close

Вход

Забыли?

вход по аккаунту

?

j.tecto.2017.10.026

код для вставкиСкачать
Accepted Manuscript
Exhumation and topographic evolution of the Namche Barwa
Syntaxis, eastern Himalaya
Rong Yang, Frédéric Herman, Maria Giuditta Fellin, Colin Maden
PII:
DOI:
Reference:
S0040-1951(17)30451-1
doi:10.1016/j.tecto.2017.10.026
TECTO 127664
To appear in:
Tectonophysics
Received date:
Revised date:
Accepted date:
18 May 2017
27 September 2017
22 October 2017
Please cite this article as: Rong Yang, Frédéric Herman, Maria Giuditta Fellin, Colin
Maden , Exhumation and topographic evolution of the Namche Barwa Syntaxis, eastern
Himalaya. The address for the corresponding author was captured as affiliation for all
authors. Please check if appropriate. Tecto(2017), doi:10.1016/j.tecto.2017.10.026
This is a PDF file of an unedited manuscript that has been accepted for publication. As
a service to our customers we are providing this early version of the manuscript. The
manuscript will undergo copyediting, typesetting, and review of the resulting proof before
it is published in its final form. Please note that during the production process errors may
be discovered which could affect the content, and all legal disclaimers that apply to the
journal pertain.
ACCEPTED MANUSCRIPT
Exhumation and topographic evolution of the Namche Barwa Syntaxis,
eastern Himalaya
Rong Yanga,c,*, Frédéric Hermanb, Maria Giuditta Fellinc, Colin Madenc,
aDepartment
of Earth Surface Dynamics, University of Lausanne, CH-1015,
Switzerland
cDepartment
SC
RI
PT
bInstitute
of Earth Sciences, Zhejiang University, Hangzhou 310027, China
of Earth Sciences, ETH Zurich, Sonneggstrasse 5, 8092 Zurich,
Switzerland
NU
*Corresponding to: royang1985@zju.edu.cn
Abstract
MA
The Namche Barwa Syntaxis, as one of the most tectonically active regions,
remains an appropriate place to explore the relationship between tectonics,
ED
surface processes, and landscape evolution. Two leading models have been
PT
proposed for the formation and evolution of this syntaxis, including the tectonic
aneurysm model and the syntaxis expansion model. Here we use a multi-
CE
disciplinary approach based on low-temperature thermochronometry, numerical
AC
modeling, river profile and topographic analyses to investigate the interactions
between tectonics, erosion, and landscape evolution and to test these models.
Our results emphasize the presence of young cooling ages (i.e., < 1 Ma) along the
Parlung River, to the north of the syntaxis. Using numerical modeling we argue
that a recent increase in exhumation rate is required to expose these young ages.
Our river analysis reveals spatial variations in channel steepness, which we
interpret to reflect the rock uplift pattern. By establishing the relationship
between erosion rates and topographic features, we find that erosion rates are
1
ACCEPTED MANUSCRIPT
poorly to weakly correlated with topographic features, suggesting that the
landscape is still evolving. Altogether, these results seem better explained by a
mechanism that involves a northward expansion of the syntaxis, which causes
high rock uplift rates to the north of the syntaxis and a transient state of
topography adjusting to an evolving tectonic setting.
SC
RI
PT
Keywords: Namche Barwa Syntaxis; low-temperature thermochronometry;
topographic analysis; thermokinematic modeling; transient landscape; syntaxis
AC
CE
PT
ED
MA
NU
expansion
2
ACCEPTED MANUSCRIPT
1. Introduction
The Namche Barwa Syntaxis in the eastern Himalaya reaches elevations
over 7600 m, which stands 5000 to 6000 m over the Yarlung River. It exposes
deep crustal rocks that were exhumed at extremely high rates, close to 10 mm/a,
during the last 10-5 Ma. How such extreme rates and relief can be sustained, and
SC
RI
PT
for how long, are challenging questions that have motivated numerous studies
(e.g., Burg et al., 1997;Zeitler et al., 2001, 2014; Seward and Burg, 2008;
Finnegan et al., 2008; King et al., 2016; Bracciali et al., 2016; Wang et al., 2014;
Wang et al., 2017; Stewart et al., 2008; Schmidt et al., 2015; Lang et al., 2013,
NU
2014, 2016;Enkelmann et al., 2011). Two leading models have been proposed for
the development of this syntaxis. (1) The “tectonic aneurysm” model (Zeitler et
MA
al., 2001, 2014; Finnegan et al., 2008) highlights the coupling between surface
process and tectonics in the Namche Barwa Massif. In this model, intense erosion
ED
leads to rapid heat advection that thermally weakens the upper crust. Such an
PT
effective coupling leads to focused deformation within the core of the syntaxis,
which in turn enhances erosion and maintains topographic relief. (2) The
CE
“expansion of the syntaxis” model (Seward and Burg et al., 2008; King et al., 2016)
AC
emphasizes tectonics as responsible for topographic growth of the syntaxis. This
model is consistent with a range of mechanical models (Burg and Podladchikov,
1999; Bendick and Ehlers, 2014) and emphasizes the importance of strike-slip
structures in the formation of the Namche Barwa antiform (Burg et al., 1998).
These two models predict different exhumation patterns and topographic
evolution. The “tectonic aneurysm” predicts high rock uplift and fast exhumation
in the high stream-power zone. Therefore, this model implies localized
exhumation and rock uplift in the Namche Barwa Massif since about 5 Ma
3
ACCEPTED MANUSCRIPT
(Zeitler et al., 2014). In contrast, the “expansion of the syntaxis” model predicts
that exhumation patterns and topography evolve as the antiform grows. Thus,
this model indicates an evolving exhumation pattern and topography that
migrates towards the north, towards the Asian Plate (Seward and Burg, 2008;
King et al., 2016). It is also worth stressing here that this model does not exclude
SC
RI
PT
that extreme erosion rates must result from an efficient coupling between
erosion and tectonics. It just questions the idea that erosion and tectonics end up
being pinned to a single location.
In this paper, we use a multi-disciplinary approach to estimate the
NU
exhumation and rock uplift patterns and the topography evolvement by using
low temperature thermochronometry, numerical modeling, river analysis, and
MA
topographic analysis. We find that exhumation and topography are more likely
ED
evolving towards the north to the Po Tsangpo and the Parlung rivers.
PT
2. Geologic and Geomorphologic setting
The Namche Barwa Syntaxis (Fig. 1), as the eastern termination of the
CE
Himalayan orogenic belt, is a 30-40 km wide, northeast-plunging antiform (Burg
AC
et al., 1997). While the structures in this region remain relatively poorly mapped
and understood due to its inaccessibility, previous studies have proposed several
strike-slip fault zones and thrust faults as the dominating features in the syntaxis,
accommodating
northeastward
indentation
of
India
by
ductile-brittle
deformation. To the west of the syntaxis, the NE-SW oriented Dongjiu-Milin fault
zone(DMFZ) acts as the western bounding structure of the syntaxis, which
exhibited left-lateral movement at ~31-25 Ma (Xu et al., 2010). This zone links
with the north-dipping Yarlung-Tsangpo Canyon thrust zone (YTCT) to the north
4
ACCEPTED MANUSCRIPT
(Ding et al., 2001).To the east of the syntaxis, the NE-SW oriented Aniqiao-Motuo
fault zone (AMFZ) acts as the eastern bounding structure of the syntaxis,
exhibiting right-lateral movement at 29-28 Ma (Dong and Xu, 2016). The northdipping Namu-La thrust (NLT) lies in the core of the syntaxis. The kinematic of
this fault remains unconstrained, but cooling rates of 50-100 °C/Ma during Late
SC
RI
PT
Miocene for samples north of this fault have been reported (Ding et al., 2001).
Outside the syntaxis, the SE-NW oriented Jiali-Parlung fault zone(JPFZ) lies to the
north. This zone was characterized by right-lateral motion between~22-12 Ma
(Lin et al., 2009; Booth et al., 2004; Lee et al., 2003).
NU
The metamorphic history in the syntaxis is complex with multiple stages.
The high-grade metamorphic rocks in the core of the syntaxis record peak
MA
metamorphic conditions between 8 and 18 kbar and 700 and 900°C from the
Early Miocene to the Early Pliocene (Booth et al., 2009; Xu et al., 2010; Burg et al.,
ED
1998; Ding et al., 2001). This time is broadly coincident with the Late Miocene to
PT
Late Pleistocene emplacement of granitic dykes and leucosomes in the Namche
Barwa Massif (Burg et al., 1998; Ding et al., 2001; Booth et al., 2004; Zeitler et al.,
CE
2014) and rapid exhumation of the syntaxis (Burg et al., 1997; Seward and Burg
AC
et al., 2008; Zeitler et al., 2014).
