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Journal of the Geological Society

Late Neogene slow exhumation of the Greater Himalaya in

Yadong area, the transition between the Central and the

Eastern Himalaya

An Wang, Kyoungwon Min, Guocan Wang, Kai Cao, Tianyi Shen, Pengfei

Jiang & Jiangwei Wei

DOI: https://doi.org/10.1144/jgs2018-186

Received 18 October 2018

Revised 11 May 2019

Accepted 14 May 2019

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Late Neogene slow exhumation of the Greater Himalaya in Yadong area, the transition

between the Central and the Eastern Himalaya

An Wang1,2

*, Kyoungwon Min3, Guocan Wang

1,2, Kai Cao

1,2, Tianyi Shen

1, Pengfei Jiang

1,

Jiangwei Wei1

1. School of Earth Sciences, Center for Global Tectonics, China University of Geosciences,

Wuhan, China

2. State Key Laboratory of Geological Processes and Mineral Resources, China University of

Geosciences, Wuhan, China

3. Department of Geological Sciences, University of Florida, Gainesville, Florida, USA

*Correspondence ([email protected])

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Abstract: The Yadong area, at the geographic boundary between the Central and Eastern

Himalaya, contains the largest along-strike structural discontinuity in the Himalaya. We

conducted zircon fission track (ZFT) and apatite (U-Th)/He (AHe) dating along a 50-km

transect of the Greater Himalaya Sequence (GHS) to investigate cooling and exhumation of

this structural discontinuity. New ZFT (14.0 - 8.2 Ma) and AHe (11.1 - 4.2 Ma) data suggest

fast tectonic exhumation of the GHS in the Middle Miocene. After this pulse of rapid

exhumation, the area remained in a long-term slower and steady exhumation (~0.32 km/Ma

since ~7.7 Ma). New stream topographic analysis also confirms this slow surface erosion in

hinterland, whereas the frontal range underwent enhanced erosion possibly due to southward

up-thrusting along the Main Boundary Thrust and Main Frontal Thrust. Our data underline

distinctive exhumation patterns between the Eastern and Central Himalaya, and suggest that

exhumation of the Himalaya was primarily driven by tectonics associated with the underlying

Main Himalaya Thrust (MHT). The long-term, slow and steady exhumation in the Yadong

area and the Eastern Himalaya hinterland since Late Neogene is consistent with a gentle dip

of the MHT, supporting a slab tear of the subducting Indian Plate.

Keywords: Fission track; (U-Th)/He; exhumation history; Himalaya; Yadong;

The Himalaya, one of the most actively growing orogenic belts on the earth (Bilham et al.

1997; Lave & Avouac 2000; Yin 2006; Berthet et al. 2014) serves as a natural prototype to

investigate the tectonic and topographic developments of continental collision, as well as the

dynamic interaction between the earth tectonics and surface processing (Molnar 1990; Willett

1999; Beaumont et al. 2001; Zeitler et al. 2001; Zhang et al. 2001; Herman et al. 2014). For

the past decades, numerous studies focused on the tectonic and chronological significance of

large-scale, orogenic-parallel boundary structures, including the Main Central Thrust (MCT),

South Tibetan Detachment System (STDS), Main Boundary Thrust (MBT) and the Main

Frontal Thrust (MFT). These studies can be classified into three major groups regarding the

deformation pattern and its driving force. The first group of studies suggests that the major

thrust faults, including the MCT, MBT and MFT, were developed as a piggy-back

propagation in deformation (Le Fort 1975; Arita et al. 1997; Lave & Avouac 2000;

McQuarrie et al. 2014). This view implies that most of the recent deformation occurred in the

frontal range where the MFT is located (Lave & Avouac 2000; Berthet et al. 2014). The

second group of studies are primarily based on structural and geochronological evidence in

the hinterland of the Himalaya (Copeland et al. 1991; Harrison et al. 1997; Catlos et al. 2001;

Whipp et al. 2007; Herman et al. 2010), and suggests an out-of-sequence deformation in the

Himalaya. This view implies that a previously unidentified large-scale thrust fault, which

approximately lies at the toe of the High Himalaya coinciding with the range physiographic

transition zone (Hodges et al. 2001; Wobus et al. 2003; 2005), accommodated the current

shortening in the Himalaya. The out-of-sequence thrusting fault is further supported by field

evidence of thrust faults in Central Himalaya (Hodges et al. 2004). The third group of studies

accepts the general concept of the out-of-sequence deformation, but further proposes that the

Himalaya hinterland deformation was driven by a buried ramp in the Main Himalaya Thrust

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(MHT) (Jackson & Bilham 1994; Cattin & Avouac 2000; Coutand et al. 2014; Landry et al.

2016), which may have formed the Lesser Himalaya duplex (Schelling & Arita 1991;

Robinson et al. 2003; Bollinger et al. 2004; 2006). Concerning the driving force of rock

uplift in the Himalaya, the last two models prefer that the Himalaya hinterland was driven or

motivated by the climate-induced, intense surface denudation although its detailed

mechanism is yet to be understood (Beaumont et al. 2001; Zeitler et al. 2001).

It is apparent that a spatially and temporally comprehensive exhumation history provides

important clues for reconstructing tectonic developments of the Himalaya. The Yadong

transect, which is located at the transition between the Central and the Eastern Himalaya,

provides a unique opportunity to investigate structural variations along strike of the Himalaya.

In this paper, we report new zircon fission track (ZFT) and apatite (U-Th)/He (AHe)

thermochronological data obtained from the Greater Himalaya Sequence (GHS) transect

spanning ~50 km in the Yadong area. These data present evidence for slow and steady

exhumation in the GHS since ~7.7 Ma, which is distinctive from the rapid exhumation

identified at the equivalent locations along strike in the Central Himalaya (Copeland et al.

1991; Harrison et al. 1997; Catlos et al. 2001; Whipp et al. 2007; Herman et al. 2010).

Available thermochronological and topographic data support that the exhumation of the

Himalaya was more significantly controlled by tectonics, which varies both parallel to and

normal to the orogen, rather than by climatic factors.

