Silicon-rich continental crust is unique to Earth. Partial melting during high- to ultrahigh-temperature metamorphism (700 °C to >900 °C) promotes the long-term stability of this crust because it redistributes key elements between the crust and mantle and ultimately produces cooler, more-differentiated continents. Granulites—rocks formerly at high- to ultrahigh-temperature conditions—preserve a record of the stabilization of Earth’s continents, but the tectonic mechanisms that drive granulite formation are enigmatic. Here we present an analysis of lower-crustal xenoliths from the Rio Grande Rift—a nascent zone of extension in the southwestern United States. Uranium–lead geo- and thermochronology combined with thermobarometric modelling show that the lower 10 km of the crust currently resides at granulite-facies conditions, with the lowermost 2 km at ultrahigh-temperature conditions. Crust and mantle xenoliths define a continuous pressure-and-temperature array, indicating that a thin lithospheric mantle lid mediates elevated conductive heat transfer into the crust. These findings establish a direct link among ultrahigh-temperature metamorphism, collapse of the Laramide orogen and lithospheric mantle attenuation. Other indicators of modern ultrahigh-temperature metamorphism are consistent with these conditions prevailing over thousands of square kilometres across the US–Mexico Basin and Range province. Similarities between the pressure-and-temperature path from the Rio Grande lower crust and those from exhumed granulite terranes imply that post-thickening lithospheric extension is a primary mechanism to differentiate Earth’s continental crust.

富含硅的大陆壳是地球独有的。高温至超高温变质（700°C至> 900°C）期间的部分熔化促进了该地壳的长期稳定性，因为它在地壳和地幔之间重新分配了关键元素，最终形成了更凉爽，分化程度更大的大陆。花岗岩-以前是在高温到超高温条件下的岩石-保留了地球各大洲稳定的记录，但驱动花岗岩形成的构造机制是令人迷惑的。在这里，我们对来自美国西南部新生延伸区里奥格兰德大裂谷的下部地壳异岩进行了分析。铀铅地球年代学和热年代学与热压法模型相结合表明，目前地壳的下10 km位于粒岩相条件下，在超高温条件下最低2公里。地壳和地幔的异种岩定义了一个连续的压力和温度阵列，这表明岩石圈薄的地幔盖介导了向地壳内传导的热传导。这些发现在超高温变质，拉拉酰胺造山带崩塌和岩石圈地幔衰减之间建立了直接联系。现代超高温变质作用的其他指标与这些条件在美国–墨西哥盆地和兰治省的数千平方公里中普遍存在。来自里奥格兰德下地壳的压力和温度路径与发掘出的粒状花岗岩的压力和温度路径之间的相似性表明，增稠后的岩石圈扩展是区分地球大陆壳的主要机制。