This region is dominated by high relief and a peculiar drainage pattern. Two
of the highest peaks of the Himalayan orogen, the Namche Barwa and the Gyala
Peri, with elevations over 7000m, characterize the Namche Barwa Massif. This
massif is deeply incised by the Yarlung River, which flows parallel to the
Himalayan orogenic belt for ~1700 km before entering the syntaxis. At the
syntaxis, it suddenly becomes narrow and deeply entrenched and turns
clockwise to cut across the syntaxis. Over a distance of ~150 km, this river drops
5
ACCEPTED MANUSCRIPT
in elevation from ~3000m down to ~1000 m before leaving the syntaxis, cutting
deep gorges into the syntaxis. The Yigong and Parlung rivers flow eastwards and
westwards, respectively, roughly parallel to the JPFZ and join the Yarlung River
at the northern end of the syntaxis through the Po Tsangpo River.
SC
RI
PT
3. Low-temperature thermochronometric ages and modeling
3.1 New apatite (U-Th-Sm)/He thermochronometric data
We analyzed seven new samples using apatite (U-Th-Sm)/He (AHe) dating
(Fig. 1, Table 1). One sample is located within the syntaxis. The rest of the
NU
samples are located outside the syntaxis, with four of them from northeast of the
syntaxis and two from west of the syntaxis. Apatites were separated by
MA
conventional heavy liquid methods and then individual apatite grains were
handpicked for AHe measurement. At least three grains with euhedral
ED
morphology and no visible inclusions were selected for each sample. For samples
PT
that did not yield enough suitable apatite grains, only two grains were measured.
The dimensions of each apatite grain were measured and only grains at least >
abundances, each grain was degassed by wrapping it in a platinum foil and
AC
4He
CE
60 μm in both length and width were considered for (U-Th-Sm)/He dating. For
then heating it with a diode laser to a fixed temperature (in the range of 800900°C). The released helium was measured on a magnet sector field mass
spectrometer equipped with a Baur-Signer ion source at ETH Zurich. After
degassing, each crystal was weighted before and after adding the U-Th-Sm
isotope spike. The same grain was then dissolved in HNO3. The U-Th-Sm
concentration of each dissolved grain was then measured on an inductively
coupled plasma quadrupole mass spectrometer at ETH Zurich (PerkinElmer
6
ACCEPTED MANUSCRIPT
ElanDRC-e). The α-ejection correction was performed for each single apatite
grain age according to Ketcham et al. (2011). The age error was derived from the
analytical uncertainties in U, Th, and Sm measurements, and the variance of the
single grain ages. We used the mean age of each sample for the modeling. Note
that the uncertainty related to grain size is not included in the modeling.
SC
RI
PT
However, the effect of grain size on estimating cooling rates is minimized in
rapidly cooling terrains (Meesters and Dunai, 2002).
Mean AHe ages range between 0.18 and 5.62 Ma (Table 2). Samples within
the syntaxis (NB184) and in its northern vicinity (NB124, NB142, NB135) exhibit
NU
ages < 2 Ma with the youngest age on the Parlung River (NB124). The other three
samples outside the syntaxis (NB115, NB109, NB134) show ages between 2.47
MA
and 5.62 Ma. Previous studies found low temperature thermochronometric data
including zircon fission track (ZFT), apatite fission track (AFT), zircon (U-Th)/He
ED
(ZHe), and AHe ages younger than 2Ma in a wide area extending northeast and
PT
northwest across the bounding faults of the syntaxis, while the higher
temperature thermochronometer biotite
40Ar/39Ar
(BAr) gave ages < 2 Ma only
CE
within the syntaxis (Fig. 2, Supplement Table 1). Outside the syntaxis, BAr can be
AC
as old as 90 Ma. Along the Po Tsangpo and the Parlung, they vary from 2 to 44
Ma. Moreover, some of the old and of the youngest BAr ages are found in close
proximity indicating that exhumation there occurred from depth near the limit
between total and partial retention for BAr. Our new data fit into this regional
pattern and confirm the rapid cooling in the Parlung River.
3.2Thermokinematic modeling of thermochronometric ages
7
ACCEPTED MANUSCRIPT
Among the two alternative models proposed for the evolution of the
Namche Barwa Syntaxis, the key area for testing these models lies to the north of
the syntaxis. It has been proposed that the young AHe, AFT, ZHe, ZFT ages (<
2Ma) to the north of the syntaxis may reflect either the increased geothermal
gradient due to the lateral heat flow from the rapidly exhumed massif to the
SC
RI
PT
south as predicted by the “tectonic aneurysm” model (Zeitler et al., 2014) or a
recent change in rock uplift rates as suggested by the “expansion of the syntaxis”
model (Seward and Burg, 2008; King et al., 2016). Thus, we focus on the river
courses along the Po Tsangpo above the gorge region and along the Parlung
NU
River (see location in Fig. 2). Fig. 3a shows the age distribution along the rivers.
All the young (<5 Ma) BAr ages are located along the Po Tsangpo while all the old
MA
ages (> 10 Ma) are located along the Parlung. All the young ages (<2 Ma) of the
other lower temperature thermochronometers are located within a distance of
ED
60 km downstream. Upstream from this region, ages spread over a broad range
PT
from ~1 to >10 Ma.
Here we use a modified version of the finite element model Pecube (Braun,
CE
2003; Herman et al., 2010; Braun et al., 2012) to test the influence of lateral heat
AC
flow and rock uplift on the thermochronometric ages, respectively. Since the
onset of uplift in the syntaxis has been suggested to be after 10 Ma (Zeitler et al.,
2014; Seward and Burg, 2008), we only use ages < 10 Ma for the modeling. A 200
m-resolution digital elevation model was used in the simulations. It was sampled
from the 90 m-resolution SRTM DEM data (Jarvis et al., 2008). The thickness of
the modeled domain is 40 km. We imposed a fixed temperature of 840 °C at the
base of the domain and of 5 °C on the surface, which defined an initial,
unperturbed geothermal gradient of ~21 °C/km. The thermal diffusivity is 31.6
8
ACCEPTED MANUSCRIPT
km2/Ma. Heat production is not included in the modeling since it depends on the
rock type and also the thermal boundary conditions and it is poorly constrained
in this area. However, we report the resolved near surface geothermal gradient.
A reasonable geothermal gradient would indicate a proper thermal setting for
the modeling. All these thermal parameters and boundary conditions were
SC
RI
PT
guided by the 2-D model reported by Zeitler et al. (2014). We also tested the
sensitivity of models to a temperature lapse rate of 6 °C/km such that the surface
temperature changes with elevation. Since all the samples we modeled except
one are at elevations between ~2000 m and ~2800 m (Fig. 3b), the differences
NU
between surface temperatures are small and thus no significant differences
were observed. Therefore, in the following sections we only discuss results that
MA
were simulated with the thermal parameters mentioned above.
Two end-member scenarios of exhumation history were considered,
ED
including localized fast exhumation in the Namche Barwa Massif and expansion
PT
of the high exhumation zone to the north, corresponding to the tectonic
aneurysm model and the syntaxis expansion model, respectively (Fig. 4). The
CE
present-day topography was kept constant in these two scenarios. Given that
AC
most of the AHe ages outside the Namche Barwa Massif to the west are within 610 Ma (Fig. 2), the mean exhumation rate is estimated to vary between 0.25-0.5
mm/a. This is derived by assuming a closure temperature of 70°C for the AHe
system and a geothermal gradient of ~21 °C /km that corresponds to the
unperturbed geothermal gradient imposed by our initial model parameters. The
first scenario includes two periods of exhumation (Zeitler et al., 2014): between
10 and 5 Ma, the whole region was exhumed at a rate of 0.25 mm/a and after 5
Ma, the core of the massif was exhumed at 10 mm/a while the other areas
9
ACCEPTED MANUSCRIPT
remained slowly exhumed at a rate of 0.25 mm/a. In the second scenario, the
exhumation history was divided into three periods. During the first and second
periods, exhumation rates are the same as in the first and second periods of the
first scenario, respectively, and these periods go from 10 to 5 Ma and from 5 to 2
Ma. This scenario differs only in that from 2 to 0 Ma, the high exhumation center
SC
RI
PT
expanded northwards towards the Po Tsangpo and the Parlung rivers. Thus,
during this period, exhumation rates were high in the syntaxis and to the north.
