1. The Shelf Zone:
The stable shelf zone occupies the northwestern part of Bangladesh and is composed of the Pre-Cambrian Indian platform. The area is characterized by limited to moderate thickness of sedimentary rocks above a Precambrian igneous and metamorphic basement. This unit is geologically stable that has not been affected by fold movement with some fault generated horst and grabens occur within the Precambrian basement.
The basement of the Indian platform slopes to the northwest and southeast from a central ridge, which has the shallowest occurrence (approx. 140 m) of Precambrian rocks in Bangladesh [Uddin and Lundberg, 2004]. Taken into account the basement rock depth and its nature of slope the stable shelf zone has been segmented into three parts: the Rangpur saddle, the Dinajpur slope and the Bogra slope [Bakhtine, 1966; Guha, 1978].
Rangpur Saddle represents Indian Platform and connects the Indian Shield with the shilling massif and the Mikir Hills. In Rangpur Saddle the basement is the most uplifted and is covered with thin sedimentary deposits.
The Dinajpur slope is the northern slope part of Rangpur saddle and it inhabits the north-west portion of Rangpur-Dinajpur districts and it slopes down gently near the Himalayan foredeep region.
The Northern Slope of Rangpur Saddle also known as Dinajpur Slope, inhabits the north
western part of Rangpur-Dinajpur districts and gently slopes towards the Himalayan foredeep.
Bogra Slope represents the southern slope of Rangpur Saddle which is in general plunging gently towards southeast to the Hinge Zone. This zone marks the transition between the Rangpur Saddle and the Central deep basin from depositional as well as structural point of view.
2. The Hinge Zone:
Calcutta-Mymensingh Gravity High zone reflects a tectonic element known as ‘Hinge zone’ [Sengupta, 1966] and it separates the two structural provinces in the Bengal Basin. It is about 25 km long narrow strip of flexure zone and it separates the Bengal foredeep zone and the stable shelf zone.
The NE-SW oriented hinge zone (Fig. 4) goes between the Naga-Disang-Haflong thrust zone of north-eastern Assam and the west coast of the Bay of Bengal in the south [Uddin and Lundberg, 2004].This zone is thought of as a zone of deep-seated normal faults in the basement complex rocks which divides the continental crust of Indian platform from the Bengal foredeep zone.
3. The Bengal Foredeep zone:
The Bengal Foredeep zone occupies a vast area of Bangladesh between the stable shelf zone in
the west and the Arakan-Yoma hill range in the east. It is characterized by the occurrence of deep
subsided basement complex accommodating huge thickness of sediments which started to
develop during the intense upliftment of the Arakan-Yoma anticlinorium in the Oligocene time.
Based on the geotectonic behaviour, the Bengal foredep has been subdivided into two major
parts: (a) Deep basin area and (b) The folded belt area. [Hossain, 1989]
a. Deep Basin Area:
The deep basin area of the foredeep zone is occupied by a massive thickness of sediments with two prominent depressions in this region, the Surma basin (or Sylhet trough) and the Patuakhali trough (or Hatiya trough). These two depressions are separated by a relatively uplifted zone, known as the Madhupur high. The deep basin occupies two other tectonic elements: Faridpur trough and Barisal-Chandpur high.
Faridpur Trough is situated adjacentent to Hinge Zone and is characterized by a general gravity low with development of Neogene sequence [Hossain, 1989]. Barisal-Chandpur High is located between the Faridpur and Hatiya trough of the foredeep region and this zone is described by few authors that it is attributed to a gravity and magnetic anomaly due to the presence of a magmatic body at depth.
The Patuakhali trough (or the Hatiya trough) [Bakhtine, 1966; Guha, 1978] is the
deepest trough in the foredeep zone. This zone is formed by the relative subsidence of the Precambrian basement rocks along deep-seated normal faults which then accumulates large volume of clastic sediments.
The Surma basin occupies the north-eastern part of Bangladesh and it is also known as the Sylhet
trough [Bakhtine, 1966; Guha, 1978]. This basin area is very close to the Dauki fault system in the north. Shillong Massif forms the northern boundary of Surma basin while the Dauki Fault system separates the basin from the massif part. A relatively uplifted Madhupur high separates the Surma basin from the adjacent Patuakhali trough. This Madhupur high area is believed to be uplifted in recent times as evidenced by the upliftment of two isolated uplifted Pleistocene blocks, Madhupur and Lalmai hills.
Folded belt Area:
Folded Belt embodies the most noticeable tectonic element of Bengal Foredeep with a large
number of narrow, elongated N-S trending folds in the eastern part of Bangladesh (Sylhet and
Chittagong Divisions), Tripura, part of Assam, Mizoram and Myanmar territory, parallel to the
Arakan Yoma Folded System (Fig. 4).
The folded belt area is represented by gentle box like north-south trending folds featuring numerous faults. These folds are thought to be the western extension of the Arakan-Yoma anticlinorium in the east. [Hossain, 1989]
Structural complexity of the folded flank increases from west to east and merges into the Indo–
Burman ranges farther east. And it is demonstrating the influence of the tectonic forces created by the overriding Burmese plate on the Tertiary sediments of the folded belt.
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