Geological Settings

The Himalaya is widely deemed as an archetype orogen for continental collision

(Rowley 1996; Hodges 2000; Yin 2006). As is mentioned, its structures are dominated by

four large-scale, orogenic-parallel faults of the STDS, MCT, MBT and the MFT from north

to the south, which slice the Himalaya into a series of orogenic wedges. The Tethyan

Himalaya sequence (THS), which consists of Phanerozoic Gondwana sequence, overlies the

STDS and occurs as the topmost tectono-stratigraphic unit. The GHS, which is structurally

sandwiched by the STDS and the MCT, is composed of the most highly metamorphosed and

deformed rocks (Schelling 1992; Harrison et al. 1997; DeCelles 2000; Kali et al. 2010).

Among these wedges, the GHS coincides with the high topography along strike of the

Himalaya (Fig. 1), and occurs as a key component of the Himalaya construction. It remains

puzzling that the GHS’s upper structural boundary (=STDS) shows the evidence of

large-scale NS extension, whereas its lower boundary (=MCT) as well as other common

structures yield evidence of NS convergence. A number of chronological data support that

both the STDS and MCT were active during the Early-Middle Miocene (Kellett et al. 2010;

Tobgay et al. 2012).

The Yadong transect is located at the geographic boundary between the Central and the

Eastern Himalaya, which coincides with the largest structural discontinuity along strike of the

Himalaya. The structure in Yadong area is manifested by NNE-striking grabens and rifts (Fig.

1), which constitute the southern part of the regional NS graben system, the Yadong-Gulu

Rift, extending ~550 km in southern Tibet (Wu et al. 1998). Consistent with many

NS-striking extensional structures following ductile deformation of the STDS in Middle

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Miocene in southern Tibet (Wang et al. 2006; Kali et al. 2010; Leloup et al. 2010; Kellett et

al. 2013), the Yadong rifting is suggested to have initiated before ~8 Ma based on

thermochronologies (Edwards & Harrison 1997; Kapp 2005).

The Yadong transect comprises rocks of the GHS and the THS, which are structurally

separated by the STDS. The GHS rocks along the Yadong transect consist of granitic

migmatites, gneissic rocks and schists (Fig. 1). These rocks were derived from Precambrian

pelitic protoliths with minor intermediate-basic volcanic rocks, which experienced

amphibolite-facies metamorphism at relatively high-pressure conditions. Chronological

studies of the GHS in Central Himalaya yield Neoproterozoic ages for its protolith (DeCelles

2000).

The Yadong transect experienced multi-stage ductile deformations evidenced by gneissic

foliations that are the most common penetrative structure in the area. Re-deformed hook folds

and disharmonic folds are common with axial planes parallel to the gneissic foliations,

indicating they structurally substituted the preexisting folds (possibly of the Pan-African

timing) during deformation of Neogene gneissic foliations. Although both the top and the

bottom boundaries of the GHS dip to the north, gneissic foliations throughout the GHS are

not monotonous. Numerous EW-striking folds superimposed on these penetrative foliations.

The south of the Yadong town is structurally characterized by upright folds (axial plane

vertical and hinge horizontal) with gentle-open interlimb angles (Fig. 1). Field investigations

indicate that these EW-striking folds can be well traced along strike. Along strike of the

Himalaya, the Paro window to the east in Bhutan and the Darjeeling window to the west are

probably equivalent structures of the major anticline system developed to the south of

Yadong town (Fig. 1). It is apparent that these EW-striking folds superimposed on gneissic

foliations are of NS-convergent deformation postdating the GHS ductile positioning by the

MCT and STDS.

An impressive structural feature of the GHS is the Miocene leucogranite intrusions in the

form of dykes and stocks with or without deformation (Fig. 1). These leucogranites are

considered to have been derived from melting of pelitic and gneissic rocks in the GHS,

during either crustal thickening or decompression by rapid slip of the STDS (Harris et al.

1993; Harrison et al. 1999; Zhang et al. 2004; Streule et al. 2010). Along the Yadong transect,

outcrops show two major Neogene leucogranite intrusion bodies, the Dingga intrusion in the

north and the Gaowu intrusion in the south (Fig. 1). These leucogranite intrusions occurred

during Early-Middle Miocene (Wu et al. 1998; Gou et al. 2016; Liu et al. 2017).

Leucogranite dykes developed at the structural top of the GHS are often deformed into

boudinages parallel to structural foliations, indicating ductile deformation by extension of the

STDS. Undeformed leucogranite dykes crosscutting these structural foliations yield U/Pb

ages of 15.8 – 16.3 Ma (Liu et al. 2017), suggesting a minimum age of cessation of ductile

deformation in the STDS.

The THS rocks, which are typically composed of carbonites and clastic sediments, are

characterized by flysch sequence derived from the passive margin of Indian continent. Most

THS rocks experienced low-grade metamorphism and deformation in Cenozoic, developing

EW-striking fold-and-thrust structures. In the Yadong section, Paleogene sequence preserved

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to the north of the STDS was involved into folding and thrusting, indicating significant

NS-shortening in Neogene.

Approaching to the south, the bottom of the THS are ductile-deformed by slipping of the

STDS forming mylonites. The mylonite zone of the Yadong STDS is commonly 500 – 2000

m thick, developing syntectonic dykes deformed parallel to structural foliations. Pelitic rocks

in the Yadong STDS were mylonized with foliation typically dipping 20°-30° to the north, of

which the ductile deformation was also crosscut by undeformed leucogranite dykes (Fig. 2a).

Unlike in the Central Himalaya where the STDS outcrops only at high altitudes

approximately along the main range divide, the trace of Yadong STDS and THS migrates

~50 km southward to much lower elevations. Field investigations identified ductile

deformation of the STDS shear zone to the north of the Yadong town, where leucogranite

dykes intruding into sandstones of the THS display clear evidence of deformation with

numerous boudinages parallel to structural foliations with dipping to the north (Fig. 2b). The

Yadong THS remnant can be well traced along strike to the east into Bhutan, where a series

THS klippes are scattered at similar latitudes (Fig. 1).

Based on apparent offsets of the STDS and range divide in Yadong area in plan-view,

previous study (Burchfiel et al. 1992) suggested a sinistral strike-slip fault in Yadong Cross

Structure. Alternatively, Wu et al. (1998) proposed that the sinistral offset is a visual effect of

dip fault striking NS, which was produced by a lateral ramp dipping to the west in MHT.