Both scenarios yield final near surface geothermal gradients up to 180 °C/km
beneath the deep valleys in the massif, which is in agreement with independent
NU
constraints from fluid inclusions (Craw et al., 2005).
We present the modeled ages in Fig.5. In the first scenario, all the predicted
MA
ages are around 10 Ma, much older than the observed ages (Fig. 5a). The second
model reproduces well all the lower temperature thermochronometric ages
ED
along the river course <60 km downstream of the syntaxis, but slightly
PT
underestimates the BAr ages along the Po Tsangpo (Fig. 5b). In the upper reach
of the Parlung, all the predicted ages are older than the observed.
CE
We tested the sensitivity of our models to different exhumation rates and
AC
durations of fast exhumation. First (Fig. 5c), we tested a three-stages scenario,
that is the syntaxis expansion model, for a different exhumation rate during the
last stage by increasing it in the north at 2 Ma to 5 mm/a instead of 10 mm/a.
This model predicts BAr ages along the Po Tsangpo that are older than the
observed ages, indicating that such rates result in insufficient exhumation to
expose the observed BAr ages along the Po Tsangpo. Secondly (Fig. 5d), we
tested the same scenario for a change in exhumation rate during the last stage in
the north to 9 mm/a, which is slightly lower than the exhumation rate in the
10
ACCEPTED MANUSCRIPT
syntaxis during the same period. The modeled ages match the predicted ages for
all the thermochronometers in the Po Tsangpo and the lower Parlung. Along the
upper Parlung, the predicted ages are overestimated, implying that exhumation
rates may have increased in the past 10 Ma in the upper Parlung as well, but not
as much as in the downstream region where we report the new ages. Finally, (Fig.
SC
RI
PT
5e), we imposed that the expansion of the localization of fast exhumation to the
north of the syntaxis starts at 1 Ma. During this last stage, in order to achieve the
same amount of exhumation as constrained by the previous tests, a rate of 18
mm/a is required. The predicted AHe, AFT, and ZHe ages show good matches in
NU
the Po Tsangpo and the lower Parlung while the rest of the BAr ages are
overestimated. This overestimation is because the geothermal gradient did not
MA
have time to adjust to the change of exhumation rate.
We did three additional simulations to test the impacts of relief change
ED
while the exhumation histories are the same as in the aneurism scenario (Fig. 5f-
PT
g-h). Doubling the relief towards the present topography since 2 Ma (Fig. 5f),
which is equivalent to incising ~1.2 km deep along the Parlung River, results in
CE
that all the AHe ages are around 2 Ma in contrast to the broad range of the
AC
observed AHe ages. The rest of the predicted thermochronometric ages are all
overestimated. A two-fold increase in relief at 1 Ma (Fig. 5g) predicts almost the
same age pattern as in the previous simulation, indicating no significant
influence of the time for relief change on the thermochronometric ages. If we
impose a four-fold increase of the relief at 2 Ma (Fig. 5h), all the predicted AHe
ages and only the AFT ages in the very downstream are < 2 Ma. The rest of the
ages are overestimated.
11
ACCEPTED MANUSCRIPT
4. River profile analysis in the Yarlung drainage network
The evolution of topography is coupled to changes in the river channel
network, which is part of a dynamic system adjusting towards a balance between
tectonic uplift and erosion. As a result, morphometric analysis of the river
channel is a useful tool to assess the transient state of a landscape. In particular,
SC
RI
PT
the channel steepness, which is the slope of the channel normalized by drainage
area, is predictive of channel response to uplift rate (e.g., Kirby et al.,
2003;Ouimet et al., 2009). There are two ways to calculate the channel steepness.
One way is to derive it from the channel slope and drainage area relation (e.g.,
NU
Kirby et al., 2003). These two parameters should exhibit a linear relation in the
log-transformed space for rivers at steady state and channel steepness can be
MA
derived from the interception of this linear relation. Because the accuracy of the
channel slope calculation is largely dependent on the precision of the DEM, we
ED
use the integration approach that was proposed by Perron et al. (2012), which
PT
largely overcomes this limitation. In this approach, instead of calculating the
channel slope at each point along the river channel, elevation is used as the
CE
dependent variable and an integral of drainage area over the distance of the river
AC
course, named as , is used as the independent variable. Elevation and define a
linear relation for rivers in equilibrium and the slope of this line is channel
steepness, which is a function of rock uplift rate and rock erodibility. One
advantage of this approach is that although river longitudinal profiles can vary
between rivers, all the streams at steady state with uniform rock uplift rate and
rock erodibility should collapse onto a single line in elevation-
space.
Deviations from this line indicate either spatial variations in rock uplift and
lithology or rivers in a transient state. Both cases will generate knickpoints along
12
ACCEPTED MANUSCRIPT
the river profiles separating river segments with different channel steepness.
Knickpoints formed due to contrasting rock uplift or lithology are stable and
should follow closely the structural/lithological boundaries whereas knickpoints
formed from a common origin are transient and they should propagate upstream
through different channels (Wobus et al., 2006). The vertical component of the
SC
RI
PT
knickpoint migration velocity is relevant to the rock uplift rate (Whipple and
Tucker, 1999; Niemann et al., 2001). Thus, all the transient knickpoints should be
located at the same elevation if rock uplift rate is uniform in space.
In order to characterize the rock-uplift rate pattern and the evolution of
NU
river networks in the Namche Barwa Syntaxis, we extracted and analyzed 18
tributaries in this region together with the main trunk of the Yarlung River (Fig.
MA
6a). These tributaries were picked based on the criteria that the channels are
currently not heavily glaciated. For rivers within the syntaxis only channels that
ED
are located below the highest knickpoint on the Yarlung (Fig. 6a) are analyzed
PT
because channel profiles above have been potentially graded to the local base
level which has been maintained due to glacial dams in the Quaternary (Korup
CE
and Montgomery, 2008). Of all the tributaries analyzed 11 rivers are located
AC
within the syntaxis, of which 5 are in the south (colored in red in Fig. 6a) and 6 in
the north (colored in blue in Fig. 6a). The other 7 rivers are located beyond the
syntaxis along the Yigong-Parlung river network (colored in green in Fig. 6a). We
used the SRTM3 DEM data with a resolution of 90 m and a threshold of 5 km2
was used to exclude regions that are potentially dominated by debris flows or
covered by glaciers. The concavity index is suggested to vary in a small range
between 0.35 and 0.6 in bedrock channels (Whipple and Tucker, 1999). We used
a concavity value of 0.5 for the
calculation.
13
ACCEPTED MANUSCRIPT
Fig. 6b shows the
-profiles of these tributaries together with the main
trunk river. All tributaries at all elevations deviate systematically to higher
relative to the main trunk of the Yarlung River. The southernmost river is the
only one with a linear trend in the -plot and has the lowest channel steepness.
All the other rivers have convex profiles. Except for the southernmost river, the
SC
RI
PT
-profiles of 4 tributaries in the southern sector of the syntaxis overlap with the
profiles of 4 out of 6 tributaries in the northern sector of the syntaxis. The 2
tributaries to the Po Tsangpo in the northern sector of the syntaxis and the
tributaries from the Yigong-Parlung overlap and exhibit the highest channel
NU
steepness. The main trunk of the Yarlung River shows a significant knickpoint at
MA
~2900 m and a second one at ~3500 m as observed by Schmidt et al. (2015).
These knickpoints, however, are not present on the tributaries. All the tributaries
ED
are characterized by several knickpoints.
PT
5. Erosion rates and topographic analysis
Apart from the above analysis, we also intend to explore the relationship
CE
between topographic metrics and erosion rates in this region with extremely
AC
high exhumation and high topography. A simple linear correlation between relief
(hillslope) and erosion rate was suggested by Ahnert (1970). However, an
increasing body of work suggests that this linear relation is only valid in
tectonically less-active landscape (Roering et al., 2001; Montgomery and
Brandon, 2002; Ouimet et al.,2009; DiBiase et al., 2010;Carretier et al., 2013).
Erosion rates increase with hillslopes at hillslopes <30°- 35° beyond which
this relationship breaks (Burbank et al., 1996; Binnie et al., 2007; Ouimet et al.,
14
ACCEPTED MANUSCRIPT
2009). Similarly, relief also provides a measure of the steepness of landscape, but
it is less dependent on the quality and scale of the sampled DEM and remains
positively correlated with erosion rate (Montgomery and Brandon, 2002).
