Late neogen deformation of rock successions at Renah Gajah Mati I Region Seluma Regency in Bengkulu

Studies on tectonic deformation of the Late Tertiary sequence during Late Neogene time has been carried out at the Renah Gajah Mati I region using balancing and restoring techniques, kinematic and dynamic analyses. The Late Neogene deformation was mainly controlled by compression, which has resulted in folding and thrusting. Hence, the rock sections underwent shortening that varied from 1.42-1.83 km or 0.75%-0.78% with the estimated rate of deformation 0.12-0.15 mm/year. Two types of tectonic structures developed within the region, fault propagation folds and trishear faults. The brittle deformation extended sequentially in the direction of tectonic transport, resulting in a series of faults. The generated faults constitute imbricate fans with a relatively closely-spaced thrust system. The array of thrusts is apparently associated with folds that strike NW-SE, implying the geometry of fault-linked folding. The hanging wall anticlines are generally asymmetric overturned folds, indicating a shallow ramping fault with a high shear pattern. In regional context, the general strike of structures within the study area appears relatively parallel to the NW-SE trending Barisan orogen. This feature suggests that tectonic transport was likely at the NE-SW direction.


Introduction
The present study aims to reconstruct the deformational styles that involved the Late Tertiary sequence in the Renah Gajah Mati I (RGM I) region. The study area is located between latitude 4˚54'5"-4˚13'24.8"S and longitude 102˚49'20.7"-102˚54'14.4"E. Particular areas from which observation and measurement for structural analysis were undertaken include the RGM I, Kemang Manis, Mekarsari Mukti, and Napalan districts of Seluma Regency, Bengkulu. The region is tectonically situated in the fore-arc setting of Bengkulu basin, and geologically constituted by the Tertiary rock units that have been deformed through folding and thrusting mechanism during Late Neogene time ( Figure I).
The study area comprises three lithostratigraphic units, from older to younger consecutively the Seblat Formation, the Lemau Formation, and the Quaternary Volcanic sequence. The lower succession consists mainly of sandstones intercalated with siltstones and mudstones, whilst the overlying Lemau column contains predominantly calcareous mudstones and fossiliferous mudstones. The youngest unit of rock section is constituted mainly by volcanic breccias and tuffs. The sedimentary sequence underwent tectonic compression that was coincident with the onset of Barisan orogeny in the Late Tertiary. The event allowed the formation of brittle structures in the Seblat unit, and ductile deformation in the Seblat and Lemau successions.
Regionally, the study area seems to have been controlled by the NW-SE trending alignments. This structural trend is consistent with the strike of Semangko Fault System occurring along the Barisan Mountain range. Importantly, tectonic events that have been responsible for structuring within the region  (Pulunggono, 1992;Simanjuntak and Barber, 1996). Since the Late Neogene orogeny proceeded, the older tensional structures were reactivated by compressional regime. The later deformational system is considered to have regionally resulted in dextral movement along the Sumatran fault zone, and locally formed the NW-SE striking folds and thrusts (de Coster, 1974). Therefore, it is interpreted that the observed region has been subjected to extensional and subsequently compressive strains during the Tertiary. A particular interest of the present work is to analyze tectonic structures developed throughout the Seblat and Lemau successions for the selected sections, applying the techniques discussed in the following section.

Methods
This work conducted geological mapping by using a conventional ground survey in order to observe rock types and structures exposed throughout the study area. The outcropping rocks were described and mapped to generate the 1:25,000 scaled geological map. The identification and measurement of structural elements were carried out to construct the deformational styles occurred within the region. Structural studies herein have also adopted the geometrical evaluation of surface structures and a general interpretation of fault propagation folds (Mitra, 1990;McClay, 1992). Moreover, structural analysis has applied the techniques of balancing and restoring sections crossing the A-A' and B-B' lines ( Figure 1). The lines of sections were selected with consideration of the most available outcrop data. Interpretation of the deformational styles was supported as well by the 3D configuration of Digital Elevation Model (DEM) to gain regional features, in particular the alignments of topographic expression that may characterize structural controls. In modeling fault-interactive folding mechanism, this study employed the method suggested by Cardozo et al. (2003). Subsequently, the model was analyzed by utilizing "Stereo V8" program, especially in the determination of tectonic transport, whereas fault types were identified by the computer-aided "Fault Fold V6" software (Allmendinger, 2012).
In the construction of balancing and restoring sections, two common methods are applied, the equal line-length and constant-area techniques. According to Dahlstrom (1969a), there is no significant change in rock volume, and the initial thickness and length of the beds are maintained during tectonic deformation. Therefore, this assumes that oblique shearing of sedimentary successions to bedding is negligible to the folding mechanism, thus the interpretation considers only the parallel folding of the beds. The area balancing method assumes that the rock volume remains equal during structuring episodes (Suppe and Medwedeff, 1990; Mosar and Suppe, 1992). Relying on this hypothesis, the creation of balanced section commonly applies area conservation, i.e. rock volume is conserved during deformation. Mitra and Namson (1989) distinguished the technique into the area restoration and the excess-area method. However, the present study employed the excess-area approach in determining the depth to detachment and the regional shortening. Calculation of the depth to the detachment has been illustrated by, for example, Dahlstrom (1969aDahlstrom ( , 1990) and Mitra and Namson (1989).

