Atypical Linear Tectonic Block of the Intraplate Deformation Zone in the Central Indian Ocean Basin

Abstract

The Central Indian Ocean Basin (CIOB) is distinguished by unusually high tectonic activity, setting it apart from all other passive oceanic basins. Within the interior of the Indo-Australian lithospheric plate lies a unique area of intraplate deformation. This region is characterized by the highest recorded intraplate oceanic seismicity, with earthquake magnitudes reaching up to M = 8, abnormally high heat flow—measured to be two to four times higher than background levels for the ancient oceanic lithosphere of the Cretaceous age—and, most notably, intense folding and faulting of sediments and the basement, which are typically associated only with boundary zones of lithospheric plates. This anomalously tectonically active intraplate area was studied during regular research cruises in the 1970s–1980s, after which new conclusions were mainly drawn from satellite data modeling. Substantially new geophysical data were obtained in 2017 after a long gap. Bathymetric surveys using multibeam echosounders during the 42nd cruise of the R/V (Research Vessel) Akademik Boris Petrov and the SO258/2 cruise of the R/V Sonne provided full coverage of a large portion of the intraplate deformation area in the CIOB. This confirmed the mosaic-block structure of the intraplate deformation zone in the Central Indian Ocean Basin, consisting of numerous isometrically deformed tectonic blocks. A linear block at 0.2–0.6° S, which has a branch-like shape in plain view, is morphologically distinct from these blocks. It represents a system of structural elements of different scales (folds, flexures, ruptures), which constitute a structural paragenesis formed in the mechanical environment of a dextral transpressive tectonic setting.

1. Introduction

The deep-water basins of the world ocean are characterized by a generally flat sub-horizontal bottom with rare individual seamounts rising above the surface. This structure is caused by the tectonic passivity of the oceanic lithospheric plates far from their boundaries, where the deep-water basins are located. The Central Indian Ocean Basin, which shows unusually high tectonic activity in its northern part, is an exception to this general picture. This is the area with the highest oceanic intraplate seismicity [1]. High young tectonic activity is also evidenced by anomalously high heat flux, with measured values two to four times higher than background values for the ancient Cretaceous oceanic lithosphere [2,3]. Continuous seismic reflection profiling (CSP) in this area has revealed intense fold-rupture deformation of the sedimentary cover and basaltic basement forming large bedding irregularities [2,4,5]. The complex of observed unique tectonic structures characterized by expressive geophysical anomalies makes the area of Intraplate Deformation of the Indian Ocean Lithosphere (IDIOL) in the Central Indian Ocean Basin one of the most deformed zones in the world ocean. This significantly distinguishes it from the known manifestations of intraplate activity known within the continental lithosphere in various areas of the Earth [6,7,8,9].

Although IDIOL has been studied for quite some time [2,4,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24], its structure has not yet been elucidated in detail, and its nature is still largely debatable. This is mainly due to the fact that, unfortunately, the most important full-scale geological and geophysical studies of IDIOL were carried out before the 1990s at the end of the last century, during the cruises of research vessels that were regularly conducted in the 1970s–1980s. Moreover, in them, the main method was Continuous Seismic Profiling (CSP), with a rather powerful pneumatic source, according to the data of which intraplate deformations of the basement and sedimentary cover were studied. And the references in the bibliography emphasize this. Then followed a long pause, until 2017, when we received new data during the 42nd cruise of the R/V Akademik Boris Petrov and the SO258 cruise of the R/V Sonne (Figure 1). There was multibeam bathymetry, but unfortunately, there was no more CSP (except for a small single profile of multichannel seismic profiling of the common-midpoint method (CMP) in the “Sonne” cruise), and there was shallow high-resolution seismo-acoustic profiling (Parasound), which, in the presence of a thick sediment cover, was not very informative for studying tectonics.

The seismic profiles show that the basement, together with the overlying sediments, is being “squeezed” upward with the formation of large uplifts up to 1–2 km high and up to 100–200 km in size, which are complicated by numerous faults and folds with amplitudes of tens to hundreds of meters [2,11,13,14,15,19,20,21]. Deep-sea drilling (Ocean Drilling Programm LEG 116 sites 717–719) has established that these faults and folds are about 8 Ma old, i.e., late Miocene, which determines the time of the onset of intraplate deformation of the Indian Ocean lithosphere [25]. It is also suggested that some of this deformation started earlier than ~14–15 Ma [21]. The geological and geophysical data collected in the IDIOL area until the early 1990s and the results of their processing and interpretation are summarized and analyzed in the monograph “Intraplate deformation in the Central Indian Ocean Basin” [19].

