The nappe rule: why does it work?

The nappe rule stresses that imbrication emplaces nappes with higher metamorphic grade above those with lower grade. This means that the overriding nappes must be exhuming during nappe stacking; otherwise metamorphic grade would increase structurally downwards. The high-grade metamorphic internides of many orogens are characterized by a pervasive nappe-emplacement-related subhorizontal foliation indicative of vertical ductile shortening (thinning). By quantifying finite strain in the Gran Paradiso Massif, Italian Alps, we show that vertical ductile thinning associated with a nappe-emplacement-related subhorizontal foliation caused sufficient exhumation of the overriding nappes and can thus explain the occurrence of higher-grade rocks above lower-grade ones.

推覆体法则强调，叠瓦构造会将变质级更高的推覆体（nappe）就位在变质级更低的推覆体之上。这意味着在推覆体堆叠过程中，上覆推覆体必须处于折返状态；否则变质级将会沿构造方向向下（即向地壳深部）升高。许多造山带的高级变质内部带均以广泛发育的、与推覆体就位相关的近水平面理为特征，这类面理指示了韧性垂向缩短（减薄）作用。通过对意大利阿尔卑斯山格兰帕拉迪索地块（Gran Paradiso Massif）的有限应变进行定量分析，我们证明了与推覆体就位相关的近水平面理所伴生的韧性垂向减薄，足以使上覆推覆体发生有效折返，从而可以解释高级变质岩出露于低级变质岩之上这一现象。