Morphotectonic Analysis of South Solok Area: Implication for Geothermal Manifestation and Relative Tectonic Activity

The distribution of geothermal energy in Indonesia reaches 40% of all Indonesia's geothermal reserves. Sapan Maluluang, South Solok is one of the areas that has a promising geothermal manifestation. This area has structural hills that surround the depression area which characterizes areas that have several geothermal surface manifestations. A combination of morphotectonic analysis and field observation of surface manifestation were used to understand the distribution of potential geothermal distribution in the study area. morphometric was carried out to analyze how the morphotectonic control in the research area used dem data input. Smf and Vf were used to understand the landscape evolution and tectonic control to the present morphological condition. In addition, field observation is carried out on how the condition of the source of manifestation found at the research site and observations of the lithology and geological structures control. In the dem data analysis, morphotectonic analysis elements were calculated, such as valley height-weight ratio (Vf) which resulted in 0.02 – 2.38 values of mountain front sinuosity (Smf) which had a value of 1.1-1.3. The research area is heavily controlled by active tectonics. Based on the results of field observation, it is known that there are 5 geothermal manifest sources in the form of hot springs which have a temperature of 175 ̊ C -185 ̊ C and are controlled by the lithologies of the volcanic rocks of Mount Kerinci. In addition, from the lineament analysis at the research site, it was found that the NW-SE lineament was in the same direction as the Sumatran fault zone.


INTRODUCTION
Indonesia is one of the greatest countries that have geothermal potential with a distribution of 25,875 MW or 40% of Indonesia's entire geothermal reserves (ESDM, 2010).Sapan Maluluang, South Solok is one of the areas that have geothermal manifestations.Regionally the research site is on the Bukit Barisan column which is characterized by the existence of a fault zone associated with a row of active volcanoes northwest-southeast.The Sumatra Fault Zone is composed of Pretertiary rocks to late-quarter volcanic rocks in the form of metamorphic rock complexes and volcanic rock units (Figure 1).
The collision zone of the Indo-Australian Plate and the Eurasian plate forms a cluster of rows of hills that extends from the north to the south of Sumatra Island (Barber, 2005).The active tectonics of the Indo-Australian plate move sharply against the Eurasian plate moving southward at a rate of movement of 7 cm/year (Sieh & Natawidjaja, 2000).The research area experienced several times uplifting, folding, and switching events in the Mesozoic era, resulting in the deposition of shallow seas to the deep in the eastern part and the Cretaceous in the west (Sieh & Natawidjaja, 2000).Cretaceous tectonic events were accompanied by regional uplift, alignment, and switching.The alignment that occurs controls the formation and development of basins between volcanoes.

During
Tertiary, volcanism activity produces eruptions and leads at the bottom are sea parts and at the top become sea stacking (Kusnama & Mangga, 2007).
Regional structural patterns consist of the fault and folding structures in the general northwest-southeast direction.While the direction of the anticlinal axis and sinking of Tertiary rocks is almost the same as the direction of the crease of pre-tertiary rocks that flow.Depression in the magmatic arc of Sumatra, occupying the eastern part of the Barisan Mountains and Bukit Barisan Anticline.This depression has straightness with the Ombilin Basin, Sinamar Basin, sub-Kiliran Basin along Bukit Barisan (Rosidi & Pendowo, 1996).
The main fault of part of the Sumatra fault is the shear fault and some normal faults are northwest-southeast (Sieh & Natawidjaja, 2000).The research site is in the Suliti Segment with tectonic basins located in the North Labuh Estuary.The Suliti segment has a length of 95 Km (Sieh & Natawidjaja, 2000).
In addition, the characteristics of the landscape can identify geological processes of plains, plateaus, hills, mountains, slopes and rivers represent the founding of research sites.The topography of the research site consists of hills to mountains from the southern end of Muaralabuh to the north of Muaralabuh.The area of active tectonics and quaternary volcanism is related to the shape of the manifestation of hot water and the appearance of ancient caldera seen from structural basins influenced by down warping tectonic regions which control the distribution of geothermal distribution in the study area.

METHOD
The research was done using, a combination of morphotectonic analysis and field observation of surface manifestation were used to understand the distribution of potential geothermal distribution in the study area (Figure 2).Morphometrics was carried out to analyze how the morphotectonic control in the research area used DEM data input (Noordwijk, et al., 2004).Vf and Smf were used to understand the landscape evolution and tectonic control of the present morphological condition based on watershed function (Noordwijk, et al., 2004).In addition, field observation is carried out on how the condition of the source of manifestation found at the research site and observations of the lithology and geological structures control, was carried out by observing the morphology formed by the geological structures that formed an area, that is what is called "Morphotectonic" (Hancock, 1994).The calculation of the ratio of the width of the valley floor and between the two heights, which this method was first introduced by Bull & McFadden, 1977) (Figure 3) as a geomorphic index used to discriminate between the V-shaped valley floor and the U-shaped valley floor.With the calculation of the Vf index, it can be seen that there is a uplifting process in an area. in the study area.Smf is calculation of the ratio between the face of the mountain range and the straight-line projection (Bull & McFadden, 1977) (Figure 3).So that it can detect the level of erosion formed that can be used as a parameter whether the area is controlled by high tectonics or not.The SMF concept was first introduced by Bull & McFadden, 1977).Smf =   (2) Ld (Lineameant Density) Lineament density is used to determine the relationship (Bull & McFadden, 1977) between structure density areas formed by the interconnections of fractures or lineament and faults.Ld can be used to identify permeability rate of the rocks and can be used to identify damage zone.

