Appraisal of active tectonics in Karangsambung Amphitheater: Insights from DEM-derived geomorphic indices and geological data

The morphology of the Karangsambung area shows a unique form, namely the morphology of the amphitheater. This area results from the complexity of tectonic, erosion, and depositional processes. The active tectonics in this region greatly influence the drainage system and geomorphic expression. The study area provides evidence of subduction in Java. It is an ideal natural laboratory for studying evidence of tectonic activity due to the subduction of the Indo-Australian plate with the Eurasian plate during the Cretaceous period. We evaluated active tectonics using the DEM to assess the characteristics of the geomorphic index. The results obtained from these indices were combined to produce an index of relative active tectonics (IRAT) using GIS. The average of the seven geomorphic indices measured was used to evaluate the distribution of relative tectonic activity in the study area. We defined four classes to determine the relative level of tectonic activity: class 1: very high (1.0 ≤ IRAT < 1.5); class 2: high (1.5 ≥ IRAT < 2); grade 3: moderate (2 ≥ IRAT < 2.5); and grade 4: low (2.5 ≥ IRAT). The results show that the study area was strongly deformed and was influenced by tectonic activity. Landsat imagery, DEM, and field observations also proved the presence of active tectonics in the form of uplift accompanied by high vertical erosion forming pointed hills with narrow valleys, the exposure of Cretaceous-aged rocks, amphitheater morphology, and the uplift of river terraces. The indicative IRAT values were consistent with the relative uplift levels, landforms, and geology.


Introduction
Research on tectonic activity is very important to know the tectonic evolution from the oldest to the youngest age period (Resen).Tectonic activity has formed certain geomorphology and landscapes in an area.Therefore, the analysis of the area's landscape through the measurement of the quarry pattern will provide information about the current tectonic processes and their activities.Attempts to measure tectonic deformation from landscape analysis have been ongoing for decades [1][2][3][4].Relative tectonic activity assessment can use geomorphic index parameter measurements as the object of study [1,3,5,7].The development of GIS techniques and the availability of digital data such as DEM has greatly helped to calculate and analyze tectonic activity with geomorphic indices across areas of various environments and scales [7][8][9].However, studies using geomorphic indices to explore the relative tectonic activity of the study area as a "subduction fossil" are still limited.Research on the influence of tectonic activity on the landscape form, the shape of river valleys, and river terraces as a fluvial process has never been carried out in the research area.
The subduction of the Indian-Australian Plate subducting into the Eurasian Plate during the Cretaceous-Paleocene (NE-SW) era has resulted in complex geology.The Luk Ulo mélange complex is proof of the complexity of the geology of the study area.At the Miocene age, the tectonic regime continued in the strike-slip system until a pure-compression tectonic regime occurred at the Plio-Pleistocene age [10].The tectonic process continues with a change in the direction of subduction to W-E and results in rising faults and folding in the W-E direction.
The subduction process of the Indian-Australian Ocean Plate is subducting under the Eurasian Continental Plate in the south of Java Island at a speed of 67±7 mm/year in the direction of N11°E±4° [11], indirectly affecting the deformation of the study area.The Sunda megathrust zone in southern Java is one of the world's most active tectonic plate boundaries, extending 1700 km from the Sunda Strait to eastern Indonesia [12].Regionally, the study area is crossed by the Central Java Fault (CJF), a horizontal strike-slip fault with a NE-SW direction, marking the transition from Central Java to the eastern Kendeng Basin [13].The lineament parallel to the main CJF, suggesting that the CJF is a laterally segmented system [12].
The study aimed to determine the relative tectonic activity of the Karangsambung area as a "subduction fossil" with the development of local faults that might be reactivation by the regional activity of the Indian-Australian Ocean Plate subduction with the Eurasian Continental Plate south of Java Island.

Regional tectonic and geomorphological setting
The research area results from subduction tectonic processes between the Indo-Australian and Eurasian plates in the Cretaceous -Paleocene [14][15][16][17][18].The movement of the Sundaland (Late Cretaceous) Gondwana microcontinent has caused the Indo-Australian subduction to stop on the eastern and southeastern edges [19].The formation of a new subduction zone in the southern part of the study began in the Middle Eocene.It was followed by the deposition of the Karanggulung and Totogan Formations in the Late Eocene -Miocene, above which the Waturanda Formation and Penosogan Formation were deposited at Tertiary age [20,21].Quaternary rock fills many valleys, floodplains, and river terraces (figure 1).

