) subduction age, the Raman Spectroscopy of Carbonaceous Material, 40Ar/39Ar age of mica porphyroblasts, whole-rock LuHf isochron age and thermal results as prograde peak metamorphic condition during subduction and basal accretion, and the apatite fission-track data for cooling age of the Taiwan Orogen to illuminate the subduction and collisional processes between the Eurasian Plate and the Luzon Arc. The Hsuehshan Range Nappe in the west of the Taiwan Orogen, whose subduction was initiated at 12 Ma, reached peak metamorphism with a temperature of ~250 °C to 490 °C at a depth of 8 km during the interval 6–2.5 Ma, mainly by basal superimposed accretion. The isothermal dwelling during peak metamorphism condition of the Hsuehshan Range suggests that tectonic underplating through basal superimposed accretion contribute significantly to the formation of the Taiwan Orogen, and led to a sudden increase of subduction and décollement depths, and slow downward thickening of the orogenic wedge and mountain root growth. Located in the east of the Taiwan Orogen, the Yuli Beltsubduction was initiated at 15.4–16 Ma, and attained peak metamorphism at a depth of 40 km, with a temperature of 550–560 °C and a pressure of 10.5–13 kbar during the interval 12.2–4.4 Ma due to basal superimposed accretion. The exhumation of the Hsuehshan Range commenced at ~2.5 Ma, which is later than that of the Yuli Belt at ~3.4 Ma. Such distinct and different thermal-age paths of the Hsuehshan Range and the Tananao Complex indicate that different tectonic wedges in the Taiwan Orogen are formed through different subduction and accretional processes.

Late Cenozoic exhumation and erosion of the Taiwan orogenic belt: New insights from petrographic analysis of foreland basin sediments and thermochronological dating on the metamorphic orogenic wedge

Late Cenozoic orogen-derived sediments recorded in the pro- and retro-foreland basins on both sides of the Taiwan orogenic wedge provide important constraints on sediment provenance evolution, and further outline the exhumation and erosion histories of the orogenic belt. We present 172 sandstone petrographic data from seven representative regions on the pro- and retro-foreland basins, which we coupled with previous thermochronological data from the pro-wedge of the Hsuehshan Range and the retro-wedge of the Backbone Range to elucidate the source-sink relationships between a source orogenic wedge and its associated foreland basin. Our data analysis and estimation provide evidence that the initial depositional age of orogen-derived sediments in pro- and retro-foreland basins is consistent with the exhumation and erosion histories of the adjacent orogen, indicating that the Backbone and Hsuehshan Ranges started to build up at ~12 and 5–6 Ma, respectively. The diachronous onset of orogenic sediment deposition and orogenic uplift also suggest a tendency for progressively southward and westward migration of the foreland basin, and asymmetric exhumation and erosion of the orogenic wedge over time due to oblique collision. The timing of the onset of the Taiwan orogeny formed the retro-wedge of the Backbone Range at ~12 Ma, which is much older than the previously determined onset age of Taiwan mountain building of ~6 Ma. We divide the Taiwan orogenic evolution into two stages: the first stage comprises the extrusion and exhumation of the subduction wedge formed as the Yuli Belt of the Backbone Range prior to the collision during the middle Miocene. The second stage comprises the arc-continent collision, producing doubly vergent orogenic wedge extrusion, leading to a diachronous uplift history of the Hsuehshan and Backbone Ranges since late Miocene.

Seismic reflection profiling of the first deep geothermal field in Taiwan

The first deep geothermal energy development project in Taiwan was launched at the Ilan Plain in northeastern Taiwan, where metamorphic rock is covered by sediments and detailed subsurface structures were poorly known. An eastward plunging anticline containing a fault system was effectively imaged in our seismic sections. However, since some rock fragments recovered from the exploration wells were still questioned, two fault structure models were proposed. One is an extensional strike-slip fault system, and the other is an active normal fault system, which depends on whether the rock fragments were affiliated with the Hsuehshan Range or with the Backbone Range.

