Study on the Occurrence of Double Bottom Simulating Reflectors in the Makran Accretionary Zone

Abstract

A Bottom Simulating Reflector (BSR) is a seismic feature closely related to marine gas hydrate as it is usually regarded as the seismic response of the base of the gas hydrate stability zone in seismic profiles. BSRs are widely distributed in the Makran accretionary wedge, and double BSRs are observed at some locations. Double BSRs usually appear on seismic profiles as two layers of BSRs located at distinct depths but with large lateral seismic amplitude variations. Based on the multi-channel seismic reflection data acquired over the Makran accretionary wedge, this work studies the origin of the double BSR in the Makran accretionary wedge and its association with fluid escape events. Our modeling suggests that double BSRs correspond to both the paleo-seafloor and modern seafloor caused by late sedimentary activities. Also, the residual paleo-BSR migrates upward due to the increase in local geothermal gradient caused by diapirs and gas chimney thermal fluids.

1. Introduction

A Bottom Simulating Reflector (BSR) was first associated with the presence of marine gas hydrates by Shipley et al. [1] and generally has the following characteristics when interpreted in seismic reflection data: (1) the BSR is a seismic event roughly parallel to the strong reflector corresponding to the seafloor [2]; (2) the seismic reflection polarity of the BSR is opposite to that of the seafloor [3]; (3) the BSR cuts through sedimentary strata [2]. Despite its distinct characteristics, the BSR is difficult to identify in multi-channel seismic reflection profiles in the presence of geological formations dipping with gentle slopes or when the seismic reflections are cluttered. In addition to the characteristics previously mentioned, BSRs might also be associated with seismic amplitude anomalies, such as bright spots, acoustic blanking, and enhanced reflections, which are located above and below the BSR on the seismic profile.

Although BSRs are widely distributed on continental margins, double/multiple BSRs are relatively rare [4,5]. Their origin is still not fully understood and therefore modeling the origin and the development of double BSRs is an international research frontier and hotspot [6]. On the same seismic profile, the double BSRs appear as two layers of BSRs located at different depths [7], which are mostly irregularly distributed. Research reports on double BSRs were first interpreted on the Storegga margin in western Norway [8]. In the past decade, with the increased interest about unconventional energy, such as gas hydrates, and the development of seafloor detection technology, double/multiple BSRs were observed on both active and passive continental margins, such as the South China Sea [9], Antarctica [10], Black Sea [2,11], hydrate ridges [12], the western Norwegian continental margin [13], and the northern margin of Hikurangi [4].

Double BSRs are important to interpret and model as they are not only a reflection of the dynamic changes in the gas hydrate system [4] but also of great significance for studying the characteristics of the gas hydrate system during its evolution in time [14]. Due to the large uncertainties associated with past climate change and tectonic history modeling, the formation mechanism of double BSRs is not well understood and in some cases controversial [4]. Originally, researchers agreed with both the “residue theory” and “multiple gas source components theory” [8,13]. In recent decades, with the continuous investigation and research, a large number of studies have revealed that the occurrences of double BSRs include, but are not limited to, the following three aspects: (1) changes in temperature and pressure conditions (e.g., climate change, tectonic activities) [12,15]; (2) differences in gas source components of natural gas hydrates [8,13]; (3) changes in formation lithology or minerals [3,16,17,18,19].

Since the 1990s, domestic and foreign research institutions have conducted several marine geological surveys in the Makran accretionary wedge [20,21] and found that double BSRs also exist in this region [22,23]. Like the Hikurangi margin, the Makran accretionary wedge is located in a subduction zone, and both have the same continental margin tectonic background and thus can be considered analogue. Han et al. [4] found multiple double BSRs in the northern part of the Hikurangi margin and found that the formation of double BSRs is closely related to the extensive subduction process. Rapid sedimentation, tectonic uplift, and overpressure/thermal advection caused by fluid migration change the temperature and pressure conditions of shallow seafloor sediments, which is beneficial to the formation of double BSRs [4]. The tectonic activity of the Makran accretionary wedge is active [24]. Regarding the formation mechanism of double BSRs, Ding et al. [25] identified the double/multi-BSR phenomenon but did not study it further. Gong et al. [26] proposed that the formation of double/multi-BSRs in the Makran accretionary wedge may be related to the presence of slumps or tectonic uplift, which still needs further confirmation and explanation. Wu et al. [22] speculated that the multiple double BSRs found in the mud diapir development area were formed by the upward migration of thermal fluids resulting in the decomposition of hydrates. Liu et al. [23] used high-resolution seismic data to identify three phenomena of the double BSRs in the Makran accretionary wedge and speculated that the formation mechanism of double BSRs was most likely associated with the presence of slumps or rapid sedimentation.

In the work presented herein, we analyze the origin of double BSRs identified in multi-channel seismic data from the Makran accretionary wedge by using numerical simulation and further explore the relationship between the seafloor fluid escape activities and the occurrence of the BSR, so as to provide reference for future research on the origin of the double BSRs and related seafloor fluid escape activities in the study area or other accretionary wedges.