Besides hillslope and relief, the channel steepness index has been proposed as a
better predictor for erosion rates (DiBiase et al., 2010; Ouimet et al., 2009).
index continues to increase with erosion rate.
SC
RI
PT
Unlike hillslope, which is limited by the threshold value, the channel steepness
We used the SRTM3 DEM data (with a resolution of ~90m) in the Namche
Barwa Syntaxis. Hillslope angles were calculated for each 3x3 grids (8 cells) and
NU
averaged within a 10 km x 10 km window. By using such an analysis window, the
width of the largest valleys in the syntaxis can be spanned so that the measured
MA
hillslope angles represent regional topographic features. The same size of the
window was applied for the relief calculation. The channel steepness was
ED
calculated from the slope of the -profiles as described in the previous section.
PT
Estimation of erosion rates from thermochronometric ages in this area has
been performed by King et al. (2016). Here we use the same approach(Fox et al.,
CE
2014; Herman and Brandon, 2015) and the same parameters by incorporating
AC
our new data together with the previous data (Fig. 2) to derive the erosion rate
pattern. These parameters include an a priori exhumation rate of 0.5mm/a with
a variance of 2.5 mm/a, a spatial correlation length of 20 km, and a time interval
of 2 Ma. The a priori erosion rate is approximate to the mean exhumation rate
outside the syntaxis as estimated above. Exhumation rates are represented by
the a priori value when there is no age constraint but it exerts little control when
there are enough data to resolve the exhumation rates (Fox et al., 2014). Both a
priori variance and spatial correlation length act as trade-off parameters
15
ACCEPTED MANUSCRIPT
between solution resolution in space and the need to average out noise. The
kinetic parameters for helium diffusion in apatite and zircon are from Farley
(2000) and Reiners et al. (2004), respectively, and for fission track annealing in
apatite and zircon from Ketcham et al. (1999) and Brandon et al. (1998),
respectively, and for argon diffusion in biotite from Grove and Harrison (1996).
SC
RI
PT
We ran the inversion for a 20 Ma erosion history but exhumation rates before 10
Ma are poorly constrained due to the limited number of ages that are older than
10 Ma. Since we aim to investigate erosion and its correlation with modern
topographic features, we only focus our analysis on the last 2 Myr. Mean-basin
10Be
(Finnegan et al., 2008) are also used for
NU
wide erosion rates derived from
topographic analysis.
MA
Fig. 7 shows the erosion pattern during the last 2 Myr. Erosion rates,
hillslope, relief, and channel steepness were extracted at locations where the ZHe
ED
samples were collected (Fig. 2). These samples have a good spatial coverage in
PT
and outside the syntaxis, and erosion rates at these locations cover a broad range
from 0.26 mm/a to ~6 mm/a, enabling us to analyze the connection between
CE
erosion and landscape in both slowly and quickly eroded regions.
AC
Fig. 8 shows the results of the morphometric analysis. Determinations of
mean slope angles vary between 16°- 34° (Fig. 8a) and the mean local relief
varies between~1300m and ~3500 m (Fig. 8b). The relief in the Namche Barwa
syntaxis is higher than in other tectonically active regions, such as New Zealand,
the European Alps, or Taiwan, where relief is generally< 1500 m (Montgomery
and Brandon, 2002). It appears that the bedrock erosion rate is weakly
correlated with both local slope and relief, with R2=0.16, p<0.01 and R2=0.34,
p<0.01, respectively. Measurements of channel steepness range from 150 to
16
ACCEPTED MANUSCRIPT
~320 (Fig. 8c). Since not all the locations analyzed are on the river channels, we
only plot the relation between bedrock erosion rate and channel steepness for
locations that are on the river channels. These two parameters are weakly
correlated with R2=0.28, p=0.18. The mean, basin-wide erosion rate shows a
similarly weak relationship with channel steepness (Fig. 8d). In general, our
SC
RI
PT
results reveal poor to weak correlations between erosion rates and topographic
features in the syntaxis.
6. Discussion
NU
6.1 Implication of rock uplift pattern in the syntaxis
The two main knickpoints at ~2900 and 3500 m on the main trunk of the
MA
Yarlung (Fig. 6) are not at the same elevation as on the tributaries. Schmidt et al.
(2015) suggested that the higher knickpoint along the Yarlung aligns with most
ED
of the tributaries that they analyzed and in turn interpreted this observation as
PT
indication of a common rock-uplift rate change or base-level fall. We do not
observe any common knickpoint. Instead, we observe variations in channel
CE
steepness among all the rivers.
AC
The channel steepness is clearly different between rivers in the north along
the Po Tsangpo and Yigong-Parlung networks, as well as in the rivers in the
south. Contrasts in channel steepness values can arise from variations in rock
erodibility, rock uplift rate, or precipitation rate. However, the rivers we
analyzed have similar lithologies of predominant gneisses and metasediments.
Precipitation rate varies from > 1000 mm/a in the eastern part of the syntaxis to
< 400 mm/a in the western part (Anders et al., 2006). Almost all the tributaries
analyzed are from the high precipitation area with small variations in
17
ACCEPTED MANUSCRIPT
precipitation. Therefore, we suspect that the influence of precipitation on river
slope is small. Instead, we propose that the south to north variation in channel
steepness indicates a similar pattern in rock uplift rate with high rock uplift
within the massif and to its north, and low uplift south of the massif. Such a
gradient in rock uplift rate may be associated with the strain partition along
SC
RI
PT
these faults in this region. The southernmost river has the lowest steepness and
this can be reconciled with the fact that this river is located close to the southern
extent of the syntaxis where the topography becomes less rugged and where
erosion rates are likely to be low, as suggested by the detrital ZFT ages of the
NU
same river (Enkelmann et al., 2011).
One limitation of our analysis is that we could not preclude the impact of
MA
glaciations in modifying the river profiles, in particular, in the high reaches of the
rivers. Although we only selected rivers that are currently not heavily glaciated,
ED
we do not have information on their past glacial history. The unusual high
PT
channel steepness of the main trunk may be associated with the frequent
occurrence of floods due to breaks of glacial dams in the Quaternary above the
CE
gorge region (Montgomery et al., 2004; Korup and Montgomery, 2008; Lang et al.,
AC
2013), introducing more water into the gorge region and variations in the stream
power law.
6.2 Implication of landscape evolution
Our results reveal weak correlations between erosion and topographic
features. Such an observation is not new. Burbank et al. (1996) have also
observed independence of erosion rates on slope angles in the Nanga Parbat,
western Himalaya syntaxis. Li et al. (2014) found no statistically significant
18
ACCEPTED MANUSCRIPT
correlation between basin-wide erosion rates and mean hillslope angles in
Kunlun Shan, northern Tibet. Safran et al. (2005) reported a weak correlation
between mean basin erosion rates and mean basin slopes and reliefs in the
Bolivian Andes. Several other studies reported no correlation between erosion
rates and mean hillslope angles in the Central, Western, and Eastern Alps
SC
RI
PT
(Wittmann et al., 2007; Delunel et al., 2010; Norton et al., 2011; Glotzbach et al.,
2013). Similar to the Nanga Parbat syntaxis, the Namche Barwa syntaxis is also
characterized by high rock uplift rates and rapid erosion. In such a high uplift
area, the weak correlation between erosion rate and topography may have
NU
multiple causes. One possible cause is cyclic glaciations, which may influence
both mountain height and volume (Champagnac et al., 2012; Egholm et al., 2009),
MA
irrespective of the rate of tectonic processes involved (Brozovic et al., 1997). The
Namche Barwa syntaxis was affected by the Pleistocene glacial/interglacial
ED
cycles (Owen et al., 2008; Montgomery et al., 2004) and the syntaxis is currently
PT
still glaciated. Thus, with the influence of climatic cycles on landscape, the
patterns.
CE
topographic metrics have limited potential in predicting the erosion rate
AC
Alternatively, the weak correlation between bedrock erosion rates and
landscape indices may imply an evolving landscape which is still adjusting to
changes in rock uplift rate. The erosional response to rock uplift in a landscape
includes both bedrock river erosion and hillslope erosion with the former
process setting the boundary condition for the latter. Therefore, it is possible
that the response of the hillslope erosion lags behind the bedrock erosion. This
may be especially the case for a young topographic feature with ongoing
deformation. In places where the increase in rock uplift occurred earlier, the
19
ACCEPTED MANUSCRIPT
landscape has already scaled with bedrock erosion rate, whereas in other places,
where the increase in rock uplift occurred later, landscapes have not yet adjusted
to the new bed rock erosion rate. However, the time for landscape to equilibrate
with rock uplift also depends on erosional efficiency which is a function of
climate conditions and rock erodibilities (Whipple and Meade, 2006). In our
SC
RI
PT
analysis, erosion rates across the whole region do not show a good correlation
with landscape indices, suggesting a combined effect of rock uplift, climate, and
rock erodibilities.
