Assessment of geomorphic processes and active tectonics in Con Voi mountain range area (Northern Vietnam) using the hypsometric curve analysis method

Vietnam Journal of Earth Sciences, 38(2), 202-216, DOI: 10.15625/0866-7187/38/2/8602 202 (VAST) Vietnam Academy of Science and Technology Vietnam Journal of Earth Sciences Assessment of geomorphic processes and active tectonics in Con Voi mountain range area (Northern Vietnam) using the hypsometric curve analysis method Ngo Van Liem*1, Nguyen Phuc Dat2, Bui Tien Dieu3,4, Vu Van Phai5, Phan Trong Trinh1, Hoang Quang Vinh1, Tran Van Phong1 1Institute of Geological Sciences, Vietnam Academy Science and Technology 2Vietnam Institute of Geosciences and Mineral Resources, Ministry of Natural Resources and Environment 3Geographic Information System Group, Department of Business Administration and Computer Science, University College of Southeast Norway 4Faculty of Geomatics and Land Administration, Hanoi University of Mining and Geology 5Faculty of Geography, VNU University of Sciences Received 25 January 2016. Accepted 7 June 2016 ABSTRACT The main objective of this study is to assess geomorphic processes and active tectonics in the Day Nui Con Voi (DNCV) area of Vietnam. For this purpose, a spatial database was collected and constructed, including DEM (Digital Elevation Model) and a geological map. The hypsometric curve (HC) analysis method and its statistical moments were adopted to use for the assessment. These methods have been widely used for the assessment of geomorphic processes and active tectonics in many areas in the world showing promising results. A total of 44 sub-basins of the Red River and the Chay river were analyzed. The result shows that 3 curve-types such as "straight- shape", "S- shape", and concave were found; with the concave curve being the dominant and widely distributed in the northeast side and in the south of the southwestern side of the study area. The hypsometric integral (HI) values are rather small with the largest value is 0.37 and the smallest one is 0.128. Other statistical moments of the hypsometric curve, i.e. skew (SK), kurtosis (KUR), and the density function (density skew - DSK and density kurtosis-DKUR) show great values, which increased in the south direction of the area study. Accordingly, recent active tectonics (uplift-lower) in the study area is generally weak; however, they are also not completely homogeneous and can be distinguished by different levels. The southwestern side is being lifted higher than the northeastern side. The northern part is being lifted larger than the southern part. In the region, the uplift activities were increased gradually in the Pliocene- Quaternary and could have stopped at certain time in the past. The current geomorphic processes are mainly headward erosion in the upstream. Keywords: Geomorphic index; Hypsometric curve; Statistical moments; Active tectonics; Red River fault; Day Nui Con Voi. ©2016 Vietnam Academy of Science and Technology 1. Introduction* The Red River shear zone (RRSZ) extends *Corresponding author, Email: liem.igsvn@gmail.com over a length of 1000 km from Tibet to the East Vietnam Sea. Along the shear zone, four narrow massifs of high-grade metamorphic complexes, the Day Nui Con Voi in Vietnam, N.V. Liem, et al./Vietnam Journal of Earth Sciences 38 (2016) 203 Ailao Shan, Diancang Shan and Xuelong Shan in Yunnan, China are considered as the "axes" of the RRSZ - important geological boundaries in Asia. The Day Nui Con Voi range is in the southeasternmost part of this shear zone (Figure 1). This area has been re- ceived attentions of many geoscientists and seen as a key to understand the geodynamics of the RRSZ (Leloup et al., 1995; 2001; Le et al., 2004). The achieved results have contrib- uted to the explanation and clarification of many issues in geology, tectonics and geomorphology. However, some points are not consistent and disputed (e.g. Tran et al., 1999; 2002; Le, 2003; Le et al., 2001; Phan et al., 2004; Wang et al., 2000; Leloup et al., 2001. Studies of tectonics in this area have not paid much attention to the role and significance of geomorphology; especially, the lack of quantitative analyses of landscapes using various geomorphic indices. Geomorphic indices have been found to be useful in identifying areas experiencing tec- tonic activity because they facilitate rapid evaluation of large areas (Strahler, 1952; Bull and McFadden, 1977; Keller and Pinter, 2002; Joshi et al., 2013). Furthermore, active faults and growing folds commonly have topogra- phy that is useful in