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Regular spacing of deformation bands in sandstone: Layer-thickness control or constitutive instability?
Journal of Structural Geology ( IF 2.6 ) Pub Date : 2021-04-07 , DOI: 10.1016/j.jsg.2021.104335
K.E. Elphick , A. Lv , H. Roshan , C.R. Sloss , K. Regenauer-Lieb , C.E. Schrank

We examine whether bedding thickness or constitutive properties correlate with the spacing of equally spaced shear-enhanced compaction bands and compactional shear bands in Miocene sandstones of the Whakataki Formation, North Island, New Zealand. Quantitative spacing analysis reveals a positive correlation between bedding thickness and the spacing of strata-bound deformation bands, as is commonly observed for veins, joints, and normal faults. The observed spacing to thickness ratio (STR) ranges from 0.02 to 0.14 and is considerably smaller than observed for veins, joints, and normal faults (usually ≥ 1). Modelling compaction bands as stiff tabular inclusions in a layered sequence, simple elastostatic numerical simulations show that they act as stress amplifiers, which may explain STR ≪ 1. Additionally, we obtained laboratory estimates of rock permeability and strength to test if the constant band spacing can be explained with a constitutive instability proposed for compaction bands: the stationary cnoidal wave. The results from this study indicate that this model fails to explain the observed spacing if we use the material properties obtained in the laboratory. This discrepancy likely arises from the following: [1] laboratory estimates of host-rock properties are not the same as those at depth; [2] the used semi-analytical solution for the stationary cnoidal wave employs an incorrect value (m = 3) of the stress exponent; and [3] the equation does not account for coupled layered media. While we cannot conclusively falsify the cnoidal-wave model for compaction band spacing per se, it appears likely that the existing solution should not be used for predicting band spacing in layered porous sandstones.



中文翻译:

砂岩中变形带的规则间距:层厚控制还是本构不稳定性?

我们研究了地层厚度或本构性质是否与新西兰北岛瓦卡塔基组中新世砂岩中等距切变增强压实带和压实剪切带的间距相关。定量间距分析揭示了层理厚度与地层约束形变带间距之间的正相关关系,这在静脉,节理和正断层中很常见。所观察到的间距与厚度之比(STR)在0.02至0.14的范围内,并且比在静脉,关节和正常断层(通常≥1)上所观察到的要小得多。通过将压实带建模为分层序列中的刚性表格夹杂物,简单的弹力数值模拟表明它们可作为应力放大器,可以解释STR≪ 1。我们获得了岩石渗透率和强度的实验室估算值,以测试恒定的带间距是否可以用压实带的本构不稳定性来解释:平稳的弦波。这项研究的结果表明,如果我们使用实验室中获得的材料特性,则该模型无法解释观察到的间距。这种差异可能是由于以下原因引起的:[1]实验室对岩石性质的估计与在深处的估计不相同;[2]所使用的半静态平稳波半解析解采用了不正确的应力指数值(m = 3);[3]该方程式没有考虑耦合的层状介质。虽然我们不能最终确定压实带间距本身的正弦波模型,

更新日期:2021-04-19
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