Figure 1. General tectonic setting of the Central Indian Ocean Basin underlain by satellite-derived bathymetry (a). Bottom relief map of the CIOB according to [26], with the addition of new multibeam echo-sounder data from the 42nd cruise of the R/V “Akademik Boris Petrov [27] and from R/V Sonne SO258/2 [28]. The map shows the focal mechanisms of recent earthquakes with a magnitude greater than 5. Black and white beach-balls are thrusts; red and white—shear; and orange circles—mechanism undetermined [29,30,31]. Fracture zones (FZ) are shown by dash lines [32,33]. Magnetic anomalies are shown according to [32,34]—dark blue line is a 34 anomaly with an age of 83 Ma; violet line—33 anomaly with an age of 79 Ma [24]. ODP LEG 116 sites are marked by a red star [25]. ANS—Afanasy Nikitin seamount. The black box notes the study area shown in Figure (b). Black solid lines—seismic profiles (lines 1–6) from the SO258/2 cruise of the R/V Sonne [35]; black dot-dashed line—seismic profile from the 22nd cruise of the R/V “Professor Shtokman” [11,19].

Figure 1. General tectonic setting of the Central Indian Ocean Basin underlain by satellite-derived bathymetry (a). Bottom relief map of the CIOB according to [26], with the addition of new multibeam echo-sounder data from the 42nd cruise of the R/V “Akademik Boris Petrov [27] and from R/V Sonne SO258/2 [28]. The map shows the focal mechanisms of recent earthquakes with a magnitude greater than 5. Black and white beach-balls are thrusts; red and white—shear; and orange circles—mechanism undetermined [29,30,31]. Fracture zones (FZ) are shown by dash lines [32,33]. Magnetic anomalies are shown according to [32,34]—dark blue line is a 34 anomaly with an age of 83 Ma; violet line—33 anomaly with an age of 79 Ma [24]. ODP LEG 116 sites are marked by a red star [25]. ANS—Afanasy Nikitin seamount. The black box notes the study area shown in Figure (b). Black solid lines—seismic profiles (lines 1–6) from the SO258/2 cruise of the R/V Sonne [35]; black dot-dashed line—seismic profile from the 22nd cruise of the R/V “Professor Shtokman” [11,19].

Modern remote sensing techniques have made some contribution to the study of the intraplate deformation zone. A bathymetric map has been constructed from satellite altimetry (ETOPO1) and shipboard data [36,37], which clearly shows the complex structure of the Central Indian Ocean Basin, with variations in the elevation of the ocean floor up to almost four kilometers within the study area (Figure 2a).

Gravity anomaly maps describe variations in Earth’s gravity that result from differences in the density of the Earth’s crust. These variations can reveal the distribution of subsurface masses and help understand the structure and composition of the Earth’s crust, as well as the geological processes that have formed it. The free-air and complete Bouguer gravity anomalies, which we obtained from satellite/shipborne data (EIGEN-6C4), reflect the differentiated density structure associated with the anomalous region of the lithosphere (Figure 2b,d) [38,39]. Within the polygon studied in detail (Figure 1; 0.2–0.6° S and 81–82° E), the complete Bouguer anomaly is characterized by values of 260–270 mGal and a general northwest trend. The morphological structure itself is weakly expressed in the gravity field. However, the residual gravity in this area corresponding to the morphological uplift reveals a narrow negative anomaly up to −90 mGal. In addition, the gravimetric isostatic anomaly from WGM2012 provides insight into the differential vertical motions of the crust and lithosphere under unstable isostatic equilibrium (Figure 2c). Isostasy refers to the gravitational balance between the densest and least dense parts of the Earth’s crust, and theoretically, at the center of a level plateau, both gravity anomalies should be approximately equal. In the studied local area, isostatic anomalies are negative and have values of the order of −50 mGal.

In order to better understand the nature of the studied morphostructure, we analyzed the magnetic field of the area using the EMAG2v3 model. Regionally (Figure 3a), the magnetic anomaly reduced to Ecuador can be divided into three areas (magnetic domains), the Northeast, Central, and Southwest domains. The NE and SW domains are characterized by positive anomalies isometric in plan with values from 0 to115 nT. The central one is dominated by the NW trend, with narrow, elongated, sinuous, variable anomalies from +100 nT to −110 nT.

