Springer-Verlag
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1 Background
Rivers play a dominant role in the earth surface process system on the continents, providing
the major pathways for both the water and sediment fluxes, causing material transport from
land to ocean. In response to changes in discharge, slope and sediment load, a river can incise
or aggrade. The related evolution of river systems is controlled by internal processes and
external forces induced by climate change, tectonic movements and anthropogenic influences.
Unravelling the relative contributions of these factors is one of the most challenging
goals of fluvial geomorphological research (Schumm, 1965; Vandenberghe, 1995a; Bridgland
et al., 2000; Bridgland and Westaway 2008; Wang et al., 2015; Cordier et al., 2017;
Rits et al., 2017). Alternations of aggradation and incision, and the formation of fluvial terraces
may reflect the different impacts of individual driving forces (e.g. Mather et al., 2017).
As concerns climate forcing, morphological cyclicity in temperate and cold regions has
generally been attributed to cold (glacial)-warm (interglacial) alternations in settings with a
background tectonic uplift (e.g., Büdel, 1977; Vandenberghe, 1993, 1995b; Van den Berg,
1996; Maddy et al., 2001; Starkel., 2003; Lewin and Gibbard, 2010; Bridgland et al., 2017).
But, similar climate-driven terrace staircases do exist also in other climatic environments
(e.g., Bridgland and Westaway,2008; Pan et al., 2009; Wang et al., 2013; 2015; Gao et al.,
2016; Jia et al., 2017).
The climate-driven model fails to explain the progressive Quaternary valley incision,
which is suggested to be the result of fluvial system adjustment to long-term regional uplift.
The degree of regional uplift is a key issue in the generation and subsequent preservation of
terrace flights (Maddy et al., 2001; Bridgland and Westaway, 2008; Wang et al., 2014, 2015), as
it can force a river system to incise during each climate cycle to separate terrace levels adequately.
Terraces can be generated only when the regional uplift rate is sufficiently high (e.g.,
Pan et al., 2009). Based on the assumption that terrace surfaces record net incision driven by
tectonic uplift, the vertical separation and the longitudinal profiles of terrace surfaces have
widely been used to infer tectonic uplift rate (e.g., Van Balen et al., 2000; Maddy et al., 2001;
Van den Berg and Van Hoof, 2001; Peters and Van Balen, 2007; Demoulin et al., 2017).
In contrast to large-scale tectonic movements, tectonic differentiation at small scale may
lead to a variegated fluvial morphology. For example, in the Huangshui catchment (Northeastern
Tibetan Plateau) areas of fluvial incision and aggradation spatially alternate and erosion
and accumulation terraces simultaneously develop at relatively short distances indicating
effects of relative uplift and subsidence on relatively small spatial scales (Wang et al.,
2010, 2014; Vandenberghe et al., 2011).
The relation between forcing intensity by climate and tectonics and fluvial response is not
linear due to the preponderant role of delay effects and thresholds (Schumm, 1979; Knox,
1972; Vandenberghe, 2002; Veldkamp et al., 2017). For instance, fluvial incision may lag
behind uplift as the river may be dependent on a climatic change to enable incision to take
place (Maddy et al., 2001). Tectonic movements and climate are independent forcing factors
and their interplay may result in conflicting, or at least complex, effects on the fluvial morphology
(Wang et al., 2015; Van Balen et al., 2016). Until now, both forcing factors have
been approached as separate as possible. However, it is challenging to investigate the complex
result of their interference.
The Northeastern Tibetan Plateau (NETP) (Figure 1) is one of the tectonically most active
regions in the world. However, it also experienced drastic climate changes. For example,
during the LGM mean annual temperatures were at least 7° less than at present leading to the
expansion of glaciers and the initiation of periglacial processes (Wang et al., 2013). Thus,
geological and geomorphological records of river evolution in the NETP are ideal for studying
the impacts of the coupled effects of tectonic motions and climate changes on fluvial
processes. At here we review the studies of the fluvial morphological development and the
resulting sedimentary architecture of fluvial deposits in the NETP, mainly based on the case
of the Huangshui catchment.