Recent geophysical and chronological data support that the MHT dip varies along-strike of

the Himalaya (Coutand et al. 2014; McQuarrie & Ehlers 2015; Landry et al. 2016). At

equivalent locations along strike, the GHS rocks are widely exposed in the Central Himalaya,

whereas THS remnants are more common in Yadong and the Eastern Himalaya (Fig. 1),

indicating differential exhumation between these areas.

Methods

Along the GHS transect outcropped in China, we collected twenty samples (Table 1)

spanning a NS distance of ~55 km, most of which distribute along the Yadong/Amo River.

Our NNE-striking sampling transect is ~10 km west away from the nearest parallel rifting

fault, which implies that our samples are within a single coherent block of hanging/west wall,

thus unlikely displaced by any major rift structures. At the southern tip of the section, a

vertical transect was defined by five samples (Y3 - Y7) with a total elevation difference of

~600 m. The collected samples were crushed, seized and processed with standard mineral

separation to yield zircon and apatite grains. We conducted ZFT and AHe analysis for all of

these samples.

Zircon Fission Track

ZFT samples were prepared following the standard external detector method (Hurford &

Green 1983). Detailed laboratory procedures can be found in similar work (Garver et al. 1999;

Bernet 2005). Zircons were mounted in Teflon mounts, which then were polished to reveal

scratch-free surfaces close to their maximum grain section for etching. Etching of polished

zircon grains was carried out in a NaOH and KOH eutectic mixture at a constant temperature

of 228 °C in a thermostatically controlled oven for 25-30 h. Thermal neutron irradiation was

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conducted at the China Institute of Atomic Energy with a nominal neutron fluence of

1×1015

cm-2

, which was monitored by a couple of CN1 standard glasses mounted at ends of

sample column. Low-uranium mica was used as external detector, and after irradiation these

were etched in 48% HF at room temperature for 18 min. A zeta value of 106.66 ± 2.4 was

obtained using the Fish Cannon Tuff zircon standard from multi-irradiated sample columns.

Fission tracks were counted under a Zeiss Axioplan 2 microscope at a magnification of 1000.

Because the expected fission track ages are relatively young, we counted more than 1000

spontaneous semi-tracks in ~20 grains for most of the samples to obtain reliable ages.

Apatite (U-Th)/He

AHe dating was performed at University of Florida. Euhedral apatite grains were

selected and examined under a binocular stereomicroscope at a magnification of 160 to

exclude grains with apparent inclusions. Alpha ejection correction factor (FT) for each grain

was calculated based on measured linear dimensions (Farley et al. 1996). Single or multiple

apatite grains were wrapped in Pt tubes, and heated using a diode laser under high vacuum.

The extracted gas was mixed with 3He spike, then analyzed using a quadrupole mass

spectrometer. All the samples were degassed at least twice to ensure complete extraction of 4He. The degassed sample packets were mixed with U-Th-Sm spike and dissolved in 5%

nitric acid at 120 °C overnight. The U-Th-Sm isotopic compositions of the sample solutions

were measured using an Element2 ICP-MS (Inductively Coupled Plasma Mass

Spectrometer).

Stream topography analysis

In stream topography analysis, we employed data of ASTER Global Digital Elevation

Model (2nd version released in 2011), which are widely used in digital topographic analysis.

These data have an average precision of 1 arc-second in horizontal (~ 30 m) and < 15 m in

vertical, quality of which has been substantial improved compared to the previous version

(Tachikawa et al. 2011). We extracted swath topographic and channel longitudinal profiles

for topographic analysis. Data processing was carried out with tools on ArcGIS.

Fluvial incision rate in bedrock is dominated by the stream power expenditure along

channel, which is commonly expressed as a function of discharge (often parameterized as the

upstream-drainage area A) and channel gradient S (Howard & Kerby 1983; Whipple &

Tucker 1999):

E = KAmS

n

where K is an erosion coefficient, m and n are positive constants. Absolute incision rates

along stream can be predicted once these constants are calibrated. It is apparent that these

parameters depend on hydrology and thus may vary between drainages according to local

factors, including structures, climate and lithology. However, empirical studies indicate that

the ratio of m and n (m/n) stated as concavity is stable within a narrow range of 0.35-0.6

(Whipple & Tucker 1999; Kirby & Whipple 2001). In Siwalik Hills of central Nepal, Kirby

and Whipple (2001) obtained a concavity of ~0.46 for rivers transecting an active fault-bend

fold.

Due to the lack of well calibration of these parameters, prediction of absolute incision

rates remain rare. However, a quantitive proxy for channel incision rate based on stream

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power model can be developed (e.g. Finlayson et al. 2002). To investigate the relative

incision rate along stream in this study, we employ (E/K)1/n

, which equals Am/nS, as a proxy

for channel incision (PCI). Since K and n are constants in specified drainages, the PCI serves

as an index for relative incision intensity along stream, of which the pattern provides

encouraging prospects for interpretation of driving forces.

Results

Zircon Fission Track

Collected samples include gneiss from GHS, Miocene leucogranites as well as the

bottommost part of the THS that are intensively deformed by ductile shear of the STDS

(Table 1). Nineteen samples yielded effective ZFT ages in the range of 14.0 - 8.2 Ma. These

cooling ages are consistent with our expectation that these samples would yield cooling ages

of Neogene that are younger than the crystallization age of leucogranite in this area. Table 2

shows pooled ages for samples with Chi squared probabilities higher than 1%, or central ages

if the samples’ Chi squared probabilities are lower than 1%. In fact, because of the large

number of spontaneous and induced tracks counted, there are very minor differences between

the pooled ages and the central ages.

The ZFT ages plotted as a function of latitude (Fig. 3) appear a faint northward younging

pattern for the southern segment of the transect (latitude < 27.6 degree), and then an increase

to the north in the northern segment. The northernmost 3 samples (Y18-2, Y19, Y20) yielded

ages between 11.9 - 14.0 Ma. Note that the Y18-2 was collected from an undeformed

leucogranite vein (Fig. 2a), whereas Y19 and Y20 were from intensely deformed gneissic

rock of GHS. These ZFT ages, with no systematic variation between gneiss and intruded

leucogranites, indicate that these samples were thermally equilibrated before ~11.9 - 14 Ma,

and experienced the same cooling history since then.

