Neo tectonic activity around dhaka city.

Neotectonics concerns the study of horizontal and vertical crustal movements that have occurred in the geologically recent past and which may be ongoing today. Though most crustal movements arise directly or indirectly from global plate motions (i.e., tectonic deformation), neotectonic studies make no presumption about the mechanisms driving deformation. Consequently, ‘movements’ is a vague catch-all term that encompasses a myriad of competing deformation processes, such as the gradual pervasive creep of tectonic plates, discrete (seismic) displacements on individual faults and folds, and distributed tilting and warping through isostatic readjustment or volcanic upheaval. The phrase ‘geologically recent past’ is also appropriately vague. Early attempts to define the discipline by arbitrary time windows (e.g., Late Cenozoic, Neogene, or Quaternary) have given ground to a more flexible notion that envisages neotectonism starting at different times in different regions. The onset of the neotectonic period, or the ‘current tectonic regime’, depends on when the contemporary stress field of a region was first imposed. For instance, the current tectonic regime began in the Middle Quaternary (∼700 000 years ago) in the Apennines of central Italy, and even more recently (<500 000 years ago) in California; in contrast, in eastern North America, the present-day stress regime has been in existence for at least the past 15 million years.

Typically then, neotectonic movements have been in operation in most regions for the past few million years or so. Over such prolonged intervals, neotectonic actions are revealed by the stratigraphic build-up of sediments in inland and marine basins, the burial or exhumation histories of rocks, and the geomorphological development of landscapes. Geological studies of palaeobotany and palaeoclimate, numerical models of landscape evolution, and techniques such as fission-track analysis and cosmogenic dating are among the disparate tools unravelling this long-term tectonic activity. Over periods of many tens of to several hundreds of thousands of years, the actions of individual tectonic structures (faults and folds) can be determined, unmasked by their deformation of geomorphic markers, such as marine and fluvial terraces, and tracked with reference to the Late Pleistocene glacial–eustatic time-frame. The apparently smooth deformation rates discerned over intermediate time-scales are revealed to be episodic and irregular when faults and folds are examined over Holocene (10 000 years) time-scales. Over millennial time-scales, secular variations in the activity of tectonic structures can be gleaned from a diverse set of palaeoseismological approaches, from interpreting the stratigraphy of beds that have been affected by faulting, to detecting disturbances in the growth record of trees or coral atolls.

1. MD. MAHFUZ ALAM B.S. 4 th Year A systematic review of Submarine Canyon

2. Submarine canyons are a conduit that transfer sediment to a submarine fan and important geomorphic feature for evolution for continental margin (Canals et al., 2013 as cited by Micallef et al., 2014) WHAT IS A SUBMARINE CANYON Figure-1: Submarine Canyons Worldwide (Ryan et al., 2009) The pioneer of submarine canyons studies is laid by (Dana, 1863). Daly R. A. (1936) was the first researcher showed submarine canyons are evolved by turbidity currents ( Baztan et al., 2005) METHODOLOGY AND TECHNIQUES 2 D and 3D seismic data ( jobe et al., 2011), Multibeam echo-sounder data ( taviani et al., 2023) Remotely operated vehicles ( rovs ) ( tyler et al., 2009).

3. S ubmarine canyons are created from multiple origins like 1) Tectonics and 2) Eustatic Sea Level Change. ORIGIN OF SUBMARINE CANYONS Tectonics Eustatic Sea Level Change Active margins compose a significant number of canyons right now (Harris and Whiteway, 2011 T he initial phase of the (CCS) Central Canyon System can be formed and evolved by tectonic transformation in the Qiongdongnan Basin situated in northern South China Sea. Another example is the Rose Canyon Fault Zone tectonic disturbances in association with La Jolla and Scripps submarine canyons (Le Dantec et al., 2010). In seismic profile narrow, nearly linear course, V-shaped morphology and steep canyon walls indicates tectonic influence. In November 2016, Kaikōura earthquake magnitude 7.8 hit New Zealand which created turbidity current and canyon flushing in surrounding canyons systems. According to (Mulder et al., 2003) submarine canyons would form by large riverine erosion during relative sea-level fall in low stands S ea level change results greater subaerial exposure originated by the Mid-Pleistocene transition, which resulted in flushing of turbidity current onto the shelf edge. It dissected the shelf edges and was responsible for generating an incised canyon (Allin et al., 2018) Figure-4: Sea level and submarine canyon evolution ( Twichell & Roberts, 1982)