identifying different geomorphic or structural segments along the fault and estimating the most active segments (Azor et al., 2002; Font et al., 2010; Joshi et al., 2013). Segments along a morphostructure may be outlined and identified to determine the relative intensity of tectonic activity along a fault by utilizing a detailed study of drainage anomalies coupled with geomorphic indices (Azor et al., 2002; Keller and Pinter, 2002; Joshi et al., 2013). Moreover, with the current development of GIS, the calculation of geo- morphic indices has become easier (Troiani and Della Seta, 2008; Pérez-Peña et al., 2009; Joshi et al., 2013). So, the geomorphic indices have been widely used in geomorphology and active tectonics (e.g., see in the above refer- ences). In Vietnam, despite some initial geo- morphic indices also to be used quite success- fully in several studies such as Nguyen et al. 1999; Phung, 2011; Phan, 2014; Nguyen, 2015. However, most of the calculations in these studies were manually carried out based on topographic maps and satellite images; so the results often depend on the ability to estimate, sight and experience of experts who conducted these studies. Therefore, the analy- sis and assessment of geomorphic indices have not been shown clearly roles, the signifi- cances, and its relationship to the geomorpho- logical processes and active tectonics. In this paper, we present quantitative analyses and assessments of the hypsometric curve (HC) and its statistical moments in rela- tionship between geomorphic processes and active tectonics in the DNCV area. The HC index is one of the geomorphic indices that has been considered as a powerful tool for quantifying the topographic features and differentiate zones deformed by active tectonics (Keller and Pinter, 2002; Chel et al., 2003; Pérez-Peña et al., 2009; Pedrera et al., 2009; Mahmood and Gloaguen, 2012). However, in Vietnam, this is the first time the method is adopted for the assessment of the active tectonics in the Lo River fault zone and the Tam Dao area (Ngo et al., 2016), but statistical moments of the hypsometric curve has not been analyzed and assessed. 2. Tectonic, geologic, and geomorphic settings The Day Nui Con Voi (DNCV) mountain is less than 10 km wide and more than 250 km long, extending from Lao Cai to Viet Tri, and appearing as an elongated NW-trending core of metamorphic rocks (Tran et al., 1998) (Figure 1). The altitude of the mountain is peaked at Nui Lai of 1450 m, then descending to the northwest and southeast. This mountain is characterized by three main strips, with the NW-SE direction and separated by the parallel lines with the Red River. The topography in this area is asymmetry: slope of the northeast- ern side is smaller than the southwest side; on the northeastern side have some narrow strips extending along the main mountain; the southwest side is divided into individual peaks. The center strip of the DNCV is uplifted (500-1000 m) compared with the two sides (<500 m) (Le et al., 2004). Vietnam Journal of Earth Sciences, 38(2), 202-216 204 Figure 1. (a). The Red River shear zone in Asia, (b) geological sketch map around the Day Nui Con Voi (Modified after Tran et al., 1998; 2003) Hoang Sa Truong Sa N.V. Liem, et al./Vietnam Journal of Earth Sciences 38 (2016) 205 Figure 2. Geological strength level map in the Day Nui Co Voi and surrounding area As for the Ailao Shan, the DNCV is a narrow high-grade metamorphic rocks and are mapped as Proterozoic (Phan et al., 1994; 2012). It is composed chiefly of garnet- biotite-sillimanite gneiss and garnet-biotite gneiss, and minor two-mica schists with garnet (Figure 2). The DNCV also includes amphibolite layers, migmatites, mylonite bands and small lenses of marble. This rock assemblage suggests that the DNCV was formed with severe deformation and deep metamorphism of sedimentary rocks (Tran et al., 1998; Phan et al., 1994; 2012). The rocks within the DNCV are strongly foliated. The foliation, which is marked by the preferred orientation of planar minerals (biotite and amphibole) and by flattened quartz or feldspar ribbons, commonly strikes parallel to the local trend of the gneiss core and dips steeply (~70º) to the northeast. The lineation is deduced by elongated quartz and feldspar ribbons, long tails of feldspar porphyroblasts, stretched leucocratic veins and preferred orientations of sillimanite crystal shapes all locally plunge to the northwest in a range of 5-20º (Tran et al., 1998). A mylonite band about 200-500 m wide is well exposed in the center of the northeastern flank of the