2. Tectonic Setting

Located in the northern Arabian Sea, the Makran accretionary wedge is at the intersection of the Arabian, Eurasian, and Indian plates and is the result of the Arabian plate subducting the Eurasian plate northward at an average rate of 4 cm/a throughout the Cenozoic (Figure 1) [23,27,28]. Large sedimentary thickness (up to 7 km) and low subduction dip (less than 3°) are the two distinctive features of the Makran accretionary wedge [29,30].

The Makran accretionary wedge has mainly experienced three evolutionary phases. From the late Oligocene to the middle Miocene, it was dominated by turbidite deposits of sand, mainly composed of quartz, and mud. Between the Late Miocene and Middle Pleistocene, the upper part of these sediments was displaced by the underlying bed, while the lower part was subducted to greater depths and subsequently covered by the shelf and slope sands [31,32]. The accretionary wedge was accreted seaward through the accretion of trench-filling sediments, as well as the progression of continental slopes, shelves, and coastal plains. Since the Middle Pleistocene, the nearshore part of Makran has undergone uplift and normal faulting, while accretion of the seaward part has continued to the present [27]. The narrow shelf, rapid tectonic uplift, and intense erosion led to extremely high deposition rates (0.2 - > 1 mm/a) of the Makran accretionary wedge. Large amounts of terrigenous and organic matter are accumulating in the wedge [33], culminating in one of the largest wedges.

The Makran accretionary wedge is about 400 km wide from north to south and consists of two parts, onshore and offshore, of which about two-thirds is located onshore [34]. The offshore accretionary wedge is about 100 km wide, bounded by the Oman deep-sea plain in the south, with a maximum water depth of approximately 3500 m. It consists of three parts: a narrow continental shelf, a broad continental slope, and an accretionary foreland [30,32]. Among them, the sediments of the accretionary front are approximately 7 km thick and are divided into two parts, which can be characterized from bottom to top as: the lower sediments are about 4 km thick, from the Himalayan turbidite in the eastern Indus fan; the upper sediments are about 3 km thick, from the northern Makran sand [29]. The Makran sand lies unconformably on top the Himalayan turbidite, which is also covered by a thin layer of shelf sediments. The Makran accretionary wedge is 800 km long from east to west [29], and the west is connected with the Minab fault in the east, and the east is connected with the Omach-Nal fault in the west [24].

The Makran accretionary wedge is mainly characterized by a ridge–valley form, with a large number of folds and imbricate thrust faults (Figure 1b) [26,35]. From north to south, the thrust fault activity of the Makran accretionary wedge increases gradually [24], and at the southernmost end, some thrust faults are exposed on the seafloor [36]. From west to east, the activity of thrust faults gradually weakens [25]. Sufficient gas sources and related fault structures demonstrate that the Makran accretionary wedge has good prospects for gas hydrate exploration [35].

3. Data and Methods

To identify the formation mechanism of the double BSRs observed in multi-channel seismic reflection data acquired over the Makran accretionary wedge, we leverage high-resolution seismic data acquired during the China–Pakistan joint marine scientific cruise onboard the R/V Haiyangdizhi 10 held from December 2018 to February 2019. The seismic data were acquired with two guns with a capacity of 420 cubic inches, fired simultaneously at 25 m intervals. The receiver array has 96 channels with 12.5 m receiver spacing. The minimum and maximum offsets are 112 m and 1330 m, respectively. The sampling rate of the seismic data is 1 ms and the record length is 8 sec. The processing flow of the acquired data includes applying the acquisition geometry to the seismic file headers, noise suppression, velocity analysis, and pre-stack time migration. Simultaneously to the seismic data acquisition, high-resolution multibeam data were acquired using a ship-mounted EM302 multi-beam system working at frequencies between 26 kHz and 34 kHz. The raw multi-beam data were processed using Caris HIPS and SIPS 8.1 software.

The work presented herein focuses on seismic profile 21, which was acquired along the north–south direction in the perpendicular direction of the Makran accretionary wedge (Figure 2). We assume that the geothermal gradient does not change considerably along the seismic profile, and we fitted a geothermal gradient to the modern seafloor to simulate the related parameters of the double BSR. The modeling procedure followed this sequence of steps:

• (1). Retrieve the two-way travel time (TWT) of the double BSR, modern seafloor, and basal sliding surface according to the seismic profile (Figure 2b);

• (2). Time-to-depth conversion of the TWT picked up in (1);

• (3). Predict the geothermal gradient fitted to the modern seafloor;

• (4). Model the depth of the paleo-seafloor using the geothermal gradient fitted to the modern seafloor and the depth of BSR1 (Figure 2b);

• (5). Model the depth of BSR1 based on the depth of the basal sliding surface interpreted directly from the seismic profile;

• (6). Estimate the BSR-derived heat flow and geothermal gradient using the observed BSR1 and BSR2.

