I am not very sure what is termed as 'reverse / thrust faults' in pahoehoe sheet lobes. A photograph would help.

But, pending that, take the following into account:

In a compound lava system (whether at the terminal end or near the proximity of the vent during the quenching phase), younger lava would be emplaced (with a degree of force) into / above / under an existing semi-consolidated lava, with sharp contacts. This could yield an appearance of 'thrusts', but cannot be technically called as a 'thrust / reverse fault'. (for definition of fault - see any structural geology book!). The resulting geometry is a product of the 'syn-volcanic / eruptive' history of the lava that is being emplaced in batches and yielding a multiplicity of inter-relations between various 'lobes'.

Faults necessary evolve (following an initial brittle failure) after a set of strata have formed and consolidated. Not during. 

Therefore, (and I am basing this on some publications that have recently come claiming to be 'thrusts in the Deccan Trap basalts') I am of the opinion that these features are emplacement-related features and not faults sensu at all!.

It is as absurd as treating a sheet-lobe of lava that is pushed forcefully during its emplacement, under an existing lobe to be an 'underthrust' sheet!!! 

Tectonic faulting and thrusting has to be one that manifests the stress imposed on a rock-body after its formation - not during its formation.

Thank you Sir.

He who laughs the last laughs the best.

I will meet you and discuss in detail.

Hello Raymond,

I agree with Vivek - the features at the terminal ends of lobes can exhibit structure like small-scale thrust faults, for sure. You can also get similar examples at the terminus of landslips, or push moraines. I guess it is the point at which rheology changes from essentially plastic to brittle, but the force behind continues.

On lava flows it is not uncommon to get all types of 'push-up' slabs, spines and tumuli which, on examination contain structures that are to all intents and purposes reverse faults and thrusts developed over several metres. However, sensu-stricto, I suspect they are not what a structural geologist would consider true thrusts or faults. I wonder whether we should develop a more appropriate terminology for such lava features (there may even already be one!).

I believe I have seen what you describe developed on flows in Iceland (Laki, for instance, and cetinly on rhyolite examples which are by their nature 'sticky'), but on basalt flows I would call this jumble of thrust and faulted slabs simply and collectively 'rubbly pahoehoe'. There are some good papers by Guillbard et al., and Self and Blake - which I'm sure you know. The difficulty with the Deccan examples is, however, the state of preservation - and sadly many of these nuances of detail are lost - except in the examples exhumed in quarries and road cuts.

Best wishes,

Mike.

Indeed Mike. 

We now have several instances where we find that at the terminal ends of large-volume and thick slabby pahoehoe lava sheets, features suggesting forceful injection (into the upper lobe) and local ejecta emplacement are present.

A photo on one such feature occurring along the fringe of the DT province is attached as an example. The basalt of the 'upper lobe' has been contorted during the emplacement of the tachylitic vesicular material (red) that appears to have been achieved with a 'upwelling push' into the upper lobe. 

Such features could be mis-interpreted as structural 'faults / thrusts' if viewed in isolation. 

Hello Mike,

Thanks for the thought provoking reply to my question. The 'thrust'  I am referring to range from 2 to 10 m and displace sheet pahoehoe slabs that are10's of meters in length and can be seen in road sections. I appreciate that we have a problem with 'terminology' here and simple cannot refer to these as "slabby pahoehoe" or 'rubbly pahoehoe' due to issues related to dimensions. I will go to the field shortly and map/sketch some of these so that we can have a fruitful discussion.

Warm regards

Raymond

Hi Renie,

Excellent!!! Thats what I was talking about. This is example two. We should meet up and discuss and collaborate so that we can write it together.

I have this one figured out.

Cheers!!!

Nice photo Renie.

To my mind, this is another example (very well captured in your photo - since these features are often not so well exposed) of quasi-ductile deformation occurring in an inflated crust of a compound pahoehoe sheet flow at its distal ends.

To my mind, it could result from

The photo that you have given indicates the latter possibility to be stronger, although the petrography of the 'red tachylitic horizon' that displays the dislocation and its comparison with the host basalt would be critical in determining with any degree of certainty. 

In any case, is a syn-volcanic feature. Not a superimposed structural deformation of a basaltic flow. Similar (but less sharply preserved due to weathering and other aspects) features and discordant relations including 'step-faulting' have been recorded by us in the distal parts of inflated pahoehoe flow sheets in the Deccan Traps.

As discussed in the earlier comments, this does NOT qualify as a 'structural thrust' in the strictest sense.

Shall be happy to hear counter-views.

All the best.

Nice find and photo by Rennie, and a good discussion. Here is my view, briefly.

A scale object would have been helpful. Nevertheless, the outcrop to me seems to show small-scale lobes, and to show one of two things: (i) Some kind of invasive lobe, injected from the left (into what?). Close-ups would be helpful. (ii) A thrust, as Renie suggests. But note that the red horizon (palaeosol? glassy altered base or top of a flow?) is clearly affected by it, and so the thrust could not have formed during the late-stage emplacement of the lava flow, under ductile conditions, as Renie and Raymond suggest, but had to form much later, following solidification and even weathering, as a typical brittle-behaviour thrust, and even came up to the surface. It appears that the "thrust" doesn't extend into the overlying flow, so the thrusting has to have occurred before the overlying flow was emplaced. What might cause such thrusting needs to be considered as well, but this means that local compressional and tensional environments may have existed in between eruptions. Thanks for reading.

Thanks Hetu. I agree with you completely. The thrusting has to occur 1) after  emplacement of the lobe, 2) and before the upper lobe is emplaced.. I think I have figured out the mechanism here and elsewhere. But what do you think we should call such thrust? so that we we distinguish it from the tectonic element..

In deed a very interesting discussion and a very impressive photo by Renie. I feel this to be a primary feature rather that a tectonic one since on zooming Renie's photo, the plane of dislocation, appears to be a wavy plane rather than a sharp straight plane as expected during a brittle deformation (at least for the scale of this photo). I feel during the emplacement of the new flux, the lava has occupied the space available to it thus giving rise to this wavy plane.  Can this be called as Synvolcanic Deformational Features some thing similar to Synsedimentary Deformational features, wherein all the features do not necessarily indicate tectonic deformation.