19 03 2020 V 2 A

Welcome back. So, in previous lectures we discussed about fluvial landforms like terraces and alluvial fan, and then we also discussed about the false curves, back tilted terraces or we can say alluvial fan surfaces. Now, these are the most common features which you will come across when we are looking for the active tectonic deformation or the manifestation of the tectonic deformation on the surface. Now, apart from this we also looked at some details of different type of channel patterns, mainly straight, braided and meander. And I will try to give one lecture if possible on the tectonic geomorphology part, where we can utilize the importance of the different channel patterns straight, braided and meander, and how they change over from one to another depending on the change in the base level or change in the slope gradient or we can say the river profile. Other than that we have another one is the basin that is drainage basin, usually we take into consideration and different type of drainage pattern. So, basin drainage basin is consisting is will be consist of the main stream and the tributaries, and within the main basin you will have the sub basins also. And the drainage divide usually is been marked, where we have an slightly elevated portion which separates the flow direction of two drainages on either side, and those drainages are contributing to the different drainage basins. So, this is what has been shown here, so this portion is your drainage divide, and you have the drains and streams flowing in different directions to respective drainage basins. And this also shows the major drainage basin is marked by this yellow line, and then red shows the sub basins or the sub basins which are related to the tributaries. Now as we have discussed about the terraces and all that, so this is what we came across on a beach in Andaman. So, water was coming in and there is a beach area where the tidal water comes and goes back, and it has an slope break here. So, how the landforms are been formed, this you can visualize on a miniature scale. So, you have a meander here, so this is an erosional side, and then you have the depositional side here, and at the same time there is an incision which is going on which is resulted into the formation of terraces. So, this is terrace 1 here, terrace 2 here, and then finally the top one here. So, at least we have T 0, T 1 and T 2 here on the depositional side. At the same time what we see is that when there is an slope break or the break in the slope, you have the incision is more here, and the channels are getting distributed, and we have in between marked by the braid bars. So, these are all channel braid bars. So, this is a close up of that. So, you can see the terraces here, and the incised channel in this location here. Now further looking to the drainage, one can classify the different type of drainage. So, one is your consequent drainage and subsequent drainage. We see in the consequent drainages mostly the consequent drainage will be they follow this the slope developed initially, because of the sudden tectonic uplift. So, the streams which will develop on the surface will follow the slope, and in terms of the subsequent drainage, it will not exactly follow this slope, because the slope here is if you take the fold here, then the slope is in this direction as well as the slope is in this direction here, but the stream is incising across the slope. So, the stream that develops later curve the softer rocks and flow almost right angle to the this original slope. So, the main difference between the consequent stream and the subsequent stream is that they follow the slope, and development of the slope may be because of the sudden uplift, whereas here there is a subsequent stream which follows almost at right angle to the original slope. Then we have also the term called as antecedent streams. Now antecedent streams are those which will cross across or flow across the uplifted part. So, they are capable enough to erode the uplifted area. So, for example, if we are having the folds which are in a developing stage due to ongoing deformation, then the stream which is flowing in that area will be able to cross the or incise and flow through the uplifted portion. Those streams are termed as antecedent streams. And then another one is your superposed streams which are the streams where the underlying rocks with different lithologies have been seen, and they will flow in direction eroding the flat lying surfaces or flat lying strata as well as inclined strata. So, these are been termed as the superposed streams. Another important part which usually we come across because the drainages usually is been seen developed like you have an slope, then the streams will or the drainage will keep developing in the headward direction. So, this is the pattern where we see the development of the basin will be because stream will keep adding from the headward side. So, the basin will extend towards the headward side. So, we usually say that the basin develop towards headward side. So, if you are having for example, the basin which is adjacent to this, then sometime this erosion because it is eroding it in the headward direction, then it may preach the other streams or the basin and will capture the other basin and the water flows in that basin. So, basically what we see is that through rapid headward erosion, a stream may preach the divide. So, if you have a divide here for example, and then you are having another divide. So, another important aspect what usually we come across in the landscape, in the rural landscape is the what we call stream piracy. Now, the drainage and then if you recall the first two slides which we were talking about the divide here and the streams are flowing into one basin and here the streams are flowing in another basin and meeting the main streams. So, this becomes your drainage divide. Now, the drainage usually evolve towards headward side. So, due to headward erosion, the basin is that is your drainage basin will evolve. So, you have one stream here, then it will second if you see then it will keep on adding here like this and then further it keeps going up. So, you have the addition