A back thrust would be directed in the opposite direction of foldbelt propagation. The fault would be directed towards the hinterland and away from the foreland. If you are looking at a backthrust in the field you will find older, stratigraphically lower rocks in the hanging wall on the foreland side of the fault, juxtaposed onto younger rocks in the footwall on the hinterland side of the fault.

This is a basic explanation, and in some high complexity scenarios, this may not hold true (with polydeformational structures, ductile duplexes, etc), but in most cases the above statements hold true.

Hope this helps!

Dear Ahmet

In Himalaya we identify back thrusts by their propagation direction which is opposite to the fold thrust belt and also the foliations. The foliations form due to 'fore-thrusts' which propagate in the direction of the fold-thrust belts. But a regional perspective of the foliation is important because they may be affected by local structures, such as isoclinal and recumbent folds.

All the best !

Dear Ahmet

In many thin ­skinned fold and thrust belts, most of the fold and thrust structures have a definite, consistent convergence to them. That is, the sense of overturn on the folds and the dip and transport direction on the faults suggest consistent transport of material towards the foreland. A back thrust is a thrust fault that dips in a direction opposite to that of most of the structures in the belts. Superposition concept and its interpretation are so significant. In other words, Thrust faults usually emplace older rocks on top of younger rocks, with the exceptions of when the strata dip more steeply in the same direction as the fault or in areas of complex deformation where the stratigraphy is overturned or folded. Therefore field determining of back thrust fault requires startigraphic data base.

Fore Thrust is actually described: Foreland fold and thrust belts" typically involve shelf rocks or a former continental margin (or miogeocline).

They often also involve strata of the early part of the foreland basin that have later been incorporated into the orogen.

Regards

Massih

Massih

I agree with Massiah. Superposition is  a different  concept.

For field evidence of back thrust in Quaternary terrain (Himalayan foredeep) the paper on Sarpang re-entrant (Tectonophysics, 2013; available in RG) may be consulted.

In active continental fold-thrust belt significant criteria for thrust fault is the fault scarp. In Himalaya for identification of back thrust one has to look for North facing scarp; while for fore-thrust (hinterland dipping) the scarp will be South facing.

Hello,

As many colleages have already pointed out, identification of a backthrust as such requires a good knowledge of the regional geology of the orogen. That is because backthrust, in words of Saklani (2008), can be defined as a thrust that has a displacement sense contrary to the tectonic transport direction within the orogen (that is, it is directed towards the hinterland instead of the foreland).

This of course is appliable to any other compressive environment, it is not restricted to orogens themselves. One may find perfectly developed backthrusts in slumped sediments, for example.

Therefore, identification of a thrust as a backthrust is difficult and risky when observing at the outcrop scale, because the mechanics of both features are exactly the same, only that, as exposed, the sense of displacement is (approximately) the opposite.

I hope this helps.

Cheers,

Manuel

Ref: Saklani, P.S. (2008). Glossary of Structural Geology and Tectonics. Satish Serial Publishing House.