Hi!
In response to the query below there is a straightforward solution in Bell
& Johnson (1989, 1992)
"This brings to mind nappe structure. Almost inevitably, if
one limb of a nappe is sheared out, it is the lower limb not
the upper limb, rather like the dough sticking to the bench.
Why is this? Is it some function of nappes being recumbent
antiformal structures? I can imagine a situation where the
"back end" of a packet of layered rocks is "picked up" and
folded into a recumbent synform, but this appears not to be
the norm. If it did this instead, would the upper limb be
sheared out in preference to the lower limb?"
To form thrust nappes, the rocks have to be structurally in a part of the
orogen where, if porphyroblasts were growing, spiral inclusion trails
would develop. The shear on shallow foliations has to be top out from the
orogen core. The shear on events with steep axial planes has to uplift the
core to give the gravitational head to drive thrusting (e.g. Price, 1973;
Elliott, 1976). Orogenesis resulting from such successions will
predominantly produce high shear strain on the lower limbs of nappe
anticlines. The figures in the B & J papers show how.
Cheers
Tim
Prof Tim Bell
School of Earth Sciences
James Cook University
Townsville
QLD 4811
Australia
ph: +61 7 47814766
fax: +61 7 47251501
email: [log in to unmask]
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