Two contrasting Phanerozoic orogenic systems on Earth revealed by Hf isotope data from zircon

Arthur Holmes would be fascinated by how we can now use a combination of isotopic dating and tracing from the simple mineral zircon, a production of granite magmatism, to present a global view of plate tectonic process for the entire Phanerozoic. We already know that two fundamentally different orogenic belts presently exist on Earth--the dominantly accretionary Circum-Pacific and the dominantly collisional Alpine-Himalayan-Indonesian Orogens--but we can show through zircon studies that they are part of only two orogenic systems that have persisted for approximately 550 million years. Through a global compilation of zircon Hf isotope analyses (>3200 datapoints), we show that contrasting Hf isotope arrays for both orogenic systems are caused by differing subduction symmetry, which controls the nature of the lower plate, potential magma sources, and orogen dynamics. Within the external (Circum-Pacific) system, ancient lower crust and subcontinental lithospheric mantle (SCLM) are progressively removed or isolated, primarily by thermal erosion and subduction retreat respectively, and replaced with juvenile crust. By contrast, within the internal (including Alpine-Himalayan) system, they are replaced by ancient crust and SCLM usually similar to that already removed during or following terminal continental collision. This model reinforces the concept that the planet can be divided into two mantle supercells, separated by the Circum-Pacific subduction system, and that consistent, but contrasting, long-term deep-mantle flow patterns can be deduced from the history of these two orogenic systems. The model introduces a new method for reconstructing supercontinental cycles and provides a different basis for unraveling the global geodynamic evolution of the ancient Earth.