Orogeny and mantle dynamics : Holmes (1931) revisited

Orogeny, i.e. the formation of mountains, represents perhaps the most spectacular surface expression of plate tectonics. Orogenic belts are regions in the continental lithosphere where horizontal shortening results in crustal thickening, accompanied by magmatism, metamorphism, and the formation of Earth's highest topographic reliefs. The search for the dynamic mechanisms underlying orogeny has been ongoing since the early days of geology. Building upon the influential work of Holmes (1931), we delve into the relationship between orogeny style and mantle dynamics. Here, I propose to distinguish between two types of orogeny: "subduction orogeny," associated with one-sided upper mantle subduction, and "mantle orogeny," linked to larger mantle convection cells. Extreme crustal thickening is a hallmark of the latter. Our proposition suggests that mantle orogeny is triggered by the penetration of slabs into the lower mantle, altering convection length scales. Numerical dynamic models support this idea, demonstrating that upper plate compression is linked to slab penetration into the lower mantle. Slabs can further induce buoyant plume upwelling intensifying whole mantle convection and upper plate compression. To validate this model, we examine the geological and topography evolution record. Present-day compressional backarc regions often coincide with slabs subducting into the deep lower mantle. The temporal evolution of Nazca and Tethyan slabs, along with associated Andean Cordillera and Tibetan-Himalayan orogenies, suggests that extreme crustal thickening beneath Bolivia and the Tibetan plateau occurred during slab penetration into the lower mantle. This thickening in the Tertiary period bears similarities to Pangea assembly events. We propose that Late Paleozoic large-scale compression is related to a shift from transient slab ponding in the transition zone to lower mantle subduction. If our model holds true, the geological record of orogeny in continental lithosphere can be a valuable tool for understanding time-dependent mantle convection. Episodic lower mantle subduction may, in turn, be causally linked to the supercontinental cycle.