We report palaeomagnetic and 40Ar/39Ar dating results from two sequences of basaltic lava flows deposited at the same locality in western China, yet separated in time by ~50 Myr: one set lies within the Cretaceous normal superchron at 112–115 Ma and a second at 59–70 Ma spanning the Cretaceous‐Palaeogene boundary. We find that magnetic field directions during the superchron exhibit bimodal populations: one with inclinations representative of a dipolar field and another with shallow inclinations that could reflect a more complex, multipolar field. However, the time‐dependent variability in field directions was 50% lower during the superchron than after, which implies greater field stability during the superchron. Our results suggest that episodes of less dipolar field behavior occurred within the Cretaceous superchron and raise the question whether a second, more multipolar, field state is more persistent than previously thought.

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Geophysical imaging of the Yellowstone supervolcano shows a broad zone of partial melt interrupted by an amagmatic gap at depths of 15-20 km. We reproduce this structure through a series of regional-scale magmatic-thermomechanical forward models which assume that magmatic dikes stall at rheologic discontinuities in the crust. We find that basaltic magmas accumulate at the Moho and at the brittle-ductile transition, which naturally forms at depths of 5-10 km. This leads to the development of a 10-to 15-km thick midcrustal sill complex with a top at a depth of approximately 10 km, consistent with geophysical observations of the pre-Yellowstone hot spot track. We show a linear relationship between melting rates in the mantle and rhyolite eruption rates along the hot spot track. Finally, melt production rates from our models suggest that the Yellowstone plume is~175°C hotter than the surrounding mantle and that the thickness of the overlying lithosphere is~80 km. Plain Language SummaryWe present a series of supercomputer models which we use to investigate the origins of the two-level magmatic system revealed by recent geophysical observations of the Yellowstone supervolcano. We show that the distribution of melt which matches these observations arises when we assume that rising magmas preferentially accumulate at depths where there are strong contrasts in the ratio of melt overpressure to the effective viscosity of the surrounding crust. Melt accumulates at the major rock strength discontinuities which occur at the base of the crust and at the brittle-ductile transition which forms above the developing magmatic system at approximately 10-km depth. This second boundary captures the considerable majority of the melt and produces a basaltic sill complex which resides between depths of 10 and 25 km and which provides heat which melts the surrounding crust. This sill complex cools and solidifies and separates the partially molten crust above and below it into the two magmatic systems seen in the geophysical images. Finally, we are able to constrain the temperature of the Yellowstone mantle plume to be 175°C hotter than the surrounding mantle and the thickness of the overlying lithosphere to be approximately 80 km. Model SetupWe performed several dozen 2-D numerical experiments exploring a broad parameter space using a 1,000 × 300-km finite difference grid with a resolution of 2 km, with 16 or more Lagrangian markers per cell, a time step of 5 kyr, and a total duration of 7-8 Myr. We are interested in the behavior of the crustal magmatic system which develops over a relatively stable mantle plume tail, rather than over the plume head which is COLÓN ET AL. 3873

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