shear zone. Foliation and lineation within the mylonite band are parallel to those of the host gneisses. Numerous kinematic indicators suggest a left-lateral shear movement of this mylonite band (Phan et al., 1995; Tran et al., 1998). The foliation of gneisses is then cut by two sets of steep conjugate faults, N10ºE striking dextral and more numerous N110ºE Vietnam Journal of Earth Sciences, 38(2), 202-216 206 striking sinistral, indicating N60ºE shortening. It shows that a successive deformation with ENE shortening (Tran et al., 1998). From Vietnam-China border, at Lao Cai, the Red River valley fault splays into two roughly parallel strands, the Chay River and Red River faults, which bound the DNCV to the north and south, respectively. Currently, both fault-strands appear to slip mostly right- lateral slip, with variable components of normal slip (Allen et at., 1984; Phan et al., 1994, 2004, 2012). Narrow straight ‘grabens’, which are traced along the Red River and Chay River faults, are filled with Late Mio- cene sediments containing abundant pebbles of gneisses and mylonites, being interpreted as a synorogenic formation resulting fromthe reversal of fault movements from left-lateral to right-lateral about 5 m.y. ago (Leloup et al., 1994). On the SW and NE sides of the DNCV also exist some small faults run nearly parallel with the Red River and Chay River faults, respectively (Le et al., 2004). 3. Data and methods To determine the hypsometric curve and its statistical moments for the study area, we used Digital Elevation Model (DEM) with 30 m resolution which is provided by the United States Geological Survey (USGS). The DEM is analyzed by ArcGIS software; it is useful tools to ensure accuracy, quick and less expensive in the calculation of morphology parameters. The calculation in this study is carried out automatically using the extension tools of ArcGIS 10.1 software (Pérez-Peña et al., 2009). Geological map of the study area was constructed using the digital Geological and Mineral Resources maps at the scale of 1:200,000 (The Department of Geology and Minerals of Vietnam). We used the active faults from the Phan et al. (2004, 2012), Ngo et al. (2006, 2011), and Le et al. (2004). 3.1. Hypsometric curve and hypsometric integral The hypsometric curve describes the dis- tribution of elevations across an area of land with different scales from one drainage basin to the entire planet. The curve is created by plotting the proportion of total basin height (h/H = relative height) against the proportion of total basin area (a/A = relative area) (Strahler, 1952; Keller and Pinter, 2002) (Figure 3). The shape of the hypsometric curve is related with the stage of geomorphic development of the basin. Convex hypsometric curves are typical of a youthful stage; S-shaped curves are related to a maturity stage, and concave curves are indicative of a peneplain stage (Strahler, 1957; Gardner et al., 1990; Delcaillau et al., 1998; Keller and Pinter., 2002; Pérez-Peña et al., 2009) (Figure 3). Figure 3. Basic hypsometric curves and its geomorphological development cycles (Modified after Strahler, 1952; Pérez-Peña et al., 2009; Mahmood and Gloaguen, 2012) N.V. Liem, et al./Vietnam Journal of Earth Sciences 38 (2016) 207 A simple way to characterize the shape of the hypsometric curve for a given drainage basin is to calculate its hypsometric integral (HI). The integral is defined as the area under the hypsometric curve and can be calculated (Keller and Pinter, 2002): HI = (Hmean - Hmin) / (Hmax - Hmin)     (1) where HI is hypsometric integral, Hmax is maximum elevation, Hmin is minimum eleva- tion, and Hmean is mean elevation. The parameters in the formula (1) can be identified by analyzing the DEM with the GIS software. The HI index has been used, as well as the hypsometric curve, to infer the stage of development of a basin. The values of the HI always vary from 0 to 1. Values near 1 indi- cate a state of youth and are typical of convex curve. However, in the mature stage of the ba- sin, it has a lot of S-shape and concave shape but the HI values often similar. Meanwhile, to distinguish or assessment correlate between the basins, we often base on the statistical indices are given below. 