Figure 1. Map of the location and geology of the research area
3 [22] stated that the East Java micro-continent collided with the island of Java at the Paleocene age so that the structural patterns trending Northeast -Southwest (Meratus Pattern) stopped and changed into structural patterns trending West-East (Javanese Pattern).[23] reconstructed a subduction line in Java in the Early Cretaceous, where Luk Ulo in Karangsambung was the collision zone between the East Java micro-continent and the western part of Java, probably, Karangsambung was also the impact zone of the collision.The evidence for the suture zone has been disclosed by [24], which mention the existence of a sinistral fault line trending around 70° east longitude.The N 70°E fault is likely to have significantly exposed old Cretaceous rocks in the Karangsambung area.

Methods
Tectonic geomorphology is used to determine the relative tectonic activity in a large area with a fast time [3,25,26].In this study, several geomorphic indices were used with the primary data in the form of DEM (figure 2).Data processing using Arcgis software.The measurement results of the geomorphic index parameter are verified with geological structure data and river terrace data.Here are some geomorphic index calculations as follows:

Drainage Density (Dd)
The drainage density of the deformed tectonic area causes rocks that are not resistant and are easily eroded [27].The ratio of the length of the river segment to the area of the watershed is the drainage density value (Dd).The Dd value is determined by the amount of river drainage in the watershed [28].The Dd formula is as follows: Where ∑L is the total length of the river (km) and A is the area of the watershed (km 2 ).

Basin Asymmetry (Af)
Tectonic activity influences river flow patterns in a watershed; therefore, tectonic activity affects the watershed pattern and geometry.One of the geomorphic index parameters that can see the relationship between tectonic activity and watershed geometry is the asymmetry factor (Af) (figure 3).Af is larger or less than 50, which gives us a tilting due to tectonic activity.

Basin Shape (Bs)
Tectonic activity can be reflected in elongated or rounded geometric shapes [8,30]: Watersheds influenced by active tectonics tend to have an elongated shape following the differences in watershed topography between upstream and downstream [25].The initial elongated shape tends to develop into a circular shape following a decrease in tectonic activity towards stability [25].

Mountain Front Sinuosity (Smf)
The Smf index is associated with active fault zones, and each active fault shift will cause a change in the sinuosity of the mountain face.The sinuosity value of this mountain face is measured as the Smf index value.The Smf with low values is related to active tectonics and uplift.When the uplift rate decreases, erosion will increase and erode the mountain face resulting in irregular mountain face sinuosities and a significant value of the Smf index.

Integral Hypsometric (HI)
Integral Hypsometric (HI) is an index that describes the distribution of the topographical relief of an area [30].Sl can be calculated using the following formula [31] is

Figure 6. Integral hypsometry calculation
Where hmax, hmin and hmean are the maximum, minimum, and average elevation.The hypsometric curve reflects the topographic relief, divided into 3 levels, namely young, medium, and old stages [7].Geological factors will affect the morphological form, which is reflected in the HI value, and rock resistance and geological structure affect the value of HI.Therefore, the erosion process produces new incisions forming topographical relief and affects the value of the HI index [8].

Ratio of Valley Floor and Valley Height (Vf)
The morphogenetics of river valley shapes and alluvial deformation into asymmetrical folds are strongly influenced by tectonic activity associated with active fault movements [32].Wide river valleys are Ushaped, indicating low uplift, and horizontal erosion erodes the sides of the river valley so that the river becomes wider and shallower.The low uplifting process can be seen from the high Vf value.Whereas low Vf values would reflect the deep valley and reflect the increased river activity, it is associated with the velocity of uplift.

Streams Length -Gradients Index (SL)
The correlation between active tectonics, bedrock resilience and topographical relief is reflected in the value of the SL index [33].Changes in river flow patterns will affect the value of the SL index.The existence of an active fault affects the flow pattern of the river, affecting the value of SL index.
= (/) ⨯  (7) Figure 8. SL calculation, where the elevation (ΔH) is different from the point of being counted, the ΔL is the length of the river to the point of being counted and L is the total length of the river up to the point of being counted.The level of lifting activity is reflected in the value of the SL index [33].

Index Relative Activity Tectonic (IRAT)
Each geomorphic index parameter has a tectonic active class value that reflects different morphological characters.The relative tectonic activity index (IRAT) is the sum of all geomorphic index parameter measurements.The IRAT formula is: Where S is the sum of the class values of each geomorphic index, and nis the number of geomorphic indices.Tectonic activity class (IRAT) is divided into 4 classes, class 1 value 1<S/n <1.5 very high), class 2 value 1.5<S/n <1.5 (high); class 3 value 2 <S/n <2.5 (medium), and class 4 value S/n > 2.5 (low) [31].

