Principle and geomorphological applicability of summit level and base level technique using Aster Gdem satellite-derived data and the original software

This article presents principle and geomorphological applicability of summit level technique using Aster Gdem satellite-derived topographic data. Summit level corresponds to the virtual topographic surface constituted by local highest points, such as peaks and plateau tops, and reconstitutes palaeogeomorphology before the drainage erosion. Summit level map is efficient for reconstitution of palaeosurfaces and detection of active tectonic movement. Base level is the virtual surface composed of local lowest points, as valley bottoms. The difference between summit level and base level is called relief amount. These virtual maps are constructed by the original software Baz. The macro concavity index, MCI, is calculated from summit level and relief amount maps. The volume-normalised three-dimensional concavity index, TCI, is calculated from hypsometric diagram. The massifs with high erosive resistance tend to have convex general form and low MCI and TCI. Those with low resistance have concave form and high MCI and TCI. The diagram of TCI vs. MCI permits to distinguish erosive characteristics of massifs according to their constituent rocks. The base level map for ocean bottom detects the basement tectonic uplift which occurred before the formation of the volcanic seamounts.

Summit level map (seppômen) represents the virtual topographic surface constituted by local highest point, such as peaks and plateau tops.It shows the palaeo geomorphology before vertical erosion of valleys and drainages.Base level map (sekkokumen) constructed by the surface composed of the local lowest points, as valley and basin bottoms.It predicts the future geomorphology which will be formed by lateral erosion of the drainages.Either summit level or base level maps are elaborated from topographic map.
Summit level technique has been developed since decade of 1950, mainly by Japanese researchers of natural geography.At the present, it is used by Japanese municipal and provincial governments as a fundamental instrument for natural disaster predictions and mitigations, especially for landslides and debris flows caused by heavy rainfalls.
In Brazil, summit level technique was introduced for the morphologic analyses of Poços de Caldas alkaline intrusive complex, State of Minas Gerais, in order to examine if the present morphology is originated from a volcanic caldera or differential erosion of felsic alkaline intrusive rocks.The Mendanha alkaline intrusive body, State of Rio de Janeiro, was studied using summit level technique to verify if it is a volcanic edifice with cone and crater or an intrusive massif originated from differential erosion of felsic alkaline rocks (MOTOKI et al., 2008a).Recently, the felsic alkaline intrusive complexes of Morro do São João and Tanguá, State of Rio de Janeiro (MOTOKI et al., 2014a;2015a and b), were studied for the purpose of morphologic comparison with gneissic and granitic massifs.Couto et al. (2012) applied summit level technique for Quaternary tectonic movement in the Paraná Plateau border areas.
This paper shows the principle and methodology of summit level and base level technique and their applicability for morphological analyses based on the available published articles, submitted papers, and new data of the studies in development.In addition, the authors discussed possible future technical developments, proposing new parameters.

Principle of the techniques
In the initial stage of the erosion cycle of Davis (1899), narrow and deep drainages are formed grooving original elevated peneplain.This phase is called young stage and the original surface is preserved in a large and continuous area.Therefore, the original elevated peneplain can be reconstituted by drainage fill.Summit level map was created for this purpose (Figure 1).
In an area with more relevant erosion, the original surface is preserved only at small and separated areas.Even in this case, summit level map can reconstitute the original plain, but less precisely.The older morphologic information is less preserved.
In stage of more advanced erosion, called the mature stage, the original surface is no more preserved and reconstitution of the original surface is not possible.In an area of much more advanced erosion, called old stage, vertical erosion is not important and lateral erosion becomes expressive.In the peneplain stage, summit revel map show the virtual surface formed by the top of monadonocks, indicating erosive resistance of the constituent rocks.
Summit level map is useful to detect vertical displacement of active fault, especially in areas of the young stage.It is efficient also for the reconstruction of original morphology of eroded volcanic edifices and impact craters.On the other hand, base level map shows erosive tendency indicating three-dimensional longitudinal section of valleys, being useful to study knickpoints.In the oceanic regions, it expresses basement height of seamounts, with similar effect of the Wiener lowpass filter (SMITH;SANDWELL, 1994).