3.2. Statistical moments of the hypsometric curve In addition to analyzing hypsometric inte- gral (HI) index, we also calculate and analyze other statistic moments of hypsometric curve (HC): skewness of the hypsometric curve (hypsometric skewness, SK), kurtosis of the hypsometric curve (hypsometric kurtosis, KUR), skewness of the hypsometric density function (density skewness, DSK), and kurto- sis of the hypsometric density function (densi- ty skewness, DKUR). Harlin (1978) developed a technique that treated the hypsometric curve as a cumulative probability distribu-tion and used its statistic moments to describe it quantitatively. It con- sists of the hypsometric curve by a continuous polynomial function with the form (Harlin, 1978) (Figure 3). f(x) = a0 + a1x+ a2x2+ + anxn            (2) and HI can be defined: HI = ∬ ݀ݔ݀ݕோ (3) where R is the region under the hypsometric curve, x is relative area, and y is relative height. Skewness of the hypsometric curve is defined by: SK = µ3/(µ21/2)3 (4) where µ3 and µ2 are the third-order and second-order moment about x, µ3 = ଵୌ୍ ∬ ሺݔ െ ߤଵሻଷ ݀ݔ݀ݕ (5) µ2 = ଵୌ୍ ∬ ሺݔ െ ߤଵሻଶ ݀ݔ݀ݕ (6) where μ1 is the fist-order moment or x mean or x centroid, µ1 = ∬ ݔ݀ݕ݀ݔோ (7) Kurtosis of the hypsometric curve is defined by: KUR = ఓరሺఓమଵ/ଶሻర (8) where µ4 is fourth-order moment about x, µ4 = ଵୌ୍ ∬ ሺݔ െ ߤଵሻସ ݀ݔ          (9) Density skewness (DSK) and density kurtosis (DKUR) are defined similarly except that now y is the first derivative of the hypsometric curve, i.e., the density function of the hypsometric curve (replacing y with y’). These definitions are chosen so that they are consistent with Harlin’s original work (Harlin, 1978). In statistics, skewness and kurtosis de- scribe the shape of a distribution relative to the normal distribution and are dimensionless. Skewness characterizes the degree of asym- metry of a distribution around its mean. A positive value of skewness (SK>0) signifies a distribution with an asymmetric tail extending out toward a more positive x (skewed to the right); a negative value (SK<0) signifies a distribution whose tail extends out toward a more negative x (skewed to the left); and the skew is zero (SK=0), when the variable distribution is symmetrical. Kurtosis measures the relative peakedness or flatness of a distribution, relative to a normal distribution. Larger kurtosis (KUR>3) indicates a "sharper" Vietnam Journal of Earth Sciences, 38(2), 202-216 208 peak than normal distribution (the same Luo, 2000 and Pérez-Peña et al., 2009, under the definition used in this paper, the kurtosis of a normal distribution is 3); smaller kurtosis indicates "flatter" peak than normal distribu- tion. These statistics are applied to the distribu- tion function of the hypsometric curve order to explain the erosion and slope basins and has been tested by Harlin., (1978); Luo., (1998, 2000); Pérez-Peña et al., (2009). Accordingly, the hypsometric skewness repre- sents the amount of headward erosion in the upper reach of a basin (Figure 4); density skewness indicates slope change; a large value of kurtosis signifies erosion on both upper and lower reaches of a basin, and density kurtosis delineates midbasin slope. Figure 4. Schematic diagram showing the relationship between the shape of the hypsometric curve and its integral, skewness, and density skewness (Luo, 2000) These statistical moments can be used to describe and characterize the shape of the hypsometric curve and, hence, to quantify changes in the morphology of the drainage ba- sins. In many cases, these parameters are very useful for the hypsometric analysis, especially in basins with similar hypsometric integrals but different shapes (Pérez-Peña et al., 2009). 4. Results In the DNCV area, the hypsometric curve analysis method and its statistical moments are used for assessment at 44 sub-basins of the Red river and the Chay river. In which, 30 sub-basins are located in the Red River (from the basin 1 to 30) and 14 sub-basins are located in the Chay River (from the basin 31 to 44) (Figure 5). The results are showed on Table 1, Figures 5 and 6. In the study area, the hypsometric curve can be grouped into 3 curves: "straight- shape", "S- shape", and concave curves (Figs. 6a, 6b and 6c,d, respectively) and no convex curve. Accordingly, concave curve has the largest proportion (26/44 basins), followed by the S-shape (10/44 basins) and final are straight-shape (8/44 basins). Consistent with them, the HI indices are also very small, the largest value is the basin No.13 (HI = 0.37) and the smallest is the basin No.28 (HI = 0.128). In which, the basins with "straight- shape" have the HI values are greater than 0.3; the "S-shape" have HI values are greater than 0.25 and the concave curves with largest HI value is 0.28 (Table 1). The results shown in Table 1 show that the skew values are from 0.45 to 1.3 and these values do not change much in the basins with straight-shape of the hypsometric