Figure 1 (a) Simplified map of major tectonic boundaries and Tertiary faults in the Northeastern Tibetan Plateau (after
Dai et al., 2006); (b) Morphologic characteristics of the NETP derived from Aster Global Digital Elevation Model data
2 Geological and geomorphological background
The NETP, undergoes coeval crustal shortening and left-lateral strike-slip faulting, related to
the northeastward growth of the plateau margin as a result of the collision of the Indian and
1328
Asian plates. The crustal deformation primarily due to these relative plate motions is expressed
by folding and faulting of the Paleozoic to Cenozoic bedrock (Figure 1). Within the
NETP, also syn-orogenic basins were formed, such as the Gonghe basin (Craddock et al.,
2010), Linxia basin (Li et al., 1997), Baode basin (Pan et al., 2010; Hu et al., 2017) and
Xining basin (Figure 1). The basins have been filled with thick continental clastic red beds,
while simultaneously the surrounding highs were bevelled (as in the areas surrounding the
Xining basin, see further discussion below). Subsequently, the area was incised by rivers in
response to regional tectonic uplift of the Tibetan Plateau and local deformation affecting
even the red basin fills, probably during the Pliocene and Pleistocene (Li, 1997; Fang et al.,
2005; Gao et al., 2008; Stroeven et al., 2009; Craddock et al., 2010; Hu et al., 2017).
The deformation in the study area was controlled by two major large sinistral strike-slip
faults, the Altyn-Haiyuan Fault in the north and the Kunlun Fault in the south (Figure 1;
Tapponnier et al., 2001; Dai et al., 2006). The offset along these faults decreases gradually
in eastward direction, which is accommodated by internal compressive deformation (Tapponnier
et al., 2001; Dai et al., 2006). This deformation has resulted in a tectonic mosaic,
consisting of open folds, and reverse and normal, dextral- and sinistral strike-slip faults,
leading to the formation of alternating subsided and uplifted blocks of more limited extent
than the large Mesozoic-Early Cenozoic basins, resulting in a fragmentation and compartmentalization
of the former basins and inter-basin areas (Figure 1).
The Yellow River and its tributary, such as Huangshui River which is the focus of the
study, follows a series of those uplifted and subsided blocks that resulted from the fragmentation
of the Xining and other basins. The basement of the Xining Basin consists of Proterozoic
gneisses and schists, Cambrian gray limestones and green basalts and Mesozoic clastic
sediments (QBGMR, 1991; Dai et al., 2006). Cenozoic successions of playa to fluvio-lacustrine
environments, concordantly overlying the Cretaceous clastic sediments or
discordantly older basement
rocks, have been subdivided
into the Xining and Guide Groups.
In addition, ‘fanglomeratic’
rocks unconformably overlie
the Guide Group (QBGMR,
1991; Dai et al., 2006). The
Huangshui cut ~600 m into the
Cenozoic strata and the Proterozoic-Mesozoic
basement
rocks, and meanwhile built a
well-shaped staircase of terraces
in the Xining basin
(Figure 2) (Lu et al., 2004a;
Vandenberghe et al., 2011).
The youngest deposits consist
of aeolian sediments that
blanket the landscape.
Figure 2 Terrace sequence along a schematic section in the Xining
basin (modified after Lu et al., 2004a, and Vandenberghe et al., 2011)
3 Quartz Optically Stimulated Luminescence (OSL) dating of fluvial sediments
in the NETP
Datings are the prerequisite to unravel the details of the relationship between the fluvial
events (as terrace formation, processes of erosion and deposition) and climate changes, and
they are also significant to examine the neo-tectonic process in the NETP. It is difficult to
establish a reliable chronological control by 14
C dating for the last-glacial events in the study
region because of the dating limit (<45 ka) and the lack of suitable dating materials in this
arid and cold area. The typical fluvial channel sands, floodloam sediments, outwash fans and
glacial- and fluvio-glacial sediments were successfully dated using quartz optically stimulated
luminescence (OSL) with the single aliquot regenerative-dose (SAR) protocol (Wang
et al., 2013, 2014, 2015).