Apatite (U-Th)/He

For most of the samples, three replicate AHe ages were determined (Table 3). Out of 37

newly determined AHe ages, six are exceptionally old, some of which are even older than

their ZFT ages. Such exceptionally old ages are commonly found in (U-Th)/He analysis

(Biswas et al. 2007; Landry et al. 2016; Wang et al. 2016; Ketcham et al. 2018), and are

often explained with various potential reasons such as eU effect (Flowers et al. 2009),

high-U-Th inclusions (Ehlers & Farley 2003), U-Th zonation (Ault & Flowers 2012), alpha

implantation from neighboring phases (Murray et al. 2014), fragmentation (Brown et al. 2013)

or microvoids (Zeitler et al. 2017). We exclude these apparent outliers in further discussion.

The (U-Th)/He ages are in the range of 11.1 - 3.2 Ma with intra-sample means of 11.1 -

4.2 Ma. The AHe age vs. latitude relationship is very similar to the ZFT ages: almost random

or slightly decreasing ages to the north in the southern segment (latitude < 27.6 degree), and

increasing ages to the north in the northern segment (Fig. 3a). The vertical transect (Y2 - Y7;

Fig. 3c) at the southern tip of the Yadong section presents a good positive relationship

between age and elevation (Fig. 3b), which yields an apparent exhumation rate of 0.32

km/Ma between 7.7 - 5.9 Ma.

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Using 1D simulation (Willett & Brandon 2013), a cooling age can be translated into an

average exhumation rate since cooling below its closure temperature assuming steady state

exhumation. At an assumed higher geothermal gradient of 40 °C/km due to high vertical

advection rate of GHS by the STDS and MCT (Johnson et al. 2001; Robinson et al. 2003;

Tobgay et al. 2012), and common high radioactive heat production in the GHS (Whipp et al.

2007; Landry et al. 2016), the AHe and ZFT samples in the vertical transect (Table 4) yield

average exhumation rates of 0.321 ± 0.063 and 0.655 ± 0.096 km/Ma, respectively. It is clear

that the study area experienced relatively rapid cooling when cooling through the ZFT

closure temperature in the Middle Miocene.

Drainage topography

The swath topography across the Yadong Himalaya (Figs. 4a and 4b) presents an overall

very gentle southward-declining trend at the northern segment, which is contrasted to its

southern segment spanning ~20 - 40 km where altitudes drop more sharply shaping the

steepened frontal range.

The gentle-steep-slope pattern observed in swath topography is consistent with the

channel longitudinal profile (Fig. 4 c), which is divided by a major knick point (~5 km to the

south of sample YD1) into two sub-segments. The well-developed downstream sub-segment

channel profile is concave with channel gradient decreasing downstream. Note that

immediately downstream the knick point, a high-channel-gradient anomaly zone occurs,

which defines a knick zone. In spatial, this knick zone coincides well with the steep slope in

swath topography.

The up-stream subsegment occurs as an overall linear channel longitudinal profile. In the

middle of the upper segment, an alternative minor high-channel-gradient anomaly zone exists

approximately at between latitudes 27.5° and 27.6°. This could be related to a local structure

or a short-term channel perturbation possibly driven by local landslides.

Simulated PCI (E/K)1/n

along the Yadong River (Fig. 4c) indicates that the knick zone of

high channel gradient anomaly is characterized by strengthened channel incision, while its

upstream and downstream sub-segments yield overall much lower PCI.

Discussion

The ZFT cooling ages (14.0 - 8.2 Ma) are only slightly younger than the zircon U/Pb

ages (~12-17 Ma) obtained from leucogranite intrusions in this area (Wu et al. 1998; Gong et

al. 2012; Gou et al. 2016). In addition, biotite 40

Ar/39

Ar ages from the Yadong GHS are 11.0

- 11.5 Ma (Gong et al. 2012), which are similar to our ZFT ages within uncertainty,

suggesting that the area experienced rapid cooling over the range between the corresponding

closure temperatures (~350 ˚C for biotite 40

Ar/39

Ar; ~240 ˚C for ZFT). This implies rapid

cooling to below the ZFT closure temperature in the Middle Miocene. This fast cooling is

also evidenced by the steep slope in age-altitude relationship for ZFT data compared to the

AHe data (Fig. 3d, Table 4). Available apatite fission-track cooling ages in the same transect

(Gong et al. 2012) indicate that the fast cooling continued until the temperature approached

the AHe closure temperature (~70 °C) (Fig. 3d). The limited variation of ZFT ages in the

~50-km NS section indicates that the fast cooling occurred in the whole root-zone GHS. We

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attribute the large-scale fast cooling to a tectonic exhumation driven by extension of the

STDS and thrusting of the MCT. This is supported by the extensional slipping of STDS in

this area during the Early-Middle Miocene (Wu et al. 1998; Cooper et al. 2015), and in the

Central and Eastern Himalaya during the same period (Edwards et al. 1996; Searle & Godin

2003; Searle 2003; Wang et al. 2006; Yin 2006; Liu et al. 2007; Leloup et al. 2010; Wang et

al. 2010). The MCT, which was activated before 12 - 9 Ma in western Bhutan (McQuarrie et

al. 2014), was synchronous in activation with the STDS (Johnson et al. 2001; Robinson et al.

2003; Tobgay et al. 2012).

Age-altitude relationship and the 1D steady-state simulation (Willett & Brandon 2013)

document that exhumation rate has been almost constant since ~7.7 Ma in the Yadong section.

We notice that the documented exhumation rates are very similar to the modern erosion rates

obtained at equivalent locations to the east in the western Bhutan, where in situ-produced 10

Be and 26

Al in fluvial sediments yield an average erosion rate of 0.388 ± 0.032 km/Ma in

Puna Tsang Chhu catchment (Portenga et al. 2015).

The constant exhumation rate since 7.7 Ma documented in Yadong section is contrasted

with a number of previous studies in Central Himalaya at the Nyalam area (Wang et al. 2010),

the Everest (Streule et al. 2012) and the Ama Drime Range (Jessup et al. 2008; Kali et al.

2010; Wang et al. 2016), which obtained very-young cooling ages emphasizing an enhanced

Late Neogene-Quaternary exhumation (Burbank et al. 2003; Thiede et al. 2004; Wang et al.

2010; Streule et al. 2012; McDermott et al. 2013). In Central Himalaya accelerated cooling

of the Himalaya Orogen in Late Neogene and Quaternary is often ascribed as driven by

climate agencies, possibly due to the significant synchronous global cooling (Zhang et al.