of the drainage or smaller by smaller stream and you keep developing that. Now, suppose you are having a slope that you will understand in the next slide. Suppose you are having a slope this I am putting profile here. So, if you see this one here like this. So, you have the streams flowing along the slope. So, you have the streams which are going like that and then here the streams are flowing in this direction because the slope is in this direction here, the slope is in this direction here and this will mark your drainage. Take into consideration the headward erosion and the drainage basin evolution, then what will happen that this will keep eroding the side and the time will come this will connect and that what we call is the stream piracy or even we call this as a stream capturing. So, the whole drainage basin will be captured by either of one basin and then we will have the flow getting into the one single basin. So, this usually happen where you have the erosion is going on and erosion is high in the region. So, such features are also important in understanding that how the drainage basin evolved and whether there was any sudden change in the overall flow patterns of the respective drainage basin. Through rapid headward erosion, a stream may breach a divide intersecting another channel and capturing its flow or stream capture is the interception and diversion of one stream by another stream that is extending its basin by erosion in the headward direction. So, it keeps eroding towards the headward direction and finally, it will reach to that extent that it will capture the stream. So, diversion of the flow in new channel may leave a wind gap along the ridge. So, this one can look at in terms of the wind gaps if they are available. So, suppose you are having an uplift going on here. So, this is the basically the drainage divide over this part and as we were talking about that the drainage will evolve in the headward direction. So, the time will come it will breach and it will try to capture the whole drainage from the side. Now, other than that what we also look at is the drainage pattern and the channel morphology. So, I will talk about a few example from this portion that is again in the northwest Himalaya in Punjab region and this is Hoshiarpur and the feature which you see here is the Janauri antic line and you can easily mark the drainage divide which goes like that. So, this is the top ridge line and the slope in different in opposite direction. So, this drainage are flowing like that and the streams are flowing from here in this direction into the Indo-Gangetic plane and this is the major streams. So, we will see what drainage has to tell us about the subsurface topography. So, what we found was that of course, there is an very sharp distinct geomorphic boundary here which marks the antic line and the Indo-Gangetic plane and as we were looking in the previous slide that we have the trenches flowing in this direction and this direction here and this is the ridge line, but in this portion what we found is in the area which was devoid of drainage and which is when we moved in field it was as good as flat area which you see when the like in similar to the Indo-Gangetic plane. So, our interpretation was that this whole area was uplifted and a very quickly in a very short time which did not allow the stream to flow through or the deeper major river because shuttledge flowed through this area here and this was the remnant of the paleo shuttledge which was connected through this and flowing through this area here. So, this is our interpretation, but because of the rapid uplift of this area it did not allow or shuttledge was not able to cope up the uplift hence it took the return or deflection and it started flowing through a gap which was available and this area is now left out as an flat uplifted surface. So, because of the devoid of drainage here we were able to pick up that this is a flat area and that what our interpretation supported when we looked at or identified the paleo channel here. Now, similarly if we look at in this portion this portion we were able to see that there is a very distinct drainage which is flowing in all direction here hence we interpreted this as a warp area. So, if you are having suppose a slope like this what do you find is that you have the streams flowing in either directions. So, this becomes a sort of an the warped area or deformed area. So, this also it helped in identifying that what are the features we see on the surface and what the drainage has to tell us about the subsurface features. Another important part which we were able to pick up from the drainage itself that we found some wind gaps and this is from central Himalaya this portion is this city is Ramnagar and Nainital some sit somewhere over here. So, what we found was that this drainage which is right now flowing from this portion earlier flowed through this one because this marks the wind gap here and then later on it shifted its course here then further here and then finally, it is flowing here. But later in future if there is a tectonic moment along this fault line then it may go and merge the main Kosi river this is this stream is a Dabka river and this is Kosi river. So, this also we can one can interpret and identify that what happened in the past and reconstruct the tectonic geomorphic history of any region. So, this is the close up of that which you can see the stream is has left out the remnants of the paleo channel of what we have marked as an wind gap. So, we marked as an wind gap 1, wind gap 2, wind gap 3 and then finally, it is flowing here. And similarly in one of our research article we have also tried to highlight that there was another stream which used to flow and has left out the wind gaps in this area also and that was Bo river. So, again coming to the north west Himalayan part what we did was we quickly extracted this the drainage using SRTM data and the dark blue lines are showing the higher order streams or the main drain drainage or the main trunk stream and the smaller are all tributaries. So, as we were talking about that we can easily mark the drainage divide if we carefully take into account the in which direction the