Geomorphic Indices
The results of the calculation of the geomorphic index are used for morphotectonic analysis and to determine the level of relative tectonic activity in the study area.Calculations and morphometric analysis were carried out in 52 sub-watersheds in the Karangsambung amphitheater, namely the Cacaban watershed, the Gebang watershed and the Welaran watershed.The Karangsambung amphitheater is an anticline valley in which various pre-Tertiary to Quaternary lithologies are exposed.Pre-Tertiary rocks consisting of chert, schist, gabbro, basalt, and greywacke are exposed in the western part of the study area.While the southern and eastern parts are composed of breccias and sandstones as well as limestone blocks and conglomerates of the Tertiary age, and the middle part is Quaternary deposits [14,20,34].
The correlation between the hypsometric curve and the calculation of the tectonic class shows that the study area is affected by tectonic activity, which is offset by erosion.The resulting topographical relief is a product of balanced tectonic and erosional activity.These results cannot conclude the tectonic activity of the study area.Correlation of all geomorphic index parameter calculations is needed to produce a relatively active tectonic index.
The research results in the form of Dd calculations show that almost all sub-watersheds are in the middle tectonic class category and a small part are in the high and low categories (figure 9).This value reflects that each sub-watershed is affected by the same tectonic activity resulting in a geological structure that is almost similar in the form of joints.In addition, the rock resistances also have similarities, so the tectonic response that affects them produces the same geological structure.
The calculation results (figure 8) show that the AF values vary with different classes of tectonic activities.The resulting value is a response to the form of a symmetrical or asymmetrical sub-watershed.A low AF value with a tectonic class in the low category (figure 8) indicates a symmetrical valley, meaning that the river is almost in the middle of the sub-watershed and there is no significant tilt effect (sub-watershed 2, 8, 9, 11, 15, 16, 30, 35, 39, 40).The asymmetrical sub-watershed will be reflected by a high AF value and is a high tectonic class.In the field, the asymmetrical sub-watershed is composed of blocks of the Luk Ulo melange complex, and there is evidence of faulting in the form of intensive slicken-side and joints (sub-watershed 45, 46, 47, 48, 49).
The BS values in the study area ranged from 0.81-4.30.About seventy-five percent of sub-watersheds are in classes 2 and 3 (figure 10).The results of field research show that low Bs values dominate the sub-watershed with the same constituent lithology, namely the volcanic breccia of the Waturanda.Although the lithology is the same, several sub-watersheds show different Bs values due to the difference in length of the sub-watersheds forming a more elongated sub-watershed.This happens because of the significant elevation difference between the upstream and the downstream.The classification of Smf classes according to the level of tectonic activity is class 1 (Smf<1.1),class 2 (1.1<Smf<1.5)and class 3 (Smf>1.5). ) [31].Of Smf classes according to the level of tectonic activity, class 1 (Smf<1.1),class 2 (1.1<Smf<1.5)and class 3 (Smf>1.5)[31].The results of the SMF calculation show that almost all sub-watershed categories are classified as low tectonic (figure 10).This value is closely related to the absence of evidence of steep mountain front curvature, such as triangular facet formation.The shape of the triangular facet hills will occur if there is fault activity that cuts or affects the shape of the curvature of the face of the mountains.High Vf values in the eastern part are due to resistant rocks, namely andesitic breccias of the Waturanda Formation.The influence of fault and joint structures causes the rock to lose its strength to become nonresistant, so it is easily eroded and causes high vertical erosion to produce V-shaped valleys indicated by low Vf values (figure 11).In the study area, the V-shaped valleys are composed of melange rock blocks with a claystone matrix that is easily eroded.The results of the SL calculation show low, medium, and high tectonic classes.The SL tectonic class is the river's response to the difference in elevation between the upstream and the downstream.The fault line's presence can also influence each river's different responses.Several sub-watersheds with SL values are categorized as high tectonic classes located on steep or steep rivers with hard lithology such as volcanic breccias, for example, sub-watersheds 2, 4, 7, 8, 10, 24, 25, and 34 (figure 12).The value of SL is also influenced by geological structures such as faults or joints.Sub-watershed 48 is classified as a high class because its position is close to the fault line, so it develops intensive jointing resulting in a large river density (figure 12).