Finally, a relatively thin sedimentary layer overlying a complexly constructed geologic basement provided an excellent opportunity to demonstrate the effectiveness of tilt derivative techniques applied to magnetic satellite, ship, and airborne data (EMAG2v3) as an interpretative tool in an area of subtle magnetic signals (Figure 3b) [40,41,42]. This is a simple and fast method for locating vertical contacts where zero radian contours delineate the spatial location of the magnetic source edges; the method in its simplest form assumes that the source structures have vertical contacts, that there is no remanent magnetization, and that the magnetization is vertical. This map shows that magnetic bodies are oriented in a preferential NW-SE direction and a complementary ENE-WSW direction. The tilt-depth method only depends on mapping specific contours of magnetic inclination angles, while the depth to the source is the distance between the zero contour and the −45° or +45° contour or their average.

Based on the rare seismic profiles and relatively low-detail satellite measurements of the gravity field, it has been suggested that meridional compressional stress within the Indo-Australian plate as a result of the continental collision of India with Eurasia caused the crust of the Central Indian Ocean Basin to buckle, forming a series of extended conjugate latitudinal ridges and troughs [2]. The oceanic lithosphere of the Central Indian Ocean Basin was approximated as a homogeneous thin elastic slab. Under the influence of meridional horizontal compression, long-period folds—“ridges and troughs”—oriented in a sublatitudinal direction were formed within this plate. This type of long-wavelength regular folding of the lithosphere in the region of intraplate deformation in the CIOB served as the basis for simplified models of this process [18,43,44,45]. Such concepts of the corrugated structure of the lithosphere in the IDIOL region were conditioned by the prevailing views at that time about the simple structure of the deep-sea basin floors of the World Ocean, which were associated with their poor exploration and the low level of detail of regional geophysical data at the end of the last century.

These first schematic views of the tectonic structure and the lateral distribution of deformed uplifts based on rare seismic profiles and satellite measurements of the gravity field have survived up to the present day (Figure 2) [21]. However, the first detailed geophysical survey during the 31st cruise of the R/V “Dmitri Mendeleev” in 1984 corrected these ideas. It was found that the highly deformed tectonic block in the area of 4° S and 80° E has an isometric rhomboid shape, bounded on all sides by faults of two generations [11,12,19,46]. It is bounded in the west and east by ancient meridional paleotransformations—the Indrani at 79° E and the unnamed one at 80.5° E—and from the north and south by a series of sublatitudinal echelon faults of the late Miocene age. Instead of the wavy structure of the IDIOL, created by a series of extended latitudinal rises and troughs, a mosaic-block structure of the intraplate deformation region was proposed, created by chaotically distributed isometric deformed tectonic blocks [27,46]. Physical modeling of the compressive deformation of the Indo-Australian plate during the continental collision of India with Eurasia demonstrated that this is precisely how the plate deforms in the presence of meridional weakened zones of transform faults [47].

The geophysical survey carried out later in the 22nd cruise of the R/V “Professor Shtokman” (1989) at three polygons in the CIOB corroborated the isometric shape of isolated deformed uplifts and the mosaic distribution of such tectonic blocks [10,11,19]. Simultaneously, this survey revealed that these blocks differ from each other in morphology and exhibit highly individual tectonic structures [11]. Continuous seismic profiling revealed the structure of the sedimentary cover in this area within the IDIOL to have a thickness of 2.5–3 km [10,11,19,21]. The data indicate that the sediments and basement are affected by significant conformational folding and fracture disturbances, which are characteristic of the entire IDIOL in the Central Indian Ocean Basin [11].

In cruise SO258/2 of the R/V Sonne (2017), geophysical work was carried out in the Central Indian Ocean Basin spanning the band from 6° N to 3.5° S from 81° to 84° E [35]. Eight meridional profiles of 600 miles each were completed in a corridor of approximately 140 km in width. Two NW-SE profiles (250 miles each), two SW-NE profiles (100 miles each), and several short linking profiles were also completed. A new detailed map of the bottom topography, based on the results of a bathymetric survey with a multibeam echosounder, revealed a clear, chaotic mosaic distribution of isometric rises in the CIOB [27]. This distribution differs from the previously depicted extended latitudinal ridges [2,48]. However, this original scheme of conjugate latitudinal ridges and troughs is still accepted by numerous researchers [21], which renders our paper pertinent. This paper discusses the results of the SO258/2 cruise survey on an atypical linear deformed IDIOL linear tectonic block at 0.5° S, which exhibits a morphology and deformation character that differs markedly from all other isometric tectonic blocks (Figure 1). We refer to this block as “the branch”.