2001; Burbank et al. 2003; Thiede et al. 2004; Streule et al. 2012; Herman et al. 2014).

However, the Yadong area experienced relatively slow and monotonous exhumation since

the Late Miocene without any acceleration, implying a long-term decoupling between the

rock exhumation and the climate change in Himalaya. We therefore propose that the

long-term cooling and exhumation in the Yadong area was more likely controlled by

tectonics rather than climate change, although the latter can affect surface erosional

efficiency in a shorter-term.

In Central Himalaya the hinterland fast exhumation was proposed to be driven by an

active out-of-sequence thrusting at physiographic transition zone (Hodges et al. 2004; Wobus

et al. 2005), or by a buried ramp in the MHT as suggested by a number of recent

thermo-kinematic modeling (Herman et al. 2010; Thiede & Ehlers 2013; Landry et al. 2016).

Although the detailed tectonic settings are still controversial, tectonics are an indispensable

driving force for the observed age distributions.

The sharp contrast in exhumation patterns between the Central and the Eastern Himalaya

(e.g. Thiede & Ehlers 2013; Coutand et al. 2014; Landry et al. 2016) implies distinctive

tectonic settings dominating the long-term exhumation of the Himalaya. We suggest that the

slower exhumation in the Yadong and Eastern Himalaya since Late Miocene was modulated

by a gentler dip of the MHT underneath the range hinterland than in the Central Himalaya. A

lower angle of the MHT in the Eastern Himalaya allows the foreland migration of

deformation front. This is supported by older cooling ages commonly identified in the

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hinterland (the Eastern Himalaya; Fig. 1), and a sudden decrease of the cooing ages in the

south near the fault traces of MBT and MFT (Coutand et al. 2014; McQuarrie et al. 2014).

Thermo-kinematic modeling of cooling ages in Bhutan yields long-term exhumation rates of

1 - 3 km/Ma and 0.7 km/Ma for frontal range and hinterland, respectively (Coutand et al.

2014). This first-order cooling and exhumation pattern across the Eastern Himalaya suggests

that intense exhumation occurred at its frontal range. The intense exhumation at frontal range

in the Eastern Himalaya is consistent with the Late Neogene activation of the MBT and the

MFT (Lave & Avouac 2000; Mukul et al. 2007; Berthet et al. 2014).

The suggested exhumation pattern is consistent with the observed Yadong River channel

topography. The channel longitudinal profile of Yadong River presents a major knick point

located ~5 km to the south of sample Y1. The knick point divides the whole Yadong River

channel longitudinal profile into two natural segments (Fig. 4c). A most apparent explanation

for this double-segment pattern is the upstream-wall uplifting by southward up-thrusting

along a north-dipping fault, which modulated a high-channel-gradient anomaly zone in

hanging wall downstream the knick point (Fig. 4c) (Brookfield 1998). This is also supported

by the spatial pattern of simulated PCI predicting abnormally high erosion rates at the knick

zone, which implies a latest focus of tectonic activity migrating to the frontal range. Here we

exclude erosional competency from potential reasons in channel bed gradient anomaly

because the GHS rocks that the Yadong River incises are uniform in lithology with similar

erosional competency.

Due to the lateral variation of MHT dips and subduction angles (Xiao et al. 2007; Chen

et al. 2015; Duan et al. 2017; Wang et al. 2017), the subducting slab tears underneath the

Eastern and the Central Himalaya. The NS-striking Yadong graben forms in hanging wall as

a result of this slab tear, which might have promoted the slow cooling of the Yadong area. A

simplified structural model is constructed for the slow exhumation in the Eastern Himalaya

hinterland (Fig. 5).

The contrasting exhumation patterns between the Eastern and Central Himalaya imply

differential deformation characteristics and spatially uneven shortening within the Himalaya.

Assuming a constant gross shortening accommodation between the Indian Plate and Tibet,

the long-term slower and constant exhumation in the Eastern Himalaya hinterland supports

that a considerable shortening partition of the Indian-Eurasia convergence may have been

accommodated by the rise of the Shillong Plateau (Grujic et al. 2006).

Age-latitude plots for the ZFT and AHe data (Fig. 3a) show a slight concave shape. This

might have been caused by recent differential uplifting in hinterland during NS shortening, or

an along-strike attenuation of the fast exhumation zone adjacent in hinterland controlled by

basal structure of MHT. Structural extrusion of the GHS which is constrained before 12 – 9

Ma in this area (McQuarrie et al. 2014) is unlikely a reason, because both of the ZFT and

AHe datasets display a concave pattern, suggesting a much younger timing (< 4 – 6 Ma) for

the pattern. Our data provide no evidence for major out-of-sequence thrusting within the

Yadong Himalaya. If there was any major north-dipping thrusts cross-cutting the transect,

one can expect north-younging pattern in cooling ages (Lock & Willett 2008). However,

neither the ZFT nor the AHe ages displays such tendency, suggesting that the GHS

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deformation occurred as an overall structural-coherent block without any major

out-of-sequence thrusting across this transect. More studies are required to refine the

structural significance intra-GHS.

Conclusions

New ZFT and AHe cooling ages, collected from a ~50 km N-S transect in the Yadong

GHS, range 14.0 - 8.2 Ma and 11.1 - 4.2 Ma, respectively. These data suggest slow and

constant exhumation (~0.32 km/Ma) since 7.7 Ma in the Yadong Himalaya hinterland. The

spatial and temporal patterns of exhumation suggest that exhumation of the Himalaya is was

determined by tectonics rather than by climate changes.

Contrasting Late Neogene exhumation patterns are identified for the Central and the

Eastern Himalaya, indicating along-strike structural segmentation of the underlying MHT and

subducting Indian Plate. The long-term slow and steady exhumation of the Eastern Himalaya

hinterland and Yadong area prefers a gentler dip of the underlying MHT than in the Central

Himalaya. These results support the slab tearing of the subducting Indian Plate near the

boundary between the Central and the Eastern Himalaya.

Assuming a constant gross shortening rate between the Indian Plate and the Tibet, the

long-term lower exhumation rate in the Eastern Himalaya hinterland requires significant

shortening deformation in the frontal range probably by the MBT and MFT. In addition, the

rise of the Shillong Plateau might also have accommodated the Indian-Eurasia convergence.

