Chih-Tung Chen
email: chihtung@ncu.edu.tw

Yu-Chang Chan

Olivier Beyssac

Chia-Yu Lu

Yue-Gau Chen

Jacques Malavieille

Steven Kidder

Hao-Cheng Sun

Thermal history of the Northern Taiwanese slate belt and implications for wedge growth during the Neogene arc-continent collision

A significant issue in the study of orogenic systems concerns the roles played by frontal and basal accretion in the construction of orogenic wedges. These different accretion mechanisms result in different thermal histories, with underplated materials experiencing significant heating and deformation during tectonic burial. This work provides new thermal data from Raman spectroscopy of carbonaceous material (RSCM) in combination with structural and stratigraphic observations of the northern Taiwan slate belt to address these questions of wedge development. Sedimentary rocks of the Northern slate belt were deposited on the Chinese continental margin immediately before the onset of the Neogene Taiwan arc-continent collision. In the slates of the northern Hsüehshan Range, a large-scale pop-up structure on the prowedge of the Taiwan Orogen, syn-orogenic metamorphism has been investigated through analyses of peak temperatures and metamorphic field gradients. Results indicate underthrusting of the margin sediments to ~8 km depth with significant folding in two major duplexes occurring before underplating. Such basal accretion is considered responsible for the distinct culmination of the Hsüehshan Range in central Taiwan and its relative uplift with respect to the Backbone Range to the east along the Lishan Fault. A similar underthrusting scenario is also suggested for the Backbone Range Slate Belt. We propose that basal accretion is the predominant mechanism in the growth and evolution of the Taiwan orogenic wedge, and may have been achieved through inversion of a graben system on the ancient passive margin during continental subduction.

Introduction

Patterns of metamorphism recorded in mountain belts reflect the underlying tectonic processes driving rock deformation and exhumation (e.g. [START_REF] Huerta | The effects of accretion, erosion and radiogenic heat on the metamorphic evolution of collisional orogens[END_REF][START_REF] Jamieson | Interaction of metamorphism, deformation and exhumation in large convergent orogens[END_REF][START_REF] Rosenberg | Relating orogen width to shortening, erosion, and exhumation during Alpine collision[END_REF]. One important debate related to orogenic belts is the dominant mechanism by which material enters an accretionary wedge. Two endmember scenarios have been proposed: frontal accretion of material at the toe of the wedge (e.g. [START_REF] Barr | Brittle frictional mountain building 3: Low-grade metamorphism[END_REF], or basal accretion by duplexing of underthrust footwall rocks along the main detachment [START_REF] Van Gool | Frontal vs. basal accretion and contrasting particle paths in metamorphic thrust belts[END_REF][START_REF] Gutscher | Episodic imbricate thrusting and underplating: analog experiments and mechanical analysis applied to the Alaskan Accretionary Wedge[END_REF][START_REF] Kukowski | Mechanical decoupling and basal duplex for-mation observed in sandbox experiments with application to the Western Mediterranean Ridge accretionary complex[END_REF]. These contrasting wedge formation mechanisms influence a mountain belt's structure as well as its metamorphic and thermal configuration (e.g. [START_REF] Dahlen | Brittle frictional mountain building 1. Deformation and mechanical energy budget[END_REF][START_REF] Bollinger | Thermal structure and exhumation history of the lesser Himalaya[END_REF][START_REF] Simoes | Investigating the kinematics of mountain building in Taiwan from the spatiotemporal evolution of the foreland basin and western foothills[END_REF][START_REF] Bonnet | Interactions between tectonics, erosion, and sedimentation during the recent evolution of the Alpine orogen: analogue modeling insights[END_REF][START_REF] Chen | Nappe structure revealed by thermal constraints in the Taiwan metamorphic belt[END_REF][START_REF] Konstantinovskaya | Thrust wedges with decollement levels and syntectonic erosion: A review from analog models[END_REF][START_REF] Beyssac | Peak metamorphic temperature and thermal history of the Southern Alps (New Zealand)[END_REF][START_REF] Molli | RSCM thermometry in the Alpi Apuane (NW Tuscany, Italy): New constraints for the metamorphic and tectonic history of the inner northern Apennines[END_REF]. Terranes composed of frontal-accreted sediments should reveal mainly static diagenesis and retain the thermal signature of the original depositional setting; whereas basal-accreted rocks are expected to exhibit a strong synorogenic (dynamic) metamorphic overprint acquired during the underthrusting stage [START_REF] Glodny | Internal dynamics of a paleoaccretionary wedge: insights from combined isotope tectonochronology and sandbox modeling of the South-Central Chilean forearc[END_REF][START_REF] Konstantinovskaya | Erosion and exhumation in accretionary orogens: Experimental and geological approaches[END_REF]. Regional analyses of metamorphic terranes can provide insights into the contribution of these contrasting accretion pathways. Given that it is a type locality for the understanding of wedge mechanics [START_REF] Davis | Mechanics of fold-and-thrust belts and accretionary wedges[END_REF][START_REF] Barr | Brittle frictional mountain building 3: Low-grade metamorphism[END_REF], it is important to determine, as best as possible, the accretion mechanisms involved in the active arc-continent collision system of Taiwan.

The Taiwan mountain belt results from oblique convergence between the Eurasian and the Philippine Sea plates. Eastward subduction of the South China Sea beneath the Manila Trench at a rate of ~8 cm per year [START_REF] Yu | Velocity field of GPS stations in the Taiwan area[END_REF][START_REF] Lin | GPS crustal deformation, strain rate, and seismic activity after the 1999 Chi-Chi earthquake in Taiwan[END_REF]; Fig. 1 inset) led to the oblique collision of the Chinese Continental Margin with the Luzon Arc. This process has been ongoing since the late Miocene [START_REF] Suppe | Mechanics of mountain building and metamorphism in Taiwan[END_REF](Suppe, , 1984;;[START_REF] Teng | Geotectonic evolution of late Cenozoic arc-continent collision in Taiwan[END_REF]. Given its well-constrained and relatively simple tectonic setting, Taiwan offers the opportunity to determine how an extended continental margin covered with a thick pile of sediments [START_REF] Lin | Cenozoic stratigraphy and subsidence history of the South China Sea margin in the Taiwan region[END_REF] transforms into an arc-continent collisional prism (e.g. [START_REF] Simoes | Mountain building in Taiwan: a thermokinematic model[END_REF][START_REF] Simoes | Late Cenozoic metamorphism and mountain building in Taiwan: A review[END_REF][START_REF] Brown | The structure and kinematics of central Taiwan mountain belt derived from geological and seismicity data[END_REF]. The Taiwanese orogenic wedge-in which both the basement and cover sediments of the continental margin are bulldozed against the indenting arc-was traditionally considered to have grown by frontal accretion with only minor (<25%) underplating via basal accretion (e.g. [START_REF] Dahlen | Brittle frictional mountain building 1. Deformation and mechanical energy budget[END_REF]. It follows that thrusts, and associated fault-bend folds [START_REF] Suppe | A retrodeformable cross section of northern Taiwan[END_REF][START_REF] Yue | Structural geology of a classical thrust belt earthquake: the 1999 Chi-Chi earthquake Taiwan (Mw=7.6)[END_REF], are directly rooted into a low-dipping detachment fault throughout the orogen [START_REF] Carena | Active detachment of Taiwan illuminated by small earthquakes and its control of first-order topography[END_REF]. This inference generally holds true for the outer fold-thrust belt of the orogeny in the Western Foothills. There, the deformed sedimentary successions display burial diagenesis and an absence of syntectonic prograde metamorphism [START_REF] Chen | Explanatory notes for the Metamorphic Facies Map of Taiwan, 2 nd ed[END_REF]. However, in the interior part of the mountain belt, orogen-related heating and cooling is recognized in rocks of prehnite-pumpellyite facies up to lower amphibolite facies (e.g. [START_REF] Ernst | Crustal accretion and metamorphism in Taiwan, a post-Palaeozoic mobile belt[END_REF][START_REF] Lo | Rejuvenation of K-Ar systems for minerals in the Taiwan Mountain Belt[END_REF][START_REF] Liu | Thermo-kinematic evolution of the Taiwan oblique-collision mountain belt as revealed by zircon fission track dating[END_REF][START_REF] Fuller | A thermomechnical wedge model of Taiwan constrained by fission-track thermochronometry[END_REF][START_REF] Lee | Mountain building mechanisms in the Southern Central Range of the Taiwan Orogenic Belt -From accretionary wedge deformation to arc-continent collision[END_REF][START_REF] Lee | Simultaneous mountain building in the Taiwan orogenic belt[END_REF][START_REF] Beyssac | Late Cenozoic metamorphic evolution and exhumation of Taiwan[END_REF][START_REF] Chen | Nappe structure revealed by thermal constraints in the Taiwan metamorphic belt[END_REF][START_REF] Chen | Basal accretion, a major mechanism for mountain building in Taiwan revealed in rock thermal history[END_REF]. The distribution of metamorphic grades across the orogen does not exhibit a linear increase from the west (foreland) to the east (hinterland against the backstop) as envisaged in frontal-accreted wedges [START_REF] Barr | Brittle frictional mountain building 3: Low-grade metamorphism[END_REF]. Instead, a high metamorphic temperature anomaly is present in the prowedge Hsüehshan Range (Fig. 1 inset; [START_REF] Beyssac | Late Cenozoic metamorphic evolution and exhumation of Taiwan[END_REF][START_REF] Simoes | Mountain building in Taiwan: a thermokinematic model[END_REF][START_REF] Simoes | Late Cenozoic metamorphism and mountain building in Taiwan: A review[END_REF]. Further complexity arises from the subducted Chinese continental margin, which possesses numerous Paleogene rifts and grabens related to the opening of the South China Sea [START_REF] Teng | Tectonic aspectsof the Paleogene depositional basin of northern Taiwan[END_REF][START_REF] Teng | Geotectonic evolution of Tertiary continental margin basins of Taiwan[END_REF][START_REF] Teng | Cenozoic tectonics of the China continental margin: insights from Taiwan[END_REF]. Hence, we find it unlikely that a single, continuous master detachment surface exists beneath the entire orogen.