2. Materials and Methods

The continuous geophysical survey conducted on the R/V Sonne during cruise SO258/2, which included multibeam bathymetry, seismo-acoustic profiling, magnetometry, and gravimetry, was carried out along the entire ship’s route at a speed of 10–13 knots [35]. Six of the eight meridional profiles crossed the considered atypical linear tectonic block IDIOL, which is the subject of this study (Figure 1).

The bathymetric survey was acquired with a Kongsberg EM122 shipboard deep-sea multibeam echosounder, which was equipped with a 16 × 8 m antenna with up to 288 beams focused. The longitudinal and transverse beam widths were 0.5° and 1.0°, respectively, with a viewing angle of 140°. This approach ensured high data density and resolution, as well as a large coverage width in a six-depth water band (up to 30 km).

The seismo-acoustic survey was conducted using a shipboard Atlas Parasound DS3 (P70) narrow-beam parametric profiler. This device emits acoustic signals with a power of 70 kW, which provides a maximum penetration depth of up to 200 m in weakly consolidated sediments. In the course of the SO258/2 cruise, the penetration depth did not exceed 100 m at a signal frequency of 4 kHz.

In order to characterize the structure of the entire sedimentary section, the results of continuous pneumatic source seismic profiling, which were previously obtained in this area during the 22nd cruise of the R/V “Professor Shtokman”, were utilized. The aforementioned data were recorded in analog form.

The multibeam echosounder data were processed in GenericMapping Tools software, with the visualization performed in GlobalMapper-22 and Golden Software Surfer-21. The seismo-acoustic data were processed using the RadExPro-2019 software package, with Kingdom Suite 2017 used for data interpretation. The archived seismic data recorded in analog form were interpreted in CorelDRAW-22 software.

A new high-resolution bathymetric map of the “branch” study area, volumetric models, and a series of seismic profiles from different years were used as the data for morphostructural analysis. Detailed morphostructural analysis became possible only after the creation of high-precision relief models showing even the smallest features.

3. Results

3.1. Morphostructural Analysis

The studied area represents a hilly elevation with a highly dissected bottom relief and appears as branch-like in plan (Figure 4a). The northern edge of the entire structure is raised by 70–80 m relative to the south along a narrow–elongated ridge extending in the southeast–northwest (120°) direction, with a height of over 100 m. Along its entire length, a series of short sub-latitudinal ridges adjoin it from both sides, with the largest located approximately equidistantly (7–10 km). These regular short ridges are clearly manifested in the northern block, where they are the same height—over 100 m (Figure 4b). In the southern block, its own system of ridges has formed, which is practically independent of the northern system. This new interpretation of the bottom relief is radically different from the previous one, which depicted three large latitudinal ridges instead of a series of small latitudinal ridges [19,49], with no signs of an oblique ridge.

The studied block has a complex morphology, and a detailed analysis allows us to consider it as a structural paragenesis [50,51] of structural elements (folds and various types of faults) at different scales. As mentioned earlier, the defining structural element of the block is the complexly constructed Main Ridge, which has a strike azimuth of about 120° and is located on a hypsometric step, where the northern block is raised by 70–80 m relative to the southern block.

From a structural point of view, the Main Ridge is a long narrow Main Anticline of a cylindrical type with a sharply asymmetric structure. Its northern limb is a gentle slope, while the southern limb is complicated by a left-stepping series of short faults along the entire length of the fold. These faults are morphologically expressed as deep furrows and represent tension cracks oriented at an angle of about 45° (±10°) to the strike of the anticline (azimuth 335–355°). All tension cracks are arranged in a series of left-stepping en echelon arrays and form three domains.

