Formation of en-échelon pull-apart arrays in pure-shear dominated transpression

We report on a series of small-scale, en-échelon faults in limestone (Les Matelles, France) that result in the formation of a series of linked pull-aparts (en-échelon pull-apart arrays) that have been infilled with calcite. The formation of stylolite seams and isolated veins occurs adjacent to these fault zones, which constrain the direction of minimum infinitesimal stretching axis (commonly interpreted as maximum compressional stress). We observe that en-échelon pull-apart arrays occur when the maximum infinitesimal shortening direction is at 55° or greater to the enveloping surfaces of the en-échelon fault arrays. This situation differs from en-échelon vein arrays, in which the maximum horizontal infinitesimal shortening direction occurs at 55° or less to the enveloping surfaces. We interpret these features with respect to transpressional strain modeling, which distinguishes between wrench (simple shear) dominated transpression and pure shear dominated transpression. Pure shear dominated transpression occurs if the maximum infinitesimal shortening direction is at 55° or greater to the enveloping surfaces, and results in a maximum infinitesimal stretching direction that is vertical. The formation of en-échelon vein arrays with vertical enveloping surfaces require a horizontal infinitesimal stretching direction, which is incompatible with pure shear dominated transpression. Rather, en-échelon pull-apart arrays form to accommodate the wrench component of deformation in pure-shear dominated transpression.