Active shortening on the island as revealed by GPS measurements [START_REF] Yu | Velocity field of GPS stations in the Taiwan area[END_REF][START_REF] Lin | GPS crustal deformation, strain rate, and seismic activity after the 1999 Chi-Chi earthquake in Taiwan[END_REF] is localized along the western prowedge and eastern retro-wedge toes rather than being evenly distributed across the entire system as predicted by classical wedge theory [START_REF] Avouac | Mountain building, erosion, and the seismic cycle in the Nepal Himalaya[END_REF][START_REF] Simoes | Investigating the kinematics of mountain building in Taiwan from the spatiotemporal evolution of the foreland basin and western foothills[END_REF]. Additionally, the vertical displacement field across Taiwan suggests that tectonic underplating is responsible for the spectacular uplift in the Central Range [START_REF] Simoes | Mountain building in Taiwan: a thermokinematic model[END_REF][START_REF] Ching | Modern vertical deformation rates and mountain building in Taiwan from precise leveling and continuous GPS observations, 2000-2008[END_REF].

The above arguments have prompted studies on the nature of accretion in the Taiwan wedge with results advocating substantial tectonic underplating [START_REF] Fuller | A thermomechnical wedge model of Taiwan constrained by fission-track thermochronometry[END_REF][START_REF] Simoes | Mountain building in Taiwan: a thermokinematic model[END_REF][START_REF] Simoes | Late Cenozoic metamorphism and mountain building in Taiwan: A review[END_REF][START_REF] Malavieille | Impact of erosion, sedimentation, and structural heritage on the structure and kinematics of orogenic wedges: Analog models and case studies[END_REF][START_REF] Chen | Nappe structure revealed by thermal constraints in the Taiwan metamorphic belt[END_REF][START_REF] Chen | Basal accretion, a major mechanism for mountain building in Taiwan revealed in rock thermal history[END_REF][START_REF] Brown | The structure and kinematics of central Taiwan mountain belt derived from geological and seismicity data[END_REF]. Other studies, however, have questioned the presence of Neogene prograde metamorphism in the exhumed continental margin basement rocks, as well as the overall significance of underplating [START_REF] Yamato | Taiwan mountain building: insights from 2-D thermomechanical modeling of a rheologically stratified lithosphere[END_REF][START_REF] Wintsch | Geochronologic evidence for a cold arc-continent collision: The Taiwan orogeny[END_REF].

In this study, Raman spectroscopy of carbonaceous material is used as a geothermometer (RSCM; [START_REF] Beyssac | Raman spectra of carbonaceous material in metasediments: a new geothermometer[END_REF][START_REF] Lahfid | Evolution of the Raman spectrum of carbonaceous material in low-grade metasediments of the Glarus Alps (Switzerland)[END_REF] on Paleogene to Miocene continental margin metasediments exhumed in the Taiwanese Central Range slate belt. Unlike the poly-phase metamorphosed continental margin basement now exposed in the Tananao Complex of the eastern Central Range (Fig. 1 inset; [START_REF] Yui | A speculative tectonic history of the Tananao Schist of Taiwan[END_REF][START_REF] Yui | Late Triassic-late Cretaceous accretion/subduction in the Taiwan region along the eastern margin of South Chinaevidence from zircon SHRIMP dating[END_REF], these rocks are too young to have experienced tectonic events prior to the late Cenozoic arc-continent collision. The highest recorded temperatures of the late Tertiary rocks should therefore reflect either pre-orogenic burial in margin rift basins, or heating during the Neogene orogeny [START_REF] Beyssac | Late Cenozoic metamorphic evolution and exhumation of Taiwan[END_REF][START_REF] Chen | Nappe structure revealed by thermal constraints in the Taiwan metamorphic belt[END_REF][START_REF] Chen | Basal accretion, a major mechanism for mountain building in Taiwan revealed in rock thermal history[END_REF]. While most past efforts have focused on the central part of the island (e.g. [START_REF] Dahlen | Brittle frictional mountain building 1. Deformation and mechanical energy budget[END_REF][START_REF] Fuller | A thermomechnical wedge model of Taiwan constrained by fission-track thermochronometry[END_REF][START_REF] Simoes | Mountain building in Taiwan: a thermokinematic model[END_REF], we present new RSCM measurements in the northern part of the slate belt along a NW-trending transect, the Northern Cross-Island Highway and the Taipingshan Road (Fig. 1).

According to the oblique convergence tectonic framework [START_REF] Suppe | Mechanics of mountain building and metamorphism in Taiwan[END_REF], the northern part of Taiwan has undergone a longer collisional process than further south, and exposes rocks which were once located deeper within the orogenic wedge. Below, we document the history of the northern slate belt using RSCM data on peak thermal conditions together with structural, stratigraphic and published thermal history constraints. This approach allows a quantitative estimate of synorogenic dynamic metamorphism and sheds light on the extent of basal accretion. Structures responsible for underplating are then evaluated, leading to a better understanding of basin inversion atop the subducted continental margin, and a refined tectonic interpretation of the Taiwan arc-continent collision.

Regional background

The slate belt of Taiwan outcrops on the prowedge side of the Taiwanese orogenic wedge, and is sandwiched between deformed, un-metamorphosed continental shelf sediments of the Western Foothills and the exhumed continental margin basement of the Tananao Complex. The slate belt comprises two north-south trending zones: the Hsüehshan Range and the Backbone Range Slate Belt (Fig. 1). The Hsüehshan Range contains a continuous succession of Eocene to mid-Miocene continental margin sediments [START_REF] Teng | Tectonic aspectsof the Paleogene depositional basin of northern Taiwan[END_REF] deposited in eastward-deepening rift grabens [START_REF] Teng | Geotectonic evolution of Tertiary continental margin basins of Taiwan[END_REF] that were associated with the opening of the South China Sea [START_REF] Teng | Cenozoic tectonics of the China continental margin: insights from Taiwan[END_REF].

The Backbone Range Slate Belt is found east of the Hsüehshan Range and comprised mainly of Miocene pelagic sediments. The Hsüehshan Range and Backbone Range are separated by the Lishan Fault in northern Taiwan. The Backbone Range Slates are juxtaposed to the east against the schist-grade Tananao Complex along a basal unconformity which was reworked as a major shear zone [START_REF] Suppe | Observation of some contacts between basement and Cenozoic cover in the Central Mountains, Taiwan[END_REF][START_REF] Fisher | Taiwan Slate Belt: Insights into the ductile interior of an arc-continent collision[END_REF].