Acknowledgements

This study was financially supported by the Natural Science Foundation of China

(40902060, 41672195, 41702208), the China Geological Survey Institute (1212010610103)

and Fundamental Research Funds for the Central Universities, China University of

Geosciences (Wuhan) (CUGCJ1701, CUGCJ1802). We express special appreciation to John

I. Garver providing assistance in zircon fission track dating and analysis. Appreciation also

goes to two anonymous reviewers and handling editor Yuntao Tian for many constructive

suggestions to this paper.

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ACCEPTED MANUSCRIPT



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ACCEPTED MANUSCRIPT



ACCEPTED MANUSCRIPT



Tab

le 1

. B

asi

c g

eolo

gic

al

info

rma

tio

n o

f co

llec

ted

sa

mp

les

Sam

ple

L

ong

itu

de

(°)

Lat

itu

de

(°)

Ele

vat

ion

(m)

Lit

holo

gy

T

ecto

no

-str

atig

rap

hic

unit

s

Y1

8

8.9

509

27

.40

81

28

36

Gn

eiss

G

HS

Y2

8

8.9

331

27

.42

09

28

41

Gn

eiss

G

HS

Y3

8

8.9

239

27

.43

78

28

68

Gra

nit

ic G

nei

ss

GH

S

Y4

8

8.9

098

27

.43

44

34

95

Gn

eiss

G

HS

Y5

8

8.9

118

27

.42

91

34

01

Gra

nit

ic G

nei

ss

GH

S

Y6

8

8.9

147

27

.42

75

32

40

Gra

nit

ic G

nei

ss

GH

S

Y7

8

8.9

184

27

.43

59

30

67

Gra

nit

ic G

nei

ss

GH

S

Y8

8

8.9

164

27

.45

26

29

10

Gra

nit

ic G

nei

ss

GH

S

Y9

8

8.9

063

27

.48

60

29

55

Gra

nit

e

γN

Y1

0

88

.90

32

27

.50

38

30

28

Gra

nit

e

γN

Y1

1

88

.91

47

27

.52

24

31

29

Gra

nit

e

γN

Y1

2

88

.92

66

27

.53

84

31

81

Gra

nit

e

γN

Y1

3

88

.93

05

27

.55

51

32

58

Gra

nit

e

γN

Y1

4

88

.91

50

27

.56

99

32

96

Gra

nit

ic G

nei

ss

γN

Y1

5

88

.90

90

27

.58

84

34

13

Gn

eiss

G

HS

Y1

7

88

.92

29

27

.63

02

37

52

met

asa

nd

ston

e

ST

DS

/TH

S

Y1

7-3

8

8.9

276

27

.62

29

36

93

Gra

nit

e

γN

Y1

8-2

8

8.9

766

27

.75

34

39

90

Und

eform

ed G

ran

ite

γN

Y1

9

88

.99

93

27

.79

12

42

56

Gn

eiss

G

HS

Y2

0

88

.98

82

27

.83

43

45

13

Gn

eiss

G

HS

ACCEPTED M

ANUSCRIPT

by

gues

t on

July

11,

201

9ht

tp://

jgs.

lyel

lcol

lect

ion.

org/

Dow

nloa

ded

from


Tab

le 2

. Z

irco

n f

issi

on

tra

ck d

ata

res

ult

s

Sam

ple

N

ρs(

Ns)

ρ

i(N

i)

ρd(N

d)

U

(pp

m)

P

(χ2

)

%

Ag

e

Ma ±

2σ

Y1

2

0

3.8

8 ×

10

6 (

15

13

) 8

.25

× 1

06 (

32

17

) 4

.22

7 ×

10

5 (

427

8)

77

6.9

± 3

4.1

2

2.3

10

.6 ±

0.8

Y2

2

0

2.8

0 ×

10

6 (

14

64

) 7

.65

× 1

06 (

40

01

) 4

.24

7 ×

10

5 (

427

8)

71

7.1

± 2

9.5

0

.

0

8.4

± 0

.9

Y3

2

0

3.8

9 ×

10

6 (

23

38

) 8

.79

× 1

06 (

52

84

) 4

.26

7 ×

10

5 (

427

8)

82

0.3

± 3

1.3

0

.

0

10

.0 ±

0.9

Y4

3

0

3.0

0 ×

10

6 (

62

1)

7.1

5 ×

10

6 (

14

81

) 4

.28

7 ×

10

5 (

427

8)

66

4.4

± 8

.8

0.

4

9.7

± 1

.3

Y5

2

0

3.5

0 ×

10

6 (

23

75

) 8

.07

× 1

06 (

54

74

) 4

.30

7 ×

10

5 (

427

7)

74

6.3

± 8

.4

2.

0

10

.0 ±

0.7

Y6

2

0

3.3

7 ×

10

6 (

18

01

) 6

.52

× 1

06 (

34

87

) 4

.32

7 ×

10

5 (

427

7)

59

9.7

± 2

6.0

0

.

0

12

.0 ±

1.2

Y7

2

0

4.2

2 ×

10

6 (

23

54

) 1

.06

× 1

07 (

58

99

) 4

.34

7 ×

10

5 (

427

7)

96

8.2

± 3

6.5

0

.

0

9.2

± 1

.0

Y8

2

0

3.6

6 ×

10

6 (

18

82

) 8

.35

× 1

06 (

42

93

) 4

.36

8 ×

10

5 (

427

7)

76

1.2

± 3

1.2

0

.

0

10

.2 ±

1.0

Y9

2

0

3.5

4 ×

10

6 (

14

87

)

8.7

9 ×

10

6 (

36

90

) 4

.38

8 ×

10

5 (

427

7)

79

7.1

± 3

4.2

2

.

5

9.4

± 0

.8

Y1

0

2

0

2.1

8 ×

10

6 (

10

55

) 5

.40

× 1

06 (

26

18

) 4

.40

8 ×

10

5 (

427

7)

48

7.5

± 2

3.3

0

.

0

9.4

± 1

.4

Y1

1

2

0

3.2

8 ×

10

6 (

10

66

)

6.8

0 ×

10

6 (

22

09

) 4

.42

8 ×

10

5 (

427

6)

61

1.1

± 3

1.1

0

.