The first domain is distinguished in the western part of the Main Anticline, where it is significantly narrower than in other parts (Figure 5). The tension cracks here are oriented at the steepest angles to the strike of the fold (about 35°). In places, they are accompanied by fractures parallel to the strike of the anticline, most likely shear fractures (strike-slip faults). Such a combination of morphostructural elements is typically formed in a dextral simple shear environment. In several places between the tension cracks, relative depressions in the topography are observed. As a result, structures of the pull-apart type arise, bounded by pairs of opposite normal faults and fractures. The hinge point of the fold is represented by a narrow pericline with a gentle plunge of the hinge to the west–northwest.

The second domain is distinguished in the central part of the Main Anticline. Here, the normal faults often form paired tension structures, resulting in the formation of small grabens (pull-apart type basins) bounded by shear fractures parallel to the axis of the Main Anticline (Figure 6). The tension cracks in this part are oriented fairly strictly at an angle of 45° to the strike of the Main Anticline and are usually associated with longitudinal shear fractures, which are morphologically expressed as long narrow furrows or steep scarps. Such a combination of morphostructural elements is typically formed in a dextral simple shear environment.

The most complex structure is found at the eastern end of the Main Anticline (Figure 7). The fold hinge rises sharply here, and the fold is truncated by a transverse fault and lacks a pericline. Three sets of narrow, N-S trending gaps are identified north of the transverse normal fault. These do not belong to the previously described fracture sets and compensate for the meridional compression. In the most uplifted part, the tension cracks across the entire anticline, not just its southern limb. In this place, they clearly underwent a counterclockwise rotation. Such a combination of morphostructural elements is typically formed in a simple shear environment. However, such an interpretation encounters a number of contradictions, which are discussed below.

Two series of sublatitudinal ridges are associated with the central ridge, which from a structural standpoint represent linear and brachyform anticlines. One series is located within the northern limb of the block, while the other is located within the southern limb. All folds have approximately the same morphology—gentle northern limbs and steep southern ones, i.e., the folds have inclined axial planes with southern vergence. The southern limbs of the folds are complicated by faults, expressed as deep narrow depressions in the relief. Despite the similarities described, the northern and southern groups of folds have fundamentally different structures.

The southern group consists of five folds (a1–a5 in Figure 8), associated with the Main Anticline (Figure 8). In addition to the common parameters (inclined axial surfaces, vergence), the folds share another feature. All of them formed on convergent bends of a series of faults that dichotomize from the southern edge of the Main Anticline, dipping northward and forming a “horse-tail” structure. The faults are morphologically expressed as sharp scarps and, less frequently, as broad depressions. Furthermore, each of the folds in the southern group has specific features that distinguish it from the others.

Fold “a1” has a curved axial surface, convex to the north (Figure 8). At the bend, the hinge of the anticline undulates, and a saddle is formed on the fold. The western pericline of the fold is separated from it by a wide strait, most likely formed in a tension zone. The part of fold “a1” adjacent to the Main Anticline is disrupted by a series of tension cracks, similar to those localized within the Main Anticline.

Fold “a2” is also adjacent to the Main Anticline and is bounded to the south by an arcuate fault (Figure 8). Morphologically, the fold represents a gently dipping anticline that virgates westward. The part of fold “a2” adjacent to the Main Anticline is disrupted by a series of tension cracks, similar to those localized within the Main Anticline.

Fold “a3” is very similar in structure to fold “a1” (Figure 8). The axial surface of the anticline is also curved, and the fold is bounded on the south by an arcuate fault. Directly at the bend, the hinge undulates, resulting in a deep saddle on the anticline. The part of fold “a3” adjacent to the Main Anticline is disrupted by a pair of tension cracks, similar to those localized within the Main Anticline itself.

The anticline “a4” is fundamentally different from the others. The axial surface of the fold and the fault bounding it to the south are S-shaped (Figure 8).

The anticline “a5” is the simplest of the folds in the southern group. It is short and very slightly curved, although in the fracture zone, the fold is disrupted by a large tension crack common with the crack on the southern limb of the Main Anticline (Figure 8).

The northern group of folds (a6–a11 on Figure 8) consists of at least seven anticlines, morphologically expressed as relatively wide ridges up to 100 m high. Unlike the folds in the southern group, they are arranged similarly: gently inclined anticlines and gently dipping anticlines, sometimes with weakly curved axial surfaces. The folds differ in the curvature of their axial planes. Fold “a6” has a gentle northward curvature, fold “a7” has a gentle southward curvature, fold “a8” is a straight axial plane, and fold “a9” exhibits a left-lateral offset of its axial plane, likely related to antithetic faults R’.