Following the cessation of Mesozoic paleo-Pacific subduction in southeastern China, the continental margin remained subaerial until the early Eocene. Subsequently, grabens developed in the middle part of the margin were drowned, allowing coastal to shallow marine sediments to accumulate (e.g. [START_REF] Lin | Cenozoic stratigraphy and subsidence history of the South China Sea margin in the Taiwan region[END_REF]. During the Oligocene, the grabens deepened and widened in response to the opening of the South China Sea [START_REF] Briais | Updated interpretation of magnetic anomalies and seafloor spreading stages in the South China Sea: Implications for the Tertiary tectonics of Southeast Asia[END_REF][START_REF] Yeh | Tectonic evolution of the Northeastern South China Sea from seismic interpretation[END_REF] and thick successions of shelf and distal marine deposits accumulated [START_REF] Teng | Geotectonic evolution of Tertiary continental margin basins of Taiwan[END_REF]. Following the latest Oligocene, marine sediments draped most of the continental margin. The rifted outer-most rim of the former margin [START_REF] Lu | Tectonic evolution of the Taiwan mountain belt[END_REF][START_REF] Shyu | Tandem suturing and disarticulation of the Taiwan orogen revealed by its neotectonic elements[END_REF] was covered by minor Eocene deposits and Miocene pelagic shale sequences [START_REF] Teng | Tectonic aspectsof the Paleogene depositional basin of northern Taiwan[END_REF]. During the Neogene Taiwan Orogeny some of the rift grabens situated in the distal portions of the continental margin were inverted as a large-scale pop-up structure forming the Hsüehshan Range [START_REF] Clark | Kinematic analyses of the Hsueshan Range, Taiwan: a large-scale pop-up structure[END_REF][START_REF] Tillman | Kinematic analysis of the Taiwan Slate Belt[END_REF][START_REF] Brown | The structure and kinematics of central Taiwan mountain belt derived from geological and seismicity data[END_REF]. The Backbone Range Slate Belt is made up of the deformed margin rim cover sediments. Due to the oblique nature of the Taiwanese convergence, the northern parts of the ranges are considered more evolved than their southern counterparts. Exhumed sedimentary successions are not identical along the mountain belt [START_REF] Teng | Geotectonic evolution of Tertiary continental margin basins of Taiwan[END_REF] indicating the involvement of different basins along the continental margin.

From bottom to top, the meta-sediments in the northern Hsüehshan Range comprise the following units: Eocene Hsitsun Formation, containing phyllitic slate and intercalated meta-sandstone layers (Fig. 2C [START_REF] Chang | Eocene/Miocene hiatus and N conglomerate in the Central Range of Taiwan[END_REF] were from tectonic blocks embedded in the Miocene pelagic mud matrix [START_REF] Lu | Tectonic evolution of the Taiwan mountain belt[END_REF].

The sedimentary succession in the northern Hsüehshan Range was thrust over the Western Foothills along the Chuchih Fault and deformed into a series of NE-trending, NW-vergent faults and folds including the Shihtsao Fault, the Loshan Fault, the Chatienshan Anticline, the Tahan Fault, and the Hsitsun Anticline (e.g. [START_REF] Lin | Degrees of metamorphism in the northern Hsüehshan Range: a study of vitrinite reflectance in metapelites along the Northern Taiwan East-west Cross-Island Highway[END_REF]Fig. 1). The Hsüehshan Range is uplifted relative to the Backbone Range along the subvertical E-vergent Lishan Fault which has a complex kinematic history [START_REF] Lee | Polyphase history and kinematics of a complex major fault zone in the northern Taiwan mountain belt: the Lishan Fault[END_REF]. Exhumation increases eastward within the Hsüehshan Range as Miocene rocks are found on the flanks of the Chatienshan Anticline and are absent southeast of the Tahan Fault, while Eocene strata crop out only in the core of the Hsitsun Anticline.

Degree of metamorphism also increases eastward within the Hsüehshan Range as slaty cleavage is developed in all rocks southeast of the Junghua Dam on the northwestern limb of the Chatienshan Anticline (the 'onset of cleavage' in Fig. 1) with a NE-trending, inclined to subvertical attitude (Fig. 3A), while the Oligo-Miocene indurate argillite in the exterior Hsüehshan Range possesses only pencil cleavage (Fig.

2

). Metamorphic conditions of the Hsüehshan Range span from prehnite-pumpellyite grade to greenschist facies in the Hsitsun anticline [START_REF] Clark | Kinematic analyses of the Hsueshan Range, Taiwan: a large-scale pop-up structure[END_REF][START_REF] Chen | Explanatory notes for the Metamorphic Facies Map of Taiwan, 2 nd ed[END_REF][START_REF] Beyssac | Late Cenozoic metamorphic evolution and exhumation of Taiwan[END_REF][START_REF] Chen | Nappe structure revealed by thermal constraints in the Taiwan metamorphic belt[END_REF]. Muscovite K-Ar and zircon and apatite fission track ages from the greenschist facies rocks are younger than 5 Ma, indicating cooling during the Taiwanese orogeny [START_REF] Liu | Thermo-kinematic evolution of the Taiwan oblique-collision mountain belt as revealed by zircon fission track dating[END_REF][START_REF] Fuller | A thermomechnical wedge model of Taiwan constrained by fission-track thermochronometry[END_REF]Beyssac et al., 2007).

Based on its thermo-kinematic history, the Hsüehshan Range can be subdivided into upper and lower nappe units [START_REF] Chen | Nappe structure revealed by thermal constraints in the Taiwan metamorphic belt[END_REF]. The current study area in the northern Hsüehshan Range represents the lower nappe unit, in which synorogenic metamorphism is interpreted to overprint earlier lower grade diagenesis (see below).

The central and southern Hsüehshan Range belong to the upper nappe unit which is characterized by higher grade, statically-developed diagenetic metamorphism [START_REF] Beyssac | Late Cenozoic metamorphic evolution and exhumation of Taiwan[END_REF]. The cleavage developed in the northern Hsüehshan Range has been constrained from syn-kinematic mica porphyroblasts found in concordant mafic pyroclastic units within the Tatungshan Formation to have been acquired 6 -2.5 Ma at ~250˚C [START_REF] Chen | Basal accretion, a major mechanism for mountain building in Taiwan revealed in rock thermal history[END_REF].

The Backbone Range Slate Belt is more penetratively deformed, with nappe stacks containing gently-dipping layer-parallel foliation (Fig. 3A) indicating the dominance of top-to-the-northwest shear [START_REF] Lu | Tectonic evolution of the Taiwan mountain belt[END_REF][START_REF] Tillman | Kinematic analysis of the Taiwan Slate Belt[END_REF][START_REF] Fisher | Taiwan Slate Belt: Insights into the ductile interior of an arc-continent collision[END_REF]. Overall the Backbone Range Slate Belt to the west of the Tananao Complex consists of a synform or synclinorium [START_REF] Yang | Geologic structures of the Tananao Schist and its cover in the Tayuling area, Central Range of Taiwan[END_REF]. The metamorphic grade in the Backbone Range Slate Belt increases eastward from prehnite-pumpellyite facies at its western border with higher-grade Hsüehshan Range rocks across the Lishan Fault, to biotite grade greenschist facies at its eastern boundary with the schist basement [START_REF] Ernst | Crustal accretion and metamorphism in Taiwan, a post-Palaeozoic mobile belt[END_REF][START_REF] Beyssac | Late Cenozoic metamorphic evolution and exhumation of Taiwan[END_REF].

Quantitative assessment of the peak metamorphic conditions, especially the maximum temperature experienced by the rocks of the northern Hsüehshan Range and Backbone Range Slate Belt, is lacking [START_REF] Chen | Nappe structure revealed by thermal constraints in the Taiwan metamorphic belt[END_REF] and addressed here by applying the RSCM method.

