You could try this paper: "Vorticity analysis in shear zones: A review of methods and applications" by Xypolias, 2010. I believe you will find what you seek

Kind regards

Ioannis Alexandridis

you can find the equation in the book of earth structures, 2nd edd. 2004 of VAN DER PLUIJM & STEPHEN MARSHAK

Dear Seba, I was thinking about your question.

I don't have the answer, but I have another question:

What is the meaning of simple shear percent? If there is one?

After Bobyarchick (Tectonophysics, 1986)

Wk = cos (arctan (2 epspt / gammapt)

where epspt= pure shear strain rate and gammapt = simple shear strain rate

and

epspt/gammapt = ½ tan (arcos Wk)

After Tikoff and Fossen ( Tectonophysics, 1993)

Wk = cos (arctan (2 ln(k_total) / gamma_total))

and we can assume also that

ln(k_total) / gamma_total = ½ tan(arcos Wk)

Therefore, it is easy to relate Wk and the ratio between  pure/simple shear strain rates or with the total amount of pure and simple shear (at least in 2D), but what is the meaning of simple shear percent and the direct relation if there is any with Wk?

From the above written equations is clear that in order to have constant Wk we need constant ln(k_total)/gamma_total

best regards

pedro