3.1. Methods

3.1.1. Time-to-Depth Conversion

The TWTs of modern seafloor, basal sliding surface, and double BSRs were picked from the seismic profile shown in Figure 2b. The TWTs of basal sliding surface and modern seafloor are 2.77~3.36 s and 2.72~2.98 s, respectively; the TWT ranges of BSR1 and BSR2 are 3.47~3.77 s and 3.3~3.53 s, respectively. We assume a sound velocity water column in the Makran accretionary wedge of 1500 m/s. Kaul et al. [37] propose an empirical formula for estimating the velocity of the sedimentary formation ($V_z)$:

(1)$V_z=\left\{\begin{array}{c}1500+3.451\ast z, 0\le z\le 100,\\ 1845.1+0.783\ast z, 100<z\le 1000\end{array}\right. (\mathrm{m}/\mathrm{s}),$

where $z$ is the depth of the sediment below the seafloor in meters below seafloor (mbsf), from which the velocity–depth relationship can be converted into a time–depth relationship.

3.1.2. Simulation of the Depth of BSR

In seismic exploration, the BSR is usually regarded as the base of the gas hydrate stability zone (BGHSZ). In this work, we use the depth of the BSR to approximate the simulation of the BGHSZ, which is usually calculated based on the gas hydrate P–T curve, seafloor temperature, and geothermal gradient. Below we describe in detail the approach followed herein.

Physical samples of shallow hydrates below the seafloor of the Makran accretionary wedge have been obtained, but their specific components are not clear. Based on these preliminary results we assume that the gas hydrates present in this region are composed of pure methane. We opted for the P–T curve of seawater gas hydrate proposed by Miles [38]:

(2)$P=2.8074023+aT+b{T}^2+c{T}^3+d{T}^4,$

where $a=1.559474 \ast {10}^{-1}$, $b=4.8275\ast {10}^{-2}$, $c=-2.78083\ast {10}^{-3}$, $d=1.5922\ast {10}^{-4}$; P is the pressure in MPa; T is the temperature in °C.

We also follow the approximation of Miles [38] to calculate the hydrostatic pressure at the depth of the BSR:

(3)$P=P_{atm}+\left[\left(1+C1\right)\ast H+C2\ast H^2\right]\ast {10}^{-2},$

where $P_{atm}$ is the atmospheric pressure, 0.101325 MPa; $H=h_{sf}+z$, $h_{sf}$ is the depth of the seafloor in meters; $C1=(5.92+5.25\ast \left[si{n}^2\left(Lat\right)\right]\ast {10}^{-3}, C2=2.21\ast {10}^{-6}$, $Lat$ is the latitude.

According to the measured value of the geothermal gradient, combined with the seafloor temperature, the temperature of the sediment layer at any depth below the seafloor can be obtained:

(4)$T_Z=T_{sf}+GTG\ast Z,$

where $T_{sf}$ is the seafloor temperature, which is estimated from CTD casts (SO123, SO124) acquired during the seismic acquisition survey [39,40], GTG is defined as the geothermal gradient.

Assuming that the temperature gradient in the seawater before and after the occurrence of slumps does not change considerably, the corresponding seafloor temperature can be estimated according to the depth of the seawater. The data about the temperature gradient were retrieved from the Global Heat Flow Database (GHFD: http://www.ihfc-iugg.org/ (accessed on June 2022)). The average range of measured heat flow is 40~45 mw/m², and the average range of geothermal gradient value is 36~36.5 °C/km.

In summary, Equations (2)–(4) allow prediction of the depth of the BSR.

3.1.3. Estimated Geothermal Gradient and Heat Flow from BSR Depth

Since temperature is one of the key controlling factors for gas hydrate formation, the BSR essentially represents an isotherm associated with the depth of the seafloor. Therefore, BSR locations have been used to estimate geothermal gradients and obtain information about local conduction heat flow [41]. The $\mathrm{GTG}$ retrieved from the interpreted BSR depends on the seafloor ($T_{sf}$) and BSR ($T_{BSR}$) temperature [42].

(5)$\mathrm{GTG}={\displaystyle \frac{\left(T_{BSR}-T_{sf}\right)}{(Z_{BSR}-Z_{sf)}}},$

Yamano et al. [43] first proposed a method to calculate heat flow using the gas hydrate P–T curve and the depth of the BSR. Heat flow (HF) is the product of the geothermal gradient and thermal conductivity.

(6)$\mathrm{HF}=k\ast \mathrm{GTG},$

where $\mathrm{HF}$ is the heat flow in mw/m²; $k$ is the thermal conductivity of the seafloor sediment expressed in W/(M·°C). Since the measured thermal conductivity of seafloor sediments is difficult to obtain, the algorithm proposed by Davis et al. [44] was used to calculate the average thermal conductivity from the seafloor to the BSR, which can be expressed by a simple empirical equation:

(7)$k=1.07+5.86\ast {10}^{-4}\ast z-3.24\ast {10}^{-7}\ast z^2,$

3.1.4. Inversion of Water Depth from BSR Depth

This inversion method does not require any prior information [4] and can be defined by:

(8)$Z_w=h_{BSR}-{\displaystyle \frac{T_{BSR}-T_{sf}}{\mathrm{GTG}}},$

where $Z_w$ is the depth of the inverted seafloor in meters below sea level (mbsl); $h_{BSR}$ is the observed BSR depth in mbsf; $T_{BSR}$ is the temperature of the BSR depth in °C. Assuming a relatively stable geothermal gradient, the geothermal gradient fitted to the modern seafloor and BSR1 is used to estimate the water depth of the paleo-seafloor.