Thermometry

The Raman spectroscopy of carbonaceous material (RSCM) geothermometer

Organic matter present in rocks is transformed into graphitic carbonaceous material during metamorphism in a systematic process that can be used as a geothermometer (e.g. [START_REF] Beyssac | Raman spectra of carbonaceous material in metasediments: a new geothermometer[END_REF][START_REF] Beyssac | Contrasting degrees of recrystallization of carbonaceous material in the Nelson aureole, British Columbia and Ballachulish aureole, Scotland, with implications for thermometry based on Raman spectroscopy of carbonaceous material[END_REF]. The graphitization process is irreversible, such that retrograde metamorphic reactions do not affect temperature estimates. The RSCM thermometer is based on the quantification of the degree of ordering of carbonaceous material and has an intrinsic calibration error of ~50 °C due to the petrological data used for calibration. Relative accuracy is better at about 15 °C [START_REF] Beyssac | Thermal metamorphism in the Lesser Himalaya of Nepal determined from Raman spectroscopy of carbonaceous material[END_REF]. For temperatures below 330 °C, [START_REF] Lahfid | Evolution of the Raman spectrum of carbonaceous material in low-grade metasediments of the Glarus Alps (Switzerland)[END_REF] performed a systematic study of the evolution of the Raman spectrum of carbonaceous material in low-grade metamorphic rocks in the Glarus Alps (Switzerland). They showed that the Raman spectra of lower temperature carbonaceous material has additional peaks from the spectra observed at higher temperature, and established a quantitative correlation between the degree of ordering of carbonaceous material and temperature.

Although this calibration is not universal (see discussion in [START_REF] Lahfid | Evolution of the Raman spectrum of carbonaceous material in low-grade metasediments of the Glarus Alps (Switzerland)[END_REF], we use it to obtain first-order insights regarding the thermal evolution of rocks in the study area.

At low temperatures (<330 °C), Raman spectra of carbonaceous material exhibits a graphitic band composed of the G and D2 bands, a defect D composite band made up of the main defect band D1 with the D4 band shoulder, and a small D3 band in between [START_REF] Lahfid | Evolution of the Raman spectrum of carbonaceous material in low-grade metasediments of the Glarus Alps (Switzerland)[END_REF]Beyssac and Lazzeri, 2012). The structural organization of carbonaceous material can be quantified using the R2 and RA1 parameters which reflect the proportion of the D composite band area within the entire measured spectra

(R2=D1/[G+D1+D2], RA1=(D1+D4)/[G+D1+D2+D3+D4]
, where all variables represent band areas). A linear correlation of RA1 with metamorphic temperature is established in the range 210-330°C [START_REF] Lahfid | Evolution of the Raman spectrum of carbonaceous material in low-grade metasediments of the Glarus Alps (Switzerland)[END_REF].

Raman spectra analyses were carried out at Tatung University in Taipei, and ENS, Paris, both using a Renishaw (Wutton-under-Edge, UK) InVIA Reflex microspectrometer equipped with a 514-mm Spectra Physics (20 mW) argon laser. A DMLM Leica (Wetzlar, Germany) microscope with a 50× (100× at ENS) objective (NA=0.90) was utilized to focus the laser on the sample. The laser power at the sample surface was set to be around 1 mW using neutral density filters. The Rayleigh diffusion was eliminated by notch filters, and to attain the optimal spatial resolution the spectrometer entrance slit was closed down to 10 -15 µm to reach a nearly confocal configuration. The signal was dispersed using an 1800 gr/mm grating and analyzed by a Peltier cooled CCD detector. Calibration of the spectrometer with a silicon standard was carried out before each session. The analytical procedure described in [START_REF] Beyssac | Raman spectra of carbonaceous material in metasediments: a new geothermometer[END_REF][START_REF] Beyssac | Experimental study of the microtextural and structural transformations of carbonaceous materials under pressure and temperature[END_REF] was adhered to prevent analytical pitfalls.

Measurements were done on polished thin sections cut perpendicular to the schistosity, and the carbonaceous material was systematically analyzed below an adjacent transparent mineral, usually quartz. At least ten spectra were recorded for each sample to smooth out within-sample structural heterogeneity in the extended scanning mode (800 to 2000 cm -1 ), thereby allowing clear imaging of the entire background signal and proper defining of baseline. The acquired spectra were then processed using the Peakfit software using a five-band solution with Lorentzian band profile, and the parameter RA1 was calculated and utilized to estimate peak temperature [START_REF] Lahfid | Evolution of the Raman spectrum of carbonaceous material in low-grade metasediments of the Glarus Alps (Switzerland)[END_REF]. Spectra of the higher temperature sample NCIH-23 were analyzed using a four-band solution following [START_REF] Beyssac | Raman spectra of carbonaceous material in metasediments: a new geothermometer[END_REF], and the peak temperature was inferred through the R2 parameter. As exhibited in Fig. 4 and described in detail below, the carbonaceous material found within metasediments of the northern Taiwan slate belt has a wide range of degree of graphitization similar to that reported in [START_REF] Lahfid | Evolution of the Raman spectrum of carbonaceous material in low-grade metasediments of the Glarus Alps (Switzerland)[END_REF].

Results from the northern Hsüehshan Range

A total of 24 samples from the Hsüehshan Range were collected along the Northern Cross-Island Highway and analyzed. Sample spacing was about 1 km in the northwest direction, i.e. perpendicular to the structural grain of the region. The dense sampling was chosen in order to best constrain the thermal history of each strata unit and structure along the transect. Most sampled rocks are massive slates and argillites, along with a few thin slate layers from sandier units (e.g. the Szeleng Sandstone).

Representative field and microscopic images are shown in Fig. 2. Sample locations and measurement results are listed in Table 1 and

Results from the northern Backbone Range Slate Belt

A total of six samples were collected from within the Backbone Range along the Taipingshan Road. The area is composed predominantly of massive slate which grade to phyllite near Cueifong Lake. A few, sometimes thick, lenses of meta-sandstone are also present. Sample locations and measurement results are listed in Table 1 and projected on the NW-trending profile shown in Fig. 3B.

The RSCM temperature of sample TPS01 is similar to that of samples NCIH29 and 31 on the southeastern end of Northern Cross-Island Highway, implying some structural continuity with the Hsüehshan Range. A significant RSCM temperature drop occurs in the low-hill area near the Lanyang River at Tuchang from ~270°C (TPS01) to ~230°C (TPS02), coinciding with a microstructural change from well-developed slaty cleavage to argillitic texture with incipient slaty cleavage (Fig. 2G andH). Further southeast the estimated peak temperature increases from ~230°C to ~240°C at Jhongjian (TPS04), and ~270°C to ~300°C at Taipingshan and the Cueifong Lake (TPS08, 10 and 13) where phyllitic texture appears (Fig. 2I). The increase of metamorphic temperature towards the southeast with increasing elevation indicates the presence of an inverted metamorphic field gradient also seen in the central Backbone Range [START_REF] Beyssac | Late Cenozoic metamorphic evolution and exhumation of Taiwan[END_REF] and likely resulting from intense thrust-fold stacking of the Backbone Range slates [START_REF] Lu | Tectonic evolution of the Taiwan mountain belt[END_REF].

Discussion

New quantitative constraints on temperature and comparison with existing data

The peak temperature estimates using the RSCM method in the northern Hsüehshan Range fit well with patterns exhibited by other geological constraints (Fig. 3B). Zircon fission track ages are completely reset only in the Eocene and lowermost

Oligocene rocks along the highway; and, in contrast, younger formations yield partly-reset and unreset ages [START_REF] Liu | Thermo-kinematic evolution of the Taiwan oblique-collision mountain belt as revealed by zircon fission track dating[END_REF]. The Hsitsun Formation and Szeleng Sandstone are thus both the oldest and highest-grade rock units in the region. The trend of rising temperature between the Shihtsao Fault and the Chatienshan Anticline axis is revealed in both the vitrinite reflectance [START_REF] Chen | Nappe structure revealed by thermal constraints in the Taiwan metamorphic belt[END_REF] and RSCM data sets. The metamorphic temperature assessed from the metamorphic assemblage of a meta-basalt on the northwest limb of the Chatienshan Anticline [START_REF] Shau | Petrology of basaltic rocks from Junghua, Taoyuanhsien, northern Taiwan[END_REF] is also close to RSCM and vitrinite reflectance-derived temperatures of nearby samples. Illite crystallinity data [START_REF] Chen | Application of K-mica crystallinity to the study of stratigraphy and structures in the Hsüehshan and Central ranges of Taiwan[END_REF]) also exhibit a pattern that mirrors the RSCM results: higher temperatures in the cores of the Chatienshan and Hsitsun

Anticlines relative to their flanks, and an increase in grade from hanging wall to footwall across the Tahan Fault (Fig. 3B).

