New evidence from seismic tomography reveals a unique mineral fabric restricted to the thick mantle lithosphere beneath ancient continental cratons, providing an important clue to the formation of these prominent and influential features in Earth’s geological history. Olivine, the dominant mineral of Earth’s upper mantle, has elastic properties that differ along its three crystallographic axes, and preferential alignment of individual olivine grains during plastic deformation can affect the bulk nature of seismic-wave propagation. Surface-wave tomography has shown that over most of Earth, deformation of the mantle lithosphere has oriented olivine crystals with the fast axis in the horizontal plane, but at depths centered at ∼150 km within cratonic continental-lithosphere roots, the fast crystallographic axis is preferentially aligned vertically. Because of the high viscosity of the cratonic roots, this fabric is likely to be a vestige from craton formation. Geochemical and petrological studies of upper-mantle garnet-peridotite nodules demonstrate that the cratonic mantle roots are stabilized by their reduced density, which was caused by melt removal at much shallower depths than those from which the nodules were subsequently extracted. The mineral fabric inferred from surface-wave tomography suggests that horizontal shortening carried the depleted zone downward after the melt-depletion event to form the thick continental roots, stretching the depleted material in the vertical dimension by pure shear and causing the fast crystallographic axis to be aligned vertically. This seismological fabric at ∼150 km is evidence of the shortening event that created the cratonic roots.

中文翻译：

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大陆根的形成

地震层析成像的新证据表明，一种独特的矿物结构仅限于古代大陆克拉通下面的厚地幔岩石圈，这为形成地球地质历史中这些突出而有影响的特征提供了重要线索。橄榄石是地球上地幔的主要矿物，其弹性特性沿其三个晶体学轴有所不同，并且塑性变形过程中单个橄榄石颗粒的优先排列会影响地震波传播的整体性质。面波层析成像显示，在地球的大部分区域，地幔岩石圈的变形已经定向了橄榄石晶体，其快轴位于水平面，但在克拉通大陆-岩圈根中约150 km处的深度集中，快结晶轴为优先垂直对齐。由于克拉通根的粘度很高，这种织物很可能是克拉通形成的痕迹。上地幔石榴石-橄榄岩球节的地球化学和岩石学研究表明，克拉通地幔根的密度降低是稳定的，密度的降低是由于熔体在比随后从中提取结节的深度浅得多的深度被去除。从表面波层析成像推断的矿物织物表明，水平缩短缩短了融化耗尽事件后使贫化带向下形成粗大的大陆根，通过纯剪切在垂直方向拉伸贫化了的材料，并使快速结晶轴成为垂直对齐。这种在约150 km处的地震构造是形成克拉通根的缩短事件的证据。