4. Results

The entire fullstack seismic profile used to model the double BSR is shown in Figure 3b. The region exhibits east–west ridges and north–south canyons along the continental slope. The numerous amphitheater-shaped head scarps along the incision west of the canyon indicate that a mega-slump caused the long N–S-oriented straight depression with a “blind” northern end. The seismic profile passes through the intersection of the sediment channels, highlighted by the white dashed circle in Figure 3a.

The white dashed circle in Figure 3a and the black dashed circle in Figure 3c reveal the interaction of sedimentary processes. Towards the south of the black dashed circle, this region is interpreted on the seismic profile due to a series of chaotic, or random, seismic reflectors. In contrast, the northern region exhibits relatively continuous internal seismic reflections, reflecting the deposition processes dominant in that area. The seismic characteristics within the circle represent a transitional zone, combining features from both the northern and southern regions. The mega-slump (up to 120 m thick) totally filled the former channel of canyon 1 and even removed by truncation the top of the tectonically deformed well-stratified sediments of the anticline (Figure 3b,c). In addition, a contribution to the slump of the southern ridge could be expected from the seismic profile and the bathymetric map.

The marked discordance between the truncated top section of the well-stratified sediments of the piggyback anticline and the chaotic sediments of the slump must be recognized. Therefore, the unconformity surface does not represent the paleo-seafloor. However, it represents the basal gliding or slip plain of the slump, which also corresponds to the discordance associated with the northern anticline. At the southern side of the piggyback basin, the former paleo-surface might still be preserved.

The unconformity surface (Figure 4) indicated that the paleo-seafloor was eroded by late gravity flow deposition and manifested as a cut-off of the anticline event. The seafloor physiography obtained from the multi-beam data shows that tectonic movements played an important role in the ridges observed. Canyons were formed at a later stage and were less affected by tectonic activity. In the late stage, it can be considered that the tectonic geological background does not change much. Therefore, under the assumption that the geothermal gradient does not change considerably, the geothermal gradient fitted to the modern seafloor can be used to estimate the depth of the paleo-seafloor combined with the observed BSR1 (i.e., the interpreted horizon corresponding to BSR1, as shown by the cyan dashed line in Figure 4). The simulation results show that the estimated paleo-seafloor water depth is generally consistent with the basal sliding surface interpreted directly from the seismic profile but shallower near the location of the northern anticline and the gas chimney at the southern side of the piggyback basin.

Figure 5 shows the position of the modeled BSR1 (interpreted as the red dotted line) according to the observed basal sliding surface from the seismic profile. Compared with the observed BSR1, the modeled BSR1 on the northern anticline and in the vicinity of the gas chimney all migrated deeper. Figure 6 shows the heat flow and geothermal gradient inversion based on the depth of BSR picked from the seismic profile. Compared with BSR2, the BSR1-derived heat flow and geothermal gradient on the northern anticline and near the gas chimney are larger.

5. Discussion

Geothermal gradients provide critical insights into the behavior of BSRs [45]. According to the gas hydrate phase equilibrium curve, under the condition of constant geothermal gradient, the depth of the modern seafloor becomes shallower, resulting in BSR2 corresponding to the modern seafloor being located at a shallower depth than BSR1 corresponding to the paleo-seafloor. However, the anomalous migration of the modeled paleo-seafloor and BSR1 in the northern anticline area and near the gas chimney at the southern side of the piggyback basin is mainly controlled by the increase in the local geothermal gradient caused by the fluid activity in the deep diapir in the northern anticline and the gas chimney at the southern side of the piggyback basin. The local deep thermal fluid activity resulted in an increase in the geothermal gradient, which in turn led to a feature of BSR1 migrating to a shallower position compared to the paleo-seafloor (indicated by the green line in Figure 7). The simulation results demonstrate that: (1) BSR1 is indeed the residual paleo-BSR corresponding to the paleo-seafloor; (2) due to the influence of deep thermal fluid activities, BSR1 has migrated to a shallow depth compared with the BGHSZ corresponding to the paleo-seafloor. In addition, in the northern anticline region, the migration of BSR1 is also influenced by gravity-driven processes such as large-scale slumping. The sedimentary gravity flows and associated slumping in this region contribute to the downward movement of the sediment column, thereby enhancing the shallowing of the seafloor and further facilitating the upward migration of BSR1. This combined effect of thermal fluid activity and gravity flows in the northern anticline likely explains the observed anomaly in the migration depth of BSR1 in this area. In contrast, near the gas chimney at the southern side of the piggyback basin, the influence of thermal fluid activity is the dominant factor controlling the migration of BSR1. Despite these regional differences, the overall migration of BSR1 remains closely tied to the variations in geothermal gradient, highlighting the significant role of deep thermal fluid activity in altering gas hydrate stability zones.