Although the thermal history of the sedimentary succession in the northern Hsüehshan Range had already been investigated using illite crystallinity [START_REF] Chen | Application of K-mica crystallinity to the study of stratigraphy and structures in the Hsüehshan and Central ranges of Taiwan[END_REF], zircon fission track dating [START_REF] Liu | Thermo-kinematic evolution of the Taiwan oblique-collision mountain belt as revealed by zircon fission track dating[END_REF], and vitrinite reflectance (Chen et al., 2011), the RSCM data reported here allows a complementary, locally more precise, documentation of peak temperature which conforms well with geological observations. Illite crystallinity data revealed a thermal trend similar to RSCM, but failed to identify the increasing metamorphic temperatures in the Miocene rocks southeast of the Chatienshan Anticline axis. An increase in temperature there is also suggested by the occurrence of penetrative slaty cleavage absent from argillitic counterparts to the northwest that contain only pencil cleavage (Fig. 3B). The vitrinite reflectance (VR) data clearly indicate the greater extent of heating in the Miocene strata southeast of the Chatienshan Anticline than in the exterior portion of the Hsüehshan Range where peak temperatures are 200°C or lower. The diverse peak heating conditions experienced by the rocks east of the Chatienshan Anticline axis was therefore less discriminated in the VR data, and the peak temperatures from VR are exaggerated west of the anticline axis (Fig. 3B). Such discrepancy between VR and RSCM datasets probably arise because the VR calibrations [START_REF] Barker | Influence of time on metamorphism of sedimentary organic matter in liquid-dominated geothermal systems, western North America[END_REF][START_REF] Barker | Fluid-inclusion technique for determining maximum temperature in calcite and its comparison to the vitrinite reflectance geothermometer[END_REF]) applied in Chen et al. ( 2011) for all the VR data southeast of the cleavage front were poorly constrained, with few constraints for Ro over 3.

Consequently, the VR-temperature calibration is saturated in this temperature range (above ~200-250°C) and becomes insensitive. The new RSCM temperatures based on the calibration of [START_REF] Lahfid | Evolution of the Raman spectrum of carbonaceous material in low-grade metasediments of the Glarus Alps (Switzerland)[END_REF] provide more realistic and reliable peak temperature estimates that agree with structural observations (Fig. 2). In summary, RSCM thermometry within the meta-sediments appears to sensitively reflect variations of maximum temperature, and is therefore useful for estimating peak burial/metamorphic temperatures in the range ~200°C to ~700°C [START_REF] Beyssac | Raman spectra of carbonaceous material in metasediments: a new geothermometer[END_REF][START_REF] Lahfid | Evolution of the Raman spectrum of carbonaceous material in low-grade metasediments of the Glarus Alps (Switzerland)[END_REF]. When coupled with vitrinite reflectance data for rock units heated under ~200°C, the carbonaceous material geothermometers form valuable tools in understanding thermal metamorphism across orogens and basins.

Extent of synorogenic metamorphic overprint

The eastward-and down-section-increasing pattern of peak temperature in the Hsüehshan Range may be interpreted as a combination of static (basin burial) and dynamic (orogen-related) metamorphism. This is best illustrated by the thermal evolution of the Miocene strata. At the start of the Miocene, the Chinese continental margin switched from rifting to drifting, which caused shelf-wide thermal subsidence and facilitated a widespread draping of laterally-continuous margin sediments over the more heterogeneous older units [START_REF] Lin | Cenozoic stratigraphy and subsidence history of the South China Sea margin in the Taiwan region[END_REF][START_REF] Teng | Cenozoic tectonics of the China continental margin: insights from Taiwan[END_REF]). The Miocene sequence, which lacks abrupt lateral facies changes, can be correlated from the Taiwan Strait all the way through the Coastal Plain, and Western Foothills, to the Hsüehshan Range, then as thinner pelagic shale in Backbone Range Slate Belt [START_REF] Teng | Tectonic aspectsof the Paleogene depositional basin of northern Taiwan[END_REF]. Considering the depositional configuration and timing of deposition just prior to the inception of the Neogene arc-continent collision (foreland development at ~6.5

Ma; [START_REF] Lin | Origin of the West Taiwan basin by orogenic loading and flexure of a rifted continental margin[END_REF], these c. 2 km thick Miocene strata most likely underwent only minor diagenetic heating prior to structural disruption. For example, even where their stratigraphic equivalents in the Coastal Plain and the Western Foothills were later covered by foreland molasse sediments [START_REF] Lin | Origin of the West Taiwan basin by orogenic loading and flexure of a rifted continental margin[END_REF][START_REF] Simoes | Investigating the kinematics of mountain building in Taiwan from the spatiotemporal evolution of the foreland basin and western foothills[END_REF], Miocene sediments were not heated above 100°C (e.g. [START_REF] Chen | Explanatory notes for the Metamorphic Facies Map of Taiwan, 2 nd ed[END_REF][START_REF] Kuo | Application of vitrinite reflectance on hydrocarbon exploration in western Taiwan[END_REF]. The ~240°C peak temperature (~200°C temperature increment) recorded by RSCM in the Miocene Sule Formation along the Northern Cross Island Highway is therefore best explained by orogenic loading, since it is equivalent to unmetamorphosed Miocene Mushan to Nankang formations in the Western Foothills (e.g. [START_REF] Teng | Tectonic aspectsof the Paleogene depositional basin of northern Taiwan[END_REF]. Assuming a geothermal gradient of 25~30°C/km, ~8 km of overburden would be needed to match the observations. This geothermal gradient is consistent with the change of 25-30°C/km in RSCM temperature with stratigraphic depth in the central HR range [START_REF] Beyssac | Late Cenozoic metamorphic evolution and exhumation of Taiwan[END_REF], the average thermal gradient in exploration wells in Taiwan [START_REF] Zhou | Modeling of thermo-rheological structure of lithosphere under the foreland basin and mountain belt of Taiwan[END_REF], and it yields reasonable results in thermomechanical models [START_REF] Simoes | Mountain building in Taiwan: a thermokinematic model[END_REF] and thermobarometry (e.g. Kidder et al., 2013a).

Depositional patterns of the Eo-Oligocene strata in the Hsüehshan Range indicate isolated eastward-deepening half-grabens with considerable regional variation [START_REF] Teng | Cenozoic tectonics of the China continental margin: insights from Taiwan[END_REF], much different from their Miocene counterparts. The eastward rise of RSCM temperatures in these rocks may correspond in part to increased basin burial depths (Fig. 3B), thus the relative roles of static and dynamic metamorphism cannot be clearly discerned for these rocks.

Additional salient information regarding the extent of the synorogenic metamorphic overprint in the northern Hsüehshan Range metasediments is the anomalously low apparent geothermal gradient derived from RSCM data. Along the investigated Northern Cross-Island Highway transect, the sediment succession from the Eocene Hsitsun Formation to the Miocene Sule Formation is estimated to be about 6 km thick [START_REF] Teng | Tectonic aspectsof the Paleogene depositional basin of northern Taiwan[END_REF]. The apparent paleothermal field gradient calculated from the RSCM temperatures in the region is below 20°C/km for both the SE limb of the Chatienshan Anticline (based on values of ~270°C in the lower Oligocene Kankou Formation and ~240°C in the Miocene Sule Formation) and from Paling to the core of the Hsitsun Anticline (using values of ~350°C in the Eocene Hsitsun Formation to ~240°C in the Miocene Sule Formation; Fig. 3B). Such values are improbably small for an actively rifting continental margin, since for example, the contemporary geothermal gradient measured and integrated in the Qiongdongnan and Pearl River

Mouth basins along the northern border of the South China Sea is mostly above 30°C/km [START_REF] Yuan | Uniform geothermal gradient" and heat flow in the Qiongdongnan and Pearl River Mouth Basins of the South China Sea[END_REF]. We conclude that the apparent geothermal gradient in northern Hsüehshan Range has thus been reduced by about one-third its original value.