Construction method of the virtual maps
There are two main methods for the construction of summit level maps, which are grid method (Hogan-hô) and valley-fill method (maikoku-hô).The grid method is convenient for the data of digital topographic model, called DEM, and digital computing.The summit level map by the grid method is confected by the following steps (Figure 2): 1) Divide the original topographic map into small square areas by a grid of defined interval; 2) Mark the highest point of each square; 3) Construct a new topographical map using the marked points.Base level map is elaborated by similar processes but using the lowest point of the squares.The difference between them is called relief amount and it is related to local declivity.The authors adopt Aster Gdem moderated by SRTM for the original topographic data, which are released from the United States Geological Survey.The virtual maps are constructed by the original software Baz version 1.0 (MOTOKI et al., 2012a) and Wilbur version 1.0 (MOTOKI et al., 2005;2006;2007a).They run on the DOS platform by linecommand, so the operations are not familiar for Windows TM users.
Grid interval is an important factor for the virtual maps.The summit level maps based on a narrow grid show detailed palaeo-geomorphology of a near past.On the other hand, those based on a wide grid interval demonstrate palaeo-geomorphology of a remote past in ambiguous way (Figure 1).Similar tendencies occur in base level maps.The software BAZ constructs simultaneously the virtual maps of summit level, base level, and relief amount of various grid intervals, revealing geomorphologic evolution history.
The valley-fill method is convenient for analogue data treatment by manual works from the paper-printed topographic map (MOTOKI et al., 2008a;2009a).The summit level maps are elaborated filling the drainages narrower than the threshold width (Figure 3).The base level map is elaborated eliminating the ridges narrower than the threshold.The threshold length of valley-fill method has similar significance of the grid interval of grid method.

Palaeo-surface reconstitution
Reconstitution of palaeo-surfaces is the primary and most important function of summit level technique and detects Cenozoic tectonic movement form the topographic characteristics.The Figure 4 presents summit level map for the central region of State of Rio de Janeiro, Brazil, based on grid interval of 4 km (AIRES et al., 2012).The Figure 5 shows the morphotectonic interpretation with special attention of Cenozoic tectonism.Almeida and Carneiro (1998) and Zalán and Oliveira (2005) interpreted that Serra do Mar Scarp was formed by vertical displacement of the normal fault related to the continental rift of the Atlantic Ocean opening during the Cretaceous to the early Cenozoic.The fault movement and the consequent fault scarp formation were originated from the continental crust thinning.The tectonism occurred mainly in a period of 52 to 40 Ma (FERRARI, 2001;RICCOMINI et al., 2004).
The fault displacement divided the original surface of the Cretaceous into two levels, Fluminense Lowland (Baixada Fluminense) and Teresópolis Highland (Planalto de Teresópolis).The basement level of Fluminense Lowland is represented by the summit level surface of 100 m of altitude.The Teresópolis Highland is represented by the summit level surface of 1300 m of altitude.This plateau surface shows gentle tilting of 0.9º to the north-west, called the Paraíba do Sul tilted block.The Serra do Mar Scarp is originated from the vertical displacement of 1200 m.Along the scarp, some mega-landslide blocks are observed in the summit level map of the grid interval of 2 and 1 km.After the events of continental thinning and the consequent opening of Atlantic Ocean in the late Mesozoic, this area has been uplifted by isostatic compensation.The fission-track dating for apatite permits to estimate the total uplift vertical movement as ca. 3 km (MOTOKI et al., 2012b;2007b).
The summit level map is quite efficient for the areas of Quaternary tectonism.The neotectonic study of the Mount Rokko, Kobe, Japan (HUZITA; KASAMA, 1977), was a pioneer work using summit level technique.The Figure 6 shows the summit level map of grid interval of 490 m using the updated topographic data of the Aster Gdem.
The elevated peneplain of Mount Rokko has maximum altitude of 932 m and its southeast border is limited by a series of NE-SW trend composed of Suma, Suwayama, and Gosukebashi fault (LIN et al., 1998;MARUYAMA;LIN, 2000).They are named collectively Kobe Fault System and have right-lateral component with total displacement of 1.7 km (MOTOKI, 1979;1994).The horizontal displacement component is 4 times larger than the vertical one.The vertical component is small in the western span, about 200 m along Suma Fault, and becomes larger to the east, about 550 m, along Gosukebashi Fault.This map detects 4 levels of the elevated peneplain, called Rokko, Okuike, Taishaku, and Suzurandai surface, with respective altitudes of 900, 550, 550, and 400 m.All of them show general tilting to the west, with approximate declivity of 2º.
The summit level technique is very important for the areas of Holocene tectonism.At the Saint Peter and Saint Paul Islets of the Equatorial Atlantic Ocean, the most intense active uplift of Brazil is observed (MOTOKI et al., 2009a;MOTOKI et al., 2014a ).In this area, the ENE-WSW trend transform fault is exceptionally not parallel to the E-W ward relative motion between South American Plate and African Plate, with maximum angular discordance of 25º (SICHEL et al., 2011).The directional discrepancy generates near N-S compression stress, and it has squeezed the mantle peridotite out of the ductile deformation depth (SICHEL et al., 2008a) up to the sea level.The Figure 7 presents the summit level map for the Saint Peter and Saint Paul Islets, which was confected by valley-fill method with the threshold width of 10 m.The map shows two levels of wave-cut bench, called the Upper bench, from 7 to 10 m above sea level, and the Lower bench, from 4 to 6 m.The Upper bench was generated during the Flandrian Transgression in ca.6000 years ago, and indicates average uplift rate of 1.5 mm year -1 .The 14 C datings for the biogenic carbonates support this rate (CAMPOS et al., 2010).O the other hand, the Lower bench is under the on-going wave-cut erosion.