The weak dependence of bedrock erosion rate on channel steepness is,
NU
however, surprising as among all the other topographic metrics, channel
steepness is supposed to be the best proxy in predicting erosion rate. In such a
MA
region with repeated stochastic flood events, this non-linear behavior in the
relation between erosion rates and channel steepness might reflect the impacts
ED
of floods on river channels. This process has, however, not affected the small
PT
tributary basins where the mean basin-wide erosion rates were measured
(Finnegan et al., 2008). Given the similar weak dependence of erosion on
CE
hillslope and relief, we favor that the variations in the relation between channel
AC
steepness and erosion rates reflect a transient landscape that is still adjusting to
new tectonic conditions.
6.3 Implication of tectonic model in the syntaxis
Our thermokinematic modeling (Fig. 5) suggests that lateral heat flow due
to localized fast exhumation in the Namche Barwa Massif and relief increase
alone could not resolve the young ages along the Po Tsangpo and the Parlung
rivers. Instead, an increase in exhumation rate is required. Zeitler et al. (2014)
20
ACCEPTED MANUSCRIPT
also performed numerical modeling to qualitatively examine the influences of
lateral heat flow from the Namche Barwa Massif on the isotherms in adjacent
regions. We expanded the numerical analysis and applied it to predict the
thermochronometric ages. By presenting the predicted ages, we show that
exposure of the young ages along the Po Tsangpo and the Parlung depends on
SC
RI
PT
the magnitude of the exhumation rate and on the duration of high exhumation.
Among all the simulations, an initiation of rapid exhumation at 2 Ma with a rate
of 9 mm/a along the Po Tsangpo and the Parlung provides the better match with
the observed young age pattern (Fig. 5d), even though there is complexities in
NU
the age pattern that we could not explain with our thermo-kinematic modeling. A
similar magnitude of exhumation along the Parlung has also been reported by
MA
King et al. (2016) where exhumation rates were estimated from multi-OSL
thermochronometry data. Combined with our river analysis and the
ED
morphometric analysis, the high exhumation and high rock uplift to the north of
PT
the syntaxis and the decoupling between erosion rates and topographic features
can not be explained by the “tectonic aneurysm” model as localized exhumation
CE
and rock uplift in the Namche Barwa Massif would be expected by this model.
AC
Instead, we suggest that these observations result from the growth of the
antiform due to the continuous northward indentation of the India plate through
which the high rock uplift rate center has expanded to the north of the syntaxis.
It is possible that redistribution of deformation would cause the deformation and
reorganization of river drainage networks, i.e., the capture of the Parlung by the
Yarlung river as proposed by previous studies (King et al., 2016; Lang and
Huntington, 2014) which would in turn introduce more variations in the
exhumation pattern and in the landscape response.
21
ACCEPTED MANUSCRIPT
7. Conclusions
We present new AHe ages in the Namche Barwa Syntaxis and
demonstrate that recent changes in exhumation rate is required to expose the
young ages on the Po Tsangpo and the Parlung rivers. River profile analysis of
SC
RI
PT
the syntaxis reveals a spatially variable rock uplift pattern with high rock uplift
in the north along the Po Tsangpo and the Parlung rivers. Combined with the
existing thermochronometric ages, we derive erosion rates during the last 2 Ma
and we observe that erosion rates are poorly to weakly correlated with
NU
topographic features. We interpret such a correlation as an indication of a
transient state of landscape. Altogether, these results indicate a northward
MA
expansion of the Namche Barwa Syntaxis as responsible for variable rock uplift
ED
pattern and topographic evolution.
This
work
was
PT
Acknowledgements
supported
by
Zhejiang
University
(grant
number
CE
107401*172210163), National Science Foundation of China (grant number
AC
41472182), and the Fundamental Research Funds for the Central Universities
(grant numbers 2017FZA3008 and 2017XZZX007-01). The authors are grateful
to J.P. Burg for his courtesy for providing the samples dated by this study. The
paper benefited greatly from two anonymous reviewers, Peter Zeitler and the
editor Jean-Philippe Avouac.
22
ACCEPTED MANUSCRIPT
Reference:
Anders, A.M., Roe, G.H., Hallet, B., Montgomery, D.R., Finnegan, N.J., and Putkonen,
J., 2006. Spatial patterns of precipitation and topography in the Himalaya, in
Willett, S.D., Hovius, N., Brandon, M.T., and Fisher, D., eds., Tectonics, Climate,
and Landscape Evolution: Geological Society of America Special Paper 398, p.
SC
RI
PT
39–53, doi: 10.1130/2006.2398(03).
Ahnert, F., 1970.Functional relationships between denudation, relief, and uplift
in large, mid-latitude drainage basins. American Journal of Science268, 243–
NU
263.
Bendick R., Ehlers T. A., 2014. Extreme localized exhumation at syntaxes initiated
MA
by subduction geometry. Geophysical Research Letters41, 5861–5867.
ED
Binnie, S.A., Phillips, W.M., Summerfield, M.A., and Fifield, L.K., 2007.Tectonic
uplift, threshold hillslopes, and denudation rates in a developing mountain
PT
range.Geology35, 743-746.
CE
Booth, A.L., Chamberlain, C.P., Kidd, W.S.F., Zeitler, P.K., 2009. Constraints on the
metamorphic
evolution
of
the
eastern
Himalayan
syntaxis
from
AC
geochronologic and petrologic studies of Namche Barwa.Geological Society of
America Bulletin 121, 385-407.
Booth, A.L., Zeitler, P.K., Kidd, W.S., Wooden, J., Liu, Y., Idleman, B., Hren, M.,
Chamberlain, C.P., 2004.U-Pb zircon constraints on the tectonic evolution of
southeastern Tibet, Namche Barwa area.American Journal of Science 304,
889-929.
23
ACCEPTED MANUSCRIPT
Bracciali, L., Parrish, R.R., Najman, Y., Smye, A., Carter, A., Wijbrans, J.R., 2016.
Plio-Pleistocene exhumation of the eastern Himalayan syntaxis and its domal
‘pop-up’. Earth-Science Reviews 160, 350-385.
Brandon MT, Roden-Tice MK, Garver JI. 1998. Late Cenozoic exhumation of the
SC
RI
PT
Cascadia accretionary wedge in the Olympic Mountains, Northwest
Washington State. Geol. Soc. Am. Bull. 110, 985–1009.
Braun J., 2003. Pecube: a new finite-element code to solve the 3D heat transport
equation including the effects of a time-varying, finite amplitude surface
NU
topography. Computers & Geosciences29, 787-794.
MA
Braun, J., van der Beek, P., Valla, P., Robert, X., Herman, F., Glotzbach, C., Pedersen,
V., Perry, C., Simon-Labric, T., Prigent, C., 2012. Quantifying rates of
ED
landscape evolution and tectonic processes by thermochronology and
numerical modeling of crustal heat transport using PECUBE. Tectonophysics
PT
524-525, 1–28. doi:10.1016/j.tecto.2011.12.035
CE
Brozović N, Meigs A J., 1997. Climatic Limits on Landscape Development in the
AC
Northwestern Himalaya.Science276, 571-575.
Burbank, D.W., Leland, J., Fielding, E., Anderson, R.S., Brozovic, N., Reid, M.R., and
Duncan, C., 1996.Bedrock incision, rock uplift and threshold hillslopes in the
northwestern Himalayas.Nature379, 505–510.
Burg, J.-P., Davy, P., Nievergelt, P., Oberli, F., Seward, D., Diao, Z., Meier, M., 1997.
Exhumation during crustal folding in the Namche‐Barwa syntaxis. Terra
Nova9, 53–56.
24
ACCEPTED MANUSCRIPT
Burg, J.-P., Podladchikov, Y., 1999. Lithospheric scale folding: Numerical
modeling and application to the Himalayan syntaxes: International Jour- nal
of Earth Sciences88, 190–200.