All folds are bounded on the south by faults, which are expressed as deep valleys and escarpments. A distinctive feature of the fold distribution is that the intervals between folds gradually decrease from west to east, and in the easternmost folds, individual transverse tears of a meridional strike appear.

3.2. Seismic Profiling

Previous CSP surveys revealed a 2.5–3 km thick sedimentary cover in the studied area of the Central Basin [10,11,19]. CSP with a powerful airgun source showed that the sediments and basement are disrupted by pronounced conformable folds and faults, typical of the entire IDIOL. Figure 9 shows a fragment of one of the CSP profiles which was acquired using a less powerful airgun source. It is one of the seismic profiles of the 22nd cruise of the R/V “Professor Shtokman” that traverses the linear tectonic block, which is discussed in this article (the profile’s location is shown on Figure 1b). The oceanic basement is not visible on this profile, because the penetration was performed with a low-power pneumatic source of 0.3 L. However, the data clearly demonstrate tectonic disturbances in the upper layer of the sedimentary cover up to 1 km thick. The structural unconformity “A”, which marks the main phase of intraplate deformation in the Late Miocene (~8 Ma), is clearly visible. Unconformity “AA,” which separates the deformed Late Cretaceous–Early Pliocene complex from the weakly deformed Late Pliocene–Quaternary complex, is also evident. This unconformity is dated to the Early Pliocene (4.0–3.5 Ma) [11].

The maximum signal penetration depth on the seismic profiles of SO258/2 R/V Sonne during the cruise does not exceed 100 m (Figure 10). Within this depth interval, the wave pattern on the seismic acoustic section is fairly laterally homogeneous. However, the bottommost parts of the section are high-amplitude, while deeper levels are characterized by moderate to low amplitudes. The folded zone is also covered by a sedimentary cover, which is clearly visible on the gentle flanks of the folds. On steeper flanks and slopes of small local morphostructures, the apparent thickness of the layered stratum is significantly reduced.

Within the sedimentary cover on the polygon “branch”, three seismic units were identified on the profiles based on the seismostratigraphic criteria (Figure 10) [52]. Seismic unit 1 (SU1) exhibits high-amplitude, continuous reflections forming a layered wave pattern. It is distinguished on almost all seismic sections, especially in the bottom relief depressions, lying in places with a structural unconformity (overlying) on the underlying deposits. The thickness varies from 0 to 6 m. It is very rare in the folded zone of the polygon. Below follows seismic unit 2 (SU2), which is characterized by high-amplitude layered discontinuous horizontal boundaries. The thickness varies from the first meters in the folded parts of the polygon to 35–40 m in the gentle ones. Within SU2, high-amplitude individual horizons are distinguished, which are confidently traced in the gentle parts of the seismic section and gradually pinch out as they approach the folded blocks. However, in general, SU2 is characterized by a highly heterogeneous lateral wave pattern. At the same time, in some places, SU2 is also characterized by an unconformity of the overlying type at the base. Further down is seismic unit 3 (SU3), the base of which cannot be determined due to the insufficient penetrating ability of the profiler. It is distinguished everywhere and is represented by a thin-layered alternation of low- and medium-amplitude reflectors. The visible parts of the cross-sections of the folded zones are also composed of SU3. However, in some areas in the folds, there is an alternation of zones with different degrees of stratification.

Figure 10. Main seismic units (SU1, SU2, SU3) within the study area and around the oceanic drilling sites ODP 717−719 (location of line 2 is shown in Figure 1b). In the inset, there is the correlation of the seismic section with the sediment density curve of site 717 according to [53]. The boundary between SU1 and SU2 is shown in orange; the boundary between SU2 and SU3 is shown in green; black short narrow—approximate position of site 717 on line 2.

Figure 10. Main seismic units (SU1, SU2, SU3) within the study area and around the oceanic drilling sites ODP 717−719 (location of line 2 is shown in Figure 1b). In the inset, there is the correlation of the seismic section with the sediment density curve of site 717 according to [53]. The boundary between SU1 and SU2 is shown in orange; the boundary between SU2 and SU3 is shown in green; black short narrow—approximate position of site 717 on line 2.