Reconstitution of volcanic calderas
On the Earth's surface, there are circular morphologic depressions larger than 1 km of diameter.Most of them are originated from volcanic caldera or impact carter.There also are the geologic structure originated from caldera or meteorite crater.Among these morphologic depressions, the former two have similar characteristics, but the latter is notably different.According to the passage of time, the original morphology deteriorates by erosion and become ambiguous.But the original surface can be reconstituted, at least partially, by summit level technique.
The Figure 8 presents summit level maps for the Aso Caldera, Kyushu, Japan, Valles Caldera, New Mexico, USA, Kilauea Caldera, Hawaii, USA, and Poços de Caldas felsic alkaline intrusive body, Minas Gerais State, Brazil, which represent respectively Krakatoa type caldera, Valles type caldera, Kilauea type caldera, and felsic alkaline intrusive rock body.The Figure 9 shows their cross section.The Aso Caldera takes place in the central region of the Kyushu Island, Japan.The circular depression has 25 x 18 km with depth of about 400 m.The caldera bottom is flat due to old caldera lake deposits.This lake was broken by the drainage of western border of the caldera, called Shirakawa River.There are E-W trend central cones with maximum relative height of 1200 m.The last caldera formation event of explosive eruption occurred at 90 Ka (WATANABE, 1978).The summit level map of grid interval of 2 km shows a closed depression area.The caldera wall is steep, of about 30º of declivity.The areas out of the caldera wall are covered by the Aso IV pyroclastic flow deposit in a wide area with extension more than 20 km.Its surface is eroded by small drainages.The original surface reconstituted by summit level technique has gentle outward declivity of 1 to 3.5º (Figure 9A).Such a declivity is common in pyroclastic plateau surfaces (SUZUKI-KAMATA; KAMATA, 1990).
The Valles Caldera occurs in State of New Mexico, USA, which has 19 km of diameter and 400 m of depth.The last stage of the caldera formation eruption took place at 1.2 Ma.There are central resurgent lava dome, called Redondo Peak, with relative height of 700 m, and seven post-caldera cones of ring layout (SMITH; BAILEY, 1966;MACDONALD;PALMER, 1990).The volcanic morphology in the caldera is complex and the caldera bottom is divided into 3 basins.The old caldera lake was broken through the drainage of south-western border, called Valle Grande.The caldera wall is steep with 30º of declivity.The areas out of the caldera wall are underlain by the Bandelier Tuff pyroclastic flow deposit in a wide area with extension more than 30 km.Its surface shows more advanced erosion than the case of Aso Caldera because of the older age.The pyroclastic deposit surface reconstituted by summit level technique expresses outward declivity of 1 to 3º (Figure 9B).
The Krakatoa type and Valles type calderas are formed by explosive eruption of felsic magma and have negative Bouguer gravimetric anomaly (YOKOYAMA; TAJIMA, 1959;1960;YOKOYAMA;SAITO, 1965;YOKOYAMA, 1965;1969) due to pyroclastic fallout deposits.On the other hand, Kilauea type calderas are formed by basaltic effusive eruption with positive Bouguer anomaly because of the subjacent basaltic intrusive body.The Kilauea Caldera occurs in the south region of Hawaii Island and has extension of 3 x 4 km and depth of 120 m.The Halemaumau Pit Crater, which is filled by the lava of the eruption in 1974, has 500 m of diameter and 60 m of depth.The caldera bottom is flat representing lava lake surface.The lavas in the caldera depression flowed out crossing over the south-western border.The caldera wall is sub-vertical.The areas out of the caldera wall are covered by young lava flows almost without erosion effect.Therefore, the topographic surface and summit level one are almost the same.The lava surface demonstrates outward declivity of -1 to 2º (Figure 9C).
The above-mentioned examples express the following morphologic characteristics of volcanic calderas: 1) Caldera bottom is flat; 2) Only one outlet drainage is present; 3) Caldera wall is steep with declivity of about 30º; 4) The areas out of the caldera wall are covered by extensive pyroclastic deposits or lava flows with extension of more than 20 km; 5) The surface of these eruptive deposits has gentle outward declivity of about 3º.
On the other hand, Poços de Caldas felsic alkaline highland (Planalto de Poços de Caldas), with intrusive age of about 85 Ma (ULBRICH, 1984), is situated at the limit between the State of Minas Gerais and São Paulo, Brazil.The massif has geologic and morphologic characteristics widely different from the above-mentioned volcanic calderas.The main constituent rocks are holocrystalline aegirine phonolite, so-called tinguaite, nepheline syenite, and vent-filling intrusive pyroclastic rocks.The summit level map of 2 km of grid interval shows that its morphologic features are unlikely for a volcanic caldera.
The circular morphologic depression is 30 x 28 km of extension with maximum depth of 300 m.The alkaline massif has 5 outlet drainages.The basin depth of 300 m is too small in comparison with the 30 km of diameter.The basin bottom is not flat but characterised by small hills with relative height of 150 m.This height corresponds to a half of the basin depth.The southern half of the basin is about 100 m higher than northern half.Along the northern and southern border of the intrusive body, there are arc-like ridges, so-called Castle Wall, which mark the border of the basin.The western border is ambiguous and the eastern border has no morphologic characteristics.The inner scarp of the Castle Walls has declivity of 15º and the outer scarp has declivity of 35º, which is steeper than the inner scarp.The areas out of the Castle Wall expose metamorphic rocks of the basement, without the extensive pyroclastic deposits with gentle declivity surface.
On the contrary of the volcanic calderas and meteorite craters, the circular morphologic structure is about 350 m higher than the surrounding areas.That is, the morphology of Poços de Caldas highland is not a crater-like circular depression but a wood stock-shaped circular protrusion with shallow saucer-shaped hollow on the top surface (Figure 9D).Theses morphologic characteristics are similar to those of felsic alkaline intrusive massifs of Poços de Caldas-Cabo Frio magmatic alignment, which are originated from differential erosion, and not to those of the volcanic calderas.

