the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
Response of modern fluvial sediments to regional tectonic activity along the Min River, Eastern Tibet
- 1State Key Laboratory of Earthquake Dynamics, Institute of Geology, China Earthquake Administration, Beijing 100029, China
- 2Lhasa Geophysical National Observation and Research Station, Institute of Geology, China Earthquake Administration, Beijing 100029, China
- 1State Key Laboratory of Earthquake Dynamics, Institute of Geology, China Earthquake Administration, Beijing 100029, China
- 2Lhasa Geophysical National Observation and Research Station, Institute of Geology, China Earthquake Administration, Beijing 100029, China
Abstract. The deposition of fluvial sediments in tectonically active areas is mainly controlled by tectonics, climate, and associated Earth surface processes; consequently, fluvial sediments can provide a valuable record of changes in regional climate and tectonic activity. In this study, we conducted a detailed analysis of the grain‒size distribution in modern fluvial sediments from the upper Min River, Eastern Tibet. These data were combined with regional information about climate, vegetation, hydrology, geomorphology, lithology, and fault slip rate, and together indicate that modern regional tectonic activity along upper Min River can be divided into three segments. Specifically, fluvial sediments in the segment Ⅰ are dominated by fine silts (<63 μm: 70.2 %), agreeing with a low‒runoff and low‒rainfall in this segment and revealing a windblown origin influenced by the arid and windy climate. These observations are consistent with the segment’s low hillslope angle and low relief, all indicating weak activity along the Minjiang Fault. The coarse‒grained fraction (>250 μm) of fluvial sediments in the segments Ⅱ ‒ Ⅲ increases in a stepwise fashion (A = 6.2 %, B = 19.4 %, C = 33.8 %) downstream, although runoff and rainfall do not change significantly from segment Ⅱ to segment Ⅲ. These patterns correlate well with an increase in both regional relief and hillslope angles. Together, these observations imply that regional tectonic activity along Maoxian‒Wenchuan Fault becomes more pervasive downstream along the Min River. Fluvial sediments in segment Ⅳ are well sorted and well rounded, which is expected due to significant increases in rainfall and runoff in this segment. This study marks the first development of a new and important research approach that can characterize regional tectonic activity by analysis of fluvial sediments collected from tectonically active regions, combined with regional conditions in geology and geography.
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Wei Shi et al.
Status: open (until 07 Sep 2022)
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RC1: 'Comment on esurf-2022-34', Anonymous Referee #1, 19 Jul 2022
reply
Shi et al. present combined analyses of grain-size distribution and magnetic susceptibility in modern fluvial sediments from the upper Min River, Eastern Tibet to distinguish regional tectonic vs climatic influences on fluvial deposition. Their study indicates that tectonic faulting exerts a first-order control on fluvial deposition in tectonically active regions, which are well supported by regional relief and hillslope angle data. As far as I know, this study proposes a new and plausible method to characterize the development of tectonic activity along a transect, which is distinctly different from the traditional low-temperature thermochronologic dating and seismic methods as we often see. Therefore, I recommend this manuscript for publication. In the meantime, there are some minor issues that are needed to be clarified before this manuscript can be completely accepted.
P33-34: The occurrence of well-sorted and well-rounded sediments may be related to a significant increase in rainfall and runoff at the source area with more erodible bedrocks, or to long-distance transport which may experience multiple sediment recycling.
P109-110: Which analyses of river sediments should be specified here.
P118-119: …and tectonic activity is tectonically controlled by….
P141: The location of Fig. 1a was not shown in the inset map.
P200: the bedrock is naked well exposed.
P286: what does the dotted line represent at the upper left part (from the top to about No 50) of the SUS data?
P435: a higher of rivers incision rates???
P488: seem a little contradict with???
At last, I personally think the clarity of Figs 3-7 could be improved significantly. It is better to mark in color mode.
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AC1: 'Reply on RC1', Wei Shi, 01 Aug 2022
reply
We are grateful to Reviewer 1 for giving us detailed comments to improve our manuscript. The responses to the comments from Reviewer 1 are addressed point by point as follows.
Comments
Shi et al. present combined analyses of grain-size distribution and magnetic susceptibility in modern fluvial sediments from the upper Min River, Eastern Tibet to distinguish regional tectonic vs climatic influences on fluvial deposition. Their study indicates that tectonic faulting exerts a first-order control on fluvial deposition in tectonically active regions, which are well supported by regional relief and hillslope angle data. As far as I know, this study proposes a new and plausible method to characterize the development of tectonic activity along a transect, which is distinctly different from the traditional low-temperature thermochronologic dating and seismic methods as we often see. Therefore, I recommend this manuscript for publication. In the meantime, there are some minor issues that are needed to be clarified before this manuscript can be completely accepted.
Thank you very much. Revisions have been made below according to your comments.
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AC2: 'Reply on AC1', Wei Shi, 01 Aug 2022
reply
Comments
P33-34: The occurrence of well-sorted and well-rounded sediments may be related to a significant increase in rainfall and runoff at the source area with more erodible bedrocks, or to long-distance transport which may experience multiple sediment recycling.
Yes, we've also considered the possibility you mentioned. The segment Ⅳ is located in the downstream of the Dujiangyan, and large amounts of coarse debris (>250 μm) was captured in Zipingpu Reservoir. Well-sorted and well-rounded (pebbles) of fluvial sediments in segment Ⅳ may be related to long-distance transport which may experience multiple sediment recycling, further to say that it must be related to the long-time of scouring and sorting by rivers. We have corrected it in line 33-34.
P109-110: Which analyses of river sediments should be specified here.
Yes, we have specified that in line 111.
P118-119: …and tectonic activity is tectonically controlled by….
Thanks, we have corrected it in line 120.
P141: The location of Fig. 1a was not shown in the inset map.
Thanks, we have marked it in the inset map.
P200: the bedrock is naked well exposed.
Thanks, we have corrected it in line 203.
P286: what does the dotted line represent at the upper left part (from the top to about No 50) of the SUS data?
The dotted lines represent the average value of the whole sequence. We have specified that in the figure 3 caption (line 289).
P435: a higher of rivers incision rates???
We mean that active tectonics activity forced a higher regional denudation rate, rather than the river incision rates. We have corrected it in line 439.
P488: seem a little contradict with???
We mean that the hillslope angles and local relief gradually increase downstream along the Minjiang Fault are decoupled with the high and stable proportion of fine‒grained background dust in the fluvial sediments of segment Ⅰ. We have corrected it in line 492-494.
At last, I personally think the clarity of Figs 3-7 could be improved significantly. It is better to mark in color mode.
Thanks, we have modified Figs 3-7.
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RC2: 'Reply on AC1', Anonymous Referee #1, 01 Aug 2022
reply
Thanks for your careful revisions and modifications on the early version of the manuscript. I have no further comment on this well-revised manuscript, which can therefore be accepted as it is now.
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AC2: 'Reply on AC1', Wei Shi, 01 Aug 2022
reply
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AC1: 'Reply on RC1', Wei Shi, 01 Aug 2022
reply
Wei Shi et al.
Wei Shi et al.
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