1

11

.5 ±

1.4

Y1

2

21

.63

× 1

06 (

65

7)

4

.35

× 1

06 (

17

49

) 4

.44

8 ×

10

5 (

427

6)

38

9.4

± 2

1.6

0

.9

.1 ±

1.3

ACCEPTED MANUSCRIPT

by

gues

t on

July

11,

201

9ht

tp://

jgs.

lyel

lcol

lect

ion.

org/

Dow

nloa

ded

from


0

2

Y1

3

2

0

4.2

8 ×

10

6 (

12

36

)

1.2

4 ×

10

6 (

35

84

) 4

.46

8 ×

10

5 (

427

6)

11

05

.0 ±

48

.3

5

2.5

8.2

± 0

.7

Y1

4

2

0

3.3

2 ×

10

6 (

13

26

)

8.8

6 ×

10

6 (

35

37

) 4

.48

8 ×

10

5 (

427

6)

78

6.3

± 3

4.6

5

1.5

9.0

± 0

.7

Y1

5

2

0

2.1

2 ×

10

6 (

77

1)

5

.98

× 1

06 (

21

78

) 4

.50

8 ×

10

5 (

427

6)

52

8.4

± 2

7.2

0

.

2

8.8

± 1

.1

Y1

7

-3

2

0

4.0

3 ×

10

6 (

17

51

)

8.0

9 ×

10

6 (

35

17

) 4

.54

8 ×

10

5 (

427

6)

70

7.7

± 3

1.4

1

.

3

12

.1 ±

0.9

Y1

8

-2

2

0

3.7

8 ×

10

6 (

17

13

)

6.8

7 ×

10

6 (

31

12

) 4

.56

8 ×

10

5 (

427

5)

59

8.7

± 2

7.6

2

9.6

13

.4 ±

1.0

Y1

9

2

0

3.4

6 ×

10

6 (

10

58

)

6.2

2 ×

10

6 (

19

02

) 4

.60

8 ×

10

5 (

427

5)

53

7.0

± 2

9.3

0

.

1

14

.0 ±

1.7

Y2

0

2

0

4.4

4 ×

10

6 (

20

88

)

9.2

0 ×

10

6 (

43

26

) 4

.62

9 ×

10

5 (

427

5)

79

1.6

± 3

3.6

0

.

0

11

.9 ±

1.1

Note

: N

is

nu

mb

er o

f gra

ins

anal

yze

d.

ρs

is s

po

nta

neo

us

track

den

sity

(cm

-2);

Ns

is n

um

ber

of

spon

tan

eou

s tr

ack

s; ρ

i is

indu

ced t

rack

den

sity

(cm

-2);

Ni is

nu

mb

er o

f in

du

ced t

rack

s; ρ

d i

s tr

ack

den

sity

on f

luen

ce m

onit

or

(cm

-2);

Nd i

s tr

ack

s co

unte

d o

n f

luen

ce m

onit

or.

U ±

2σ

is

the

aver

ag

e u

raniu

m c

on

centr

atio

n (

pp

m).

P(χ

2)

is C

hi-

squ

ared

pro

bab

ilit

y.

Ag

es (

Ma)

are

det

erm

ined

usi

ng t

he

Zet

a m

eth

od a

nd c

alcu

late

d u

sin

g

the

com

pu

ter

pro

gra

m a

nd e

qu

atio

ns

by

Bra

nd

on

(1

99

2).

All

lis

ted

ag

es a

re p

oole

d a

ges

or

centr

al a

ge

wit

h 2

σ e

rro

r. S

ee c

onte

xt

for

oth

er l

ab

par

amet

ers

and p

roce

sses

.

ACCEPTED M

ANUSCRIPT

by

gues

t on

July

11,

201

9ht

tp://

jgs.

lyel

lcol

lect

ion.

org/

Dow

nloa

ded

from


Tab

le 3

. A

pa

tite

(U

-Th

-Sm

)/H

e d

ata

res

ult

s

Sam

ple

N

F

T

4H

e

(fm

ol)

U

(p

pm

) T

h

(pp

m)

Sm

(pp

m)

Ag

e (1

σ)

Mea

n (

SD

)

Y2

1

0.8

4

17

.33

2

6.9

7

4.0

4

19

2.1

7

6.9

8 ±

0.1

7

5.9

± 1

.0

1

0.8

4

10

.94

2

5.7

5

1.7

5

26

6.4

9

4.8

9 ±

0.1

2

1

0.8

6

20

.20

4

1.1

5

2.6

6

36

5.6

0

5.7

9 ±

0.1

5

Y3

1

0.8

7

21

.14

1

8.1

8

1.6

6

31

0.4

3

6.1

5 ±

0.1

4

6.6

± 0

.7

1

0.8

6

15

.68

2

3.1

2

1.7

4

38

5.1

8

7.1

± 0

.17

2

0.8

2

38

.87

2

5.5

9

0.3

2

44

9.2

0

13

.1 ±

0.3

2

Y5

1

0.9

1

36

1.5

8

23

.69

28

.52

32

5.4

8

18

.39

± 0

.41

7.7

± 0

.4

1

0.8

9

49

.64

21

.32

1.1

2

35

3.6

4

7.3

7 ±

0.1

8

1

0.8

8

13

5.4

4

82

.72

1.6

1

54

5.0

7

7.9

6 ±

0.2

1

Y6

2

0.8

1

65

.60

10

8.0

7

12

.90

46

4.1

8

6.5

5 ±

0.1

6

7.0

± 0

.5

2

0.8

0

43

.97

79

.95

1.4

6

49

8.7

0

6.8

8 ±

0.1

7

2

0.8

0

46

.40

82

.17

2.4

4

43

0.6

9

7.6

± 0

.19

Y7

1

0.8

1

19

.18

69

.15

5.2

8

26

8.4

5

5.4

6 ±

0.1

6

6.4

± 1

.6

2

0.7

8

15

.06

43

.96

6.1

3

33

8.6

6

5.3

8 ±

0.1

3

1

0.8

6

59

.28

45

.01

5.3

4

30

8.5

5

8.2

5 ±

0.2

Y8

1

0.8

8

44

.39

19

.24

3.4

4

37

7.4

3

8.8

4 ±

0.2

1

6.2

± 2

.3

1

0.8

9

15

.94

9.1

2

1.6

7

41

2.1

2

5.0

7 ±

0.1

4

1

0.8

8

77

.35

69

.26

20

.78

36

4.7

1

4.5

4 ±

0.1

1

Y9

2

0.8

1

79

.66

61

.79

30

.58

75

6.9

3

11

.01

± 0

.25

7.0

± 0

.4

2

0.8

0

11

4.3

5

94

.57

69

.90

10

11

.36

12

.46

± 0

.3

2

0.7

8

30

.19

58

.28

45

.32

10

29

.63

6.9

9 ±

0.1

6

Y1

0

1

0.8

6

8.7

4

21

.08

1.3

1

20

2.9

3

3.9

± 0

.09

4

.2 ±

0.4

2

0

.79

13

.33

14

.06

0.4

8

15

3.1

1

14

.7 ±

0.3

4

ACCEPTED MANUSCRIPT

by

gues

t on

July

11,

201

9ht

tp://

jgs.

