GEOLOGICAL STRUCTURE

7.1 Mahabharat Thrust (MT)

The Mahabharat thrust lies in the south of the main central thrust and towards the north of the main boundary thrust. It separates Nuwakot complex and Kathmandu complex. It is produced due to pushing of the Indian plate on the Tibetan plate. The reverse fault in which the hanging wall is actually moved up relative to the footwall is known as thrust fault or simply thrusts. The fault angle is quite low in thrusts. These are very common occurrence in folded mountains and seem to have originated as a further step in the process of adjustment of rocks to the imposed stresses.

It the field we observed that the dipping of the strata suddenly changed form 50°/82°SE to 244°/44°SE. This gives the idea that there is a low angle reverse fault or thrust. Also rock sequence that we collected gives the idea that there is a reverse metamorphism in this area due to thrust. We also found gouge and braccia. Braccia are the angular unconsolidated material consisting of fragments of rocks, mostly black coloured and occur on either or both sides of fault plane, or may form a zone. Gouge is very fine grained, unconsolidated material consisting of pulverized rock. The region consisting of gouge is the most stressed zone. Due to intense stresses here Braccia are formed. So this is also another evidence of presence of fault. This gives the evidence that there is a thrust in this area.

 

7.2 Fault

Faults are well- defined cracks along which the rock- masses on either side have relative displacement along the fracture plane. The attitude of faults is defined in terms of their strike and dip. The strike and dip of a fault are measured in the same way as they are defined in terms of their strike and dip .The strike and dip of fault are measured in the same way as they are for bedding.

7.3 Fold

The bending of rock strata due to compressional forces acting tangentially or horizontally towards a common point or plane from opposite directions is known as fold. It results in the crumbling of strata, forming wavy undulations on the surface of earth. Folds are best displayed by stratified formations such as sedimentary or volcanic rocks or their metamorphic equivalents. But any layered or foliated rock, such as granite and gneiss, may show folds. Some folds are a few miles across. The width of other is to be measured in feet or inches or even fractions of an inch. Folds of continental proportions are hundreds of miles wide. The ultimate shape of fold depends upon a number of factors like the nature, magnitude and the direction of and duration for which these forces act upon the rocks and also the nature of the rocks being affected.

 Recognition of folds in the field:

1)      The easiest and simplest way is the eye inspection. If we found any geological structure according to the normal definition of a fold then it can be identified as fold only by direct observation.

2)      The repetition and absence of beds also indicates the presence of fold.

3)      It is usually observed that streams follows the axis portion of the anticline ridges and high lands and damped-structures occur along the axis of the synclines. So; sometimes the direction flow of stream or river shows the presence of a fold.

4)      When the types of folds are to be recognized on the basis of the age consideration of the beds, the top and bottom of beds are determined by taking into account the nature, and form of the features occurring on the beds itself. Accordingly some types of folds have been inferred.

In the field we observed a fold around 100 m upstream from the old bridge. This fold is asymmetrical plunging anticline fold. The attitude of right limb is 85°/49°NE and that of left limb is 84°/89°NW. Crack was observed in the crest of the fold. We also found another fold which was syncline fold but it was under the surface. It doesn’t have much information about that fold as it was beneath the surface.

Recognition of fault in the field

To recognize the faults in the field, a number of criteria are used. The faults may be directly seen in the field, particularly in artificial express such as river-cuttings, road Cuttings, etc. But is majority of cases, faults are recognized by stratigraphic and physiographic evidences as:

1. Discontinuity of structures

2. Repetition or omission of strata

3. Solidification and mineralization

4. Presence of features characteristic of fault-planes (slickenside, gouge, fault breccia)

In the field we found fault in various parts. In the location 1 across Trisuli at around 190 m upheal from the suspension bridge we observed a fault. It was inactive normal fault. The fault plane was intruded with schist. We observed a low angled reverse fault at 500m upstream of Malekhu River from the old broken bridge. The low angled reverse fault is called thrust. Infact the whole area runs a Mahabharat Thrust of Nepal. This was observed by the sudden change in dipping of the strata from 50°/82°SE to 244°/44°SE. At 100m upstream of the Malekhu River from the old broken bridge we found another fault. We saw braccia formed in between gouge. Braccia was formed due to high stress in between the gouge layers and the stress is imposed due to fault. Due to fault the hanging wall and foot wall move opposite each other and because of this there is friction causing intense stress of temperature and pressure ultimately forming braccia, which is black in colour.

7.3 Unconformity

An unconformity is defined as a surface of erosion or non depositing occurring within a  sequence of rocks. It indicates a gap or interval of time in the geological history of the area during which the normal process of deposition was interrupted. It is a structural feature in the sense that rock formations lying above and below it generally represent different conditions under which they have been formed.

Recognition of unconformity in the field

1.      Visual inspection of the angular relation between the sequences.

2.      Presence of Basal Conglomerate.

3.      Availability of the residual soil within the sequence of rocks.

4.      Exposure of the erosion surface.

5.      Contrasting behavior of the rocks from a particular surface.

6.      Variation or gap in the geological age of the rocks of the sequence

8.Collection of Engineering Data

Collection of engineering data from outcrops for certain purpose is called engineering geological data collection. Engineering data are those, which are purposed specific and quantitative. These data collected from outcrops are attitude of rock (i.e. dip and strike).

Objectives:-

The objectives of rock mass classifications are to:

ü  Identify the most significant parameters influencing the behaviour of a rock mass.

ü  Divide a particular rock mass formulation into groups of similar behaviour – rock mass classes of varying quality.

ü  Provide a basis of understanding the characteristics of each rock mass class

ü  Relate the experience of rock conditions at one site to the conditions and experience encountered at others

ü  Derive quantitative data and guidelines for engineering design

ü  Provide common basis for communication between engineers and geologists

Benefits:-

The main benefits of rock mass classifications:

ü  Improving the quality of site investigations by calling for the minimum input data as classification parameters.

ü  Providing quantitative information for design purposes.

ü  Enabling better engineering judgement and more effective communication on a project.