Burg, J.-P., Nievergelt, P., Oberli, F., Seward, D., Davy, P., Maurin, J.-C., Diao, Z., and
SC
RI
PT
Meier, M., 1998.The Namche Barwa syntaxis: evidence for exhumation
related to compressional crustal folding. Journal of Asian Earth Sciences16,
239–252.
Carretier, S., Regard, V., Vassallo, R., Aguilar, G., Martinod, J., Riquelme, R., Pepin,
NU
E., Charrier, R., Herail, G., Farias, M., Guyot, J.L., Vargas, G., Lagane, C.,
Chile. Geology41, 195–198.
MA
2013.Slope and climate variability control of erosion in the Andes of central
ED
Champagnac, J. D., Molnar, P., Sue, C., Herman, F., 2012.Tectonics, climate, and
mountain topography. Journal of Geophysical Research Atmospheres117,
PT
140-147.
CE
Craw, D., Koons, P.O., Zeitler, P.K., Kidd, W.S.F., 2005. Fluid evolution and thermal
structure in the rapidly exhuming gneiss complex of Namche Barwa-Gyala
AC
Peri, eastern Himalayan syntaxis. Journal of Metamorphic Geology 0,
051031032640003.
Delunel, R., van der Beek, P.A., Carcaillet, J., Bourlès, D.L., and Valla, P.G., 2010.
Frost-cracking control on catchment denudation rates: Insights from in situ
produced 10Be concentrations in stream sediments (Ecrins–Pelvoux massif,
French Western Alps). Earth and Planetary Science Letters293, 72–83.
25
ACCEPTED MANUSCRIPT
DiBiase, R.A., Whipple, K.X., Heimsath, A.M., and Ouimet, W.B., 2010. Landscape
form and millennial erosion rates in the San Gabriel Mountains, CA. Earth
and Planetary Science Letters289, 134–144.
Ding, L., Zhong, D., Yin, A., Kapp, P., Harrison, T.M., 2001. Cenozoic structural and
SC
RI
PT
metamorphic evolution of the eastern Himalayan syntaxis (Namche Barwa).
Earth and Planetary Science Letters 192, 423–438.
Dong, H., Xu, Z., 2016. Kinematics, fabrics and geochronology analysis in the
Médog shear zone, Eastern Himalayan Syntaxis. Tectonophysics 667, 108-
NU
123.
MA
Egholm, D. L., Nielsen, S. B., Pedersen, V. K., Lesemann, J. E., 2009. Glacial effects
limiting mountain height. Nature 460, 884-887.
ED
Enkelmann, E., Ehlers, T.A., Zeitler, P.K., and Hallet, B., 2011.Denudation of the
Namche Barwa antiform, eastern Himalaya. Earth and Planetary Science
PT
Letters307, 323–333.
CE
Farley, K., 2000. Helium diffusion from apatite: General behavior as illustrated by
AC
Durango fluorapatite. Journal of Geophysical Research: Solid Earth (1978–
2012) 105, 2903–2914.
Finnegan, N.J., Hallet, B., Montgomery, D.R., Zeitler, P.K., Stone, J.O., Anders, A.M.,
and Yuping, L., 2008. Coupling of rock uplift and river incision in the Namche
Barwa-Gyala Peri massif, Tibet. Geological Society of America Bulletin120,
142–155.
Fox, M., Herman, F., Willett, S., and May, D., 2014. A linear inversion method to
26
ACCEPTED MANUSCRIPT
infer exhumation rates in space and time from thermochronometric data:
Earth Surface Dynamics 2, 47-65.
Glotzbach, C., van der Beek, P., Carcaillet, J., and Delunel, R., 2013. Deciphering
the driving forces of erosion rates on millennial to million-year timescales in
SC
RI
PT
glacially impacted landscapes: An example from the Western Alps. Journal of
Geophysical Research: Earth Surface118, 1491–1515.
Gong, J.F., Ji, J.Q., Chen, J.J., Sang, H.Q, Li, B.L., Liu, Y.D., and Han, B.F., 2008. The
40ar/39Ar geochronology studies of rocks in eastern Himalaya syntaxis.Acta
NU
Petrologica Sinica 24, 2255-2272 (in Chinese with English abstract).
MA
Gong, J., Ji, J., Zhou, J., Tu, J., Sun, D., Zhong, D., Han, B., 2015. Late Miocene thermal
evolution of the eastern Himalayan syntaxis as constrained by biotite
ED
40Ar/39Ar thermochronology.Journal of Geology 123, 369-384.
Grove M, Harrison TM. 1996. 40Ar∗diffusion in Fe-rich biotite. Am. Mineral. 81, 940–
PT
51.
CE
Herman F., Brandon M., 2015. Mid-latitude glacial erosion hotspot related to
AC
equatorial shifts in southern Westerlies. Geology 43, 987-990.
Herman, F., Copeland, P., Avouac, J.-P., Bollinger, L., Mahéo, G., Le Fort, P., Rai, S.,
Foster, D., Pêcher, A., Stüwe, K., Henry, P., 2010. Exhumation, crustal
deformation, and thermal structure of the Nepal Himalaya derived from the
inversion of thermochronological and thermobarometric data and modeling
of the topography. J. Geophys. Res. 115. doi:10.1029/2008JB006126
Jarvis, A., Reuter, H.I., Nelson, A., Guevara, E., 2008. Hole-filled SRTM for the globe
27
ACCEPTED MANUSCRIPT
Version 4.available from the CGIAR-CSI SRTM 90m Database (http://srtm.
csi. cgiar. org).
Ketcham, R.A., Donelick, R.A., Carlson, W.D., 1999. Variability of apatite fissiontrack annealing kinetics: III. Extrapolation to geological time scales.
SC
RI
PT
American Mineralogist 84, 1235–1255.
Ketcham, R.A., Gautheron, C., Tassan-Got, L., 2011. Accounting for long alphaparticle stopping distances in (U–Th–Sm)/He geochronology: refinement of
the baseline case. Geochimica et CosmochimicaActa 75, 7779-7791.
NU
King, G.E., Herman, F., Guralnik, B., 2016.Northward migration of the eastern
MA
Himalayan syntaxis revealed by OSL thermochronometry.Science335, 800804.
ED
Kirby, E., Whipple, K. X., Tang, W.,Chen, Z., 2003. Distribution of active rock uplift
along the eastern margin of the Tibetan Plateau: Inferences from bedrock
PT
channel longitudinal profiles. Journal of Geophysical Research: Solid Earth
CE
(1978–2012): 215-231.
AC
Korup O, Montgomery D.R., 2008. Tibetan plateau river incision inhibited by
glacial stabilization of the Tsangpo gorge. Nature455:786-795.
Lang, K.A., Huntington, K.W., and Montgomery, D.R., 2013.Erosion of the Tsangpo
Gorge by megafloods, Eastern Himalaya. Geology41, 1003–1006.
Lang, K.A., Huntington, K.W., 2014. Antecedence of the Yarlung–Siang–
Brahmaputra River, eastern Himalaya. Earth and Planetary Science Letters
397, 145-158.
28
ACCEPTED MANUSCRIPT
Lang, K.A., Huntington, K.W., Burmester, R., Housen, B., 2016. Rapid exhumation
of the eastern Himalayan syntaxis since the late Miocene. Geological Society
of America Bulletin, B31419.31411.
Lee, H.-Y., Chung, S.-L., Wang, J.-R., Wen, D.-J., Lo, C.-H., Yang, T.F., Zhang, Y., Xie,
SC
RI
PT
Y., Lee, T.-Y., Wu, G., 2003. Miocene Jiali faulting and its implications for
Tibetan tectonic evolution. Earth and Planetary Science Letters 205, 185194.
Lei, Y., Zhong, D., Ji, J., Jia, C., and Zhang, J., 2008.Fission track evidence for two
MA
Quaternary Sciences28, 584–590.
NU
Pleistocene uplift-exhumation events in the eastern layan Syntaxis.
Li, Y., Li, D., Liu, G., Harbor, J., Caffee, M., and Stroeven, A.P., 2014. Patterns of
ED
landscape evolution on the central and northern Tibetan Plateau
investigated using in-situ produced 10Be concentrations from river
PT
sediments. Earth and Planetary Science Letters398, 77–89.
CE
Lin, T.-H., Lo, C.-H., Chung, S.-L., Hsu, F.-J., Yeh, M.-W., Lee, T.-Y., Ji, J.-Q., Wang, Y.Z., Liu, D., 2009. 40Ar/39Ar dating of the Jiali and Gaoligong shear zones:
AC
Implications for crustal deformation around the Eastern Himalayan Syntaxis.