No geological fieldwork was conducted in the area of the polygon. To assign a lithostratigraphic correlation to the identified seismic units and to estimate the approximate age of the folds, we used deep-sea drilling data. Ocean Drilling Program (ODP) sites 717−719 [53] were drilled 70 km south of the studied tectonic block “branch” (Figure 10). They are located on a single conventional sub-meridional line, parallel to the meridional traverses 2 along which comprehensive geophysical studies were conducted during cruise SO 258/2 of the R/V Sonne. The sites are located at the same distance from line 2. The correlation with data from site 717 and the analysis of sections from boreholes 719 and 718 allowed us to provide an approximate lithologic-stratigraphic reference for the selected seismic units [53]. The well sections consist mainly of silty turbidites with a thin interbed of calcareous clays. SU1 represents an alternation of muds and pelagic clays of the Holocene-upper parts of the Upper Pleistocene. SU2 is composed of micaceous silty turbidites with layers of calcareous clays of the Upper Pleistocene. According to well data, the sediments of this part of the section are coarser-grained, which has affected the increase in density and decrease in porosity and has possibly caused a sharp change in the wave pattern. On seismic profiles obtained during drilling, the part of the section identified by us as SU2 and SU3 continues up to 150 m [53]. Deeper lies a complex of turbidites that is exposed in the crests of anticline folds. The age of these turbidites is also Late Pleistocene [53].

The correlation of bathymetric and seismic survey data revealed that in folded zones, within the hinges and limbs of anticlinal folds, sediments with a visible thickness of the first tens of meters outcrop (Figure 10). These sediments exhibit a clear acoustic stratification with extended parallel reflecting boundaries and are likely composed of syn-deformational turbidites of SU3.

The studied linear tectonic block is currently seismically active, with the nearest earthquake epicenters located 20 km to the northeast and 100 km to the southwest, having an average magnitude of 5 (Figure 4a).

4. Discussion

The block “branch” is located in the area of a major reorganization of the spreading system in the Indian Ocean during the Late Cretaceous period, approximately 100 million years ago [21,32,34]. It separates oceanic crust areas of the Central Indian Ocean Basin formed at spreading ridges with different orientations. It is reflected in the different orientations of linear magnetic anomalies in these regions: NE-SW in the north of the Bay of Bengal and E-W in the basin south of the equator. The different spreading directions are also evident in the orientations of paleo-transform faults trending NW-SE and meridionally, respectively. It can be assumed that the block “branch” is located in a suture zone dividing lithospheric blocks of different ages. Similar structures can form during the development of a new spreading ridge propagating into older oceanic lithosphere [54,55].

It is important to note the fundamental difference between the new structural–tectonic results presented in this paper and those previously derived from a spars grid of seismic profiles. These articles discuss the structural aspects of the area of the Intraplate Deformation of the Indian Ocean Lithosphere (IDIOL) in the Central Indian Ocean Basin showing east–west trending Riedel shears coinciding with reverse faults formed by the reactivation of ancient spreading faults [14,56]. These ideas are reiterated in a recent article by M. Desa and M. Ramana (2021), where an interpretation of a seismic profile crossing the “branch” tectonic block is provided. The reverse faults identified by these authors are also considered to have formed during the reactivation of normal spreading faults [56]. However, the described major NW-SE shear in the “branch” block does not fit into this model, as it formed later in already existing spreading crust during plate kinematics reorganization. The east–west trending right-lateral en echelon compression zones (anticlines, reverse faults) coincide with the fault trends described by Bull and Scrutton [56] but have a different origin.