A likely mechanism for the low apparent geothermal gradient is folding that predates peak metamorphism. This results in fold cores with temperatures more similar to temperatures in their limbs than would occur if metamorphism predated folding. We postulate that the presently-exposed continental margin sediment sequence of ~6 km thickness was substantially folded while being underthrust to ~8 km depth where it experienced peak metamorphism, while the vertical distance between the top and bottom of this segment (i.e. Miocene Sule and Eocene Hsitsun formations) had been reduced to ~4 km among the anticline and syncline cores (Fig. 5 A-B) to produce the low, ~20°/km apparent geothermal gradient. A simple model of fold limbs tilted at ~45° prior to peak metamorphism could reduce the apparent geothermal gradient by one-third, i.e. from 30 to 20°/km. Such treatment corresponds with ~33% shortening and ignores the effects of potential vertical thickening and time-dependent aspects of heat diffusion related to the likelihood that folding and metamorphism partly overlapped in time. Assuming that peak metamorphism roughly coincided with peak burial and underplating, a minor amount of folding and shortening followed underplating in order to bring fold limbs to their present geometries (~55° dip, Fig. 5C).

We find it unlikely that post-metamorphic thickening significantly contributed to lowering the apparent geothermal gradient because of the absence of structural or microstructural evidence for this process. For example, there does not appear to be a greater abundance of minor faults or internal penetrative deformation in the North relative to the central Hsüehshan Range, where the apparent geothermal gradient is similar to the interpreted ancient one [START_REF] Beyssac | Late Cenozoic metamorphic evolution and exhumation of Taiwan[END_REF]. Undetected penetrative thickening is, in any case, less likely in the North due to low temperatures that inhibited deformation mechanisms such as dislocation creep that facilitated penetrative strain in the central Hsüehshan Range (e.g. [START_REF] Kidder | Constraints from rocks in the Taiwan orogen on crustal stress levels and rheology[END_REF]. This interpretation of folding prior to metamorphism in the Northern Hsüehshan Range differs from what is interpreted in the central Hsüehshan Range, where a ~30°C/km gradient is preserved, and folding and faulting were interpreted to postdate peak heating [START_REF] Beyssac | Late Cenozoic metamorphic evolution and exhumation of Taiwan[END_REF].

The RSCM results in the northern Backbone Range Slate Belt along the Taipingshan Road transect are also indicative of dynamic metamorphism overprinting basinal diagenesis. As argued above for the Miocene sediments of the Hsüehshan Range, the Miocene Lushan Formation was incorporated into the Taiwan orogenic pile soon after its deposition, giving little time for diagenetic heating prior to deformation. Peak metamorphic temperatures range from ~230°C in Tuchang/Jhongjian to over 290°C around Cueifong Lake. These temperatures also indicate heating beneath ~8 km of tectonic overburden if a geothermal gradient of ~30°C/km is assumed. The Lushan formation originated as pelagic sediments covering the rifted/drifted Chinese continental margin basement, and sedimentary criteria indicate that the present thickness of the original Lushan sedimentary sequence was likely thinner than the modern 2 km thickness (e.g. [START_REF] Teng | Geotectonic evolution of Tertiary continental margin basins of Taiwan[END_REF], thus the Lushan sediments are interpreted as including several duplexes. The presence of an inverted metamorphic field gradient in an accretionary setting can also be well explained by a significant component of deformation concurrent with peak metamorphic conditions (e.g. Kidder et al., 2013b).

Nappe stacking structure of the Taiwanese slate belt and implications for wedge kinematics

The new RSCM and structural data reported in the present study, along with existing geothermometric, thermochronological, and structural data of the entire slate belt (e.g. [START_REF] Clark | Kinematic analyses of the Hsueshan Range, Taiwan: a large-scale pop-up structure[END_REF][START_REF] Liu | Thermo-kinematic evolution of the Taiwan oblique-collision mountain belt as revealed by zircon fission track dating[END_REF][START_REF] Fuller | A thermomechnical wedge model of Taiwan constrained by fission-track thermochronometry[END_REF][START_REF] Beyssac | Late Cenozoic metamorphic evolution and exhumation of Taiwan[END_REF][START_REF] Brown | The structure and kinematics of central Taiwan mountain belt derived from geological and seismicity data[END_REF][START_REF] Simoes | Late Cenozoic metamorphism and mountain building in Taiwan: A review[END_REF][START_REF] Chen | Nappe structure revealed by thermal constraints in the Taiwan metamorphic belt[END_REF][START_REF] Chen | Basal accretion, a major mechanism for mountain building in Taiwan revealed in rock thermal history[END_REF]) permit a comprehensive evaluation of the geological structures in this part of the orogenic wedge and their potential origin. The Lishan fault-which moved the Hsüehshan Range up relative to the Backbone Range-is poorly exposed in the study area, but its presence is substantiated by an RSCM temperature gap of more than 40°C in the studied transect (Fig. 3B), as well as by structural analyses [START_REF] Lee | Polyphase history and kinematics of a complex major fault zone in the northern Taiwan mountain belt: the Lishan Fault[END_REF][START_REF] Brown | The structure and kinematics of central Taiwan mountain belt derived from geological and seismicity data[END_REF]. The enhanced exhumation on the Hsüehshan Range side of the Lishan Fault is consistent with inversion of the Hsüehshan Trough basin controlled by the geometry of the underlying basement. In this scenario, the basement high east of the Hsüehshan Trough (inherited from Paleogene rifting related to the opening of the South China Sea) pushed the Hsüehshan Range sequences from the east when it subducted beneath the accretionary prism, and evolved into the Tananao Complex now exposed in eastern Central Range. We interpret that the basement-Hsüehshan Range contact developed into a major backthrust, the Lishan Fault, while the Tananao Complex is not yet exposed in the footwall due to being covered by the Lushan Formation (Fig. 6B).

The synorogenic origin of peak metamorphism hypothesized in the previous section strengthens the argument that the northern Hsüehshan Range entered the Taiwan orogenic wedge via basal accretion (as the 'lower nappe' of [START_REF] Chen | Nappe structure revealed by thermal constraints in the Taiwan metamorphic belt[END_REF] rather than frontal accretion. The present peak metamorphic temperature configuration indicates that the lower nappe was underthrust beneath the then accretionary prism (to ~8 km depth) and substantially folded before been incorporated into the orogenic wedge and exhumed. By combining RSCM temperatures, structural data (from km-scale faults and folds, to preliminary microstructural observations), and stratigraphic data, this basal-accreted terrain can be subdivided into two thrust duplexes/nappes (Fig. 6A): the exterior duplex being is the hanging wall of the Shihtsao Fault; the interior one the hanging wall of the Tahan Fault. The exterior duplex consists of relatively younger and more proximal margin sediments (the early Oligocene Kankou Formation to the Miocene Sule Formation) and exhibits generally lower RSCM temperatures (Fig. 3B). This exterior duplex has a simple, uniformly SE-dipping slaty cleavage that likely originated in the inner/proximal portion of the Hsüehshan Trough half-graben system [START_REF] Teng | Tectonic aspectsof the Paleogene depositional basin of northern Taiwan[END_REF]. In contrast, the interior duplex is composed of the older and more distal Eo-Oligocene sequence from the Hsitsun Formation to the Paling Formation (the temporal equivalent of the more-proximal Kankou, Tsuku and Tatungshan formations). The interior duplex also has higher RSCM temperatures, and stronger cleavage development and overall deformation.

These lower nappe duplexes were basally accreted into the wedge at depths of about 8 km, as determined above based in part on their RSCM temperatures.

The Backbone Range Slate Belt can be characterized as a synclinorium [START_REF] Yang | Geologic structures of the Tananao Schist and its cover in the Tayuling area, Central Range of Taiwan[END_REF], and nappe stack in which the original sedimentary succession has been multiply folded and transposed, with severely sheared and boudinaged tectonic blocks of various lithologies locally defining broken formations [START_REF] Lu | Tectonic evolution of the Taiwan mountain belt[END_REF]. Synorogenic prograde heating occurred when these pelagic sediments were underthrust along with the margin basement beneath the accretionary prism. The rapid southeastward rise of peak temperature determined by RSCM, along with the transition from slate to phyllite, indicates the progressive exhumation of more deeply buried units to the east. These were brought up by reverse faulting and shortening related to the duplexing of the Tananao Complex to the east (the purple unit in Fig. 6B; [START_REF] Fisher | Taiwan Slate Belt: Insights into the ductile interior of an arc-continent collision[END_REF][START_REF] Beyssac | Late Cenozoic metamorphic evolution and exhumation of Taiwan[END_REF][START_REF] Simoes | Mountain building in Taiwan: a thermokinematic model[END_REF][START_REF] Malavieille | Impact of erosion, sedimentation, and structural heritage on the structure and kinematics of orogenic wedges: Analog models and case studies[END_REF][START_REF] Malavieille | Deformation partitioning in mountain belts: insights from analogue modeling experiments and the Taiwan collisional orogen[END_REF], where a second, deeper underplating zone is likely located [START_REF] Simoes | Mountain building in Taiwan: a thermokinematic model[END_REF]. The rise of RSCM temperature along the Taipingshan Road likely corresponds to cryptic reverse faults which root into the basal thrust (Fig. 6A).