4. Type 1: Erosional in terms of origin Responsible for large submarine fans and aprons High coarse-grained sediment routing system Type 2: Is not associated with any kind of fluvial system Smooth u-shaped morphologies and flat bottoms It lacks deposition coarse-grained sediment Source: ( Jobe et al., 2011) Figure-2: “Bipartite Classification Scheme” 0F submarine Canyon classification by ( Jobe et al., 2011) . TYPES OF SUBMARINE CANYONS

5. Each canyon consists of 3 different parts The highest canyon, which includes of the canyon’s head and the area has higher gradients. The central canyon, where the canyon is significantly eroded and the area has milder gradient. T he lower canyon, this part of the canyons become wider and the area comprises lowest gradients (de almeida et al., 2015) . Figure-7: Otago Submarine Canyon Complex (OSCC) in New Zealand (Kumar et al., 2021) . SUBMARINE CANYON MORPHOLOGY Thalweg is deepest part of a submarine canyon system (de Almeida et al., 2015) According to (Shepard, 1954) Submarine canyons are v- Shaped. The canyon heads areas function as catchment point and deposition of sediments from shelf and river sediments ( Mullenbach et al., 2004) The submarine canyon mainly contains two discreate geomorphic features, Submarine fan and continental slope apron ( Algan et al., 2002)

6. Another stratigraphic interpretation was done in La Jolla canyon in USA, the author identified 3 stratigraphic sequences overlying in the seismic profile A) Estuarine Deposits, B) A healing-phase Wedge, and C) Massive homogeneous sands (Le Dantec et al., 2010). Figure-8: 9 facies were identified by (Gong et al., 2011) CST at Qiongdongnan Basin in China. STRATIGRAPHIC SETTINGS IN SUBMARINE CANYON Submarine canyon exhibits very complex stratigraphy. These canyons have undergone many erosional and depositional cycle during their formation ( Twichell & Roberts, n.d.) According to (Bouma, 2001) marine fans can be classified into fine grained fans and coarse grained fans.

7. Gas Hydrates: Gas hydrates deposits were detected in the Zhujiang River Canyon system and Penghu Canyon system in China (Wang et al., 2016 as cited by Wang et al., 2021) . Figure-10: The cross-sectional seismic profile across the Huaguang Channel (Wang et al., 2021). HYDROCARBON PROSPECT Pockmarks: The Waitaki Submarine Canyon in New Zealand, where pockmarks, intra-slope lobe complexes, and channel-fill deposits were found. These features are the signs of shallow hydrocarbon reservoirs nearby because pockmarks are associated with the migration of subsurface fluids (Kumar et al., 2021) A ncient buried turbidite channels , which may usually contain hydrocarbons. B iogenic petroleum system may be found where shale units act as source rocks aged middle miocene to early pliocene and turbidite sandstones acts as reservoirs aged late pliocene ( F oschi et al., 2023)

8. Two main primary channels confluence further downslope to form a channel in northeastern portion of SONG ( Kottke et al., 2003). Figure-12: Swatch of no ground ( Curray et al., 2002) SWATCH OF NO GROUND: A SUBMARINE CANYON IN BAY OF BENGAL According to LaFond (1958) suggested that SONG canyon might have developed for three reasons, A fault that possess direct linking to faults in Assam, Mass-wasting processes and the turbidity currents may be eroded the continental margin and Erosion caused by rivers during Last Glacial Maximum ( Kottke et al., 2003). Head areas of the SONG lies depth at 218270 N, 898410 E Water depth about 38 meters. Ganges and the Brahmaputra provides sediment influx. Accoording to ( Curray et al., 2002), the submarine canyon extends towards south about 160 km as straight and long depression to a depth of 1406 meters and the mean gradient is about 8.2 m/km. C hannel levee system in the deep-sea fan, i nterbedded fine to coarse-grained marine influenced fluvial deposits this also includes silty to clayey layers which are interpreted they originated due to tidal sediment flux in and graded silty sand layers are believed to be formed due to cyclonic events (Michels et al., 2003)

9. I mportant information about the history of our planet and its complex geological evolution. To obtain a concise and clear understanding of submarine canyon systems, more and more integrated interdisciplinary (sedimentology, Geology, Oceanography, ecology, and Biology) research efforts are very important for combining unknown information on submarine canyon. CONCLUSION AND SUMMERY RECOMMENDATIONS Re-excavation and superposition of canyon systems over one another is usually common. The submarine fan and buried channels within the canyon are the home potential hydrocarbon accumulation. Submarine canyons are mainly V or U shaped. Important parameter to be considered in construction of undersea naval submarine navigation, submarine cables and pipelines for transporting oil and gas. More study is required focused over sedimentological factors to evaluate hydrocarbon possibility The energy crisis at present in Bangladesh, can be solved by exploring these reservoirs

10. Thank You