Thus, the structure of the tectonic block «branch» combines several standard structural elements: (1) a main linear anticline with elements of oblique shear fractures; (2) dextral en echelon compression structures (inclined folds, reverse faults), and (3) left-stepping en echelon tensile structures (tension fractures, pull-aparts) (Figure 8). This paragenesis may correspond to a mechanical setting of simple shear, allowing for its possible interpretation within the Riedel model [57]. In this interpretation, the Major Anticline corresponds to the Riedel general right-lateral shear zone (Y in Figure 8). From the southeast to the northwest, the faults of the major shear zone bifurcate to the left (southward) at an angle of 5–15°. These synthetic faults are likely secondary Riedel shears (P). Extension fractures within the Major Anticline represent Riedel “T” fractures, which, according to this model, should be oriented at an angle of 45° to the general shear zone (Figure 8). However, this paragenesis does not strictly fit into the Riedel model, meaning its formation environment does not fully correspond to the mechanical setting of simple shear for several reasons. First, the simple shear mechanical environment assumes a constant volume of the deforming block, and compression deformations across the main shear are not accounted for in this model. In addition, despite the apparent external similarity in the structures of the northern and southern limbs of the main shear, they are, in fact, organized differently. The northern limb structure is interpreted as a dextral en echelon fold system with southward vergence, where anticlines are closely associated with the shear itself. The folds in the southern limb are not directly associated with the main shear but form compensatory structures at convergent bends of P-shears, which develop from bifurcating faults of the main shear zone (Figure 8). Morphologically, the folds of the southern limb are in many ways similar to those of the northern limb (inclined, sharply asymmetrical, with rounded hinges, bounded to the south by inclined faults), but they have less distinctness and amplitude. The main difference between the paragenesis of the northern and southern limbs lies in the identical (southern) vergence of the folds in both limbs. This does not correspond to the standard Riedel model, in which the vergence opposite limbs should be opposite. The most likely reason for this situation is the difference in the rheological properties of the rocks that make up the limbs of the shear zone. The northern limb appears to be lighter, while the southern one is heavier. As a result, the faults that bound en echelon folds to the south in the different limbs have different kinematics. In the northern limb, they formed as reverse faults (“rf” in Figure 8). Within the northern limb, they developed as reverse faults (“rf” in Figure 8) with an active hanging wall. In the southern limb, these faults formed through the underthrusting of the active footwall beneath the hanging wall (“uf” in Figure 8). These faults represent a rough kinematic analog of a subducting plate.

It should also be kept in mind that, with the general sub-meridional compression, the northern limb of the shear zone is most likely overthrust onto the southern one, that is, the general shear is not vertical but dips to the north–north–east and represents not just a shear but a reverse fault (Figure 11). Therefore, the Main Anticline has a sharply asymmetrical structure. Its southern limb is not only broken by a series of detachments; it is probably also steeper.

It is important to note that, in the Riedel shear model, the angle between the direction of compression and the general shear should be 45°, but in reality, it is even smaller when considering the angle of internal friction. In our case, the angle between the direction of compression and the general shear is significantly greater than 45°. Moreover, the direction of compression (meridional) is fixed by specific compressional structures, in our case—latitudinal anticlines, as well as strictly meridional tension fractures at the eastern end of the zone. This situation is probably explained by the fact that this shear formed on a previously existing boundary of two oceanic crust blocks, rather than being newly formed as a result of compression. Finally, the main shear zone of the block “branch” is expressed as a compressional structure (an anticline), allowing the overall block formation setting to be interpreted as transpression. This aligns with the definition by Sanderson and Marchini [58], who modeled transpression as a deformation involving shear accompanied by shortening (compression) across the fault plane and vertical extension along the plane, essentially combining pure shear and simple shear mechanisms.

Thus, the structure of the isolated block “branch” can be modeled as a transpression-related en echelon structural pattern.

5. Conclusions

For the first time, a detailed bathymetric survey of the seabed using multibeam was carried out in a local area of intraplate deformation.

In the studied area, a tectonically isolated block (branch) was identified, whose analysis allows it to be considered as a structural paragenesis of multi-scaled structural elements (folds and various types of faults).

Within the block, three structural domains were identified and mapped, characterized by structural elements such as left-stepping en echelon short fault arrays, tension cracks, left-stepping en echelon arrays, and others. The morphostructure of the domains is described in detail. Based on a detailed morphostructural analysis, a series of maps and diagrams were created to illustrate the modern morphology and evolution of the identified structures

The structure of the studied tectonic block combines several standard structural elements: (1) a major linear anticline with shear fracture elements, (2) dextral en echelon compression structures (inclined folds and reverse faults), and (3) left-stepping en echelon tensile structures (tension fractures and pull-aparts). This paragenesis largely corresponds to a mechanical setting of simple shear, suggesting its possible interpretation within the Riedel model.

Additionally, structural analysis revealed that submeridional compression in this region was compensated not by the formation of large sub-latitudinal compression zones but by the development of conjugated local oblique shear zones, which overall correspond to the mechanical setting of dextral transpression, i.e., a combination of simple and pure shear. This led to the formation of distinctive structural parageneses, in which en echelon compression and extension zones, complicated by Riedel shears, are combined.