Though BSR1 still exists with low seismic amplitude content, it has a high amplitude in the area affected by thermal fluid activity and also shifts to the shallow part, indicating that the gas hydrate of BSR1 decomposes under the influence of thermal fluid activity, and it also has the characteristics of decomposition and regeneration with the thermal fluid activity and gradually migrates to the shallow part, which indicates that the dispersing of paleo-BSR is a long-term process and is significantly affected by thermal fluid activity. However, under the condition of insignificant thermal fluid activity, the dispersing of the paleo-BSR is not significant even due to the influence of modern seafloor sedimentary activity. Due to the inability to effectively date the canyon sedimentary activities, it is still impossible to effectively quantitatively analyze the hydrate decomposition activities of BSR1, which is a very important research direction to further deepen understanding of the hydrate dissociation process in nature in geological events, so as to better assess their resource and environmental effects [46].

6. Conclusions

The double BSRs are mainly distributed in the piggyback basin, and compared with the shallow BSRs, the deep BSRs have weaker amplitudes and poorer spatial continuity. A double BSR is a BSR feature corresponding to the paleo-seafloor and modern seafloor caused by late sedimentary activities, and the paleo-BSR is affected by the activities of deep diapirs and gas chimney thermal fluids. Compared with the BGHSZ corresponding to the paleo-seafloor, the position of the lower bound of this region has migrated to the shallower part. Fluid activity in the Makran accretionary wedge is active. The double BSR analysis results demonstrate that the gas hydrate system is a dynamic system. On the one hand, the fluid migration channel provides sufficient free gas for the formation of the BSR; on the other hand, the stability of natural gas hydrate is affected by changing factors such as temperature, which leads to changes in the morphology and distribution of the BSR.

Gas hydrates are widely distributed in the Makran accretionary wedge, which have important resource prospects. The kinetics of gas hydrate and free gas formation is a complex process that requires extensive knowledge of pore water geochemistry and the internal sedimentary structure of the stratigraphic matrix. The dynamic change in the BSR in the study area is usually affected by a variety of factors. The identified BSR features should be integrated with 3D fine exploration, pre-stack elastic inversion, BGHSZ modeling, and marine drilling data to investigate their occurrence period, tectonic subsidence, and sea level changes and other marine and geological control factors affecting BSR dynamics. The dynamic evolution of BSR in the study area will be elaborated, providing deeper insights into the formation processes and mechanisms of double BSRs.

Author Contributions

Conceptualization, J.C.; writing—original draft preparation, J.C., W.Z. and B.L.; writing—review and editing, S.T., L.A., N.W., H.X., J.G., J.L. (Jing Liao) and J.L. (Jie Liang); visualization, D.L. and Y.F. All authors have read and agreed to the published version of the manuscript.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Data are contained within the article.

Acknowledgments

A great thanks to the crew members and scientists that participated in acquiring the geophysical data from the Guangzhou Marine Geological Survey.

Conflicts of Interest

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Footnotes

Disclaimer/Publisher’s Note: The statements, opinions and data contained in all publications are solely those of the individual author(s) and contributor(s) and not of MDPI and/or the editor(s). MDPI and/or the editor(s) disclaim responsibility for any injury to people or property resulting from any ideas, methods, instructions or products referred to in the content.

Figures

Figure 1. (a) Spatial distribution of the Makran accretionary wedge. The structural elements are compiled from [20]; (b) The location of the seismic profiles available for this study, the red segment represents the location of the double BSR studied.

Figure 2. Multi-channel seismic profile obtained after processing. The location of this profile is shown in Figure 1b. (a) Uninterpreted profile; (b) Black arrows indicate the presence of a double BSR.

Figure 3. (a) Seafloor bathymetry from multi-beam data and location of the seismic profile shown in (b); (b) seismic profile located along the Makran accretionary wedge showing the presence of double BSR. The black rectangle represents the location of the seismic profile shown in (c); (c) Interpreted seismic profile located along the Makran accretionary wedge.

Figure 4. Seismic profile showing the results of the inversion modeling of the paleo-seafloor water depth using geothermal gradients fitted to modern seafloor.

Figure 5. Seismic profile showing the results of the modeled BSR1.

Figure 6. (a) BSR-derived heat flow; (b) BSR-derived geothermal gradient.

Figure 7. Natural gas hydrate phase equilibrium curve.

References

1. Shipley, T.H.; Houston, M.H.; Buffler, R.T.; Jeanne-Shaub, F.; McMillen, K.J.; Ladd, J.W.; Lamar Worzel, J. Seismic evidence for widespread possible gas hydrate horizons on continental slopes and rises. AAPG Bull.; 1979; 63, pp. 2204-2213. [DOI: https://dx.doi.org/10.1306/2F91890A-16CE-11D7-8645000102C1865D]

2. Zander, T.; Haeckel, M.; Berndt, C.; Chi, W.C.; Klaucke, I.; Bialas, J.; Klaeschen, D.; Koch, S.; Atgın, O. On the origin of multiple BSRs in the Danube deep-sea fan, Black Sea. Earth Planet. Sci. Lett.; 2017; 462, pp. 15-25. [DOI: https://dx.doi.org/10.1016/j.epsl.2017.01.006]