Exposures of Structures
Regional Late Neogene tectonic events allowed the development of ductile and brittle deformation throughout the study area. The brittle type apparently develops in competent layers of the Seblat sandstones, whereas the ductile structuring most likely forms the folding patterns in the Seblat and Lemau incompetent muddy sequences. Several structural elements such as the hinge surface, microfold, and boundinage are exposed in the region (Figure 2). The structures trend NW-SE at N303ºE/72º and N301ºE/87º, and are parallel to the regional Barisan orogen striking along the western part of Sumatra. The anticlinal and synclinal features occur at the Napalan, Ijang and RGM I region. The general orientation of the folds may determine the emergence of tectonic transport that has been responsible for deformation of rock sequence (Arslan, 2013). The measured limbs of the structures vary in dips between 4˚-79˚, referred to as asymmetric overturned folds with the general hinge line oriented to the NW-SE direction. This implies that deformation of the region might have gradually progressed southwestwards, and it must have taken place in the Late Neogene, coincident with the inception of major tectonic events that allowed the building of Barisan Mountain range, which extends along the western portion of the island.  There are three contractional fractures recognized in the region, namely the Alas, RGM I, and Mekarsari Mukti thrust faults that strike to the NW-SE direction. The faults might have elevated the upper sedimentary column. The Alas faults are exposed in the northern section of the Air Alas Kanan river, associated with fault drag oriented to the N308ºE/43º direction. The RGM I thrust system occurs in the western segment of the river site, in which the fault breccias are exposed with the N302ºE measured strike. The fragmented rocks vary from 0.8 to 3 cm in diameter with the apparent thickness of about 56 cm. This particular fractured zone is interpreted here as chaotic breccias (Woodcock and Mort, 2008). Besides, shear fractures (Peacock et al., 2017) are encountered in the region, showing crosscutting geometry in the angle of 58º-69º with a general direction of approximately N250ºE/66º and N12ºE/48º. The associated structure with the thrust fault is drag fold. Stereographic analysis results in the fault strike at N302ºE, dipping to the 39ºNE direction with a net slip 09º N315ºE and rake 16º. It is therefore interpreted as a thrust right slip fault (Rickard, 1972) with a maximum stress field (σ1) orientation N213ºE. The presence of Mekarsari Mukti thrust was observed at different localities along the Air Empangan river. Structural components exposed within the river sites included fault bedding N294ºE/43º and displacement of about 33 cm, gouge, chaotic breccias, fault slip N291ºE/22º with rake 12º, and calcite veins oriented to the N30ºE strike. Overall, those thrust elements suggest a general fault strike of N298ºE, hence tectonic transport responsible for thrusting was likely from northeasterly. Additionally, structural outcrop shown by Figure 5c.

Results and Interpretations
Two balanced and restored cross-sections along the A-A' and B-B' lines ( Figure 3 and 4) have been constructed to interpret the structures of the region, which are thus geometrically admissible.
Interpretations of ramp and flat systems adopted the fundamental concepts, which suggest that ramps occur commonly in competent rock layers, whereas flats develop better in less competent beds. Within the study area, the sandstones belonging to the Seblat sequence are interpreted to be the lower competent rocks, whilst the siltstones and mudstones of the Seblat Formation are likely to be the lower incompetent beds. The mudstones of the Lemau succession are referred to as the upper incompetent units. Additionally, brittle deformation is common in competent rocks, whereas ductile structuring likely develops within less competent rocks.    Two structural models constructed in the present study show quite similar architectural styles. Both sections suggest that the region is characterized by fault-interacted folding features. The structures are observable particularly in the Early Miocene Seblat sequence and the Middle-Late Miocene Lemau succession. This phenomenon provides direct evidence that the rock units were tectonically deformed during the Plio-Pleistocene, concurrent with the uplift of the NW-SE trending Barisan orogen (Barber and Crow, 2003;Sieh et al.,2000). Thrust faults developed principally in the Seblat rock unit, whereas folds appear existed in both Seblat and Lemau sequences. In addition, it is interpreted that the progression of fault tip lines might have led to folding, thus resulted in fault-propagation folds. The antiforms developed at the hanging walls commonly show an asymmetric overturned style. This indicates that the folding might have been controlled principally by a shallow ramp with a considerably high shearing during faulting. The interpretation is consistent with the structural models suggested by Corredor (2005). Table 1 describes that folding in the region permitted the shortening of rock sequence with the estimated minimum value ranging from 1.42 km to 1.83 km, or a contraction ratio of 0.75%-0.78%. In the construction, the calculated depths to detachment (Z) varied from 5.80 km to 7.24 km, implying a shallow décollement along the lower incompetent beds within the Early Miocene Seblat Formation (Figure 6b). It is interpreted that the initiation of the incipient Ijang fault was possibly in either the Late Miocene or the Early Pliocene. As compression progressed laterally, a flat developed, subsequently the tip line of fault to some extend went up to higher stratigraphic levels forming a ramp. Since the ramping proceeded, the rock cover folded and generated the hanging wall anticlines. As a result of this structuring, fault propagation folding developed in the entire Late Tertiary successions. Such deformation likely progressed southwestwards from backlimb to forelimb, and resulted in the trishear faults at the RGM I and Mekarsari Mukti areas. This scenario is consistent with the measured maximum strain at about N213ºE, suggesting tectonic transport to the NE-SW direction (Figure 5a). Importantly, the fault propagation folding mechanism appears to have been responsible for the present topographic elevation within the presently investigated region. Additionally, structural model mechanism deformation tectonic in study area shown by Figure 5d.

Conclusions
The present study reported in this manuscript draws some concluding remarks as the following: 1. The Late Neogene deformation has involved in the Late Tertiary sequence through thrusting and folding mechanism. Hence, the rock unit shortened tectonically between 1.42 km and 1.83 km, or 0.75% and 0.78%.