streams are flowing. So, this stream for example, is flowing and joining this main stream here and these are the all streams which are flowing in this direction. So, this portion becomes here and then this portion is here drainage divide, whereas this portion is not occupied much by the drainage and this is your flat area. So, one can also take into account the drainage pattern and try to identify the what type of structure is sitting subsurface. So, this clearly marks that this portion is in folded area. So, if you take the cross section here then you will find that this is something like this. So, you have the slope in this direction you have a slope in this direction. So, you have different type of drainages and drainage pattern which if you take into consideration like dendritic pattern, radial pattern, trellis and rectangular. So, how and what is the importance of this and can we interpret the subsurface lithology or the structure based on this. Now, a rectangular drainage again will be seen in the areas which are having very typical fractures because of ongoing deformation and they will flow almost at a right angle. And this will be seen in the area where you are having homogeneous material, whereas the radial drainage will be seen in the area where you are having either volcanic cone or any domal structure, whereas trellis pattern you will be seeing where this main stream and the drain the tributaries will be joining at almost right angle. And this type of drainages are seen in the areas where you are having softer rocks and the resistive rocks. So, mostly in the folded terrain you will be able to come across and find the trellis pattern. Now, as I was talking about that we did some studies in this area and try to look at that what exactly the drainage is telling us about the land form. So, this is the map of this portion here from this area, this is the area which is been covered in this map. So, we can see here this one and this is the exit of shutlage which is coming here, this is the portion and up to this one here. Now, what we found was this region was showing us the streams which are flowing in all directions and we were not having idea that why it is showing. So, because this usually should have been a flat area because this is an area which is similar to the Pinjore dune. So, Pinjore dune is a flat region almost flat valley between the two mountain ranges that is what we call intermountain valley and dune is the term which is locally used for intermountain valleys like Dehradun, Pinjore dune and Sondun. So, we expected that this area will show flat surfaces, but this portion which has been shown here showed us that this is not exactly flat, but we see a sort of an radial drainage here. And then we also picked up some anomalies of when the channel pattern over here which I will show in the next slide. So, we looked at the lithology of course, it was and old deposits of tertiary age and then when we looked at the high resolution photographs of particularly this area, this portion and then what we see here is that the drainage here is showing a typical radial pattern. So, definitely this area should be showing a warped surface in the field. And then second one we will come to that. So, first this was the warped area. So, we marked a fault here that was responsible for deforming the surface. So, the deformation is like your sigma 1 will be like that and this area is getting deformed like this. So, this is the deformation. So, and this line which is also marks the present day road. So, when we looked at this in 3D, we were able to identify a very distinct change in elevation. So, this side is up and this side is down and this portion we marked as a fault trace. Now, when we looked at this one, what we found was that in the portion which is uplifted, the drainages are showing very tight meander, whereas it comes in the down thrown side or the foot wall side, then the drainage becomes almost straight. So, this is the portion here, this portion in the next slide I will show and the close up of this if you look at in this portion, then you will be able to make out this clear demarcation here. So, this is the fault line which goes here and the drainages which are in the up thrown side is tightly meandering, whereas this portion is almost straight. So, this again is showing very tight meanders, whereas this portion is almost straight. And then we looked at the warped area in field. So, we were able to pick up very clear and distinct topography which was showing the warping here at Dana. Now, drainage pattern usually tells us about the underlying geology. So, quickly we can look at this one. So, the dendritic pattern which is a very irregular branching tree like features and common in massive rocks or we can say that where we are having mostly the homogeneous geology. Parallel drainage again are very much similar to what we are talking about the consequent drainage, they will develop on the parallel to the slope. Then we have the radial patterns which indicate that there is an elevated portion in the center, and that has allowed the drainages to flow in either directions. And then we have rectangular drainages commonly seen in an area where you have fracture or jointed rocks, and they are almost at a right angle along its bends. Then we have trellis patterns. So, these are common in area where outcropping edges of the folded sedimentary rocks both weak and strong are seen, and mostly the tributaries will be at almost like an right angle to the mainstream. And then annular patterns are nearly circular slightly similar to your radial pattern and all that. Now, this all like understanding of drainage network or drainage pattern or channel pattern you can use to interpret the subsurface area. And if there is sudden change in the drainage pattern or there is sudden change in the channel pattern, one can interpret in terms of tectonic deformation or it could be related to the climatic change also. So, one has to be careful while talking about this. So, I will stop here, and in the next lecture we will continue on coastal landforms. And before that I will try to quickly talk about the tectonic geomorphology part if time permits. Thank you so much.