FORMATION OF EN-ÉCHELON PULL-APART ARRAYS IN PURE-SHEAR DOMINATED TRANSPRESSION

A series of small-scale, en-échelon faults and veins are reported from the limestone outcrops of Les Matelles, France. The formation of stylolite seams and isolated veins occurs adjacent to these fault zones, which constrain the direction of minimum (ISA₃) and maximum (ISA₁) infinitesimal stretching axis, respectively. En-échelon vein arrays occur when the minimum infinitesimal shortening direction is at 55° or less to the enveloping surfaces of the en-échelon fault arrays. In contrast, 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.

We compare these field results to transpressional strain modeling. Pure shear dominated transpression occurs when the minimum infinitesimal shortening direction (ISA₃) is at 55° or greater to the enveloping surfaces of a deformation zone; it also results in a vertical maximum infinitesimal stretching direction (ISA₁). Therefore, vertical vein arrays cannot form in pure shear dominated transpression, because they require a horizontal maximum infinitesimal stretching direction. Rather, the observation is that en échelon pull-apart arrays form, with the faults being reactivated stylolites. We note, however, that the en échelon stylolitesd form in front of these pull-apart arrays. We interpret that the en-échelon pull-apart arrays form in response to pure-shear dominated transpression, to accommodate the wrench component of 3D deformation. In contrast, en-échelon vein arrays can form in simple shear dominated transpression, simple shear, and all transtensional deformations. An additional prediction is that vertical stylolites seams should not form in pure shear dominated transtension, which occurs when the minimum horizontal infinitesimal shortening direction is at 35° or less to the enveloping surfaces of a deformation zone. The transtensional strain modeling suggests that the minimum infinitesimal shortening direction (ISA₃) will be vertical in this setting.