curve (the skew values range from 0.55 to 0.83) and the "S-shape" of the hypsometric curve (0.45 <SK <0.64). In contrary, the skew values have considerable variability in the basins with concave shape of hypsometric curve (the skew values range from 0.46 to 1.3). In the basins with straight-shape and s-shape of hypsomet- ric curve, the density skew values range from 0.33 to 0.96, and the basins have concave curve, this values range from ~ 0.78 to 1.58. The kurtosis values range from ~2.0 to 4.1; in there, the basins have the hypsometric curve with the “straight” and “S” shape, the kurtosis values are less than 3.0 (the kurtosis of a nor- mal distribution is 3.0). The density kurtosis values range from 1.75 to 4.87. As the skew values, the density kurtosis values are not change much in the hypsometric curve basins with the “straight” and “S” shape, and quite change in the concave shape basins. The variation values of the main statistical mo- ments indices in the DNCV are showed on Figure 7. N.V. Liem, et al./Vietnam Journal of Earth Sciences 38 (2016) 209 Figure 5. Schematic distribution of the hypsometric curve in the DNCV area 5. Discussion The hypsometric curve and its statistical moments influenced by active tectonics, are also affected by geological and regional cli- matic characteristics (Moglen and Bras, 1995; Willgoose and Hancock, 1998; Huang and Niemann, 2006; Pedrera et al., 2009). Because the study area is located almost in the center of the DNCV with a narrow range, so the climate is basically not much different. According to the geological map (1:200,000) of the Department of Geology and Minerals of Vietnam, the DNCV area has identical geol- ogy and is composed chiefly of high-grade metamorphic rocks (Figure 2). So, anomalies (if any) of geomorphic indices in this area are mainly a reflection of the recent tectonic activity. Regarding to the difference of the number basins in the northeast side (14/44) and the southwest side (30/44) of the DNCV area, be- cause in the southeastern part of this area has the Thac Ba hydropower dam, so the basins should flow directly into the lake having been changed base erosion level by the volume of water. Therefore, we did not use these basins in the calculations. On the other hand, due to Vietnam Journal of Earth Sciences, 38(2), 202-216 210 relief features of the DNCV with slopes in the southwestern side (in the Red River basin) is greater than the northeastern side (in the Chay River basin). Therefore, area of the basins in the southwestern side usually smaller than the northeastern side and opposite side, the number of basins in the northeastern side is less than in the southwestern side. The steeper and higher of the southwestern side than the northeastern side reflected lift active of the DNCV in the southwestern side is higher than the northeastern side. This will be clarified by analyze the hypsometric curve and its statisti- cal moments as below. Figure 6. Hypsometric curves of the sub-basins in the DNCV area; (A) - “Straight-shape” group; (B)- “S-shape” group; (C) and (D)- concave curves As the results presented above, in the study area, the hypsometric curve has revealed 3 curves such as "straight- shape", "S- shape", and concave curves, but no convex curve. In there, the hypsometric curve is almost con- cave curve (26/44 basins) and fit it, the HI values mainly small; maximum is 0.37 (Figure 5 and Table 1). Accordingly, the basin in this study area is mainly in the oldest stage, meaning that the basin has reached the equilibrium in the longitudinal profiles of the river (or stream). In these basins, the dominant geomorphological processes usually are lateral erosion, vertical erosion (if any) also occurs in the upstream area. Another way, the active tectonics (uplift-lower) in these basins is basically weak. However, there still exists the hypsometric curve as "straight- N.V. Liem, et al./Vietnam Journal of Earth Sciences 38 (2016) 211 shape" and "S-shape" are distributed in some parts of the study area and focused mainly in the northern part to the center of the southwestern side of the DNCV. Whereas, in the northeastern side of the DNCV, the hypsometric curve mainly is concave curve (Figure 5 and 6a,b). Tectonic activity in the study area is not fully uniform. Accordingly, uplift active in the southwestern side (Red River basin) basically is greater than that in the northeastern side (Chay River basin). In which, some of the northern segment uplifted is greater than southern segment (Figure 5). This result is consistent with Le et al. (2001, 2004). In