Macro concavity index
In areas of mature and old stages, summit level maps provide information about erosive resistance of the base rock.This map also shows the geologic anomalies reflected to the morphology.The combination of summit level and base level techniques provides quantitative index of erosive resistance of the target massif.
The macro concavity index, called MCI, indicates quantitatively general form of the massif if it is convex or concave.It is defined on the diagram of summit level vs. relief amount, called MCI diagram.The projected points are represented by second order polynomial regression.The MCI corresponds to 1000 times of the 'a' constant of the of the quadric equation y = ax 2 + bx + c (MOTOKI et al., 2012a;2014b;2015a;b).The massifs of young stage of erosion have negative MCI and those of advanced stage have positive MCI.
Nevertheless, the above-mentioned alkaline intrusive bodies stand up on the basement peneplain forming morphologic elevations with relative height of 300 to 900 m, so-called alkaline massifs.This apparently controversial phenomenon is due to clayrich impermeable regolith originated from the alkaline rocks which protects the rock body from surface water percolation, the phenomenon called weathering passivity (PETRAKIS et al., 2010;SILVA, 2010).
The Figure 10 shows the MCI diagram, which demonstrates the strong morphologic contrast between alkaline massifs and young volcanoes.The alkaline massifs have remarkable negative MSI and the convex three-dimensional landforms because of strong erosive resistance of massive plutonic rocks.On the other hand, the active and young volcanoes have positive MCI, demonstrating concave threedimensional landforms because of the original form of volcanic edifice.The silhouette of some Brazilian alkaline massifs is apparently similar to that of active volcanoes, generating local urban legends of volcano, such as the Mendanha, Morro de São João, and Cabugi Peak.The MCI parameter is strongly influenced by foothill angle of the massif and distinguishes well the both.The alkaline massifs has steep foothill slope with declivity of 30 to 35º, and their general three-dimensional landform is characteristically convex (Figure 11A).On the other hand, volcanoes has gentle foothill slope, with declivity lower than 10º, and their general form is concave (Figure 11B).
The summit level surface of alkaline massifs of the State of Rio de Janeiro is characterised by marginal scarp (MS, Figure 12), virtual plateau (VP), and top protrusion (TP).The virtual plateau is observed only on summit level maps and it is constituted, in fact, by high ridges and deep valleys.This map evidences the inexistence of volcanic cone and crater in Mendanha massif, that is, absence of 'Nova Iguaçu Volcano'.

