Microplate Solutions to Crustal Growth and Metal Endowment in Modern Back-Arc Basins and Ancient Greenstone Belts

Abstract Ore formation throughout Earth’s history has tracked major pulses of crustal growth. The spectacular endowment of some greenstone belts, in particular, has been linked to high heat flow and extensive Archean rifting. Three aspects were likely important: (1) greater numbers of plates required to dissipate the heat; (2) abundant crustal-scale transcurrent faults to accommodate plate growth; and (3) increased hydrothermal convection to cool the crust at the plate boundaries. Because the plates were smaller and more numerous than today, the total ridge length was greater, thus allowing for more efficient cooling of the newly formed crust. Mantle upwelling and rifting between microplate domains focused melts and fluids into well-mineralized corridors. This tectonic style is observed today at the Indo-Australian margin, providing clues to the crustal architecture of some well-endowed Archean terranes, such as the Abitibi greenstone belt in the Superior province of Canada. Hot, thickened oceanic crust, like that of the modern Lau basin and North Fiji basin, has strong similarities to mineral-rich greenstone belts like the Abitibi in terms of structure, kinematics, and magmatic evolution. The majority of this crust formed during a basin-wide microplate “breakout” that occurred in response to the collision of Australia with the Ontong Java and Melanesian Border plateaus in the Late Miocene. Today, the back-arc basins contain some of the fastest growing crust on Earth and an extraordinary concentration of magmatic and hydrothermal activity. In the northern Lau basin, at least seven distinct microplates formed within the last 5 m.y., with crustal growth partitioned across numerous simultaneously active plate boundaries in a complex microplate mosaic. The plates are bound by active spreading centers, ridges, and shear zones that are continuously deforming in response to plate rotation. Basin opening is dominated by many short, slow-spreading segments between large-scale transcurrent fault zones, with a combined strike length of spreading centers greater than in any other back-arc basin in the western Pacific. Seismic sections to depths of at least 20 km show that the plate boundaries are broad zones of deformation characterized by overlapping spreading centers, ridge jumps, and extensional transforms. Increased crustal permeability occurs where multiple spreading centers intersect (i.e., at triple junctions) with enhanced magmatic and hydrothermal activity at the plate boundaries. Seismic velocities and volcanic geochemistry also show large variations in crustal composition between the plates, indicating that the back-arc region is far more complex than supposed in earlier models. We suggest crustal growth and mineral endowment in some greenstone belts were similarly regulated by microplate formation. Because the microplates behave independently, often at great distances from the nearest subduction zone, their formation is akin to autochthonous growth in the Archean when subduction-zone processes were either absent or in their infancy. Compelling evidence of this architecture is now being revealed in the Abitibi greenstone belt by modeling of the Archean Moho topography.