the northeastern side, where the Chay River fault cuts across at the foot of the slope, almost of basins with hypsometric curve are concave curve, except the basin 32 and 33. This is consistent with previous studies that Chay River fault is right-lateral slip (Nguyen, 2002; Phan et al., 2004, 2012; Ngo et al., 2006, 2011). Table 1. The statistical moments of the hypsometric curve in the DNCV area (HI - Hypsometric integral, SK - Skew; KUR - Kurtosis, DSK - Density skew and DKUR - Density kurtosis No HI SK KUR DSK DKUR No HI SK KUR DSK DKUR 1 0.335 0.526 2.141 0.666 2.002 23 0.188 1.174 3.626 1.402 4.109 2 0.294 0.451 2.016 0.614 1.758 24 0.255 0.816 2.726 0.839 2.485 3 0.294 0.452 2.003 0.736 1.956 25 0.214 0.953 2.900 1.189 3.158 4 0.269 0.487 2.055 0.658 1.829 26 0.190 1.169 3.341 1.550 4.203 5 0.272 0.609 2.164 0.964 2.370 27 0.169 0.848 2.560 1.070 2.653 6 0.309 0.595 2.221 0.612 1.804 28 0.128 1.183 3.328 1.499 3.987 7 0.250 0.579 2.200 0.555 1.724 29 0.156 1.019 2.766 1.346 3.282 8 0.284 0.788 2.607 0.867 2.410 30 0.137 0.626 2.138 0.904 2.147 9 0.305 0.598 2.236 0.662 1.916 31 0.205 0.983 2.974 1.218 3.249 10 0.311 0.643 2.285 0.752 2.039 32 0.344 0.550 2.232 0.327 1.591 11 0.329 0.667 2.482 0.444 1.864 33 0.290 0.463 1.999 0.495 1.525 12 0.320 0.642 2.386 0.517 1.848 34 0.254 0.752 2.498 0.841 2.277 13 0.370 0.605 2.375 0.339 1.759 35 0.220 0.860 2.570 1.063 2.644 14 0.333 0.706 2.562 0.525 2.010 36 0.227 1.033 3.112 1.324 3.638 15 0.270 0.848 2.792 0.873 2.539 37 0.261 0.825 2.673 0.970 2.661 16 0.329 0.717 2.523 0.649 2.070 38 0.252 0.727 2.456 0.780 2.155 17 0.347 0.773 2.667 0.714 2.292 39 0.214 0.885 2.812 0.976 2.689 18 0.304 0.833 2.877 0.688 2.366 40 0.185 1.137 3.381 1.415 3.916 19 0.259 0.994 3.176 1.082 3.150 41 0.240 0.933 3.032 0.970 2.916 20 0.282 1.126 3.756 1.003 3.402 42 0.257 0.889 3.000 0.820 2.665 21 0.213 1.302 4.040 1.575 4.805 43 0.199 1.306 4.100 1.563 4.875 22 0.239 1.106 3.679 1.106 3.567 44 0.191 1.064 3.101 1.453 3.925 According to Al Hamdouni et al. (2008), the hypsometric curve often has convex curve when HI index greater than 0.5; intermediate form between the concave and convex shape (S-shape) or "straight-shape" when the HI value in the range of 0.4 to 0.5 and the HI- value less than 0.4, the hypsometric curve has a concave shape. In the study area, as the Table 1, Figure 5 and Figures 6a, b, the HI values of the hypsometric curve with straight- shape and S-shape are less than 0.4 and smallest is 0.25. Thus, when using and analyzing the HI index in different areas, need to combine with its hypsometric curve. Because in many cases, the basins with similar hypsometric integrals but different shapes (Pérez-Peña et al., 2009). In that cases, these other statistical moments are necessary to consider for the hypsometric analysis (Figure 7a, b). Vietnam Journal of Earth Sciences, 38(2), 202-216 212  Figure 7. a)  The variation of the statistical moments of the hypsometric curve (the basins in the southwestern side of the DNCV); b) The variation of the statistical moments of the hypsometric curve (the basins in the northeastern side of the DNCV) In the study area, according to the results in Table 1, Figure 7 (a, b) and in the direction from northwest to southeast, unless the HI index (downward trend), basically, other sta- tistical moments of the hypsometric curve are likely to increase in both sub-basin systems of the Red River and Chay River. As the results of skew, the values range from 0.45 to 1.3 (SK>0), this mean the basins in the study with the geomorphological processes almost are represent the amount of headward erosion in the upper reach of the basin (Harlin, 1978; Luo, 2000; Pérez-Peña et al., 2009). This trend is basically increased in the direction from northwest to southeast to the basin of the study area. Consistent with SK index, the DSK index also reflects a larger slope in the upper reach of the basin and also showed upward trend from northwest basins to the southeast basins. In addition, a larger value of kurtosis (almost is the concave shape in the southern of the DNCV in both the northeast and southwest sides; Table 1 and Figure 7) signifies erosion on both upper and lower (a) (b) N.V. Liem, et al./Vietnam Journal of Earth Sciences 38 (2016) 213 reaches of a basin (Harlin, 1978; Luo, 2000). These results also showed, in the basins which have large KUR values then so are DKUR values (Table 1 and Figure 7). This mean that, this basin also has large slope in the middle part of the basin (Harlin, 1978; Luo, 2000; Pérez-Peña et al., 2009). What makes this area contain