Abstract
Flat slab subduction is an enigmatic style of subduction where the slab attains a horizontal orientation for up to several hundred kilometers below the base of the overriding plate. It has been linked to the subduction of buoyant aseismic ridges or plateaus, but the spatial correlation is problematic, as there are subducting aseismic ridges and plateaus that do not produce a flat slab, most notably in the Western Pacific, and there are flat slabs without an aseismic ridge or plateau. In this paper an alternative hypothesis is investigated which poses that flat slab subduction is associated with subduction zones that are both old (active for a long time) and wide (large trench-parallel extent). A global subduction zone compilation is presented showing that flat slabs preferentially occur at old (> ~80-100 Myr) and wide (≥ ~6000 km) subduction zones. This is explained by the tendency for wide subduction zones to decrease their dip angle in the uppermost mantle with progressive time, especially in the center. A set of numerical subduction models confirms this behavior, showing that only the central parts of wide slabs progressively reduce their slab dip, such that slab flattening, and ultimately flat slab subduction, can occur. The models further show that a progressive decrease in slab dip angle for wide slabs leads to increased vertical deviatoric tensional stresses at the top surface of the slab (mantle wedge suction), facilitating flat slab subduction, while narrow slabs retain steep dip angles and low vertical deviatoric tensional stresses. The results provide a potential explanation why present-day flat slabs only occur in the Eastern Pacific, as only here subduction zones were old and wide enough to initiate flat slab subduction, and why Laramide flat slab subduction and South China flat slab subduction were possible in the geological past.
Highlights
The cross-sectional geometry of upper mantle slabs varies considerably on Earth, with variations in slab dip angle, bending curvature and potential deflection of the slab in the mantle transition zone (e.g., Jarrard, 1986; Lallemand et al, 2005)
The data generally plot in two clusters: (1) One cluster that plots toward the bottom left where most subduction zones are relatively narrow and young (e.g., Scotia, Manila, North Sulawesi, Nankai-Ryukyu, Puysegur), some are older but narrow (e.g., Hellenic, Cascadia, Lesser Antilles-Puerto Rico) and some are wide but young (e.g., Melanesia, Sunda, Northwest Pacific); and (2) Another cluster that is located toward the top right where subduction zones are both old and wide (e.g., South America)
Mid Triassic-Early Jurassic Flat Slab Subduction in South China A model of flat slab subduction has been proposed to explain the broad intracontinental Mesozoic orogen in South China and the postorogenic magmatism in the region (Li and Li, 2007), with flat slab subduction starting in the mid Triassic (∼230 Ma) (Li and Li, 2007) along a subduction zone in the paleo Western Pacific
Summary
The cross-sectional geometry of upper mantle slabs varies considerably on Earth, with variations in slab dip angle, bending curvature and potential deflection of the slab in the mantle transition zone (e.g., Jarrard, 1986; Lallemand et al, 2005). The. Controls on Flat Slab Subduction latter, with three slab hinges, has one convex-upward hinge near the trench, one concave upward hinge that marks the start of a flat slab segment dipping ≤10◦, and one convex-upward hinge that marks the end of the flat slab segment (Figure 1D). Controls on Flat Slab Subduction latter, with three slab hinges, has one convex-upward hinge near the trench, one concave upward hinge that marks the start of a flat slab segment dipping ≤10◦, and one convex-upward hinge that marks the end of the flat slab segment (Figure 1D) It is this subduction geometry, with three slab hinges that is most enigmatic and that is the subject of this study. Several examples have been proposed for the geological past, such as western North America, Central Andes and South China (Henderson et al, 1984; Li and Li, 2007; Ramos and Folguera, 2009)
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