Ice shelf rift propagation: stability, three dimensional effects, and the role of marginal weakening

Abstract

Abstract. Understanding the processes that govern ice shelf extent are of fundamental importance to improved estimates of future sea level rise. In present-day Antarctica, ice shelf extent is most commonly determined by the propagation of through-cutting fractures called ice shelf rifts. Here, I present the first three-dimensional analysis of ice shelf rift propagation. I present a linear elastic fracture mechanical (LEFM) description of rift propagation. The model predicts that rifts may be stabilized when buoyant flexure results in contact at the tops of the near-tip rift walls. This stabilizing tendency may be overcome, however, by processes that act in the ice shelf margins. In particular, both marginal weakening and the advection of rifts into an ice tongue are shown to be processes that may trigger rift propagation. Marginal shear stress is shown to be the determining factor that governs these types of rift instability. I furthermore show that rift stability is closely related to the transition from uniaxial to biaxial extension known as the compressive arch. Although the partial contact of rift walls is fundamentally a three-dimensional process, I demonstrate that it may be parameterized within more numerically efficient two-dimensional calculations. This study provides a step towards a description of calving physics that is based in fracture mechanics.

Projected Seismic Activity at the Tiger Stripe Fractures on Enceladus, Saturn, From an Analog Study of Tidally Modulated Icequakes Within the Ross Ice Shelf, Antarctica

“…(2019) showed that the rift WR4 is seismically active, and here we have here demonstrated that rift WR6 is seismogenic as well. Both rifts are seaward of the ice shelf compressive arch induced by the marginal stresses of the flowing ice shelf (which advects seaward at speeds in excess of 1 km/year near its ocean edge) and thus experience a background laterally biaxial extensional stress field (e.g., Lipovsky, 2020). Ice–ice discontinuities at Earth cryosphere temperatures and glacial strain rates typically show creeping (velocity strengthening) behavior which does not produce the dynamic instability that is necessary for stick‐slip seismicity (e.g., Scholz, 2002).…”

A Generalized Interpolation Material Point Method for Shallow Ice Shelves. 2: Anisotropic Nonlocal Damage Mechanics and Rift Propagation

“…Similarly, targeted basal melting could also trigger additional rifting. However, we emphasize that necking and mass balance effects should not always be necessary to initiate rifts, according to LEFM rift studies (e.g., Lipovsky, 2020 ). Encouragingly, the creep damage model can initiate realistic rifting without these additional effects (Section 4.2 ), though we acknowledge that given the idealized setting, it is difficult to determine whether or not this rifting should actually occur.…”

“…The approximated 3‐D stress also neglects the influence of processes such as necking, buoyancy forces, crevasse shielding, and rounding off of crack tips due to melt or accretion of new ice. Additionally, it neglects flexure that occurs everywhere along rift walls (Lipovsky, 2020 ). Although observational evidence suggests that rift propagation is primarily controlled by horizontal glaciological stresses from creep flow (Joughin & MacAyeal, 2005 ), flexure along rift walls may play a role in rift propagation, for example, by inducing contact between rift flanks (Lipovsky, 2020 ).…”

“…Physically, setting a value of less than unity can be interpreted as allowing some residual strength between the flanks of the rift, which can occur when rifts contain ice mélange that is structurally coherent enough to transmit stresses (Borstad et al., 2013 ; Larour et al., 2004b ; Rignot & MacAyeal, 1998 ). A complete description of rift forces should include a boundary condition on the rift flank walls similar to that at the ice front (Equation (8) , (10) ), but which also accounts for the pressure of ice mélange and potential contact between rift flanks (Larour et al., 2014 ; Lipovsky, 2020 ). For simplicity, we do not explicitly implement such a boundary condition here; rather, its effect on the rift opening rate is simply parametrized by setting the value of lower than unity.…”

“…Crevasse/rift propagation in LEFM models is based on the stress intensity factor and fracture toughness of ice, which are inherently related to strain energy release rate (where rate indicates per unit crack length) and fracture energy (i.e., the surface energy associated with the crack face), respectively. In the literature, analytical LEFM models were applied to estimate the penetration depth of crevasses (Rist et al., 2002 ; Smith, 1976 ; van der Veen, 1998a , 1998b ; Weertman, 1973 ) and to study rift propagation in ice shelves (Hulbe et al., 2010 ; Larour et al., 2004a , 2004b ; Lipovsky, 2018 , 2020 ; Plate et al., 2012 ). Moreover, analytical LEFM models were combined with ice flow models to predict iceberg calving (Krug et al., 2014 ; Yu et al., 2017 ) and to investigate ice shelf stability (Lai et al., 2020 ).…”