A small but sufficient amount of fine-sand quartz could be extracted to permit standard
SAR-OSL analysis. The measurements using small aliquots (or single grains) might be the
best to assess the degree of partial bleaching and date glacial and glacio-fluvial sediments, as
they are likely to suffer from poor or inhomogeneous bleaching. However, the measurements
show that OSL signals of most samples in the Tibetan Plateau were not bright and that their
luminescence sensitivity was low (Wang et al., 2013). The results of the measurements using
small aliquots for selected relatively bright samples show the distributions of the equivalent
doses are broad, with relative standard deviations in the range of ~16% to 26%, and display
little or no asymmetry (Figure 3) (Wang et al., 2013). In addition, the un-weighted average
De’s using small aliquots are not different from the values obtained using large aliquots.
These results indicate a well-bleached nature of glacial- and fluvial sediments in the NETP,
and do not hint at incomplete resetting as a significant source of error. Also, it has been arFigure
3 Histogram of De distribution for typical samples in Menyuan basin using small aliquots (after Wang et
al., 2013)
1330
gued (see e.g. Murray and Olley, 2002; Jain et al., 2003) that incomplete resetting is unlikely
to give rise to significant age overestimations for samples older than a few ka, even in glacio-fluvial
environments. Thus the Quartz-based ASR-OSL analysis could provide robust
age control for glacial and fluvial terrace deposits in the NETP, although they should, at least
in principle, be considered as maximum ages.
4 Morpho-tectonic and geomorphological evolution of the NETP
Several stages can be discriminated in the geomorphological evolution of the Huangshui
catchment, inferred from basin filling, planation and fluvial terraces in the Huangshui
catchment (Vandengerghe et al., 2011; Wang et al., 2012). During the first stage, the landscape
consisted of mountains and filled-up basins that were leveled synchronously, ultimately
resulting in the formation of a peneplain (Figure 4). This peneplain thus consists partly of a
bedrock surface and partly of the eroded top of the basin fills. This kind of surface has also
been documented at several other places besides in Huangshui catchment (e.g., Clark et al.,
Figure 4 Schematic diagram illustrating the geomorphologyical evolution of the Xining basin and the formation
of the relict surface (modified after Wang et al., 2012). Firstly, the tectonic basin was formed before the Mesozoic
(I). Then, the surrounding mountains eroded and the basin was filled (II,i). As a last stage in this step, limited and
very local erosion in the basin might have slightly lowered the surface formed in II (i), ultimately resulting in the
formation of a peneplain surface (II, ii). Finally, tectonic uplift caused the Huangshui to incise vertically into the
surface, producing a terrace sequence (III).
2006; Pan et al., 2007; 2010, 2012; Craddock et al., 2010; Hu et al., 2017). Indeed, two
planation surfaces on the top of folded strata from the eastern Qilian Mountains, around one
hundred km north of the Huangshui catchment have been reported by Pan et al. (2007): an
older ‘main surface’ and a younger ‘erosion surface’ (1.4 Ma). According to its age (Wang et
al., 2012), the peneplain-like surface in the Huangshui catchment might be equivalent to the
'main surface' in the eastern Qilian Mountain. During the second stage, the Huangshui incised
into the peneplain surface, producing a terrace sequence (Figure 4). The transition from
peneplain formation to incision in the study region was dated as older than 10–6 Ma using
the biochronology of micromammalian assemblages from fluvial terraces. In addition, it is
should be younger than the final filling of the tectonic basins, that is around 17 Ma according
to Dai et al. (2006).
Erosion surfaces have served as important markers of tectonic uplift and deformation (e.g.,
Epis and Chapin, 1975; Spotila and Sieh, 2000; Clark et al., 2004). The combination of rapid
fluvial incision into bedrock and relict geomorphic surfaces have been interpreted to reflect
a tectonic uplift on the southern Tibetan Plateau margin with a magnitude equal to the incision
depth (Burbank et al., 1996). At the southeastern margin of the Tibetan Plateau, relatively
flat and highly elevated surfaces of postulated pre-uplift age were also reported, which
were heavily dissected, creating steep fluvial valleys, and indicating the relative lowering of
base levels due to tectonic uplift (Clark et al., 2004, 2006; Schoebohm et al., 2004, 2006).
Thus, the end of basin filling and the start of the incision in the Huangshui catchment may
thus be attributed to an important uplift event starting at around 10–17 Ma ( Lu et al., 2004a;
Wang et al., 2012).
