1st report of Seismites from the Upper portion of Spituk Section,Leh Ladakh - Ritesh Arya

1st report of Siesmites from the Upper portion of Spituk Section

These intricate convolute structures were discovered by me while doing exploration for groundwater for the Indian army in the Spituk in 2007. At that time my emphasis was on groundwater exploration and its development to provide potable drinking water to the Indian army. As per published work, the section of glacial lake which is exposed on the road is approximately 24meters but on drilling we found the thickness to be approximately 270feet below the ground level. During lockdown I was seeing my old pictures and found these photos. I studied these and perusal of literature showed me that these convolute structures were never reported from these sections and the upper portion of Spituk section was generally considered to be deposited in calm lacustrine conditions showing thinly laminated massive clay deposits. The presence of these structures showed that these > 1 meter thick convolutes are a result of seismic activity in the vicinity along the Indus Suture Zone. 

PALEO EARTHQUAKE SIGNATURES (Seismite) in the paleolake sediments of Leh Ladakh.

Spituk paleoglaciated lake sediments under study are deposited on the Ladakh Batholith of granite origin. The river Indus flows in close proximity along the Indus Suture Zone(ISZ). ISZ represents the collision between the Indian Plate and Tibetan Plate in the geological past. The glacial lake sediments under study are remnants of the pale glacial lake formed after the Khardungla glacier had receded considerably, paying the way for the formation of these lakes in front of the receding glaciers which we see in modern times. 

Siesmites in Spituk sediments are reported from the lower portion of the 9 level with exposed thickness of  13 meters dated to be approximately 10000 to 6000 meters.  But the upper portion is generally considered to be devoid of these intricate sedimentary structures ( Phartiya et al2013) showing calm conditions of deposition. 

The upper portion of the Spituk section was studied   and these convolute structures more than  > 1 meter thick were discovered. These convolute structures were sandwiched between thinly laminated massive clay deposits at the bottom and silty clay layers at the top. The most probable explanation for the formation of these structures is the result of massive seismic activity with its epicenter in the vicinity along the Indus Suture Zone. The massive seismic activity reported for the 1st time in the upper portion of the glacial lake suggests that the neotectonic activity was very much active during the upper portion of the lake which was initially considered to be deposited in calm conditions in the absence of these structures. 

  

But their presence in the near top position of the lake shows that these structures could not have been formed of any other mechanism other than the earthquake or seismic activity induced due to the movement of the two plates Indian and Tibetan along the Indus Suture Zone.   These convolutes are the seismographs which tell the story of an earthquake that happened around 1000 years ago and are actually recorded on the sediments of the lake due to the seismic activity. They are signatures of paleoseismic activity or earthquakes that occurred in the past.  Seeing these massive seismic impressions in the section  I was convinced why the Ladakh region has been kept in a high seismic zone.  

MECHANISM - When the earthquake occurs along the suture zone the seismic waves travel in all directions. The earthquake shakes the water in the paleolake in an oscillating manner at very high speed being very near to the epicenter. As a result of which the soft sediments of silt sand and clay result in quick side to side motion in shallow waters of the lake. This causes  the top layers of the fresh sediments to fold in this very unusual manner. These bent layers are intricately folded and twisted recording the entire seismograph of the earthquake naturally on the sidewalls of the paleolake sediments. The granite wall on the back provided the stability and protected these structures till date. These beautifully artistically folded layers of the sediments gradually solidified and were preserved as Siesmites which were are seeing today.  Later when the earthquake subsided new horizontal sediments were laid on top of them. But they also show some folds which could be a ramification of the Siesmites formed in the lower layers

INTERPRETATION - The size of the intricate folds depends on the intensity of the earthquake. In the present case, more than 3 feet or approximately 1meter of folding is recorded and beautifully preserved in Paleolake deposits of Ladakh. The size of the sediments deformed more than a meter due to the seismic activity shows that the high intensity of the earthquake had occurred along the Indus suture zone at the time of the deposition of the sediments in the paleolake deposits. These lake deposits were formed due to the melting of the Khardungla glacier and as the glacier receded the paleolake was formed and these seismographs were recorded. 

Epicenter of Seismites - . The formation of convolute beddings (intricately folded, twisted, or coiled) is the result of earthquake activity within the paleolake sediments. The epicenter of the seismic activity/shaking is along the Indus Suture Zone (ISZ). ISZ  represents the culmination of suturing between the Indian plate with the Tibetan plate leading to evolution and upliftment of Himalayas and the position where present-day Indus River flows. 

Age the Spituk pale glacial deposits are dated to be 10000 to 1000 years ago Since I am finding these seismites from the upper portion of the deposits so the age is approximately around 1000 years from the present 

CONCLUSION Siesmites are deformed sedimentary beds and structures which have been formed  by the activity of earth-shaking/seismic /earthquakes 

This is the 1st record of paleoseismic activity in the Leh Ladakh region and can be related to neotectonic movement in the Himalayan orogeny. 

Citation: Phartiyal B, Sharma A, Kothrari G C. Damming of River Indus during Late Quaternary in Ladakh Region of Trans-Himalaya, NW India: Implica-

tions to Lake formation-climate and tectonics. Chin Sci Bull (Chin Ver), 2013, 58(Suppl.I): 142–155, doi: 10.1360/tb-2013-suppl008