Journal of Asian Earth Sciences 34, 674-685.
Meesters, A.G.C.A ., Dunai, T.J., 2002. Solving the production–diffusion equation
for finite diffusion domains of various shapes Part II. Application to cases
with a-ejection and nonhomogeneous distribution of the source. Chemical
Geology 186, 347–363.
29
ACCEPTED MANUSCRIPT
Montgomery, D. R., Hallet, B., Yuping, L., Finnegan, N., Anders, A., Gillespie, A., and
Greenberg, H. M., 2004.Evidence for Holocene megafloods down the Tsangpo
River gorge, southeastern Tibet. Quaternary Research62, 201-207.
Niemann, J.D., Gasparini, N.M., Tucker, G.E., Bras, R.L., 2001. A quantitative
SC
RI
PT
evaluation of Playfair's law and its use in testing long-term stream erosion
models. Earth Surface Processes and Landforms 26, 1317-1332.
Norton, K.P., Blanckenburg, F., DiBiase, R., Schlunegger, F., and Kubik, P.W.,
2011.Cosmogenic 10Be-derived denudation rates of the Eastern and
NU
Southern European Alps.International Journal of Earth Sciences 100,1163–
MA
1179.
Ouimet, W.B., Whipple, K.X., and Granger, D.E., 2009. Beyond threshold hillslopes:
Geology37, 579–582.
ED
Channel adjustment to base-level fall in tectonically active mountain ranges.
PT
Owen, L. A., 2008. Geomorphology: How Tibet might keep its edge. Nature 455,
CE
748-749.
AC
Perron, J.T., Royden, L., 2012. An integral approach to bedrock river profile
analysis. Earth Surface Processes and Landforms 38, 570-576.
Reiners, P.W., Spell, T.L., Nicolescu, S., Zanetti, K.A., 2004. Zircon (U-Th)/He
thermochronometry: He diffusion and comparisons with 40 Ar/ 39 Ar dating.
Geochimica et Cosmochimica Acta 68, 1857–1887.
Roering, J.J., Kirchner, J.W., Sklar, L.S., Dietrich, W.E., 2001.Hillslope evolution by
nonlinear creep and landsliding: An experimental study. Geology29, 143–146.
30
ACCEPTED MANUSCRIPT
Safran, E.B., Bierman, P.R., Aalto, R., Dunne, T., Whipple, K.X., and Caffee, M., 2005.
Erosion rates driven by channel network incision in the Bolivian Andes.Earth
Surface Processes and Landforms 30, 1007–1024.
Stewart, R.J., Hallet, B., Zeitler, P.K., Malloy, M.A., Allen, C.M., Trippett, D., 2008.
SC
RI
PT
Brahmaputra sediment flux dominated by highly localized rapid erosion
from the easternmost Himalaya. Geology 36, 711.
Schmidt, J.L., Zeitler, P.K., Pazzaglia, F.J., Tremblay, M.M., Shuster, D.L., Fox, M.,
2015. Knickpoint evolution on the Yarlung river: Evidence for late Cenozoic
MA
Science Letters 430, 448–457.
NU
uplift of the southeastern Tibetan plateau margin. Earth and Planetary
Seward, D., and Burg, J.-P., 2008. Growth of the Namche Barwa Syntaxis and
ED
associated evolution of the Tsangpo Gorge: Constraints from structural and
thermochronological data. Tectonophysics 451, 282–289.
PT
Tu, J.-Y., Ji, J.-Q., Sun, D.-X., Gong, J.-F., Zhong, D.-L., Han, B.-F., 2015. Thermal
CE
structure, rock exhumation, and glacial erosion of the Namche Barwa Peak,
constraints from thermochronological data. Journal of Asian Earth Sciences
AC
105, 223-233.
Wang, P., Scherler, D., Liu-Zeng, J., Mey, J., Avouac, J.P., Zhang, Y., Shi, D., 2014.
Tectonic control of Yarlung Tsangpo Gorge revealed by a buried canyon in
Southern Tibet. Science 346, 978-981.
Wang, Y., Zhang, H., Zheng, D., von Dassow, W., Zhang, Z., Yu, J., Pang, J., 2017.
How a stationary knickpoint is sustained: New insights into the formation of
31
ACCEPTED MANUSCRIPT
the deep Yarlung Tsangpo Gorge. Geomorphology 285, 28-43.
Whipple, K., Meade, B., 2006. Orogen response to changes in climatic and tectonic
forcing. Earth and Planetary Science Letters 243, 218-228.
Whipple, K.X., Tucker, G.E., 1999. Dynamics of the stream‐power river incision
SC
RI
PT
model: Implications for height limits of mountain ranges, landscape response
timescales, and research needs. Journal of Geophysical Research: Solid Earth
(1978–2012) 104, 17661-17674.
Wittmann, H., Blanckenburg, von, F., Kruesmann, T., Norton, K.P., and Kubik, P.W.,
NU
2007.Relation between rock uplift and denudation from cosmogenic nuclides
MA
in river sediment in the Central Alps of Switzerland.Journal of Geophysical
Research112, 86-98.
ED
Wobus, C., Whipple, K.X., Kirby, E., Snyder, N., Johnson, J., Spyropolou, K., Crosby,
PT
B., Sheehan, D., 2006. Tectonics from topography: Procedures, promise, and
CE
pitfalls. Geological Society of America.doi:10.1130/2006.2398(04)
Xu, Z., Ji, S., Cai, Z., Zeng, L., Geng, Q., Cao, H.,2012. Kinematics and dynamics of the
AC
namche barwa syntaxis, eastern himalaya: constraints from deformation,
fabrics and geochronology. Gondwana Research21, 19-36.
Zeitler, P. K., Meltzer, A. S., Brown, L., Kidd, W.S.F., Lim, C., Enkelmann, E., 2014.
Tectonics and topographic evolution of Namche Barwa and the easternmost
Lhasa block, Tibet.Special Paper of the Geological Society of America507, 2358.
Zeitler, P.K., Meltzer, A.S., Koons, P.O., Craw, D., Hallet, B., Chamberlain, C.P., Kidd,
32
ACCEPTED MANUSCRIPT
W.S., Park, S.K., Seeber, L., Bishop, M., 2001. Erosion, Himalayan
AC
CE
PT
ED
MA
NU
SC
RI
PT
geodynamics, and the geomorphology of metamorphism. GSA Today11, 4-9.
33
SC
RI
PT
ACCEPTED MANUSCRIPT
Fig. 1: Simplified geological map of the Namche Barwa Syntaxis (after Ding et
NU
al.,2001; Xu et al., 2012; Zeitler et al., 2014; Seward and Burg, 2008). Sample
locations of this study are shown by circles. NB: Namche Barwa peak. GP: Gyala
MA
Peri peak. DMFZ: Dongjiu-Milin fault zone. AMFZ: Aniqiao-Motuo fault zone.
JPFZ: Jiali-Parlung fault zone. YTCT: Yarlung-Tsangpo Canyon thrust zone. NLT:
AC
CE
PT
ED
Namu-La thrust. Brown indicates the gorge region.
34
NU
SC
RI
PT
ACCEPTED MANUSCRIPT
Fig.2: Compiled low-temperature thermochronometric ages in Namche Barwa.
MA
Biotite40Ar/39Ar (BAr), Zircon fission track (ZFT), Zircon (U-Th)/He (ZHe),
Apatite fission track (AFT), Apatite (U-Th)/He (AHe) are from Seward and Burg
ED
(2008), Lei et al. (2008), Yu et al. (2011), Tu et al. (2015), Zeitler et al. (2014),
PT
Gong et al., 2008; 2015 and this study. Ages from this study are outlined in red.
Symbols filled with white color denote ages older than 10 Ma. The black box
CE
denotes the locations of river profile and ages in Fig. 3. The red line indicates the
AC
topographic transect in Fig. 4. The grey dotted line shows the extent of the
Namche Barwa Massif.
35
SC
RI
PT
ACCEPTED MANUSCRIPT
Fig. 3: Thermochronometric ages plotted together with river profile (a) and
against elevation (b) for samples from the Parlung and Po Tsangpo rivers. The
locations of the river segments and samples are indicated in Fig. 2. BAr ages that
AC
CE
PT
ED
MA
NU
are older than 10 Ma are plotted on the top x-axis and filled in white in (a).
