Does thrust faulting occur in an extensional regime? Or rather reactivation of an older thrust fault plane and its turn into a normal fault? - If a change in faulting mechanism happened I would expect to observe and measure different directions for the two processes - at least in some outcrops within the exposure area of your shear zone.

You might search for studies from better known orogens to compare their results with your observations.

For the alps "sliding nappes" have been described, see e.g. http://scholar.google.de/scholar?q=%22gravity+sliding%22+nappe

And (in addition to gravity sliding) there is the concept of "lateral extrusion" of an orogen, i.e. its extension due to a nearby backrolling subduction zone with slab pull acting perpendicular to the axis of compression.

http://scholar.google.de/scholar?q=lateral+extrusion

Perhaps one further comment: To get advice from a real expert you might try out other topics as tags, such as 'structural geology', 'tectonics', 'geodynamics', 'orogen', 'thrust fault' and so on.

Usually, as said before and as rock failure theory suggest, low-angle tectonic structures, active in extensional tectonic regime, growth and accommodated their deformation exploiting pre-existing surfaces: bedding and very useful tilting thrust. Said this, although I suggest to do a detailed structural investigation along the shear zone, in many case, when a shear zone suffered a multiple reactivation, the kinematic indicators are not very clear.

I think that previous information about the fault activity derived from the cartographic-geological view of the area involved by fault. In many cases I could see that geological information, less used, represent a fundamental tool. For example the a typical geological features due to low-angle normal fault activity is the tectonic elision or lamination. This aspect usually characterized the geological/tectonic context involved by this problematic structures.

Hi Ugo, have a look at the central himalaya, where some thrusts due to the Eo-Oligocene compression and orogenic building have been subsequently reactivated as normal faults in the Miocene during the gravitational collapse stage. I am refeering especially to the south tibetan detachment. Look at the following paper  for details :Carosi et al., J. Asian Earth Sci., 1998.  Look also at works by Manatschal in the alps about the Er detachment, or the Chenaillet. You will find examples of thorough structural/petrological studies, highlighting the metamorphism related to detachment and some criteria that may help you to discriminate from thrust faults. Look also at some litterature about metamorphism along core complexes in the cyclades, this area also formed by orogenic collapse in response to slab rollback (I recommend works of B. Huet or E. Lecomte). The datations of the metamorphic episodes will be crucial to determine the chronology of faulting, but some tectono-stratigraphic relationships may further help you to unravel it. I hope my comment will help you, and that your research is going well! best regards

What are you referring to when you speak about "orogenic collapse" thrust? Are you referring to thrusts in front of a collapsing body (e.g. as in the case of passive-margins toe fold-and-thrust belts)? Or are you thinking to thrusts reactivated as (low-angle?) normal faults?

In the fist case I think a large scale mapping might be useful to understand if there are normal faults at the rear of your thrust, that elide a significant portion of the stratigraphy (like a core complex). Although, as far as I know, the gravity sliding explanation for large thrust emplacements is not very favoured by mechanics.

In the second case, you could recognise the kinematics from the fault rocks (could you describe them better? ) and by the hanging wall vs footwall stratigraphy.

At the scale of the outcrop be careful that "normal" sense structures develop also during compressive deformation (e.g. Riedel shears o C' extensional crenulation cleavage and others).

Best regards

G'day Ugo

When shortening is driven by plate-boundary forces, chance are that contractional tectonics affects the entire domains.  In contrast, when thrusts and other contractional structures develop in relation to the collapse of an orogen, then they develop at the front of collapsing orogens (at the margin orogen/foreland), and they are coeval with extensional structures in the orogen (normal faults, grabens, metamorphic core complexes).  As collapse proceeds, part of the shortened region can switch to extension, in which case thrust faults that formed because of collapse can be reactivated into normal fauts.  Cheers, Patrice

Ugo, 

in the case of collapse, i.e. sliding, I would expect that hanging wall fabric is not well preserved. Any way, collapsing is occurring also in compressional settings due to gravitational instability of orogenic wedges.

Hi Ugo,

I don't think you will be able to make the difference between the two scenarios you mention by looking at kinematic indicators and deformation around the fault plane. As Telemaco mentioned, and especially if the collapse is recent, i think the best option is to have a look at a greater area and see if thrusting could link to extensional structures on a contiguous elevated area. Check out some of the papers from Messager and Niviere on the Neuquen Basin, Argentina...might be useful!

Cheers, 

Giulio

Thank you to all of you. Indeed I am studying a larger area around the main fault plane in order to find structures that could be related to an extensive or compressive context, I think this will be the main difference between these hypothesis.

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