RESULT Geomorphology Unit
The research area geomorphology included plains, plateaus, hills, mountains, slopes, rivers, and other features.The creation of landscapes on Earth is influenced by tectonic forces.As a result, the landscape technique can detect geological activities at the research location.The study findings revealed that the study site was impacted by complicated tectonic factors, such as the construction of the morphology of the hills to the south of the Muaralabuh.Hot water manifestations are strongly tied to active tectonics and quaternary volcanism.This may be seen in structural basins influenced by the tectonic area of down warping, where the appearance of manifestations of Sapan Maluluang hot water is 850 m high (Figure 4).

RESULTS AND DISCUSION Vf (Valley Floor Widht to Height Ratio)
The morphotectonic index is calculated by analysing the ratio of the valley floor to the height of a bordering morphology.The valley morphology created in the measurement might be V or U-shaped (Bull & McFadden, 1977).The analysis of the VF index is used to determine whether or not there is a level of uplifting activity in a watershed.If the value of Vf < 0.5, then it is included in the high uplift level (High), the medium uplift level (Medium) has a value > 0.5 to < 1, and the value > 1 is included in the low tectonic level (Low) (Sukiyah, et al., 2016;Keller & Pinter, 1996).
The calculation result of Vf Batang Ampalu Watershed on the location numbers 3, 5, 6, 7, 10, and 14 showed high uplift rates with Vf values ranging from 0.18 to 0.47 (Figure 5, 6).While locations numbers 1,4,8,9,11 and 12 have moderate uplift rates with the value of Vf in a range from 0.63 to 0.94 and have lower uplift rates in locations numbers 2 and 13 with the value of Vf in the field from 1.04 to 1.35 (Figure 5,6).Next, a result of Vf Aia Manyuruak Watershed on location number 15 shows high uplift rates   Moreover, locations Batang Kapur Watershed, locations number 42,43,44, and 45, have a high uplift rate with the value ranging from 0.12 to 0.37, and location number 41 has a lower uplift rate with a value of value 1.05 (Figure 5 1 and Figure 5, 6. Smf (Mountain Front Sinuosity) The calculation of the morphotectonic index is in front of the mountain, and the back of the hill, which is seen from the tendency to erosion, so it can develop to find out how an area is controlled by high or low tectonic activity.This method was first introduced by (Bull & McFadden, 1977).Based on the calculation, the results can be categorized that an area has an active tectonic level if it has an Smf value < 1.5, while if it has a value > 3.0, it is classified as an area with a tectonic level that is no longer active (El Hamdouni, 2008).
The Calculation of Mountain Front Sinusity was done by determining the Smf by observing the topography and position of the mountain front of Batang Ampal Aia Manyurak, Batang Suliti, Batang Bangko keruh dan Batang Liki Watershed (Figure 5, 7) to know the level of tectonic activity on the research area.The Smf calculation indicates that all segment watershed in the researched area is dominated by active fault.The Overall Smf calculation result of six segments produces Smf values ranging from 1.1 to 1.3 (Figure 5, 7).Most of them are situated in an active tectonic activity level, and this can be described from Smf values of active tectonic such as Segment A (1.2), B (1.1), C (1.04), D (1.1), E (1.3) and F (1.1) (Figure 5, 7).According to SMF Index (El Hamdouni, 2008), this area is controlled by active tectonics (Table 2).Lineament density is performed to sharpen the determination of zone high dominant lineament intensity describing the permeability rate of the rocks formed (Figure 5).The Calculation result of the Lineameant density will produce contour maps based on the absolute value of lineament with Extraction in Software PCI Geomatica.
The interconnections, fractures or lineament, and faults can be used as evidence that active tectonic activity passes the area.The result calculation shows that the highest lineament density values range from 0.25 Km -5.92 Km (Figure 5).Tthe primary orientation that is northwest southeast and north-south was in the same direction as the Sumatran fault zone and then be related in the are found geothermal

Figure 3 .
Figure 3.Valley floor width to valley height ratio calculation (Vf) calculation method and MouintainFront Sinousity (Smf) calculation method.

Figure 5 .
Figure 5. Map of lineament density (Ld) shows areas that have low to high density.
, 6).At the same time, the result of Vf Batang Liki Watershed shows high uplift rates in locations number 29,30,31,32,33, and 34 with the value ranging from 0.02 to 0.40 and have lower uplift rate number in location number 28 (Figure 5, 6).At the end of Batang Bangko Watershed, locations number 45,46, 47, 51, and 52 45 have a high uplift rate with values ranging from 0.10 to 0.40.While the number of the locations 48,49,50 and 53 have moderate uplift rates (Figure 5, 6).While location numbers 50 and 53 have reasonable uplift rates with Vf values ranging from 0.59 to 0.62 and lower uplift rates with the value of Vf ranging from 1.20 to 1.76 in location numbers 48 and 49 (Figure 5, 6).The result expressed low to high uplift in the area as shown in Table