36
MA
NU
SC
RI
PT
ACCEPTED MANUSCRIPT
Fig. 4: Simplified sketch of the two kinematics discussed in this study. The
ED
aneurism model predicts two stages of exhumation within the core of the
Namche Barwa Massif with moderate exhumation rates (ė) before 5 Ma and fast
PT
exhumation rate from 5 Ma to the Present. In the surrounding regions
exhumation rates do not vary significantly during the same time frame. The
CE
syntaxis expansion model predicts, instead, three stages of exhumation involving
AC
the region to the north of the core of the Namche Barwa Massif. In the first stage
before 5 Ma, the exhumation rate is moderate everywhere like in the aneurysm
model. At 5 Ma the exhumation rate in the core of the massif increases to very
high values while in the surrounding regions it remains constant. At 2 Ma the
locus of fast exhumation rate expands to the north where the rate increases to
values close to those recorded in the core of the massif.
37
ED
MA
NU
SC
RI
PT
ACCEPTED MANUSCRIPT
PT
Fig. 5: Observed and modeled thermochronometric ages along the Parlung and
CE
the Po Tsangpo rivers. Ages older than 10 Ma were excluded from modeling.
Observed ages are filled in grey and modeled ages are filled in red. The
AC
parameters used for each model are described in the text. For a schematic
representation of the imposed exhumation histories see Fig. 4. (a) Aneurism
model: exhumation rate increases from 0.25 to 10 mm/a at 5 Ma in the core of
the Namche Barwa and it is constant at 0.25 mm/a in the neighboring regions. (b)
Syntaxis expansion model: exhumation rate increases from 0.25 to 10 mm/a at 5
Ma in the core of the Namche Barwa and at 2 Ma additionally to the north. (c)
Tests on the syntaxis expansion model for a different exhumation rate during the
38
ACCEPTED MANUSCRIPT
last stage: exhumation rate to the north of the core massif increases at 2 Ma from
0.25 to 5 mm/a. (d) Same as previous model but exhumation rate to the north of
the massif core increases at 2 Ma to 9 mm/a. (e) Tests on the syntaxis expansion
model for a different time of initiation of fast exhumation. Fast exhumation starts
AC
CE
PT
ED
MA
NU
different times and magnitude of change.
SC
RI
PT
at 1 Ma. (f), (g), (h): Simulations of a change in relief in the aneurysm model at
39
SC
RI
PT
ACCEPTED MANUSCRIPT
Fig. 6: Analyzed rivers (a) and their profiles (b). Red and blue tributaries are
within the syntaxis and they are located in the southern and northern sector of
the syntaxis, respectively; green tributaries are from the Yigong-Parlung river
NU
system; black is the main trunk of the Yarlung River. Brown denotes the gorge
region. Red star indicates the giant knickpoint on the Yarlung River at ~ 2900m.
AC
CE
PT
ED
MA
Rivers are keyed by the same color in Fig. 6b.
40
SC
RI
PT
ACCEPTED MANUSCRIPT
Fig. 7: Inverted erosion pattern in the Namche Barwa Syntaxis during the last 2
Ma (a) and its resolution (b). The variance reduction indicates how well the
erosion rate is constrained with respect to the a priori erosion rate and its
variance. Smaller variance indicates that the data is able to constrain better the
AC
CE
PT
ED
MA
NU
erosion rate.
41
MA
NU
SC
RI
PT
ACCEPTED MANUSCRIPT
Fig. 8: Relation between erosion rates (mm/a) and topographic factor. Erosion
ED
rates are derived from low-temperature thermochronometric ages. Only erosion
rates in the last 2 Ma are used to establish the correlation between erosion and
PT
topographic measurements. Erosion rates constrained by 10Be are from Finnegan
AC
CE
et al. (2008). Topographic parameters are derived from SRTM3 DEM data.
42
ACCEPTED MANUSCRIPT
Table 1. Namche Barwa sample description
Longitude
Latitude
Elevation(m)
Lithology
NB109
94.585
29.576
3520
Granite
NB135
95.697
29.74
4030
Granite
NB142
95.512
29.688
2360
Gneiss
NB134
95.719
29.762
4000
Granite
NB115
94.752
29.778
3310
NB124
95.118
30.085
2240
NB184
94.907
29.486
3870
SC
RI
PT
Sample
Granite
Gneiss
AC
CE
PT
ED
MA
NU
Gneiss
43
ACCEPTED MANUSCRIPT
Table 2. Apatite (U-Th-Sm)/He replicate analysis for the Namche Barwa sample
Sampl
Mass
He
238U
232Th
m
eU
Age
(ug)
(fmol)
(fmol)
(fmol)
(fmol)
(ppm)
(Ma)
NB10
9a2
236.7
2.13
1.55
NB10
9a4
6
25.48
233.2
1.77
1.25
5
4
185.5
18.76
3
32.18
NB13
5a3
1.54
0.51
112.6
0
6
7
209.6
743.4
6
2
NB13
ED
5
NB14
0.96
0.11
NB14
2.06
NB14
NB14
2
1.28
0.07
6
0.05
65.82
0.67
0.21
28.83
1.14
0.42
22.68
NB13
4a2
0.82
58.95
4.05
1.11
0.77
0.67
5.62±
0.34
1.67±0.
03
1.04
0.62
03
0.00
1.59±0.
1.24
0.78
09
0.58±0.
12.30
1
09
1.67±0.
0.49
0.84
211.7
3.94
5.28±0.
1.67±
16.85
2
08
04
0.70±0.
12.49
0.58
0.83
07
0.32
NB13
4a5
4.90
284.9
7.82
age
(Ma)
134.1
3
ed Age
0.96±
NB13
4a3
81.67
Mean_
AC
2a3
3.29
103.7
CE
2a2
66.71
PT
2a1
41.43
NU
0.61
865.0
MA
2.44
224.2
Correct
5.95±0.
27.30
9
NB13
FT
361.5
NB10
5a2
Raw
SC
RI
PT
e
147S
0.73
0.39
29.66
161.2
160.6
4
2
171.2
234.1
6
4
194.7
193.3
9
9
3.61±0.
23.49
0.68
10
6.92±0.
13.00
5.12
0.74
15
5.89±0.
24.15
44
2.44
3.95
0.67
16
ACCEPTED MANUSCRIPT
NB13
5.47±
4
0.98
NB11
5a2
1.28
0.97
NB11
5a3
321.6
224.2
265.3
8
7
6
118.6
1.01
0.27
2
2.77±0.
69.63
3.50
9
28.41
NB124
a2
1034.
1
5
68
118.1
1.11
1.63
4
NB12
0.94
0.02
92.91
94.57
4
0.32
0.88
0.49
6.82
AC
4a4
9.58
0.79
0.79
06
2.47±
0.29
0.23±0.
01
12.16±
0.20
0.13±0.
0.10
0.78
05
0.82
0.73
0.18±
0.05
686.7
8
2
0
182.5
156.4
524.2
7
0
7
45.34
177.7
1576.
821.3
147.6
3
34
0
6
582.3
576.6
841.4
209.4
0
1
6
3
CE
1.15
NB184
NB18
22.23
187.7
ED
1.39
NB18
a3
2
225.2
PT
NB18
4a2
28.13
0.18
387.3
NB12
4a1
7
0.79
MA
4a5
49.19
319.5
52.72
1.71
SC
RI
PT
0.10
465.3
NU
1.97
298.6
04
2.18±0.
5
4a1
0.72
241.4
NB11
NB12
2.00
1.17±0.
46.06
0.92
0.79
01
2.18±0.
1.70
0.78
04
14.75±
9.59
0.65
0.21
1.14±0.
0.79
0.69
02
NB18
1.50±
4
0.34
FT is calculated after Ketcham et al. (2011). The corrected ages and mean ages are calculated by
using FT. NB124a2 and NB184a3 are excluded from the mean age calculations since their ages are
too old compared to the rest of the single grain ages from the same sample.
45
ACCEPTED MANUSCRIPT
Highlights

New thermochronometric ages the Namche Barwa Syntaxis are presented.
Rock uplift is required to expose the young ages to the north of the
syntaxis.
 Variations in channel steepness indicate a S-to-N gradient in rock uplift
rate.
 Topographic metrics are weakly coupled with erosion rate.
A northward expansion of the syntaxis is inferred.
NU
MA
ED
PT
CE
AC

SC
RI
PT

46
Документ
Категория
Без категории
Просмотров
3
Размер файла
1 982 Кб
Теги
2017, tecto, 026
1/--страниц
Пожаловаться на содержимое документа