3. Berndt, C.; Bünz, S.; Clayton, T.; Mienert, J.; Saunders, M. Seismic character of bottom simulating reflectors: Examples from the mid-Norwegian margin. Mar. Pet. Geol.; 2004; 21, pp. 723-733. [DOI: https://dx.doi.org/10.1016/j.marpetgeo.2004.02.003]

4. Han, S.S.; Bangs, N.L.; Hornbach, M.J.; Pecher, I.A.; Tobin, H.J.; Silver, E.A. The many double BSRs across the northern Hikurangi margin and their implications for subduction processes. Earth Planet. Sci. Lett.; 2021; 558, 116743. [DOI: https://dx.doi.org/10.1016/j.epsl.2021.116743]

5. Monteleone, V.; Minshull, T.A.; Marin-Moreno, H. Seismic characterisation of multiple BSRs in the Eastern Black Sea Basin. Mar. Pet. Geol.; 2024; 161, pp. 0264-8172. [DOI: https://dx.doi.org/10.1016/j.marpetgeo.2023.106604]

6. Fabre, M.; Riboulot, V.; Loncke, L.; Ker, S.; Ballas, G.; Thomas, Y.; Ion, G.; Sultan, N. Slow dynamics of hydrate systems revealed by a double BSR. Geophys. Res. Lett.; 2024; 51, e2023GL106284. [DOI: https://dx.doi.org/10.1029/2023GL106284]

7. Zhang, W.; Liang, J.Q.; Su, P.B.; Wang, L.F.; Lin, L.; Huang, W.; Wei, J.G.; Liang, J. Research progress and prospect of relationship between double bottom simulating reflector and the accumulation of gas hydrates. Geol. China; 2020; 47, pp. 29-42. [DOI: https://dx.doi.org/10.12029/gc20200103] (In Chinese)

8. Posewang, J.; Mienert, J. The enigma of double BSRs: Indicators for changes in the hydrate stability field?. Geo-Mar. Lett.; 1999; 19, pp. 157-163. [DOI: https://dx.doi.org/10.1007/s003670050103]

9. Li, L.; Liu, H.J.; Zhang, X.; Lei, X.H.; Sha, Z.B. BSRs, estimated heat flow, hydrate-related gas volume and their implications for methane seepage and gas hydrate in the Dongsha region, northern South China Sea. Mar. Pet. Geol.; 2015; 67, pp. 785-794. [DOI: https://dx.doi.org/10.1016/j.marpetgeo.2015.07.008]

10. Geletti, R.; Busetti, M. A double bottom simulating reflector in the western Ross Sea, Antarctica. J. Geophys. Res.; 2011; 116, B04101. [DOI: https://dx.doi.org/10.1029/2010JB007864]

11. Popescu, I.; De-Batist, M.; Lericolais, G.; Nouzé, H.; Poort, J.; Panin, N.; Versteeg, W.; Gillet, H. Multiple bottom-simulating reflections in the Black Sea: Potential proxies of past climate conditions. Mar. Geol.; 2006; 227, pp. 163-176. [DOI: https://dx.doi.org/10.1016/j.margeo.2005.12.006]

12. Bangs, N.L.B.; Musgrave, R.J.; Tréhu, A.M. Upward migrations in the southern Hydrate Ridge gas hydrate stability zone following postglacial warming, offshore Oregon. J. Geophys. Res.; 2005; 110, B03102. [DOI: https://dx.doi.org/10.1029/2004JB003293]

13. Andreassen, K.; Mienert, J.; Bryn, P.; Singh, S.C. A double gas-hydrate related bottom simulating reflector at the Norwegian continental margin. Ann. N. Y. Acad Sci.; 2000; 912, pp. 126-135. [DOI: https://dx.doi.org/10.1111/j.1749-6632.2000.tb06766.x]

14. Song, H.B.; Jiang, W.W.; Zhang, L. Geophysical researches on marine gas hydrates(Ⅳ): Double bottom simulating reflections. Prog. Geophys.; 2003; 3, pp. 497-502. (In Chinese)

15. Haacke, R.R.; Westbrook, G.K.; Hyndman, R.D. Gas hydrate, fluid flow and free gas: Formation of the bottom-simulating reflector. Earth Planet. Sci. Lett.; 2007; 261, pp. 407-420. [DOI: https://dx.doi.org/10.1016/j.epsl.2007.07.008]

16. Foucher, J.P.; Nouzé, H.; Henry, P. Observation and tentative interpretation of a double BSR on the Nankai slope. Mar. Geol.; 2002; 187, pp. 161-175. [DOI: https://dx.doi.org/10.1016/S0025-3227(02)00264-5]

17. Martin, V.; Henry, P.; Nouze, H.; Noble, M.; Ashi, J.; Pascal, G. Erosion and sedimentation as processes controlling the BSR-derived heat flow on the Eastern Nankai margin. Earth Planet. Sci. Lett.; 2004; 222, pp. 131-144. [DOI: https://dx.doi.org/10.1016/j.epsl.2004.02.020]

