It is hard to judge from your description if there is really a problem with the original strain analysis.  I don't know anything about the area, but given you used the word "slate" to describe the associated rocks and the old term "shear fold" (I say old because the only time I've seen that term used was years ago in reference to folds with what people called a "slip cleavage" which has long been known to be a classic pressure solution phenomena, which fits with the association of the rocks with slates).  Additionally, development of stretching lineations parallel to fold axes is a common phenomenon--although imperfectly understood in many cases--so that is not out of line with significant strain in the rocks.  That said, I would guess that two observations can account for your conclusion the rocks aren't really deformed, and probably negate your conclusion: the absence of "deformed bedding" in clasts is typical of rocks deformed in pressure solution.  The strain is formed by volume loss, so there is no "internal" strain in the clasts.  Look for evidence of embayed cobbles, etc., to see if pressure solution operated in the rocks.  The observation of clasts with long axes perpendicular to the fabric is also completely compatible with strain of any kind--those clasts are just unfavorablly oriented, and the strains are not high enough to reorient them.  That is the classic characteristic dealt with in the Rf/phi strain analysis method. 

Hope that helps.  I suspect you'll have to look through the strain to understand your depositional system. 

Terry Pavlis
From: Tectonics & structural geology discussion list [[log in to unmask]] on behalf of Beunk, F.F. [[log in to unmask]]
Sent: Tuesday, September 22, 2015 5:39 AM
To: [log in to unmask]
Subject: clast fabric

Dear colleagues,

 

Working on deformed Proterozoic conglomerates, I measured sharp point maxima for the orientation of long axes of elongate metarhyolitic pebbles. In the ‘80s, when the art of strain analysis emerged, structural geologists went there, measured hundreds of pebbles and represented them in a Flinn diagram in the Journal of Structural Geology; apparently, their a priori assumption was ‘ductile deformation’. However, that assumption can easily be discarded from field evidence: Some elongate pebbles are oriented at about right angles to the preferred direction of the majority of them, and, most convincing, internal sedimentary bedding in pebbles, whenever present, is entirely undeformed, irrespective of its orientation. Also, the metamorphic temperature (from chlorite composition, ~300 C) was far too low for ductile deformation of these lithologies. We have evidence that the elongate pebble shapes result from (sub)glacial abrasion, in a lodgement till,  but the (mostly clast-supported) conglomerates were most likely deposited as debris flows in a glacially-influenced alluvial fan. However, clast fabrics in debris flows do often show preferred orientations, but nowhere as ‘sharp’ as seen here, to the best of my knowledge.  

The conglomerates and enclosing formations (slates, volcaniclastics) suffered from only a single phase of folding + shearing. Incidentally (?), the point maximum of the long pebble axes lies very close to the steeply plunging axis of these shear folds. Hence my question: Could mechanical rotation during (shear) folding have reoriented the pebbles into the preferred orientation they have at present? Any shared experience or hints from existing literature would be most welcome.

 

Frank

 

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Frank F. Beunk

Geology & Geochemistry Cluster, Department of Earth Sciences

Faculty of Earth and Life Sciences (FALW), room H-351

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