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.
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The MATLAB code used for the thermal modelling is available from the corresponding authors upon request.
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A.J.S. acknowledges support from the Slingerland Early Career Fellowship at Penn State and NSF grant EAR-2025122. J.M.G. acknowledges postdoctoral support from NSF Grant OISE-1545903 and Penn State. L. Jolivet, B. Hacker, R. Rudnick, L. Lavier, J. Reimink and R. Holder are thanked for comments and discussion that improved earlier versions of the manuscript. R. Rudnick and M. Ringwood are thanked for lending samples. S. Seman is thanked for assisting with fieldwork. Figure 1 was prepared using GMT 6.168.
The authors declare no competing interests.
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Yellow field shows peak assemblage; blue and red bars represent range of measured Ti-in-zircon and Zr-in-rutile T-estimates. Dashed red lines represent melt mode isopleths (in mol.%).
Extended Data Fig. 2 Mantle pseudosections for Kilbourne Hole lherzolite xenoliths KH-11, KH-29, and KLB-1.
The assemblage for KBH lherzolites (clinopyroxene-orthopyroxene-olivine-spinel) is outlined in green. Average temperatures Fe-Mg and Na exchange between coexisting clinopyroxene and orthopyroxenes are calculated using the calibration of Brey & Köhler (1990)31.
Each pattern represents a single zone from LASS-ICP-MS depth profiling as described in Methods. Zones with U-Pb ages dated to early in the RGR history have flat HREE patterns, suggesting that garnet was present in these rocks at this time — consistent with higher pressures from thicker overlying crust.
Extended Data Fig. 5 Select major and trace element diffusion length scales for Kilbourne Hole xenolith magmatic entrainment timescales.
Characteristic length scales of diffusion were calculated for key mineral chemistry used in xenolith thermobarometry: Ca-Na exchange in plagioclase70 a, Ti-in-quartz71 b, Garnet major elements at graphite-oxygen fO272 c, and Zr-in-rutile73 (d). Published magmatic heating timescales for KBH xenoliths have been estimated from Ca-in-olivine profiles36 and are shown as orange highlighted areas.
Black contours show characteristic length scale of diffusion (√κt) for range of times since eruption of KBH xenoliths. Date of eruption is from ref. 22.
Extended Data Fig. 7 Thermal-kinematic calculations of a lithospheric column undergoing lithospheric thinning.
a, evolution of lithospheric geotherm following instantaneous juxtaposition of asthenosphere and lithosphere at 55 km, 35 Ma. Black markers represent KBH PT constraints discussed in main text; geotherm plotted every 1 Myr. b, evolution of lithospheric geotherm during depth-dependent extension in which the crust and mantle are thinned by factors of 1.25 and 6, respectively.
Extended Data Fig. 8 Thermal-kinematic calculations of a lithospheric column undergoing depth-dependent extension.
Each panel shows the lithospheric geotherm (red line) and PTt paths (black markers and lines) for rocks at initial depths of 25, 30, 35 and 40 km, undergoing different combinations of mantle (δ) and crustal thinning (β). The duration of each model calculation is 30 Myr and geotherms and marker nodes are plotted at 5 Myr increments. White markers are PT estimates from a global compilation of exhumed granulites74. All calculations assume an initial lithospheric thickness of 125 km, a crustal thickness of 40 km, a mantle potential temperature of 1330 °C, an exponential distribution of heat production throughout the crust with a surface value of 3 μW.m-3 and an e-folding length of 8 km, crustal and mantle densities of 2.9 and 3.3 kg.m-3 respectively, and crustal and mantle and thermal diffusivities of 10-6 and 8x10-7 m2.s-1, respectively. Calculations do not take into account the effects of latent heat or the temperature-dependence of thermal conductivity. These models illustrate that replacement of the lowermost lithosphere by hotter, less dense asthenospheric mantle drives conductive heating of the remaining, overlying lithosphere; the timescale of metamorphism associated with this heating is controlled by the thickness of, and rate of extension within, the overlying lithosphere. Provided that the rate of crustal thinning is less than the rate of heat conduction through the crust, peak metamorphic T may occur during decompression. Many exhumed granulite terranes record a high-T clockwise-sense PT segment, and the tectonic sequence represented by these models (crustal thickening, lithospheric removal, and extension) is recognized in a number of modern orogens: (1) Tibet75, (2) Sierra Nevada76, (3) Anatolian plateau77, (4) south-central Appalachians78, (5) Alboran Sea and Betic-Rif mountains79.
Extended Data Fig. 9 BSE image showing example of Zr-in-rutile variation with proximity to zircon in sample DEKH2.
Rutiles tend to have lower [Zr] when closer to zircon and higher [Zr] when isolated from zircon.
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Cipar, J.H., Garber, J.M., Kylander-Clark, A.R.C. et al. Active crustal differentiation beneath the Rio Grande Rift. Nat. Geosci. 13, 758–763 (2020). https://doi.org/10.1038/s41561-020-0640-z