The entrenchment rate of the Huangshui was not stable. It accelerated for instance from 2
cm/ka to 19.5 cm/ka at around 2 Ma (Vandenberghe et al., 2011; Wang et al., 2012), indicating
a specific phase of accelerated uplift (Lu et al., 2004a; Miao et al., 2008; Vandenberghe
et al., 2011; Wang et al., 2012).
Previous studies near the Huangshui catchment, by Pan et al. (2010, 2012), Hu et al.
(2017) and Craddock et al. (2010), show that rivers started entrenching from a beveled (erosional)
surface at much younger ages, c. 3.7 Ma and 1.8 Ma along the Yellow River. The
discrepancy may be explained in several ways. First, the older landscapes at the other sites
may not have been preserved; they might have been eroded. Secondly, the relict beveled
surfaces may have experienced a complex history with different ages of development in different
areas (Clark et al., 2006). The late entrenching of the relict surface reported by the
former authors could have been caused by the headward erosion of rivers flowing at a lower
level and not by a river properly flowing on top of the peneplain, like in the Huangshui
catchment. Thus, a certain lag is possible because of time needed by the headward erosion.
In our opinion, the infilling of the large tectonic basins and the peneplain formation should
have occurred largely in the same time period on the NETP. But, the effective duration of
these different processes, and thus their time of termination, may have been different at different
locations. In addition, it was argued that the drainage basin integration, and related
excavation of Tertiary–Quaternary sedimentary basins along the Yellow River, led to
isostatic uplift, and the development of an internally drained basin in the NETP (Zhang et al.,
2014).
1332
5 Fluvial morphology and sedimentology response to small-scaled tectonic
movements
The general uplift at 10–17 Ma caused local fragmentation of the Huangshui catchment into
blocks of small extent, demonstrated as gorges and depressions (scale of kilometers or
maximally, a few tens of kilometers), respectively (Wang et al., 2010; Vandenberghe et al.,
2011). These blocks might have undergone relative subsidence and/or uplift (Wang et al.,
2010, 2014; Vandenberghe et al., 2011). It has been suggested that the inferred tectonic motions
are related to the transpression movements in the NETP as a result of the collision of
the Indian and Asian plates (Vandenberghe et al., 2011; Wang et al., 2012). In contrast with
the generally persisting uplift of the NETP, at the scale of individual blocks uplift sometimes
alternated with relative subsidence, leading to laterally alternating accumulation terraces and
erosion terraces (Figure 5) in those blocks (Fluvial deposition (>30 m thick) in the subsided
blocks, resulting in accumulation terraces, contrasts with entrenching (resulting in erosional
terraces with thin fluvial deposits, i.e., strath terraces) and formation of gorges (where terraces
are rare or completely absent) in the uplifted blocks (Figure 6) (Wang et al., 2010).
The different rates of uplift and subsidence in the individual blocks resulted in the simultaneous
development of erosion and accumulation terraces in different blocks within the same
catchment (Figure 6) (Wang et al., 2010; Vandenberghe et al., 2011). In other words, fluvial
aggradation may have occurred in specific blocks at the same time rivers incised in other
(adjacent) blocks.
Figure 5 Sedimentologic character of the erosion terrace (a) and the accumulation terrace (b) in the Huangshui
catchment
In the confluence region of the Huangshui and Huanghe rivers, from the Minhe depression
to the west Lanzhou depression, the average river incision rate since –70 ka was much
higher than in the up- and down-stream blocks, indicating relative uplift of the confluence
region (Wang et al., 2014). At a smaller time scale, somewhere between 20 and 70 ka, two
accumulation terraces with thick stacked fluvial deposits (>18 m) indicate two phases of
subsidence in the confluence region relative to the up- and down-stream blocks (Wang et al.,
2014). This could point to relatively short phases of tectonic stability or even subsidence
during a period of general tectonic uplift.
