In situ stress control on microcrack generation and macroscopic extensional fracture in exhuming bedrock

Abstract

Crustal stresses beneath evolving alpine landscapes result from a combination of tectonic strain, bedrock exhumation, and topography. The stress field is regulated by elastic material properties and the brittle strength of critically stressed elements, particularly those within preexisting faults and intact rock. Combining Byerlee's law for crustal stresses with a recently developed trilinear fracture envelope, we propose an extension of the critically stressed crust concept to constrain in situ stresses through microcrack generation and extensional fracture propagation. A compilation of 814 global in situ stress measurements suggests microcrack development will limit long‐term rock strength, and maximum differential stresses in the upper ~1 km of the crust can be controlled by cohesive bedrock behavior. Using an elastoplastic, 2‐D finite‐difference model, we approximate in situ stress development within landscapes undergoing high rates of exhumation in both normal and reverse tectonic regimes. Critical near‐surface stresses in these environments are defined by the microcrack initiation threshold, estimated to be roughly one third of the unconfined compressive strength of intact rock, while stresses deeper in the crust adhere to Byerlee's law. Our models indicate that exhumation‐induced stresses limited by long‐term rock strength are the primary contributor to the near‐surface stress field, while topographic relief reduces stress near valley axes. Simulating glacial erosion then allows us to illustrate a path‐dependent relationship between critical stress development, fracture formation, and geomorphic processes. We find that glacial unloading can generate new microcracks in near‐surface bedrock, resulting in unstable macroscopic extensional (or “exfoliation”) fracture propagation during incision of U‐shaped alpine valleys.

Sheeting joints and polygonal patterns in the Navajo Sandstone, southern Utah: Controlled by rock fabric, tectonic joints, buckling, and gullying

“…2, 3, 6, 13, 14). Martel's (2011Martel's ( , 2017 model for the origin of sheeting joints, which emphasizes the importance of convex land surfaces in generating tensile stress for opening-mode fractures, fits the morphology and topographic context of sheeting joints in our field areas better than the exhumation hypothesis (Leith et al, 2014a), which was developed to explain sheeting joints on the floors and lower walls of glacial valleys. We attribute the three-dimensional hexagonal pattern of cross-joints that formed above each smooth, domed (axisymmetric) sheeting joint to compressive stresses that were directed radially inward ( Fig.…”

“…He showed that topographic domes are especially likely to be underlain by sheeting joints because there, compressive stresses (σ 1 and σ 2 ) are additive. Leith et al (2014a) considered spalling in underground excavations to be closely analogous to sheeting joints. They argued that both phenomena develop at sites of high differential stress created by exhumation because with unloading, the vertical crustal stress component (S v ) typically diminishes more rapidly than the S H and S h components (Leith et al, 2014a).…”

“…Leith et al (2014a) considered spalling in underground excavations to be closely analogous to sheeting joints. They argued that both phenomena develop at sites of high differential stress created by exhumation because with unloading, the vertical crustal stress component (S v ) typically diminishes more rapidly than the S H and S h components (Leith et al, 2014a). Adams (1982), Romani and Twidale (1999), and Bourne (2003, 2009) argued that A-tents are (like sheeting joints) neotectonic manifestations of surface-parallel compressive stress.…”

“…We hypothesize that the (older) first-order cross-joints as well as the (younger) vertical cross-joints that terminate against the second-order sheeting joints were produced by the same stresses that generated the sheeting joints-land-sur- Loope and Burberry | Sheeting joints and polygonal patterns in the Navajo Sandstone GEOSPHERE | Volume 14 | Number 4 face-parallel compression ( Fig. 8; Martel, 2011, his figure 3; Leith et al, 2014a, their figure 2b; see Stresses, below). The small-scale polygons are analogs (fractals) of the large-scale polygons and formed by the same processes.…”

“…10, 11, 12) as products of compressive, land-surface-parallel stresses that generated tensile stress perpendicular to the land surface (Martel, 2011;Leith et al, 2014a;Bahat et al, 1999). The pre-existing tectonic joints greatly limited the slope-perpendicular extent of the sheeting joints.…”