The role of the Intermediate Stress Axis in forming Primary P-fracture Arrays, Cathedral Peak Granodiorite, California

Molly Egan, University of Wisconsin-Madison
Ann Everest, University of Wisconsin-Madison
Basil Tikoff, University of Wisconsin-Madison
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Abstract

Primary P-fractures arrays were observed within and adjacent to the Cascade Lake shear zone, in the Cathedral Peak granodiorite, a segment of the Sierra Crest shear zone system, Sierra Nevada, California, USA. They appear in the field as en-échelon pull-apart arrays filled with epidote and quartz with slickenfibers indicating strike-slip displacement. Rare offset markers indicate that the fault-parallel length of individual pull-aparts record both the sense and amount of shear. Enveloping surfaces of the P-fracture arrays are oriented at low angles to the maximum horizontal infinitesimal shortening direction (sigma 1 and ISA3), as determined by brittlely fractured K-feldspar megacrysts and rare P-fracture conjugate arrays. En-échelon vein arrays also occur locally on the outcrops and form conjugate sets. The angle between an individual vein and the the enveloping surfaces of the en-échelon vein arrays is lower than 45°, indicating transtensional deformation; the orientation, however, is at a higher angle to the maximum horizontal infinitesimal shortening direction relative to the P-fracture arrays.

We hypothesize that the P-fractures occur because of the role of the intermediate stress axis during transtensional deformation, in a manner similar to that described in Blenkinsop (2023). Based on the maximum compressive stress orientation relative to the enveloping surfaces of P-fractures and en-échelon veins, P-fractures in the Cathedral Peak Granodiorite are interpreted to be formed by transtensional kinematics. During transtensional deformation, the magnitude of the intermediate stress changes depending on the angle of oblique divergence (alpha); the intermediate stress axis is equivalent to the maximum stress axis when alpha = 20°. High intermediate stress axes suppress the formation of shear fractures and enhance opening-mode fractures. In contrast, wrench and pure divergence are two-dimensional deformations. We observe the formation of the primary P-fracture arrays exclusively in transtensional situations approaching pure divergence. The individual shear fractures form at a low (~15-25°) angle to the maximum horizontal compressional stress (minimum horizontal infinitesimal strain).
Reference: T. G. Blenkinsop (2023) Triaxial stress and failure modes in hydrothermal mineral systems, Australian Journal of Earth Sciences, 70:7, 947-957, DOI:10.1080/08120099.2023.2210637

Session

Deformation in the upper crust