these features (erosion process in the both of the upstream and downstream area, and addition large slope in the middle part)? According to Le (2001, 2004) and Ngo (2011), the regional topography has stepped clearly. This step by the heterogeneously raising activities and the active fault branches (of the Red River and the Song Chay Faults) on both the northeast and southwest sides of the DNCV area (Le, 2001, 2004; Ngo, 2011). According to Pérez-Peña et al. (2009), the value increases of the KUR and DKUR in- dexes (when the same hypsometric curve and hypsometric integral index) often show upward trend of recent tectonic activity. How- ever, in the study area, the higher anomalies of the KUR and DKUR indexes still lie in the basins with the hypsometric curve showing concave shape (Table 1, Figure 5 - Figure 7). Thus, if only individual basins with curve concave shape are considered, basins with larger KUR and DKUR values will show stronger tectonic activity. If all basins of the study area are considered, the higher anoma- lies KUR and DKUR indexes at the basins with concave curve shape possibly suggest the following remarks: According to basic hypso- metric curve model and geomorphological development cycles shown by the changes of the hypsometric curves (Figure 3), the hyp- sometric curves with concave curve shape are the oldest stage of geomorphological cycles, and it is the final stage of the cycle to stabilize the tectonic cycle to pass on to a new tectonic activity cycle. This means, the tectonic active in the study area possibly is the last period of stabilization tectonic cycle and the beginning of a new tectonic activity cycle. If so, the above assumption is appropriated in anticipa- tion of Allen (1984) to repeat the cycle of large earthquakes along the Red River Fault Zone is about 1800 years, while in the region of Yunnan, China had strong earthquake occurred 8.1 to 8.3 on the Richter scale and occurred approximately 1000 to 2000 years ago. In summary, the hypsometric curves in the study area are mainly concave shapes; some curves are intermediate form between the con- cave and convex shape (in "straight" and "S" shape). HI index is basically small and tends to decrease to the southeast. The skew and kurtosis and their density function are basi- cally large and increasing trend to the south- east. An overview, the recent tectonic activity (uplift - lower) in the study area is generally weak. In which, the southwestern side is being lifted higher than the north-eastern side. The northern part is being lifted larger than the southern part. In the region and surrounding area, the strong uplift activities and increased gradually in the Pliocene-Quaternary (mod- eled after Le et al., 2004) could have stopped at certain time in the past. The current geo- morphic processes are mainly headward erosion in the upstream. These results will be clarified in the next study when there is a combination of many different geomorphic indices. 6. Conclusions The hypsometric curves and its statistical moments are useful tools to assess the geo- morphological processes and recent tectonic activity of the region as well as the compari- son between different zones. The Day Nui Con Voi area has revealed 3 curves such as "straight- shape", "S- shape", and concave curves. The concave curve is the most common widely distributed in the northeast side and the southern part of the southwestern side of the DNCV area. The hypsometric integral (HI) values are rather small, the largest value is 0.37 whereas the smallest one is 0.128. Other statistical mo- Vietnam Journal of Earth Sciences, 38(2), 202-216 214 ments of the hypsometric curve i.e. skew (SK), kurtosis (KUR), and the density function (density skew - DSK and density kurtosis-DKUR) have great values and in- crease in the south direction of the area study. The recent active tectonic activities (uplift- lower) of the study area are generally weak. However, they are also not completely homogeneous and can be distinguished by different levels. The southwestern side is being lifted higher than the north-eastern side. The northern part is being lifted larger than the southern part. In the region, the uplift activities were increased gradually in the Pliocene-Quaternary and could have stopped at certain time in the past. The current geomorphic processes are mainly headward erosion in the upstream. Acknowledgments The study is a part of the research project VAST 05.02/14-15 funded by the Vietnam Academy of Science and Technology (VAST). 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