lyel

lcol

lect

ion.

org/

Dow

nloa

ded

from


3

0.7

6

7.2

3

29

.87

2.2

2

22

8.2

4

4.5

3 ±

0.1

1

Y1

1

1

0.8

5

36

.76

67

.50

2.3

6

18

1.7

8

4.8

1 ±

0.1

3

5.5

± 0

.7

1

0.8

5

19

.84

30

.36

3.0

1

21

1.9

8

6.2

± 0

.15

1

0.8

5

29

.92

55

.48

1.5

7

30

7.9

4

5.4

7 ±

0.1

3

Y1

5

1

0.9

0

7.4

9

5.6

3

2.3

3

64

.19

4.1

5 ±

0.1

4.5

± 1

.5

1

0.8

8

0.9

0

1.9

2

0.0

7

20

.89

3.1

8 ±

0.5

8

1

0.8

5

6.4

2

10

.54

0.3

2

87

.94

6.0

8 ±

0.1

6

Y1

7

2

0.7

9

0.7

9

0.5

4

0.0

9

6.4

0

24

.67

± 3

.67

7.3

± 0

.6

3

0.7

8

0.5

2

1.1

2

0.1

7

7.1

9

6.9

1 ±

1.3

4

0.7

5

0.4

4

0.6

9

0.5

0

6.3

5

7.7

2 ±

2.0

1

Y1

8-2

1

0.8

7

77

.67

38

.05

4.0

3

25

1.8

4

10

.47

± 0

.28

10

.6 ±

0.4

1

0

.88

97

.84

47

.17

4.2

1

20

1.4

4

11

.04

± 0

.26

1

0.8

6

11

4.4

2

78

.53

3.4

2

29

5.6

7

10

.17

± 0

.27

Y1

9

3

0.7

8

12

.54

16

.94

0.3

4

36

5.3

0

11

.13

± 0

.3

11

.1 ±

0.7

Note

: N

is

nu

mb

er o

f ap

atit

e gra

ins

anal

yze

d i

n a

sin

gle

pack

et;

FT

is

alp

ha

reco

il c

orr

ecti

on

fac

tor

calc

ula

ted f

rom

lin

ear

dim

ensi

on

s o

f ea

ch

gra

in;

Ital

ic f

onts

indic

ate

ou

tlie

rs t

hat

are

excl

ud

ed f

rom

mea

n a

ge

calc

ula

tio

n (

see

conte

xt

for

det

ails

). P

ack

et a

ge

erro

rs i

ndic

ated

are

add

ed 4

%

rela

tiv

e er

ror

by a

lpha c

orr

ecti

on

fact

or;

SD

is

stan

dar

d d

evia

tio

n.

ACCEPTED M

ANUSCRIPT

by

gues

t on

July

11,

201

9ht

tp://

jgs.

lyel

lcol

lect

ion.

org/

Dow

nloa

ded

from


Tab

le 4

. A

vera

ge

exh

um

ati

on

ra

tes

calc

ula

ted

fro

m A

He

an

d Z

FT

ag

es o

f ve

rtic

al

tra

nse

ct

Sam

ple

A

ltit

ud

e (

m)

AH

e (M

a)

ZF

T (

Ma)

A

ver

age

Ex

hu

mati

on R

ates

AH

e (m

m/a

) Z

FT

(m

m/a

)

35

°C

/km

4

0 °

C/k

m

35

°C

/km

4

0 °

C/k

m

Y2

2

84

1

5.9

± 1

.0

8.4

± 0

.9

0.4

48

± 0

.08

5

0.4

07

± 0

.07

7

0.9

07

± 0

.10

2

0.8

04

± 0

.09

1

Y3

2

86

8

6.6

± 0

.7

10

.0 ±

0.9

0

.39

3 ±

0.0

46

0.3

57

± 0

.04

1

0.7

54

± 0

.07

1

0.6

68

± 0

.06

3

Y4

3

49

5

9

.7 ±

1.3

0

.71

1 ±

0.1

01

0.6

22

± 0

.08

9

Y5

3

40

1

7.7

± 0

.4

10

.0 ±

0.7

0

.25

7 ±

0.0

15

0.2

27

± 0

.01

3

0.6

98

± 0

.05

1

0.6

12

± 0

.04

5

Y6

3

24

0

7.0

± 0

.5

12

.0 ±

1.2

0

.31

1 ±

0.0

25

0.2

77

± 0

.02

2

0.5

91

± 0

.06

2

0.5

20

± 0

.05

5

Y7

3

06

7

6.4

± 1

.6

9.2

± 1

.0

0.3

72

± 0

.10

9

0.3

35

± 0

.09

8

0.8

00

± 0

.09

2

0.7

06

± 0

.08

1

Av

erag

e

0.3

56

± 0

.06

6

0.3

21

± 0

.06

3

0.7

44

± 0

.10

6

0.6

55

± 0

.09

6

Note

: ex

hu

mat

ion r

ates

are

det

erm

ined

base

d o

n f

orm

ula

te f

rom

Wil

lett

& B

rand

on (

20

13

), a

ssu

min

g s

tead

y s

tate

, an

av

erag

e al

titu

de

of

35

00

m,

and a

geo

ther

mal

gra

die

nt

of

35

°C

/km

an

d 4

0 °

C/k

m r

esp

ecti

vel

y.

ACCEPTED MANUSCRIPT

by

gues

t on

July

11,

201

9ht

tp://

jgs.

lyel

lcol

lect

ion.

org/

Dow

nloa

ded

from


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