Statistical Data Analysis
The statistical test results (tabel 1) showed that the significance values for the parameters Smf, Vf and AF were <0.05, meaning that statistically, H0 was rejected so that H1 was accepted (significant or there was a significant difference), and so did the Tc value, which was greater than Tt.Smf gives a significance value of 0.00015, Vf of 0.01000 and AF of 0.01366.
Significant differences occurred in Smf, Vf and Af, while other parameters were statistically significantly different between the two blocks.The average value of Smf in the West block (2.5) is higher than in the East block (1.8) Statistical test results on the geomorphic index parameter show a difference between the West and East blocks.The Smf parameters tested on both blocks show significantly different results.Geologically, the differences in these values can be explained by considering the type of lithology and the developing geological structure.The East block has a mountainous face with more distinct facets due to the homogeneous conditions of the hills composed of the hard and compact volcanic breccias of the Waturanda Formation.Meanwhile, in the West block, the lithology that makes up the hills is more heterogeneous with different resistances, so the facets of the mountain faces are different from the eastern blocks.
The West Block is more influenced by fault activity, so lifting activity is greater and impacts intensive deformation in this area, characterized by higher vertical erosion.Morphologically, it can be seen as narrow valleys and intensive joint structure development.This geological reflection is verified by statistical tests, which are significantly different between the Vf value in the West block and the Vf value in the East block.The smaller the Vf value, the higher the tectonic activity.
Statistical test values show significantly different from the AF value.The greater the AF value, the greater the tilt formed due to tectonic influences.The Karangsambung sinistral fault influences the West Block, and the differences in the rocks composing each sub-watershed affect the deformation of these rocks.The SL, HI, Bs, and Dd values were not significantly different, meaning there were no significant differences in value in the West or East blocks.All study areas are equally affected by tectonic activity in the form of uplift, where the western block is uplifted than the eastern block.The process of uplifting the research area continued until the Quaternary period.Several indications of neotectonic activity can be found in the field, such as uplifted and deformed river terraces (figure 14).

Conclusions
Quaternary tectonic activity in the study area is influenced by North-South compression originating from the subduction activity of the Indo-Australian plate in southern Java.The neotectonic activity of the study area was confirmed by the average relative tectonic active index (IRAT), which indicated that the study area had a value of 2.37, which means that the study area belongs to the middle tectonic active class.The presence of uplifted river terraces is part of the neotectonic activity of the study area.

Figure 2 .
Figure 2. The DEM map shows the morphology of the Karangsambung Amphitheater

Figure 3 .
Figure 3.The 50 is the limit of the AF value in the stable position.Values greater or less than 50 indicate a disturbance of tectonic activity on the tilt of the watershed.Af is larger or less than 50, which gives us a tilting due to tectonic activity.

Figure 4 .
Figure 4. Tectonic influence on the shape of the watershed.The Bl is the length of the basin measured from the highest point, and the Bw is the width of the subwatershed is measured the widest

Figure 7 .
Figure 7. Measurement of the index Vf, where Vfw is the width of the valley, Eld and Erd is the left-right elevations of the valley, and Esc is the elevation of the valley floor.

Figure 9 .
Figure 9. A. Results of measurement of drainage density index (Dd).B. Results of measurement of the DAS asymmetry index (AF)

Figure 10 .
Figure 10. A. Results of measurement of the basin shape (BS) index.B. Results of measurement of the front sinuosity (Smf) index.

9 Figure 11 .
Figure 11. A. The measurement results are based on the calculation of the integral hypsometry (HI) index.B. The measurement results of the index ratio of the width of the valley floor to the height of the valley (Vf).

Figure 12 .
Figure 12. Results of measurement of the streams length -gradients index (SL) indices.

Figure 14 .
Figure 14.Active tectonic indications in the form of uplifting river terraces.A. The Karangsambung anticline valley is known as the morphology of the amphitheater, where outcrops of terraces indicate an uplift process.B. The river terrace sequences (T1 and T2) show that river deposits are in the form of boulders and gravel of igneous, metamorphic, and sedimentary rocks, with a half-angled to a rounded shape.Terrace sequence boundaries by ancient soil layers (paleosoil).C. The distribution of igneous, sedimentary, and metamorphic rocks forming parallels indicates river terrace deposits.
, meaning that the value is getting smaller towards the East block.The of AF is also getting smaller towards the east (AF East block < AF West block).However, it differs from Vf, which is getting more consideration towards the east (Vf East block > Vf West block).
10 average value

Table 1 .
[35]istical calculation of the independent samples test (T test) to find the relationship of each geomorphic index parameter Tectonic geomorphological analysis to determine the effect of tectonics on morphological forms and indications of the presence of active faults can be studied by measuring the parameters of the geomorphic index.The results of measuring the geomorphic index parameter produce IRAT values that indicate that the study area is dominated by class 3 or moderate tectonic class (figure13)[35].Several sub-watersheds are in the class 2 category or high tectonic activity, especially in the western part.The intensive development of geological structures with rock types part of the melange causes high IRAT values (high tectonic activity).The morphology of sharp hills with V-shaped valleys with narrow valley floors and steep slopes reflects the high IRAT value.The topography of the "V" valley results from an uplift process accompanied by vertical erosion.Considering the values of Vf and IRAT, it shows a more significant uplift in the west.This also allows the Karangsambung anticline to form a containment anticline, opening in the west and closing in the east (figure13).

Table 2 .
Classification of tectonic activity levels or index relative active tectonic (IRAT)