The pre-convergence setting of the Chinese continental margin is illustrated in Fig. 6C, in which the difference between Paleogene (Eo-Oligocene) and Miocene depositional patterns is highlighted. In the initial stage of collision, the rifted continental margin sliver east of the Hsüehshan Trough half-graben [START_REF] Lu | Tectonic evolution of the Taiwan mountain belt[END_REF][START_REF] Shyu | Tandem suturing and disarticulation of the Taiwan orogen revealed by its neotectonic elements[END_REF] and its overlying Miocene pelagic sediments, were underthrust and buttressed against the leading edge of the Philippine Sea Plate. This caused intense deformation and heating of the Miocene sequence (the present-day Backbone Range slate). This couplet of deformed proto-Tananao Complex and metamorphosed cover sequence then bulldozed the more inboard graben sediments as convergence continued. The deepest and easternmost distal succession within the trough was driven towards the foreland, forming the 'upper nappe' unit [START_REF] Chen | Nappe structure revealed by thermal constraints in the Taiwan metamorphic belt[END_REF] now exposed in south-central Hsüehshan Range. This foreland-ward transport of the statically metamorphosed [START_REF] Beyssac | Late Cenozoic metamorphic evolution and exhumation of Taiwan[END_REF] 'upper nappe' unit was synchronous with tectonic underplating and duplexing of the underthrust proximal Hsüehshan Trough sediments that make up the 'lower nappe' units described in this study. The timing of this event was ~6 -2.5 Ma [START_REF] Chen | Basal accretion, a major mechanism for mountain building in Taiwan revealed in rock thermal history[END_REF], and it culminated with the uplift and exhumation of the present day Hsüehshan Range. This description implies the presence of multiple active detachment faults in the formation and growth of the Taiwanese accretionary wedge [START_REF] Malavieille | Impact of erosion, sedimentation, and structural heritage on the structure and kinematics of orogenic wedges: Analog models and case studies[END_REF][START_REF] Malavieille | Deformation partitioning in mountain belts: insights from analogue modeling experiments and the Taiwan collisional orogen[END_REF]. The dynamically metamorphosed lower nappe unit is able to crop out in the northern Hsüehshan Range because the northern region has experienced a longer history of uplift and denudation due to oblique and southward-propagating convergence [START_REF] Suppe | Mechanics of mountain building and metamorphism in Taiwan[END_REF]. The upper nappe unit in the north, which presumably resembled the central Hsüehshan Range rocks, has eroded away.

Based on the thermokinematic history of the lower nappe unit rocks described above, it is likely that shortening of the Hsüehshan Range rocks likely continues at depth within the orogenic wedge. Such shortening is completed before reaching the surface, since the GPS velocity field (e.g. [START_REF] Yu | Velocity field of GPS stations in the Taiwan area[END_REF][START_REF] Lin | GPS crustal deformation, strain rate, and seismic activity after the 1999 Chi-Chi earthquake in Taiwan[END_REF] demonstrates that the Hsüehshan Range is extensional at the surface. The Backbone Range slate, however, comprising rift basement cover sediments, continues to be faulted, folded and uplifted together with the thrust stacking and doming of the Tananao Complex [START_REF] Fisher | Taiwan Slate Belt: Insights into the ductile interior of an arc-continent collision[END_REF][START_REF] Malavieille | Impact of erosion, sedimentation, and structural heritage on the structure and kinematics of orogenic wedges: Analog models and case studies[END_REF][START_REF] Yui | Overturned' marble layers: evidence for upward extruion of the Backbone Range of Taiwan[END_REF].

The temperature-time and structural evolution of the Taiwanese slate depicted above suggests that the Neogene Taiwan arc-continent collision involved significant dynamic heating, particularly by tectonic overloading of underthrusted rocks. Since the cover sediments are found to have experienced prominent synorogenic metamorphism, structurally lower basement rocks are likely to have as well. From this perspective, we find the hypothesis that the Taiwan Orogen experienced little prograde metamorphism, particularly in the Tananao Basement Complex [START_REF] Yamato | Taiwan mountain building: insights from 2-D thermomechanical modeling of a rheologically stratified lithosphere[END_REF][START_REF] Wintsch | Geochronologic evidence for a cold arc-continent collision: The Taiwan orogeny[END_REF], unlikely. In addition to inferences from the metamorphism of overlying sedimentary sequences, heating associated with the Neogene orogeny in the Tananao Basement Complex has been firmly documented in analyses of thermochronologic records (e.g. [START_REF] Lo | Rejuvenation of K-Ar systems for minerals in the Taiwan Mountain Belt[END_REF][START_REF] Fuller | A thermomechnical wedge model of Taiwan constrained by fission-track thermochronometry[END_REF] and is supported by thermal metamorphic modeling [START_REF] Beyssac | Late Cenozoic metamorphic evolution and exhumation of Taiwan[END_REF][START_REF] Simoes | Late Cenozoic metamorphism and mountain building in Taiwan: A review[END_REF]. Our results also suggest that basins atop highly extended continental margins are able to subduct as a whole to upper-mid crustal depths before being inverted and exhumed. This may have implications for the rheological and tectonic evolution of subducting continents in arc-continent and continent-continent collision zones.

Concluding remarks

New RSCM temperature data in northern Taiwan provide evidence of synorogenic metamorphism related to the recent arc-continent collision. Using thermal, structural, and stratigraphic data, we postulate that synorogenic metamorphosed slates in the Hsüehshan Range were underthrust to about 8 km depth and thickened via folding before being basal accreted into the orogenic wedge as two major duplexes. The slates in the northern Backbone Range Slate Belt were tectonically loaded under similar conditions and developed an inverted metamorphic field gradient while being exhumed with the basement Tananao Complex. The inferred deformation was likely accommodated along multiple, possibly simultaneously active, detachment levels.

Tectonic underplating is a key mechanism, along with frontal accretion and surface erosion, in the growth, uplift and deformation of the Taiwanese orogenic wedge.

Prograde heating is an integral part of the Neogene orogeny and should be considered in kinematic reconstructions.

Figure captions

Fig. 1: Geologic strip map along the investigated Northern Cross-Island Highway (Hsüehshan Range) and Taipingshan Road (Backbone Range Slate Belt) transect.

Inset shows tectonic framework of Taiwan with the slate belt highlighted.

Locations of the RSCM samples are marked in yellow and listed in Table 1. [START_REF] Chen | Nappe structure revealed by thermal constraints in the Taiwan metamorphic belt[END_REF], illite crystallinity [START_REF] Chen | Application of K-mica crystallinity to the study of stratigraphy and structures in the Hsüehshan and Central ranges of Taiwan[END_REF], zircon fission track [START_REF] Liu | Thermo-kinematic evolution of the Taiwan oblique-collision mountain belt as revealed by zircon fission track dating[END_REF] and igneous petrology [START_REF] Shau | Petrology of basaltic rocks from Junghua, Taoyuanhsien, northern Taiwan[END_REF]. Table 1: RSCM temperature data obtained along the Northern Cross-Island Highway 948 and the Taipingshan Road. Temperatures are calculated following Beyssac et al. 949 (2002) and [START_REF] Lahfid | Evolution of the Raman spectrum of carbonaceous material in low-grade metasediments of the Glarus Alps (Switzerland)[END_REF]

1 .