The lateral transition between the very erosion-resistant Palaeozoic rocks of the uplifted
block to the much younger and less resistant Tertiary clastic sediments in the depression,
such as between Laoya gorge and Minhe depression (Figure 7), normaly corresponds with a very
prominent, almost vertical escarpment
in the topography
(Figure 8), and this morphological
escarpment may be interpreted
as a fault. The vertical
erosion (with formation of the
strath terraces) obviously was
always the main fluvial activity
in the exit of the Laoya gorge,
NW of the Huangshui, as a result
of the regional tectonic uplift,
while in part of the Minhe
depression, SE of the Huangshui
vertical erosion was interrupted
by accumulation at specific
periods as a result of local
subsidence (Figure 9) (Vandenberghe
et al., 2011). It was deduced
that different tectonic
backgroud caused by faults between
the gorge and the depression
lead to different fluvial
response, a series of erosion
terraces and steep cliff formed
in the hard bedrock of the Laoya
block north of the Huangshui as
the general uplift, where erosion
and accumulation terraces developed
response to general
uplift and relatively subsidence,
in the Minhe depression, SE of
the Huangshui (Figure 9) (Vandenberghe
et al., 2011).
Figure 6 The formation model of the terrace in Huangshui catchment
(modified after Wang et al., 2010). Firstly, the fluvial balance
state is crossed because of climatic change or tectonic and the river
incisions, until reaching to a new balance state; Secondly, the tectonic
subsidence happens in the blocks, and an intensified erosion in upstream
while a huge volume of sediments supplied deposit quickly in
the subsidence depression. Finally, the river incises in the whole
catchment, caused by climatic changes such as increasing of the precipitation
and the reducing of the sediment load caused by the relevant
development of the vegetation cover or tectonic uplift, and the accumulation
terraces are formed in the subsidence depressions while the
erosion terraces or no terrace are formed in upstream blocks.
Figure 7 DEM of the Minhe subsided block with
the transition to the Laoya gorge in the left upper
corner
Figure 8 Fault scarp at the boundary between the
Laoya block composed of hard rock and the Minhe
depression underlain by soft sediments
1334
Figure 9 Terrace sequence at the exit of the Laoya gorge and the transition to the western most part of the
Minhe basin, and the relation to fault activities (modified after Vandenberghe et al., 2011). Firstly, incision occurred
from 271 to 197 m, due to regional tectonic uplift. Secondly, uplift continued in the Laoya block, while the
Minhe block subsided, leading to the accumulation of 30 m gravel (terrace at +200 m). Subsequently, uplift was
dominant everywhere, resulting in the formation of a series of erosion terraces in the Laoya block between +197
and 100 m; while only one erosional terrace was preserved in the tectonic depression at +100 m. Finally, the recent
fault activity, resulted in sharp vertical erosion in Laoya block, while subsidence prevailed in Minhe depression,
leading to the formation of two accumulation terraces at 60 and 44 m.
6 Fluvial sediment process and terrace staircases as a response to climatic
change
Incision into the former peneplain was not continuous but a staircase of terraces (consisting
of at maximum 15 to 20 individual levels in some blocks), developed as a result of climatic
influences (Figure 4) (Lu et al., 2004a; Vandenberghe et al., 2011; Wang et al., 2012) . The
sedimentary series of the different terraces are similar (Vandenberghe et al., 2011; Wang et
al., 2014, 2015). From bottom to top: fluvial gravels of various thickness, interbedded with
lenses of sands, silts, and clays, and finally topped by a horizontally laminated silt, occasionally
containing gravel strings of limited extent (Figure 10). The lowest part of the gravel
deposits consists of massive gravel (Gm) representing channel bedload, grading towards the
top into finer-grained, coarsely planar bedding (Gp) (Figure 10). The latter deposits frequently
show imbrication and small-scaled cross-bedding, which indicate deposition in lateral
and longitudinal bars. Small and shallow channels occur with increasing frequency towards the
top of the terrace deposits; they are filled with cross-bedded fine gravel or sands (planar to
low-angle trough cross-bedding) (Figure 10). The upper laminated silts are interpreted as floodloam
deposits that complete the fluvial sequence prior to or simultaneously with the abandonment
of the floodplain as a result of renewed river incision. These sedimentary characteristics
of the fluvial gravel-sand deposits, dominated by varying (often cyclic) assemblages of
gravel traction-current deposits, clearly point to the shallow gravel-bed braided river systems
(Miall, 1996; Lewin and Gibbard, 2010) that contrast with the present-day wandering or
meandering pattern of the Huangshui. At several locations, soil formation has been recognized at
the top of fluvial depositional sequences, although not being well developed (Vandenberghe et
al., 2011; Wang et al., 2014). Mostly, a gradual transition from flood loam to loess is present
and the flood loams are often red-colored, pointing to upstream removal or local reworking of
soil material.
