CONTRIBUTION OF DUCTILE THINNING TO THE EXHUMATION OF THE OTAGO SCHIST, NEW ZEALAND

The exhumation of regionally coherent high-P rocks in the fore-arc of accretionary wedges is a widely debated problem. The Otago Schist belt of New Zealand, that represents the deeply exhumed fore-arc high of the Mesozoic Torlesse accretionary wedge, is one example for exhumed high-P rocks. To address the role of ductile thinning contributing to the exhumation of the Otago Schist, we calculated finite deviatoric strain from 60 mudstone samples distributed across the Otago Schist.

Finite deviatoric strain show a strong concentration of maximum shortening axes in a subvertical orientation. Intermediate and maximum extension directions scatter in the subhorizontal.

To estimate total vertical shortening, we converted our deviatoric into absolute stretches. To do so, we postulate that orogenic deformation at the regional scale approaches plane strain, where stretch parallel to the orogen is equal to one. We name this direction ”plane strain axis”. Regional absolute strain studies in accretionary wedges from the North American west coast justify this assumption, even if deformation is typically not plane strain at the local scale.

Thus, we calculated the regional tensor average of our deviatoric stretches to check if any of the principal axes qualifies as a ”plane strain axis”. The direction of the deviatoric maximum extension axis S´x matches the required features. This allows to convert deviatoric into absolute stretches. It applies S´=S Sv ^(-1/3) (1), where S´ are deviatoric stretches, S are absolute stretches and Sv represents volume strain. In our case S´x=Sx Sv ^(-1/3) (2) with Sx=1 to fulfil the plane strain assumption, i.e. average volume strain Sv=S´x ^-3 (3). Inserting eq. (3) in eq. (1) allows to determine average absolute stretches for Otago. Using the deviatoric stretches from the tensor average we obtain the absolute stretches Sx=1, Sy=0.88, and Sz=0.41, i.e. absolute values indicate ~ 60% vertical thinning and minor horizontal shortening normal to the orogen strike. To verify our results we directly measured absolute strain in metagreywackes. They yield ~ 40% vertical contraction and again no stretch along the Sx axis, i.e. Sx~1.

Our strain data suggest that the Torlesse wedge is characterised by pronounced vertical contraction that aided the exhumation of the high-P interior of the wedge.