FLAT SLAB SUBDUCTION DYNAMICS IN A MANTLE REFERENCE FRAME: EXAMPLES FROM THE ANDES, MEXICO, AND TIBET

Flat slab subduction occurs when a slab bend is offset towards the upper plate relative to the trench. Key examples are found below the Andes, and are inferred for the Laramide orogeny in western North America. Drivers of flat slab subduction are often searched on the downgoing plate or at the plate contact – are buoyancy or suction forces preventing steep subduction? Here, we focus on the role of absolute upper plate- , and trench motion. We show that in the Andes, the South American plate has been moving westwards relative to the mantle at 2 cm/yr, leading to up to ~400 km of shortening in the Andes since ~50 Ma. The modern slab bend in non-flat slab segments is located close to the trench, showing that the subducting Nazca slab has been rolling back at a more or less similar rate. The ~250 km wide modern Peruvian and Pampian flat slab segments formed since ~11-12 Ma, during which there was ~1000 km of Nazca plate subduction. This shows that in these segments, roll-back was impeded – but subduction was not. We propose that, in concert with other drivers, an overpressured sub-slab mantle supporting the weight of the slab in an advancing upper plate-motion setting can locally impede rollback and maintain flat slabs until slab tearing releases the overpressure. Releasing the overpressure ultimately re-establishes a continuous slab and allows the process to recur, providing a mechanism for the transient character of flat slabs.

A deep-time slab-fitted mantle reference frame shows that North America also moved westward relative to the mantle since the early Mesozoic. We show that the longest slab in Earth, the Cocos slab (reaching as deep as ~2500 km), has been subducting since ~220 Ma below Mexico. During absolute westward North American plate motion, slab bend stagnation corresponds to upper plate shortening, flat slab subduction west of the bend and the formation of mid-mantle vertical slab walls. Upper plate extension coincides with episodes of roll-back exceeding rates of upper plate advance, and – counterintuitively – the formation of flat-lying slab segments in the mantle transition zone.

Finally, we show that resistance against lower mantle subduction after ~50 Ma below Tibet led to slab bend advance and slab overturning, causing flat slab subduction and upper plate shortening while the upper, Eurasian plate was nearly mantle-stationary.