Figure 10 Characteristic sedimentary sequences of terraces: (a) Planar sheets of gravel alternating with finer
grained, shallow, trough cross-bedded channels of 44 m terrace above present floodplain in Ledu basin; (b) inter-bedded
layers of horizontally-laminated silt reworked soil filling shallow channels; (c) aggradation sequence
of –40 m terrace above present floodplain in the eastern part of the Minhe depression; (d) horizontally laminated silts
occasionally containing gravel strings of limited extent of 22 m terrace above preset floodplain at east Minhe depression
Climate proxies are mostly absent within the Huangshui terrace deposits, which makes it
difficult to prove the link between climate and fluvial processes. Nevertheless, in accordance
with the reconstructed character of the river morphology as a shallow gravel-bed braided
river (Miall, 1996) that contrasts with the Holocene meandering river, the majority of the
(coarse grained) fluvial deposits of the terraces seems to date from cold periods. Subsequently,
we assume that, in line with the general model of fluvial development derived for
temperate and periglacial environments (Vandenberghe, 2008; 2015), the fine-grained sandy
gravels and sands near to the top of the sediment series were deposited during the waning of
the same glacial period as a result of the beginning incision of the river (possibly in combination
with reduced precipitation). Ultimately the fluvial sediments were capped by interglacial
soil formation. Rivers incised slightly at the transition from the glacial to the interglacial, due to
less peaked river discharge and the reduction of sediment supply to the river. As a consequence
of that new, lower position in the next interglacial, the previous coarse grained channel
deposits were flooded only during peak discharges resulting in episodic, fine-grained floodplain
deposition, while most of the time soil formation took place on the floodplain. Analogous to
the present-day river, the interglacial river was probably less energetic and dominantly meandering.
Renewed incision took place at the next interglacial-glacial transition, when flu-
1336
vial energy considerably increased and sediment transport was still limited. At that time,
loess covered the former floodplain deposits and soil without any further reworking by the
river (Vandenberghe et al., 2011)
7 Climate-dependent fluvial architecture and processes on a suborbital
timescale in areas of rapid tectonic uplift since the last interglacial
In the confluence region of the Huangshui and Huanghe rivers, from the Minhe depression
to the west Lanzhou depression, eight fluvial terraces are present (Figure 11), which are
dated at around 129–103 ka (T7), 81–73 ka (T6), 68–51 ka (T5), 42–32 ka (T4), 24–22 ka
(T3), and 14–13 ka (T2), respectively, using SAR-OSL analysis (Wang et al., 2015). The
average river incision rate was 0.87 m ka-1
since the last interglacial, which is much larger
than in the upstream Huangshui river (around 0.05–0.06 m ka-1
, Vandenberghe et al., 2011),
the downstream Huanghe (around 0.35 m ka-1
, Pan et al., 2009), and even in most catchments
in the world, where long-term average downcutting rates are less than 0.2 m ka−1
(Bridgland and Westaway, 2008), indicating relative uplift of the confluence region. This
relatively strong uplift gives more space for differentiation within the terrace staircase as a
result of climatic changes, leading to 7 terraces formed as a response to small climatic fluctuations
(103
–104
year timescale) (Wang et al., 2015). Assuming that the OSL-age is reliable,
it leads to the striking conclusion that the stronger the tectonic movement the better the climatic
imprint is expressed in the terrace development.
Figure 11 (a) Terrace distribution in the confluence region of the Huangshui and Huanghe rivers (modified after
Wang et al., 2014). (b) A selected topographic section from Figure 11a, illustrating a typical terrace sedimentary
sequence. U1, U2, U3 are the sedimentary units of the terraces (U1: stacked fluvial gravels of varying thickness with
inter-bedded sand lenses, U2: cross-stratified sands, U3: inter-bedded layers of horizontally-laminated silt and sands.)