Volume-normalised three-dimensional concavity index
The authors propose another morphologic parameter in order to represent general convexity of massifs, called volume-normalised threedimensional concavity index (TCI).This parameter corresponds to three-dimensional expansion of area-normalised stream concavity index (DEMOULIN, 1998;ZAPROWSKI et al., 2005).The TCI is calculated by the equation: TCI = 1 -2 x V, where V is the volume of massif divided by that of envelope space (Figure 14A).The data of close to the massif foot have strong influence of talus and fan deposits, and therefore, they are excluded.The data above this level represent erosive morphology and are called effective data.In practice, TCI is calculated using hypsometric diagram.In this case, the massif volume V corresponds to the sum of the normalised frequency of the effective data (Figure 14B).When TCI > 0, the massif has concave general three-dimensional form, and when TCI < 0, convex form.
These diagrams show that the data of granitic and gneissic massifs and those of syenitic massifs are distributed in two distinctly separated areas.The data of Mendanha alkaline massif is projected in the intermediate area between the syenitic and granitic trends.The fact is attributed to the fact that the analysed area has either syenitic or gneissic rocks.
The Figure 13 shows the base level map of grid interval of 28 km for Vitória-Trindade Chain, exhibiting 3 basement elevations at Davis (DVS), Jaseur (JSR), and Congress (CGS) seamounts.The dome-like basement elevations are from 100 to 150 km of diameter and from 1000 to 1400 m of relative height.These diameters are significantly larger than those of the seamount foot.Skolotnev et al. ( 2010) attributed these morphologic elevations to pre-eruptive tectonic uplift of the basement, corroborating the model of solid-state penetration of asthenospheric hot-mantle along the old abyssal fracture zone from the west to the east.This idea is supported by the low Bouguer anomaly along Vitória-Trindade Chain, indicating thin lithospheric mantle(Figures 14 and 15).