18. Musgrave, R.J.; Bangs, N.L.; Larrasoaña, J.C.; Gràcia, E.; Hollamby, J.A.; Vega, M.E. Rise of the base of the gas hydrate zone since the last glacial recorded by rock magnetism. Geology; 2006; 34, pp. 117-120. [DOI: https://dx.doi.org/10.1130/G22008.1]

19. Kroeger, K.F.; Crutchley, G.J.; Pecher, I.A. Slow response of the gas hydrate system to ridge erosion and sea-level rise: Insights from double BSRs on the southern Hikurangi margin (New Zealand). Earth Planet. Sci. Lett.; 2023; 624, 118433. [DOI: https://dx.doi.org/10.1016/j.epsl.2023.118433]

20. Zhang, Z.; Deng, X.G.; Yao, H.Q.; Yu, M.; Wang, H.F.; He, G.W.; Liu, B.; Wu, T.T.; Kahkashan, S.; Haider, S.W. et al. A preliminary study on geomorphological characteristics and genetic mechanism of pockmarks in the Makran accretionary prism, northern Arabian Sea. Geo-Mar. Lett.; 2021; 41, 27. [DOI: https://dx.doi.org/10.1007/s00367-021-00704-9]

21. Gong, J.M.; Liao, J.; Zhang, Y.X.; Liang, J.; Chen, J.W.; Khan, N.; Haider, S.W. Characteristics of major and trace elements in surface sediments of the Makran Accretionary Prism, Pakistan and their implications for natural gas hydrates. China Geol.; 2021; 4, pp. 299-310. [DOI: https://dx.doi.org/10.31035/cg2021020]

22. Wu, T.T.; Deng, X.G.; Yao, H.Q.; Liu, B.; Ma, J.F.; Haider, S.W.; Yu, Z.Z.; Wang, L.F.; Yu, M.; Lu, J.F. et al. Distribution and development of submarine mud volcanoes on the Makran Continental Margin, offshore Pakistan. J. Asian Earth Sci.; 2021; 207, 104653. [DOI: https://dx.doi.org/10.1016/j.jseaes.2020.104653]

23. Liu, B.; Syed, W.H.; Chen, J.X.; Deng, X.G.; Yang, L.; Azevedo, L.; Duan, M.L.; Wu, T.T.; Ma, J.F.; Li, K.L. Distinct BSRs and their implications for natural gas hydrate formation and distribution at the submarine Makran accretionary zone. J. Oceanol. Limnol.; 2021; 39, pp. 1871-1886. [DOI: https://dx.doi.org/10.1007/s00343-021-0293-9]

24. Smith, G.; McNeill, L.; Henstock, T.J.; Bull, J. The structure and fault activity of the Makran accretionary prism. J. Geophys. Res.; 2012; 117, pp. B07407-B07423. [DOI: https://dx.doi.org/10.1029/2012JB009312]

25. Ding, F.; Spiess, V.; Fekete, N.; Murton, B.; Brüning, M.; Bohrmann, G. Interaction between accretionary thrust faulting and slope sedimentation at the frontal Makran accretionary prism and its implications for hydrocarbon fluid seepage. J. Geophys. Res.; 2010; 115, B08106. [DOI: https://dx.doi.org/10.1029/2008JB006246]

26. Gong, J.M.; Liao, J.; Yin, W.H.; Zhang, L.; He, Y.J.; Sun, Z.L.; Yang, C.S.; Wang, J.Q.; Huang, W.; Meng, M. et al. Gas hydrate accumulation models of Makran accretionary wedge, northern Indian Ocean. Mar. Geol. Quat. Geol.; 2018; 38, pp. 148-155. [DOI: https://dx.doi.org/10.16562/j.cnki.0256-1492.2018.02.015] (In Chinese)

27. Harms, J.C.; Cappel, H.N.; Francis, D.C. The Makran coast of Pakistan: Its stratigraphy and hydrocarbon potential. Mar. Geol. Oceanogr. Arab. Sea Coast. Pak.; 1984; 3, 27.

28. DeMets, C.; Gordon, R.G.; Argus, D.F. Geologically current plate motions. Geophys. J. Int.; 2010; 181, pp. 1-80. [DOI: https://dx.doi.org/10.1111/j.1365-246X.2009.04491.x]

29. Kopp, C.; Fruehn, J.; Flueh, E.R.; Reichert, C.; Kukowski, N.; Bialas, J.; Klaeschen, D. Structure of the Makran subduction zone from wide-angle and reflection seismic data. Tectonophysics; 2000; 329, pp. 171-191. [DOI: https://dx.doi.org/10.1016/S0040-1951(00)00195-5]

30. Schlüter, H.U.; Prexl, A.; Gaedicke, C.; Roeser, H.; Reichert, C.; Meyer, H.; von Daniels, C. The Makran accretionary wedge: Sediment thicknesses and ages and the origin of mud volcanoes. Mar. Geol.; 2002; 185, pp. 219-232. [DOI: https://dx.doi.org/10.1016/S0025-3227(02)00192-5]