Refining stratigraphic ages of Northern Hsuehshan Range in northern Taiwan by detrital zircon U-Pb dating

Refining stratigraphic ages of Northern Hsuehshan Range in northern Taiwan by detrital zircon U-Pb dating

Seismic evidence for central Taiwan magnetic low and deep-crustal deformation caused by plate collision

Crustal seismic velocity structure was determined for the northern Taiwan using seismic travel-time data to investigate the northeastern extension of the northern South China Sea’s high-magnetic belt. In order to increase the model resolution, a joint analysis of gravity anomaly and seismic travel-time data have been conducted. A total of 3385 events had been used in the inversion that was collected by the Central Weather Bureau Seismological Network from 1990 to 2015. The main features of the obtained three-dimensional velocity model are: (1) a relatively high Vp zone with velocity greater than 6.5km/s is observed in the middle to lower crust, (2) the high Vp zone generally parallels to the north–south structural trending of the Chuchih fault and Hsuehshan Range, (3) at 25km depth-slice, the high Vp zone shows structural trends change from northeastward to northward in central Taiwan, where the values of high-magnetic anomalies are rapidly decreasing to low values. A combination of seismic, GPS, and structural interpretations suggests that the entire crust has been deformed and demagnetized in consequence of the collision between the Philippine Sea plate and the Asian continental margin. We suggest that the feature of sharp bending of the high Vp zone would migrate southwestward and cause further crustal deformation of the Peikang High in the future.

Topographic architecture and drainage reorganization in Southeast China: Zircon U-Pb chronology and Hf isotope evidence from Taiwan

The uplift of Tibet Plateau and the marginal sea spreading have had important influence on the tectonic, landform and drainage system in East Asia, although the marginal sea spreading in shaping the topography and drainage reorganization in East Asia has been still controversial. Here we present U-Pb age and Hf isotopic composition of detrital zircon grains from Cenozoic sedimentary rocks in Taiwan to understand how the provenance and river systems evolved. Our data show that the U-Pb age spectra of detrital zircon grains in Paleogene sandstones are dominated by Yanshanian (180–67Ma) zircon grains and with subordinate or nil Proterozoic and Archean zircon grains. These results are in contrast to those in Miocene rocks that are dominated by the Indosinian (257–205Ma) zircon grains together with Yanshanian, Proterozoic and Archean population. The initial Hf isotope ratios [εHf(t)] of the zircon grains also display systematic change in Paleogene and Neogene strata. Our data demonstrate that the Hsuehshan Range and Western Foothills in Taiwan have the same sedimentary sources. The source region of Paleogene strata was mainly located at the coast in southeast China and migrated inland over time. The source might have reached the Lower Yangtze region during early Miocene. Although the mechanism of transport of sediments from the Lower Yangtze region to Taiwan is unclear, we speculate that the Minjiang River might have been larger in Early Miocene than the present and might have delivered inland material along the boundary of Yangtze and Cathaysia Blocks to Taiwan. These were then captured by the Yangtze River systems at some time after Late Miocene. This change corresponds to the time of the drainage reorganization in East Tibet, such as Yangtze River, and the regional subsidence resulting from the opening of marginal sea. The combined effects of Tibet uplift and opening of marginal sea might have shaped the topography and river system reorganization in East Tibet. The evolution of topography and drainage systems in southeast China seems to be mainly controlled by the opening of marginal sea.

Luminescence characteristics of quartz from Hsuehshan Range (Central Taiwan) and implications for thermochronometry

The mountain building processes in Taiwan are currently among the most rapid in the world. However, the spatial and temporal dynamics of this orogen are still poorly resolved within the <0.5 Ma timescale, reflecting methodological gaps in addressing young and rapid bedrock cooling by erosion. Optically Stimulated Luminescence (OSL) from quartz is an emerging thermochronometer that could potentially provide valuable low-temperature markers on a ∼0.1 Ma timescale. Here we study four meta-sandstone samples from the Hsuehshan Range in central Taiwan. We characterize the OSL from these samples in terms of feldspar contamination, saturation dose, signal components, and thermal stability. We conclude that the OSL is dominated by the fast component, and is therefore amenable to dose measurement using the SAR protocol. Based on the luminescence characteristics and ambient dose rate, we suggest that these samples may be valuable for evaluating recent cooling rates exceeding 360 ± 100 °C/Ma, in places where the alternative thermal scenarios (instantaneous cooling or prolonged isothermal storage) may be ruled out by external geological constraints.