The dating results show that for each terrace, the lower coarse-grained sediments
(gravel and sand) were deposited during cold periods (such as the LGM, MIS3b, MIS4
and MIS5d) associated with a strong Asian winter monsoon (Figure 12). The coarse
Figure 12 Comparison of the age of terraces (T2, T3, T4, T5, T6 and T7) and periods of fluvial deposition (at
top) with climatic evolution recorded by the grain-size record of loess deposits at GL (gray) (Sun et al., 2012) and
YZ (red) (Lu et al., 2004b) in the NETP (modified after Wang et al., 2015). Green, black and blue codes of ages and
errors (2σ error bars at top) correspond to sedimentary units (U1, U2, and U3, similar to that in Figure 11), respectively.
Numbers indicate marine isotope stages and substages. Age of loess deposit sequence is based on 20
OSL datings of GL by Sun et al. (2012) and correlation to marine isotope stages in YZ (Lu et al., 2004b).
grained cold phase deposits are covered
by inter-bedded, horizontallylaminated
silt and sand (representing
flood sediments that often contain
reworked soil material), during the
(cold to warm) transitional phases.
The floodplain accumulation on the
terrace continued during the subsequent
warm period. The warm periods
(such as MIS3a, MIS3c, and MIS5a)
of the climatic cycles are associated
with a strong Asian summer monsoon
(Figure 12). Pronounced incision took
place at the subsequent warm-cold
transitions (Figure 13). After this
warm-cold transition, aeolian loess
accumulated on the abandoned terrace
without any further fluvial reworking.
It demonstrates that critical thresholds
for fluvial response can be
crossed at climatic changes on a suborbital
timescale given conditions of
accelerated tectonic uplift in the
NETP.
Figure 13 Schematic diagram illustrating the terrace formation
in response to climatic change in the confluence of
Huangshui and Huanghe. U1, U2, U3 are the sedimentary units
of the terraces, similar to that in Figure 11.
1338
8 Conclusions and remarks
The substantial tectonic uplift of the Northeastern Tibetan Plateau (NETP), together with the
major climatic changes, provides an opportunity to study the impact of tectonic and climatic
changes on the morphological development and sedimentary architecture of fluvial deposits.
The effects of these processes are revealed by a terrace staircase, together with the stratigraphy
of each individual terrace. A peneplain-like surface and the related landscape transition
from basin filling to incision happened at least before the late Miocene in the NETP,
which indicates that an intense uplift event with morphological significance happened
around 10–17 Ma in the NETP. The general uplift at 10–17 Ma caused local fragmentation
of the Huangshui catchment into blocks of small extent, demonstrated as gorges and depression.
After that, incision into the former peneplain was not continuous but a staircase of terraces,
developed as a result of climatic influences. Since the late Miocene, in spite of generally
persisting uplift of the whole region, the neighbouring tectonic blocks had different uplift
rates, and the uplift rates could change on a time scale of 10 ka. This tectonic differentiation
causes a complicated fluvial response with accumulation terraces alternating with erosion
terraces at a small spatial and temporal scale. Fluvial aggradation occurred during cold
periods in general. Rivers progressively incised, reaching only sporadically the previous
floodplain at the transition warm periods. The resuming vegetation cover reduced sediment
supply to the rivers so that. This change in fluvial activity as a response to climatic impact is
reflected in the general sedimentary sequence on the terraces from high-energy (braided)
channel deposits (at full glacial) to lower-energy deposits of small channels (towards the end
of the glacial), mostly separated by a rather sharp boundary from overlying flood-loams (at
the glacial-interglacial transition) and overall soil formation (interglacial). Pronounced incision
took place at the subsequent warm-cold transitions. In addition, it is hypothesized that
in some strongly uplifted blocks energy thresholds could be crossed to allow terrace formation
as a response to small climatic fluctuations (103
–104
year timescale).
Although studies of morpho-teconic and geomorphological evolution of the NETP, improve
understanding on the impacts of tectonic motions and monsoonal climate changes on
fluvial processes, a number of questions remain to be answered and carried out in the future:
1) How far are the peneplain and the related morphological features and can they be correlated
over large distances? 2) Morphological features as terraces and peneplains at different blocks
have to be dated more precisely to make better correlations and, therefore, firmer hypotheses
on morphological evolution and the link in different blocks and/or in different tectonic settings.
3) Can we specify in more detail to what extent and intensity tectonic movements may
influence the crossing of climatic thresholds, leading to terrace development?
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