31. Platt, J.P.; Leggett, J.K.; Young, J.; Raza, H.; Alam, S. Large-scale sediment underplating in the Makran accretionary prism, southwest Pakistan. Geology; 1985; 13, pp. 507-511. [DOI: https://dx.doi.org/10.1130/0091-7613(1985)13<507:LSUITM>2.0.CO;2]

32. Grando, G.; McClay, K. Morphotectonics domains and structural styles in the Makran accretionary prism, offshore Iran. Sediment. Geol.; 2007; 196, pp. 157-179. [DOI: https://dx.doi.org/10.1016/j.sedgeo.2006.05.030]

33. Von Rad, U.; Berner, U.; Delisle, G.; Doose-Rolinski, H.; Fechner, N.; Linke, P.; LuÈckge, A.; Roeser, H.A.; Schmaljohann, R.; Wiedicke, M. Gas and fluid venting at the Makran accretionary wedge off Pakistan. Geo-Mar. Lett.; 2000; 20, pp. 10-19. [DOI: https://dx.doi.org/10.1007/s003670000033]

34. Pajang, S.; Cubas, N.; Letouzey, J.; Pourhiet, L.L.; Seyedali, S.; Fournier, M.; Agard, P.; Khatib, M.M.; Heyhat, M.; Mokhtari, M. Seismic hazard of the western Makran subduction zone: Insight from mechanical modelling and inferred frictional properties. Earth Planet. Sci. Lett.; 2021; 562, 116789. [DOI: https://dx.doi.org/10.1016/j.epsl.2021.116789]

35. Li, Q.; Yang, T.B.; Zhuang, C.; Deng, X.G.; Wang, H.F.; Yu, M. Geochemical characteristics of the sediments at site G16 of the Makran accretionary wedge, the northern Arabian Sea, and their implications for gas hydrates. Mar. Geol. Quat. Geol.; 2021; 41, pp. 161-169. [DOI: https://dx.doi.org/10.16562/j.cnki.0256-1492.2020091802] (In Chinese)

36. Liu, B.; Chen, J.X.; Haider, S.W.; Deng, X.G.; Yang, L.; Duan, M.L. New high-resolution 2D seismic imaging of fluid escape structures in the Makran subduction zone, Arabian Sea. China Geol.; 2020; 3, pp. 269-282. [DOI: https://dx.doi.org/10.31035/cg2020027]

37. Kaul, N.; Rosenberger, A.; Villinger, H. Comparison of measured and BSR-derived heat flow values, Makran accretionary prism, Pakistan. Mar. Geol.; 2000; 164, pp. 37-51. [DOI: https://dx.doi.org/10.1016/S0025-3227(99)00125-5]

38. Miles, P.R. Potential distribution of methane hydrate beneath the European continental margins. Geophys. Res. Lett.; 1995; 22, pp. 3179-3182. [DOI: https://dx.doi.org/10.1029/95GL03013]

39. Flueh, E.R.; Kukowski, N.; Reichert, C. Fahrtbericht SO123, Mamut. 62; GEOMAR: Kiel, Germany, 1997.

40. Villinger, H. Scientific Party SO124, Fahrtbericht SO124, 4.10.—16.10.1997; 111 Fachbereich Geowissenschaften, Universitat Bremen: Bremen, Germany, 1997.

41. Shankar, U.; Riedel, M. Seismic and heat flow constraints from the gas hydrate system in the Krishna–Godavari Basin, India. Mar. Geol.; 2010; 276, pp. 1-13. [DOI: https://dx.doi.org/10.1016/j.margeo.2010.06.006]

42. Grevemeyer, I.; Villinger, H. Gas hydrate stability and the assessment of heat flow through continental margins. Geophys. J. Int.; 2001; 145, pp. 647-660. [DOI: https://dx.doi.org/10.1046/j.0956-540x.2001.01404.x]

43. Yamano, M.; Uyeda, S.; Aoki, Y.; Shipley, T.H. Estimates of heat flow derived from gas hydrates. Geology; 1982; 10, pp. 339-343. [DOI: https://dx.doi.org/10.1130/0091-7613(1982)10<339:EOHFDF>2.0.CO;2]

44. Davis, E.E.; Hyndman, R.D.; Villinger, H. Rates of fluid expulsion across the Northern Cascadia Accretionary Prism: Constraints from new heat row and multichannel seismic reflection data. J. Geophys. Res.; 1990; 95, pp. 8869-8889. [DOI: https://dx.doi.org/10.1029/JB095iB06p08869]

45. KP, L.; Singh, D.N. Unveiling the Realm of Thermo-Hydro-Mechanical Characteristics of Gas Hydrate Bearing Sediments. J. Mar. Environ. Eng.; 2024; 11, pp. 277-302.

46. Zhu, C.; Jiao, X.R.; Cheng, S.; Li, Q.P.; Liu, K.H.; Shan, H.X.; Li, C.X.; Jia, Y.G. Visualising fluid migration due to hydrate dissociation: Implications for submarine slides. Environ. Geotech.; 2020; 10, pp. 218-226. [DOI: https://dx.doi.org/10.1680/jenge.19.00068]