Analysis of Tectonic Influence on Morphological Formation: Case Study of Gapura Pemalang Area

quantitative


Introduction
Geomorphology can help trace the processes that occur on the Earth's surface through the approach of studying the visible landforms. By understanding the geomorphological conditions of a research area, it is expected to provide information for human life and serve as a reference for development in terms of disaster management (landslides or floods), infrastructure development, and other aspects. [1] proposed that Central Java was formed by two anticlinal peaks, namely the Northern Serayu Mountains and the Southern Serayu Mountains. The Northern Serayu Mountains form a connecting line between the Bogor Zone in West Java and the Kendeng Mountains in Central Java, while the Southern Serayu Mountains are elements that emerge from the Bandung Depression Zone, which extends longitudinally in West Java.
The research area belongs to the Northern Serayu Zone. The range of the Northern Serayu Mountains is a continuation of the Bogor Zone in West Java and towards the east, it borders the Kendeng Mountains in East Java. The Northern Serayu Zone stretches from west to east with a width ranging from 30 to 50 kilometers. This zone has moderately prominent relief, forming the Slamet Mountains (3428 meters above sea level) in the western part of the zone. In its eastern end, it is covered by volcanic deposits from the Rorojembangan Mountains (2177 meters above sea level), and it slopes southwards, forming a plain known as the Serayu Depression (Serayu Basin) according to Van Bemmelen. This longitudinal depression extends from west to east through Ajibarang, Purwokerto, Banjarnegara, and Wonosobo. The Serayu Basin has a width of about 15 kilometers, which widens towards Wonosobo, but in this area, the depression is covered by the volcanic cones of Sundoro (3155 meters above sea level) and Sumbing (3371 meters above sea level). The level of tectonic activity in the research area can be assessed using morphotectonics through morphometric analysis. Morphometry involves the quantitative measurement of landform morphology [2]- [4]. These measurements follow the principles of geomorphology, which serve as a basis for comparing and analyzing a region's morphology and tectonic activity levels. Morphotectonics itself is influenced by the morphological conditions resulting from past tectonic activity, and tectonics themselves express the topographic features formed and can serve as indicators of ongoing tectonic movement. In the research area, the morphology is controlled by geological structures, which is why this study is necessary to determine the extent of tectonic activity in the research area and its influence on the existing morphological features.

Figure 2.
Geological structure pattern of Java Island [11], [16] 1. Meratus Pattern: The Meratus Pattern, which trends in the northeast-southwest direction, is the oldest pattern and formed approximately 80-53 million years ago (Late Cretaceous-Early Eocene). The Meratus Pattern formed due to tectonic compression processes, which are believed to be the initial thrusting of the Indo-Australian Oceanic Plate beneath the Sunda Shelf. This pattern is evidenced by the presence of the Cimandiri Fault. 2. Sunda Pattern: The Sunda Pattern, which trends in the north-south direction, formed approximately 53-32 million years ago (Early Eocene-Early Oligocene). The Sunda Pattern resulted from extensional tectonic activity caused by the slowing collision between the Indian and Eurasian continents, resulting in rollback during the Eocene-Early Oligocene. This pattern is characterized by north-south faults, both horizontal and thrust faults, predominantly found in the western part of Java Island. 3. Jawa Pattern: The Jawa Pattern, which trends in the west-east direction, is the youngest pattern and formed during the Neogene period. The tectonic activity associated with the Jawa Pattern reactivated the previous patterns and caused the compression of Java Island with a north-south orientation. As a result of this pattern, the Baribis Fault was formed.
According to [17], the formation and development of the North Central Java Basin are influenced by the movement and collision between the Indo-Australian Plate, moving northward, and the Eurasian Plate. The collision involves the oceanic crust of the Indo-Australian Plate and the continental crust of the Sunda Plate, resulting in the formation of an island arc system known as the Sunda Arc System. From the Middle Miocene to the present, there has been an acceleration of movement with the southward motion of the Sunda Plate, during which the North Central Java Basin developed into a Back Arc System. This acceleration has led to the activation of old faults and the formation of uplifts and depressions, with evidence of faulting still occurring today.
Based on gravity data interpretation, the structural pattern in Central Java produces three main directions [18]- [20]: a. Northwest-Southeast direction, especially in the border areas with West Java. b. Northeast-Southwest direction, found in the south and east of Central Java and around Mount Muria, representing the traces of Cretaceous-Paleocene tectonics in the form of a subduction zone. c. West-East direction, indicating the influence of Tertiary subduction in the south of Java. [21] states that the relative northward movement of the Indo-Australian Oceanic Plate with respect to the Asian continental plate during the Cretaceous period resulted in the collision of these plates, affecting the conditions and development of the Tertiary basins formed in Indonesia, particularly on Java Island, which influenced the development of the geological structural pattern on Java Island. Based on the research on gravity data interpretation conducted by [22]- [24], the tectonic deformation of Java Island, resulting in fold patterns in the Central Java region with a west-east orientation, has been interpreted.
Regionally, the tectonic activity in Java Island has led to the development of various geological structures. The formed structural patterns reflect the dominant stress patterns from specific tectonic processes with varying orientations. The resulting stress patterns give rise to structures such as faults, fractures, and folds at various scales, ranging from regional to smaller scales. Java Island is controlled by several major structures that reflect its tectonic evolution [16], [25], [26] (Figure 2). The major structures of Java Island consist of the Meratus Structure, trending in the northeast-southwest direction; the Sumatra Structure, trending in the northwest-southeast direction; the Sunda Structure, trending in the north-south direction; and the Jawa Structure, trending in the west-east direction. The regional structures in these areas exhibit the Jawa pattern.

