Inverted metamorphic gradient in the Zanhuang nappe/thrust system, north China indicates large-scale thrust stacking in an Archean Orogen

Abstract

We report a dramatic inverted metamorphic gradient across an Archean suture zone in the North China craton. The upper plate of the suture experienced metamorphic conditions of 650–700 °C/7–11 kbar, whereas the lower plate experienced temperatures of 500–600 °C, and pressures of 4-6 kbar, rising slightly in the frontal thrusts of the orogen. Importantly, we show that metamorphic conditions drop from 11 kbar/700 °C to 4 kbar/500 °C over a mere 1 km across-strike, representing a depth difference of almost 20 km across the frontal thrust fault. P-T trajectories derived from the upper plate record decompression, whereas those from the lower plate record a prograde path with increasing temperature. We show that the inverted metamorphic gradient formed during emplacement of thrust sheets during an Archean arc/continent collision, accommodating horizontal transport of at least hundreds of kilometers 2.5 billion years ago. Peclet number analysis shows that the inverted gradient formed with a thrusting rate of ∼ 30 mm/yr, consistent with modern analogues. This finding enhances our understanding of Archean tectonics and highlights similar convergence rates and thermal structure of orogens between Archean and modern examples.