6.1.1 Discontinuities
Discontinuities predate the surface expression of antiscarp features and are considered necessary for the formation of antiscarps because they provide a set of surfaces with lower shear strength parameters than the bounding rock material allowing them to be a preferential zone for movement.
A question that is raised is the relationship between discontinuity sets and the length of the antiscarps formed. Where the discontinuities are laterally persistent, for example a well formed bedding surface or schistosity, the expectation is that the antiscarps will also be of significant continuous length. Where the discontinuities have low persistence, for example a jointed granite or quartz diorite, then the expectation is that there will be multiple
discontinuous antiscarps.
6.1.2 Post-glacial rebound
Post-glacial rebound is postulated as a viable mechanism for the formation of most of the antiscarps referred to in Chapter 4 in that it suggests that antiscarps form rapidly following deglaciation and they are likely to occur as single event displacements. Most of the antiscarps observed cut through or across glacial features implying that they post-date the last glacial maximum. The postulation that antiscarps are the product of a single event does not correlate
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with the proposition by the majority of the authors in Chart 1 that antiscarps are likely to be related to processes that operate at the rate of creep.
Where an antiscarp is the product of post-glacial rebound it can be inferred that the rock material within which the antiscarp has formed is highly resistant to erosion and degradation because of the length of time that it has been exposed. Typical field estimated strength of the rock material within which antiscarps were observed was R5 (Table 2.1), which is indicative of a very hard rock material.
6.1.3 Seismicity
Seismicity is not a necessary factor for the formation of antiscarps and is not deemed to be a significant factor either. While there may be some reactivation or induction of movement associated with a seismic event there is no need or requirement to have a seismic input for most antiscarps to form. The key exceptions are surface ruptures or fault scarps that have an antiscarp morphology.
Antiscarps are an unreliable proxy for use as paleoseismic indicators and it is not recommended to adopt them as such without first ascertaining that they are indeed the product of coseismic activity. The main argument against a seismic relationship for the formation or evolution of most antiscarps is that the seismically-related antiscarps typically have a singular trace associated with a fault surface, or produce ridge rents with a low amplitude antiscarp, if any (Chart 1). There are few modern analogues for relating antiscarp formation or evolution to seismic events and those that do are fault traces or ridge rents. Practitioners should exercise caution when considering the seismic significance of antiscarps, and investigation techniques similar to those used in the paleoseismic study of tectonic faults should be adopted prior to assigning a seismic source (McCalpin 1999). It is not
recommended to use antiscarps as an indicator of seismicity nor should they be perceived as representative of a hazard without detailed investigation.
6.1.4 Ridge rents
Ridge rents are a feature that have been associated with seismic activity and where observed they often exhibit small displacements, in the order of centimetres. While there is a
possibility that over time they may evolve into larger-scale antiscarps there is no evidence thus far to support such a proposition. It is highly likely that ridge rents do not develop far
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beyond minor vertical displacement unless they are the initial expression of a fault that has undergone, or will undergo, further movement.
The ability to gather observations on this possible phenomenon is limited by the frequency of the event and the need for a baseline to build successive measurements upon. Further
complications arise where slope processes occur at a greater rate than any movement on the ridge rent and either obliterate or mask them.
6.1.5 Cirques and large scale volume removal
Cirques appear in a massif as an area where there has been a large volume of material removed by glacial activity. Numerical models indicate that antiscarps can develop on the opposite side of a ridge as the massif responds to the removal of the mass by relaxing into the created void. The presence of cirques is indicative that the massif has undergone some glacial activity. Caution should be taken to ensure that the antiscarps formed are related to the cirque and not to post-glacial rebound. If there are other related antiscarps along the slope of a similar scale that do not have a relationship to any cirque, or large scale removal of material, then the antiscarps are probably associated with a different process.
6.1.6 Rock mass homogeneity
All the field studies and all of the modelling were from, or were based on, homogeneous massifs. Antiscarps related to processes such as sackung or spreading, where a less competent lower unit is required, were neither observed nor modelled during the course of this study. There are higher levels of complexity associated with modelling heterogeneous materials that could not readily be incorporated into the modelling undertaken here.