Present-day surface deformation and tectonic insights of the extensional Ilan Plain, NE Taiwan

Taiwan’s mountain belt is an ideal location to address major questions regarding mechanisms of lithospheric deformation in convergent settings, mountain building processes from oceanic subduction to continental subduction, and post orogenic extension. In the northeast of this belt, the Ilan Plain is a triangular, deltaic plain characterized by a flat topography close to the sea level, and surrounded by the high mountains of the Hsuehshan Range to the northwest, and the Central Range to the southeast. Its eastern coast faces the western tip of the Okinawa Trough, the back-arc basin of the Ryukyu subduction zone. In this study, we analyzed the present-day surface deformation of the Ilan Plain, aiming at deciphering its relationships with basement faults and the regional geodynamic setting. Our approach is mainly based on surface vertical displacements revealed by Persistent Scatterer Interferometry Synthetic Aperture Radar (PSI), which indicate that there is an area of active subsidence (∼18mm/yr) located in the southern part of the plain in probable connection with active basement faults and in agreement with previous geodetic measurements and existing geophysical data. Our PSI results also suggest that the subsidence occurring in the Ilan Plain has moved from north to south during Quaternary in relation with extrusion of the belt due to the westward propagation of the Okinawa Trough through the Taiwan Mountains.

The Shuilikeng fault in the central Taiwan mountain belt

For over 200 km along strike the Shuilikeng fault of Taiwan separates Miocene rocks of the Western Foothills from the largely Eocene and Oligocene rocks of the Hsuehshan Range to the east. Despite its importance in the Taiwan mountain belt, the structure and kinematics of the Shuilikeng fault are not well known. Here, we present results from new geological mapping along 100 km of its strike length. At the surface, the Shuilikeng fault is a steeply east-dipping brittle fault with a series of splays and bifurcations. Along its southern part, it cuts an earlier fold and fault system. Outcrop kinematic data vary widely, from thrusting to strike-slip. The surface data are integrated with a relocated and collapsed seismicity database to interpret the fault location at depth. These data indicate that the Shuilikeng fault can be traced to greater than 20 km depth. Some 260 focal mechanisms from this dataset indicate that its kinematics is overall transpressive. From a regional perspective, we interpret the Shuilikeng fault to reactivate a pre-existing rift-related basin-bounding fault to the east of which rocks in the Hsuehshan Range are being exhumed.

Fracture patterns and their relations to mountain building in a fold-thrust belt: A case study in NW Taiwan

AbstractThe main purpose of this study is to analyse striated micro-faults and other types of fractures (including tensile and shear joints, and veins), in order to elucidate their relationships with regional folds and thrusts and regional tectonic stress. We take the fold-thrust belt (i.e., the foothills and the Hsuehshan range) in NW Taiwan as a case study, which is a product of the Plio-Pleistocene arc-continent collision. A total of about 760 and 1700 faults and other fractures, respectively, were collected at 41 sites in the field. We have identified four sets of bed-perpendicular joints in the study area. The observation of joints and bedding at each site indicates that most of the penetrative joint sets developed in the earlier tectonic stage of the pre-folding/pre-tilting event, illustrating the fact that the intersection of joint sets lies along the line perpendicular to the bedding plane. We thus interpret these sets as tectonic fractures under deep-seated tectonic stress. We used the regional fold axes as reference to define the four fracture sets. However, we found that complexity in the study area makes this rather tentative. Principal stress axes σ1, σ2, σ3, were calculated by means of inversion of fault slip data at each site. The ratio Φ that defines the shape of stress ellipsoid is generally small, indicating that the value of the maximum principal stress axe σ1 is much larger compared to that of σ2 and σ3, which are approximately equal. The paleostress regime was characterized by a combination of thrust and strike-slip tectonic regimes. Based on their geometric relationships with tilted bedding, we found most of striated micro-faults were strongly related to the regional folding and can be categorized as early-, during, and late-folding stages. We characterized two major directions for the compressive event, oriented N110–120°E and N150–160°E respectively, which provide additional evidence to delineate the debates about paleostress changes in the Taiwan mountain building process.

