Dear Amar, couldn´t these structures be perhaps related to shear bands?

Dear Amar Agarwal,

are you sure if the presented structures are really "Boudins", which were  formed by extension, where a rigid tabular bodies  (Quartzite), is stretched and deformed amidst less competent, ductile surroundings (Phyllite)??

Could it be a relict of sedimentary crossbedded stratification in this obviously (?) low grade metamorphic rocks???

best regards

Andreas

Hi Sebastian, 

I am not sure since I have very little experience! which mechanism do you propose? 

It is quite difficult just with the foto, as I don´t know the regional structure, etc. I thought about shear bands as there seems to be two surfaces (the stratification and the oblique surface dipping to the left). You should examine whether this is so or is just an effect from the picture. However, one possibility would be to consider S-C´ structures. In that case, you could consider shearing parallel to S (parallel to stratification) and C´ planes (oblique, dipping to the left). Another posibilities may be plausible, just have a look on the different type of shear bands (ej, in Microtectonics, from Passchier and Trouw, or you can find many examples also online) and on your structures. 

Dear Amar,

The top image shows clearly in the interstices or pressure shadows of the competent quartzites Boudins a secondary quartz filling. Therefore, I would call this structure as a real tensional Boudinage. However, the old primary Boudinage structure was later overprinted by intense foliation and shearing in different directions. Consequently, the originally stacked Boudins have slipped into the necks (constrictions) and took the path of least resistance.

Best regards,

Guenter

Thanks allot for the explanations !

After discussing it with several scientists, in summary there could be two possible mechanisms:

1.  As suggested by Dr. Grundmann. The boudins may have formed by an early stage deformation event and were later sheared.

2. As the phyllite layers a much thinner than the quartzite,  interference may have resulted in such boudins.

Further comments and suggestions are most welcomed !

Regards

Amar 

Dear Amar,

Concerning the first two photos I think that Guenter is right. You have to be cautious, whether the boudins formed prior to shearing (seems so), or they are simply an early effect of shearing. The third photo seems very characteristic for dextral shear sense (as I tried to mark on your photo- attached). There you have upright curved structures/layers along the fracture dipping to the right (between the two red arrows) and downward curved structure on the left side of the fracture.

Best regards, Mirka

Dear Mr. Argawal,

the term boudine is hardly to be applicable to all of these structures which have much in common with phacoids well known from flysch sediments (lowermost image, in particular), among others in the Variscides. They are closely related to olisthostroms and large complexes of mass wasting and sliding accompanied by syntectonic processes as already demonstrated by the term flysch. There are some good images, cartoons and photographs presented by C.-H. Friedel (2012) from the olisthostroms of the Harz Mts., Germany. Maybe a paper in English can help elucidate the true process of the formation of these types of sedimentary structures: Festa, A., Pini, G. A., Yildirim Dilek, Y and Codegone, G (2010): Mélanges and mélange-forming

processes: a historical overview and new concepts. - International Geology Review, 52/10–12: 1040–1105. Terms like hummocky cross stratification should be handled in a very reserved manner for these structures

Best regards

H.G.Dill

Thanks a lot for the responses. 

Whatever they are (look like asymetrical boundins...) they were further shortened & sheared so that swells filled in  spaces of contiguous necks of layers above and below...it seems to me... 

Luis Alberto seems to be right. Shearing seems to have taken place after boudinage, giving this paradoxical result. You should be able to define the sense of shear. Would you agree with such a view ?

Dear Mr. Argawal,

could you please help re-focus the current discussion by providing some more information as to the sedimentary environment, the minerals in the quartzite and phyllites and the tectonic setting. I attach a set of sketches from phacoids which developed in mélanges and olistostroms of flysch sediments in a low-grade to very-low-grade regime to support my answer given previously. Based upon your new data, the ideas put forward will expectedly then be better checked for their applicability as to describe this outcrop section.

Best regards

H.G.Dill

Dear Amar Agarwal,

I agree with Mirka Trajanova. Could be that the structures are simply an early effect of shearing. I also think on dextral shearing oblique to the competent layer (in photo 3) along shear bands which primarily could be laminae of  cross-stratification, may be visible above the number "49" and  top to the right, where they are dipping flat down to the right.

Best wishes

Bankwitz

Dear All,

Apologies for the late reply. We were busy getting some more field evidences. Please have a look at the attached photographs from the same structure, but taken from much shorter distance. In several boudins the quartz veins, at the neck  demosntrate the so called "fish-mouth"  structure. Thus proving that the structures are boudins and the layers had very large competency contrast.

To answer Prof. Dill, the structure is in viscinity of South Almora Thrust in Kumaun Lesser Himaya. The rocks belong to Ramgarh Group. The quartzite can be refereed as quartz arenite with more than 90% quartz. The phyllites are meta-sedimentary and are derived from argellecious rocks. We do not have the precise minerology of the rocks, however visually there are only minor differences. The depositional environment is still not well established. 

If the boudins were initially Hummocky cross-beds. Does it imply that there was a cyclic change in the depostional environment, from stormy to calm/deep sea, and visa-versa. The stormy conditions formed hummocky cross bed and the argilleceous layers were depostied in the calm/deep sea condition ?

Many Thanks for the reply!

Amar