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A model of the subducting Farallon slab under North America. In geology, the slab is a significant constituent of subduction zones. [1] Subduction slabs drive plate tectonics by pulling along the lithosphere to which they attach in a process known as slab pull and by inducing currents in the mantle via slab suction. [2]
The subduction of bathymetric highs such as aseismic ridges, oceanic plateaus, and seamounts has been posited as the primary driver of flat slab subduction. [3] The Andean flat slab subduction zones, the Peruvian slab and the Pampean (Chilean) flat slab, are spatially correlated with the subduction of bathymetric highs, the Nazca Ridge and the Juan Fernandéz Ridge, respectively.
The flat-slab has caused an uplift of Sierras Pampeanas which begun first in the north and then moved southwards over millions of years. [2] The oldest noted uplift episode associated with Pampean flat-slab is that of Sierra de Aconquija (27 °S) from 7.6 to 6 million years ago (Ma) in the Late Miocene epoch. [2]
Flat-slab subduction is ongoing beneath part of the Andes, causing segmentation of the Andean Volcanic Belt into four zones. The flat-slab subduction in northern Peru and the Norte Chico region of Chile is believed to be the result of the subduction of two buoyant aseismic ridges, the Nazca Ridge and the Juan Fernández Ridge, respectively
[1] [11] Metamorphic dehydration reactions are prominent within the subducting slab during subduction, giving rise to liquid phases that contain fluid-mobile trace elements due to the breakdown of hydrous minerals such as phengite, lawsonite and zoisite. [14] This forms a unique type of trace element distribution pattern for arc magma. [3]
Slab pull is the concept that plate motion is driven by the weight of cool, dense plates and that heavier plates will begin to subduct. [2] Once a descending slab is disconnected there must be a force that continues subduction. Eclogitization is the mechanism for continuing subduction after slab detachment in a subduction zone. [1]
However, the phase transitions may still play a role in the behavior of slabs at depth. Schellart et al. showed that the 660-km phase transition may serve to deflect downgoing slabs. [11] The shape of the subduction zone was also key in whether the geometry of the slab could overcome the phase transition boundary. [12]
Slab suction is weaker than slab pull, which is the strongest of the driving forces. When measuring the forces of these two mechanisms, slab pull in subducting plate boundaries for upper mantle slabs is 1.9 × 10^21 N. [ clarification needed ] In comparison slab suction in the upper and lower mantle totaled 1.6 × 10^21 N. [ 3 ]