Tectonic implications of nonparallel topographic and structural curvature in the higher elevations of an active collision zone, Taiwan

Curves in the topographic grain of active orogenic belts typically parallel faults and lithologic contacts in part because of linkages between uplift rates and the structural development of the overriding plate. However, in the Taiwan collision zone, the topographic grain trends nonparallel to mapped faults and folds in the central portion of the belt along the southwest flank of the Hsuehshan Range. Here, the northern side of the Puli topographic embayment trends ∼345°, forming a topographic break that lies at a high angle to the structural grain but is nearly parallel to an underlying continental margin fracture zone in the downgoing plate. We analyzed fault-slip and other structural data, extracted normalized steepness indices from streams within the Tachia, Peikang, and Mei River basins, and integrated the results with recently published precise leveling data in order to understand the spatial and temporal variation in uplift and the structures that accommodated that uplift. Stress inversion reveals a NW-SE–trending maximum compression direction for an early-stage fault population and an ENE-WSW–trending maximum compression direction for a late-stage fault population. River steepness indices delineate a NW-trending boundary in incision rate that coincides with an increase in rock uplift rates derived from independent geodetic measurements. This NW-trending boundary is consistent with the late-stage maximum compression direction, suggesting that uplift of the Hsuehshan Range relative to the Puli topographic embayment has been accommodated by the late-stage faults. Our results suggest that a continental margin promontory has slowed underplating beneath the Puli topographic embayment and that the topographic grain of active collision zones is closely linked with the architecture of the downgoing plate.

The structure and kinematics of the central Taiwan mountain belt derived from geological and seismicity data

The structure of the Taiwan mountain belt is thought to be that of an imbricate thrust and fold belt developed in a forward breaking sequence above a shallowly dipping basal detachment. In recent years, however, a growing amount of seismicity data from the internal part of the mountain belt indicates the existence of widespread fault activity in the middle and lower crust, suggesting that deeper levels of the crust must be involved in the deformation than predicted by the shallow detachment, imbricate thrust belt model. To address this issue, we present new geological mapping, together with earthquake focal mechanism and seismic energy release data from the central part of Taiwan. We concur with the interpretation that the foreland basin part of the Western Foothills comprises an imbricate thrust system that is developing as a forward breaking sequence that is structurally and kinematically linked to a basal detachment at between 7 and 10 km depth. To the east of the foreland basin, however, in the Hsuehshan and Central Ranges, our data show the presence of two fault systems. An earlier, inactive thrust system with a well‐developed cleavage is cut by a system of steeply dipping active faults that penetrate to a depth of 25 to 30 km or more. In the Hsuehshan Range, the second fault system is best represented by a structural and kinematic model in which this part of the mountain belt forms a zone of transpression with a structural architecture similar to that of a crustal‐scale positive flower structure. Eastward, in the Central Range, Mesozoic basement rocks are over thrusting strongly folded and cleaved deep water sediments of the first, now inactive, thrust system. The involvement of deep crustal levels and Mesozoic basement in the second fault system is suggestive of the reactivation of preexisting basin‐bounding faults that were located on the Eurasian continental margin.

Linking Taiwan's subcritical Hsuehshan Range topography and foreland basin architecture

[1] The contemporary presence of the Puli Topographic Embayment within the Taiwanese thrust belt provides insight into processes that initiate and maintain a subcritical state in a thin-skinned compressive wedge. Orogen-scale analyses of Taiwan have succeeded in describing the processes and interactions that affect overall development of the thrust belt; however, relatively little is known about which processes or boundary conditions control first-order organization of strain at intermediate scales within the orogenic wedge. We investigate spatial and temporal scales of variation in the overall topographic and structural architecture of a critical wedge and explore the boundary conditions that affect very rapid shortening and erosion at intermediate scales on the order of 101 kilometers and 104–105 years. Causal links between the structural and synorogenic stratigraphic architecture of the foreland basin and coincidence of the Puli Topographic Embayment provide a valuable case study of the effects of changing boundary conditions (e.g., variable erodibility or strength of rocks along strike) controlling the evolution of critically tapered thin-skinned orogens. Deeper incision of river networks into a thicker sequence of unconsolidated synorogenic sediments in the central western foreland may affect the onset of a topographically subcritical state.