Methodology
In this research area, two methods are used, namely the calculation of the Ratio of Valley Floor width to Valley Height (Vf) and the calculation of mountain front sinuosity (Smf).

Mountain Front Sinuosity (Smf)
Mountain front sinuosity or Smf refers to the curvature of the mountain front facing the plain. Smf represents the tendency of the mountain front to follow the bends and folds of the terrain, and it is coincident with active fault zones, reflecting their active tectonic nature. A low smf value indicates active tectonics and direct uplift. As uplift decreases, the erosional processes cutting through the mountains become irregular, and the smf value increases. [27] express Smf using Equation (3)

Results and discussions
In this study, a specific investigation was conducted to examine the tectonic activity in more detail using quantitative geomorphology methods. In this method, data extraction is performed on the topographic map of the study area by drawing lines for Lmf (pink) and Ls (yellow) using the ARC GIS application. The Lmf line follows along the mountain front, while the Ls line is straight and aligned with the mountain front ( Figure 6, number 1).

Quantitative Geomorphology
In the mountain front sinuosity (Smf) method, the Arc GIS application is used to obtain the values of Lmf and Ls, which can be viewed through the table in the ARC GIS application. The lengths of Lmf and Ls lines that have been drawn are determined, and 20 data points are collected from various locations within the study area ( Figure 6). These 20 values obtained from the line drawing are then inputted into the Smf formula according to Keller and Pinter (1996) (see Appendix 2).
As for the value of 20 SMF data that has been obtained, if entered into the Keller and Pinter equation, it has the following calculation value. From the collection of 20 data points, the average value of Smf is found to be 1.45. The average value of mountain front sinuosity (Smf), when classified according to the Smf classification classes [29], falls into Class 1. This indicates that the study area is associated with wide plains, narrow valleys, and steep hills. A low value of mountain front sinuosity (Smf) represents active tectonics and direct uplift.

Conclusion
Based on the quantitative geomorphological calculations using the Vf and Smf methods, and the classification of tectonic activity [27], the study area has an average Vf value of 0.19, indicating a high uplift rate. This is reflected in the presence of deep valleys and increased river activity associated with high uplift rates. Additionally, the average Smf value is 1.45, based on the classification of mountain front sinuosity classes by [29], indicating strong tectonic activity characterized by wide plains, narrow valleys, and steep hills. Therefore, the study area, based on quantitative geomorphology, exhibits high tectonic activity.