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

Yating Zhong ^a,b, Kurt Stüwe ^b, Timothy Kusky ^a,c,*, Christoph A. Hauzenberger ^b, Simon Schorn ^b,d, Lu Wang ^a

^a State Key Lab for Geological Processes and Mineral Resources, and Center for Global Tectonics, School of Earth Sciences, China University of Geosciences, Wuhan 430074, China

^bInstitut für Erdwissenschaften, Karl-Franzens-Universitat ¨Graz, Universitatsplatz ¨ 2/II, 8010 Graz, Austria

^c Badong National Observatory and Research Station for Geohazards, China University of Geosciences, Wuhan, China

^d Institute of Geosciences, Johannes Gutenberg University, J. Becher Weg 21, D 55128 Mainz, Germany

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.

Original source: Zhong, Y. T., Stuwe, K., Kusky, T.*, Hauzenberger, C.A., Schorn, S., Wang, L. 2024. Inverted metamorphic gradient in the Zanhuang nappe/thrust system, north China indicates large-scale thrust stacking in an Archean Orogen. Earth and Planetary Science Letters 651: 119121. https://doi.org/10.1130/G49599.1.