Nappe structure revealed by thermal constraints in the Taiwan metamorphic belt

Terra Nova, 23, 85–91, 2011AbstractVitrinite reflectance and Raman spectroscopy of carbonaceous material data are used to better resolve the thermal history of the Hsuehshan Range, which is accreted between the foreland fold–thrust belt and bulldozer hinterland units in the Taiwan mountain belt. The observed thermal data indicate that the strata in the northern Hsuehshan Range underwent dynamic metamorphism during the Neogene orogeny, while the strata in the southern Hsuehshan Range may have predominantly experienced burial metamorphism during Palaeogene sedimentation. Based on the thermal constraints, the Hsuehshan Range is interpreted to consist of nappe stacks, originating from the rifted Eurasian continental margin. This interpretation is consistent with well‐documented cases in the European Alps and the Himalayas and is also shown in physical modelling and thermo‐kinematic studies invoking underplating and erosion processes.

Temporal and spatial variation of stress field in Taiwan from 1991 to 2007: Insights from comprehensive first motion focal mechanism catalog

The total amount of 4,761 focal mechanisms was determined based on P-wave first motion polarities from 1991 to 2007 in Taiwan region. This dataset offers us a good opportunity to examine temporal and spatial variability of the stress field. We find that the spatial variations of stress axes are mainly controlled by tectonic structures while the temporal changes are greatly influenced by the Chi-Chi earthquake. The orientation of the maximum horizontal compressive stress axes (S H) shows a general agreement with the direction of plate motion between a depth range of 0–30 km. The 20° anticlockwise rotation of S H from the Longitudinal Valley (LV) to western Taiwan is probably caused by the left-lateral motion on the Longitudinal Valley Fault (LVF) that has consumed part of the oblique motion of plate convergence. On the other hand, part of the oblique convergence is transferred into the Central Range and the Hsuehshan Range judging from counterclockwise rotation of S H from east to west and strike-slip faulting in the Hsuehshan Range. Most events with a depth greater than 30 km occur offshore eastern Taiwan and the azimuth of S H is close to E–W directed, different from NW–SE directed at shallow depths. This may infer the existence of the transition of lithosphere rheology in offshore eastern Taiwan. The trends of S H in the depth of 0–10 km are strongly affected by the coseismic stress change of the Chi-Chi earthquake. In the northern half of the Chi-Chi rupture area, the trends of S H rotate 30° clockwise and the stress ratio increased by a factor of six after the mainshock. The orientations of S H still differ by 30° in 2007 comparing to that in the period before the Chi-Chi earthquake. The variation of S H trend is more diverse in the southern half of the rupture area, showing 20° counterclockwise rotation immediately after the 1999 mainshock followed by a clockwise rotation. The trend of S H returns to the pre-seismic direction of 110° in 2001. These notable changes of S H before and after the Chi-Chi mainshock suggests that the magnitude of background stress in the rupture area is close to the coseismic stress drop. We also recognize a significant 10° counterclockwise rotation of S H in the entire Chi-Chi rupture area between 1991 and 1999 before the earthquake took place. However, to the south of the Chi-Chi rupture, the trends of S H remain little changed before and after the Chi-Chi earthquake.

Incision and channel morphology across active structures along the Peikang River, central Taiwan: Implications for the importance of channel width

River morphology and dynamics are strongly influenced by active tectonics. We report channel dynamics for the Peikang River, which flows through the Hsuehshan Range in central Taiwan. Using a digital elevation model and field surveys, we constrain channel morphology for an ∼90 km stretch of river to calculate unit stream power and boundary shear stress along the river path. Incision rates are estimated with optically stimulated luminescence dating of sand deposited on strath terraces. We find a strong correlation between unit stream power/shear stress and incision rate, but only if variation in channel width is considered. A calibrated river incision rule implies river incision rates of ∼9–13.5 mm/yr upstream of the Meiyuan and Tili faults and suggests that one or both of these structures are presently active. Our results indicate that the Shuilikeng fault is also actively deforming, as incision rates increase to ∼6–10 mm/yr across it, compared to 1–4 mm/yr in adjacent reaches. Prominent narrowing across the Shuilikeng fault, and the absence of significant gradient variation indicate that channel width is a first-order morphological adjustment to differential incision. Only when the channel width-to-depth ratio reaches a minimum does the channel slope significantly adjust to local changes in base level, as is the case upstream of the Meiyuan and Tili faults.