We document well-preserved extensional shear zones in a rift-related lower-crustal magmatic conduit system, the Reinfjord Ultramafic Complex. Deformation along the shear zones led to seismic events below the seismogenic zone of continental rifts (10–15 kbar and 850–1150 °C), similar to events observed along the East African Rift today. Processes leading to failure of the weakened rocks caused extremely high strain. We mapped pseudotachylytes along a network of millimeter to meter-scale transtensional shear zones associated with gabbronoritic dykes, and a 2 km long low-angle extensional shear zone. Deformation, initiated through priming of the rock by magmatic fluids, exploited subgrains and microfractures in olivine, with CO₂-bearing fluids leading to volume expanding reactions (olivine + diopside + CO₂ = dolomite + enstatite), enhancing olivine fracturing. Fragmentation of the olivine grains and the addition of weaker phases facilitated strain localization and increased the strain rate by two orders of magnitude through grain size reduction (Sørensen et al., 2019). Sudden failure under transient brittle conditions formed pseudotachylytes. We conclude that failure along shear zones in lower crustal continental rifts is possible without remote stress events, in the presence of pre-existing textural and structural heterogeneities and events of volatile fluid infiltration.

我们记录了与裂谷相关的下地壳岩浆导管系统 Reinfjord Ultramafic Complex 中保存完好的伸展剪切带。沿剪切带的变形导致了大陆裂谷（10-15 kbar 和 850-1150 °C）以下的地震事件，类似于今天沿东非裂谷观察到的事件。导致弱化岩石破坏的过程会导致极高的应变。我们沿着与辉长岩脉相关的毫米到米尺度的横向剪切带和 2 公里长的低角度拉伸剪切带绘制了假速溶物。变形，通过岩浆流体引发岩石引发，利用橄榄石中的亚晶粒和微裂缝，带有 CO ₂的流体导致体积膨胀反应（橄榄石 + 透辉石 + CO₂  = 白云石 + 顽火辉石），增强橄榄石压裂。橄榄石晶粒的破碎和较弱相的添加促进了应变局部化，并通过减小晶粒尺寸将应变速率提高了两个数量级（Sørensen 等人，2019 年）。在瞬态脆性条件下突然失效形成假速溶物。我们得出结论，在存在预先存在的纹理和结构异质性以及挥发性流体渗透事件的情况下，在没有远程应力事件的情况下，下地壳大陆裂谷沿剪切带的破坏是可能的。