Conclusion
The available data and unpublished new data of the authors indicate the following utilities of summit level and base level techniques for geomorphologic studies.
1. Summit level (seppômen) is the virtual topographic surface constituted by local highest points, as peaks and ridges.This technique reconstitutes the palaeo-geomorphology before drainage erosion.It is efficient for reconstitution of palaeo-surface, detection of active tectonic movement, and geologic anomalies reflected to the landform.
2. Base level (sekkokumen) is the virtual surface composed of local lowest points, as valley bottoms.It predicts future morphology according to development of lateral erosion of the drainages.These virtual maps are constructed from the satellite digital topographic data of the Aster Gdem and SRTM, using the original software Baz and Wilbur.
3. The height difference between summit level and base level are called relief amount (kifukuryô) and indicates the degree of erosion of massifs in form of the MCI parameter.The massifs with high erosive resistance tend to have negative MCI and those of low resistance, positive MCI.This parameter distinguishes the landforms originated from volcano and differential erosion of intrusive massifs.
4. The volume-normalised three-dimensional concavity index, the TCI parameter, also represents general convexity of massifs.The massifs with high erosive resistance tend to have low TCI and MCI, and those with low resistance, high TCI and MCI.The diagram of TCI vs.MCI distinguishes erosive characteristics of massifs according to the constituent rocks.
5. The base level technique applied to ocean bottom morphology detects basement uplift before the formation of volcanic seamounts.do Rio de Janeiro, Carlos Chagas

Figure 1 .
Figure 1.Principle of summit level and base level technique and effects of grid interval, modified from Motoki et al. (2008a).

Figure 2 .
Figure 2. Confection procedures of summit level map by means of the grid method (Hôgan-hô) after Motoki et al. (2008a).The grid interval is 500 m.

Figure 3 .
Figure 3. Confection procedures of summit level map by the valley-fill method (maikoku-hô).The threshold width for this map is 500 m.

Figure 4 .
Figure 4. Summit level map for the central region of State of Rio de Janeiro State, Brazil, based on grid interval of 4 km (AIRES et al., 2012).

Figure 5 .
Figure 5. Schematic illustration for the Cenozoic tectonism in the central region of State of Rio de Janeiro State, interpreted from the summit level maps (AIRES et al., 2012).

Figure 6 .
Figure 6.Summit level map of grid interval of 0.5 km for Mount Rokko, Kobe, Japan, based on the updated data of the Aster Gdem moderated by the SRTM.Because of the confection process of the map, the positions of coast line are not precise.

Figure 7 .Figure 8 .
Figure 7. Summit level map for the Saint Peter and Saint Paul Islets, Equatorial Atlantic Ocean, constructed by valley-fill method with threshold width of 10 m (MOTOKI et al., 2009a).

Figure 9 .
Figure 9. Vertical cross section of the summit level surface for volcanic calderas and felsic alkaline intrusive body: A) Aso Caldera; B) Valles Caldera; C) Kilauea Caldera; D) Poços de Caldas felsic alkaline intrusive body.

Figure 11 .
Figure 11.Apparent similarity of the mountain silhouette: A) Morro de São João alkaline intrusive massif, State of Rio de Janeiro, Brazil (GERALDES et al., 2013); B) Sakurajima active volcano, Province of Kagoshima, Japan.The photo B is credit of Junyoh Tanaka free of free license.

Figure 12 .
Figure 12.Summit lever surface based of grid interval of 960 m (A), its oblique view (B), and horizontal views (C), demonstrating the top protrusion (TP), virtual plateau (VP), and marginal scarp (MS).The original topographic data is originated from the Aster Gdem.

Figure 13 .Figure 14 .
Figure 13.Principle of volume-normalised three-dimensional massif concavity index (TCI): A) Three-dimensional view explaining the conception of TCI: B) Hypsometric diagram and the parameter V, which represents relative volume; C) Relation between TCI and general concavity of the massif.

Figure 15 .
Figure 15.Base level map of grid interval of 27.8 km along Vitória-Trindade Chain, South Atlantic Ocean.

Filho
de Nível Superior), and Petrobras (Petróleo do Brasil, S.A.) for the financial supports and scholarship.Reference AIRES, J. R.; MOTOKI, A.; MOTOKI, K. F.; MOTOKI, D. F.; RODRIGUES, J. G. Geomorphological analyses of the Teresópolis plateau and serra do mar cliff, State of Rio de Janeiro, Brazil with the help of summit level technique and aster gdem, and its relation to the cenozoic tectonism.Anuário do Instituto de Geociências da