 1 Figure captionsTable

  projected on a NW-trending profile in Fig.3B.The Raman spectra of carbonaceous material of latest-Oligocene to Miocene indurate argillites and siltite in the outer fringe of the Hsüehshan Range between the Chuchih and Loshan faults (around Fushin and Kaopo villages) are indicative of immature carbonaceous material suggesting peak temperature <200°C (samples NCIH05 to 06-1). These low temperatures are consistent with the argillitic texture of the samples (Fig.2A and D). Immediately to the southeast of the Loshan Fault, the temperature estimated using the RA1 parameter displays a relatively linear increase from just above 200°C to around 270°C (NCIH07 to 11) at the core of the Chatienshan Anticline (from Miocene Sule Formation down-section to the early Oligocene Kankou Formation). The onset of slaty cleavage development corresponds to an RSCM temperature of 245-265°C (NCIH07-1 and 09). The RSCM temperature drops to 230 -240°C in the slaty Sule Formation on the southern limb of the Chatienshan Anticline (NCIH12 to 16;[START_REF] Chen | Basal accretion, a major mechanism for mountain building in Taiwan revealed in rock thermal history[END_REF] but the rocks still retain slaty cleavage (Fig.2B and E). The temperature increases again to ~280°C (NCIH17, 18) in the Oligocene Paling Formation in the hanging wall adjacent to the Tahan Fault.The degree of graphitization of carbonaceous material within the slates continues to rise towards the core of the Hsitsun Anticline. The Oligo-Eocene Szeleng Sandstone at Szeleng has an RSCM temperature of 300 -310°C (NCIH21, 22 Fig.2F), and the Eocene Hsitsun Formation at Mingchih reaches 350°C (NCIH23; Fig.2C). Further southeast the estimated peak temperature is scattered between 260°C to 300°C(NCIH25, 26, 29, and 31). Overall the RSCM temperature increases down-section such that the older and deeper formations of the original sedimentary succession are hotter than overlying units. RSCM temperatures also increase eastward within the Sule Formation (between the limbs of the Chatienshan Anticline), and within the Oligocene strata exposed in the Chatienshan Anticline and hanging wall of the Tahan Fault (the Kankou-Tsuku-Tatungshan series and Paling Formation).

Fig. 2 :

 2 Fig. 2: (A~C) Macroscopic occurrence of outcrops: (A) sand-shale interbedded argillite with only faint pencil cleavage in the Miocene series northwest of cleavage front (site NCIH05-1), width of view ~1 m; (B) meta-siltstone with spaced pressure-solution foliation near NCIH15 in the Miocene series on the southeastern limb of the Chatienshan Anticline; (C) Eocene slate with strong penetrative cleavage at NCIH23. (D~I) Microscopic observation of samples, with 1 mm white scale bars: (D) non-foliated latest Oligocene argillite/siltstone (NCIH07); (E) latest Oligocene argillite/slate with initial cleavage development through pressure solution (NCIH12); (F) Eocene meta-sandstone with sutured quartz grains packed between well-developed cleavage domains (NCIH22, cross-polarized view); (G) Oligocene slate with penetrative slaty cleavage (TPS01); (H) Miocene slate east of the Lishan Fault with patchy pressure solution seams in initial slaty cleavage development (TPS02); (I) Miocene phyllitic slate with dense foliation, growths of quartz porphyroblasts and mica-chlorite aggregates (TPS13).

Fig. 3 :

 3 Fig. 3: (A) Geologic profile of the studied transect along the Northern Cross-IslandHighway (NCIH) and Taipingshan Road (TPS) (location marked in Fig.1), distribution of the RSCM samples (projected), and structural data acquired in this study. (B) Calculated maximum temperature from RSCM measurements of the transect profile, along with published data of vitrinite reflectance(Chen et al.,

Fig. 4 :

 4 Fig. 4: Raman spectra of CM obtained in the Hsüehshan Range along the Northern Cross-Island Highway (NCIH-06, 07, 09, 17, 20, 21, 23) and the Backbone Range Slate Belt (TPS-02, 04) with the D and G composite bands indicated. The RA1/R2 ratio and corresponding RSCM temperature calculated for each of the depicted spectrum are specified.

Figure 1 .

 1 Figure 1.

  Figure 2.

  Figure 3.

  Figure 4.

  Figure 5.

Figure 6 .

 6 Figure 6.

  . 950

	Sample Longitude Latitude Altitude,m Spectra # RA1/R2 ratio	sdv	T,°C	1σ, °C	se, °C
	Northern Cross Island Highway transect
	NCIH05	121.3647 24.7900	389	9	0.50	0.011	<200
	NCIH05-1 121.3580 24.7822	363	7	0.49	0.008	<200
	NCIH06	121.3603 24.7721	487	12	0.52	0.008	<200
	NCIH06-1 121.3474 24.7592	499	11	0.52	0.011	<200
	NCIH07	121.3494 24.7530	510	12	0.55	0.002	211	3	0.87
	NCIH07-1 121.3564 24.7504	587	17	0.57	0.004	244	5	1.21
	NCIH09	121.3567 24.7286	582	10	0.59	0.004	263	5	1.58
	NCIH10	121.3551 24.7151	557	17	0.59	0.004	273	5	1.21
	NCIH11	121.3622 24.7088	606	15	0.59	0.003	268	4	1.03
	NCIH12*	121.3605 24.6998	632	14	0.57	0.004	246	5	1.34
	NCIH13	121.3437 24.6991	654	14	0.56	0.004	225	5	1.34
	NCIH15	121.3812 24.6804	563	18	0.56	0.004	236	5	1.18
	NCIH16	121.3923 24.6747	676	10	0.57	0.004	247	5	1.58
	NCIH17	121.4045 24.6747	685	16	0.60	0.004	277	5	1.25
	NCIH18	121.4076 24.6717	730	12	0.60	0.004	283	5	1.44
	NCIH19	121.4098 24.6534	903	11	0.59	0.003	271	4	1.21
	NCIH20	121.4173 24.6453	919	14	0.61	0.004	292	5	1.34
	NCIH21	121.4328 24.6486	1170	15	0.62	0.004	309	5	1.29
	NCIH22	121.4418 24.6454	1112	11	0.62	0.003	302	4	1.21
	NCIH23	121.4517 24.6404	1121	15	0.65	0.022	351	10	2.58
	NCIH25	121.4838 24.6424	1203	13	0.61	0.003	298	4	1.11
	NCIH26	121.4818 24.6316	1203	13	0.61	0.004	293	5	1.39
	NCIH29	121.4942 24.6057	651	14	0.59	0.002	266	3	0.80
	NCIH31	121.5126 24.5980	412	14	0.60	0.003	281	4	1.07
	Taipingshan Road transect
	TPS01	121.4945 24.5768	394	17	0.59	0.003	269	4	0.97
	TPS02	121.5000 24.5600	504	24	0.56	0.004	227	5	1.02
	TPS04	121.5127 24.5458	739	17	0.57	0.004	239	5	1.21
	TPS08	121.5242 24.5110	1844	12	0.60	0.004	275	4	1.15
	TPS10	121.5372 24.4963	1915	12	0.61	0.004	291	5	1.44
	TPS13	121.6075 24.5110	1883	11	0.61	0.004	296	4	1.21

*: data from

Chen et al., 2018. 951

Acknowledgements

Constructive comments from Guest Editor J. Ali, G. Molli and an anonymous reviewer are deeply appreciated. Support for C.-T. Chen was supplied by MOST grants 106-2119-M-008-023 and 107-2116-M-008-004, S. Kidder was supplied by NSF grant EAR-1524602.

Cross-Island Highway in the northern Hsüehshan Range: (I) pre-orogenic setting of the strata in a ~6 km deep basin on the rifted continental margin; (II) peak thermal/metamorphic state when the continental margin was incorporated in the orogenic system and the northern Hsüehshan units were overridden by the accretionary prism, presumably the 'upper nappe unit' now found in the central-southern Hsüehshan Range; (III) retrograde exhumation after basal accretion with continued shortening. Please refer to section 4.2 for details. [START_REF] Teng | Cenozoic tectonics of the China continental margin: insights from Taiwan[END_REF][START_REF] Simoes | Mountain building in Taiwan: a thermokinematic model[END_REF][START_REF] Chen | Nappe structure revealed by thermal constraints in the Taiwan metamorphic belt[END_REF][START_REF] Chen | Basal accretion, a major mechanism for mountain building in Taiwan revealed in rock thermal history[END_REF].