An official website of the United States government
Abstract A global study of subduction zone dynamics indicates that the thermal structure of the overriding plate may control arc location. A fast convergence rate and a steep slab dip bring a hotter mantle further into the wedge corner, forming arc volcanoes closer to the trench. Separately, laboratory and numerical experiments showed that the development of a back‐arc spreading center (BASC) is driven by the migration of the subducting hinge, especially following changes in the slab geometry. As both arc location and the deformation regime of the overriding plate depend on slab kinematics and geometry, we investigate the possible correlations between BASC, the position of volcanic arcs, and slab dip at the scale of individual subduction zones. To do this, we compare the distance from trench to arc and trench to BASC at the Mariana, Scotia, Vanuatu, Tonga, and Kermadec subduction zones. In most cases, the arc and BASC are closer to the trench when the slab is dipping steeply. The correlation could result from an interplay between progressive changes in slab geometry and overriding plate deformation. This assumes, on the one hand, that the isotherm at the apex of which the arc forms is tied to a constant slab decoupling depth and, on the other hand, that back‐arc opening accommodates a change in slab dip. As slab dip decreases, both the BASC and the apex of the isotherm controlling the melt focusing move further from the trench. The observed trends are consistent with a slab anchored at 660 km depth.
more »
« less
Modeling Subduction With Extremely Fast Trench Retreat
Abstract The Tonga‐Kermadec subduction zone exhibits the fastest observed trench retreat and convergence near its northern end. However, a paradox exists: despite the rapid trench retreat, the Tonga slab maintains a relatively steep dip angle above 400 km depth. The slab turns flat around 400 km, then steepening again until encountering a stagnant segment near 670 km. Despite its significance for understanding slab dynamics, no existing numerical model has successfully demonstrated how such a distinct slab morphology can be generated under the fast convergence. Here we run subduction models that successfully reproduce the slab geometries while incorporating the observed subduction rate. We use a hybrid velocity boundary condition, imposing velocities on the arc and subducting plate while allowing the overriding plate to respond freely. This approach is crucial for achieving a good match between the modeled and observed Tonga slab. The results explain how the detailed slab structure is highly sensitive to physical parameters including the seafloor age and the mantle viscosity. Notably, a nonlinear rheology, where dislocation creep reduces upper mantle viscosity under strong mantle flow, is essential. The weakened upper mantle allows for a faster slab sinking rate, which explains the large dip angle. Our findings highlight the utilizing rheological parameters that lead to extreme viscosity variations within numerical models to achieve an accurate representation of complex subduction systems like the Tonga‐Kermadec zone. Our study opens new avenues for further study of ocean‐ocean subduction systems, advancing our understanding of their role in shaping regional and global tectonics.
more »
« less
- Award ID(s):
- 1928970
- PAR ID:
- 10558317
- Publisher / Repository:
- American Geophysical Union (United States); John Wiley & Sons, Inc
- Date Published:
- Journal Name:
- Journal of Geophysical Research: Solid Earth
- Volume:
- 129
- Issue:
- 9
- ISSN:
- 2169-9313
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
Abstract Most great earthquakes on subduction zone plate boundaries have large coseismic slip concentrated along the contact between the subducting slab and the upper plate crust. On 4 March 2021, a magnitude 7.4 foreshock struck 1 hr 47 min before a magnitude 8.1 earthquake along the northern Kermadec island arc. The mainshock is the largest well‐documented underthrusting event along the ∼2,500‐km long Tonga‐Kermadec subduction zone. Using teleseismic, geodetic, and tsunami data, we find that all substantial coseismic slip in the mainshock is located along the mantle/slab interface at depths from 20 to 55 km, with the large foreshock nucleating near the down‐dip edge. Smaller foreshocks and most aftershocks are located up‐dip of the mainshock, where substantial prior moderate thrust earthquake activity had occurred. The upper plate crust is ∼17 km thick in northern Kermadec with only moderate‐size events along the crust/slab interface. A 1976 sequence withMWvalues of 7.9, 7.8, 7.3, 7.0, and 7.0 that spanned the 2021 rupture zone also involved deep megathrust rupture along the mantle/slab contact, but distinct waveforms exclude repeating ruptures. Variable waveforms for eight deep M6.9+ thrusting earthquakes since 1990 suggest discrete slip patches distributed throughout the region. The ∼300‐km long plate boundary in northern Kermadec is the only documented subduction zone region where the largest modeled interplate earthquakes have ruptured along the mantle/slab interface, suggesting that local frictional properties of the putatively hydrated mantle wedge may involve a dense distribution of Antigorite‐rich patches with high slip rate velocity weakening behavior in this locale.more » « less
-
null (Ed.)Abstract The subducted old and cold Pacific Plate beneath the young Philippine Sea Plate at the Izu‐Bonin trench over the Cenozoic hosts regional deep earthquakes. We investigate slab morphology and stress regimes under different trench motion histories with mantle convection models. Viscosity, temperature, and deviatoric stress are inherently heterogeneous within the slab, which we link to the occurrence of isolated earthquakes. Models expand on previous suggestions that observed slab morphology variations along the Izu‐Bonin subduction zone, exhibited as shallow slab dip angles in the north and steeper dip angles in the south, are mainly due to variations in the rate of trench retreat from the north (where it is fast) to the south (where it is slow). Geodynamic models consistent with the regional plate tectonics, including oceanic plate age, plate convergence rate, and trench motion history, reproduce the seismologically observed principal stress direction and slab morphology. We suggest that the isolated ~680 km deep, 30 May 2015 Mw 7.9 Bonin Islands earthquake, which lies east of the well‐defined Benioff zone and has its principal compressional stress direction oriented toward the tip of the previously defined Benioff zone, can be explained by Pacific slab buckling in response to the slow trench retreat.more » « less
-
Abstract The formation of Lau Basin records an extreme event of plate tectonics, with the associated Tonga trench exhibiting the fastest retreat in the world (16 cm/yr). Yet paleogeographic reconstructions suggest that seafloor spreading in the Lau Basin only initiated around 6 Ma. This kinematics is difficult to reconcile with our present understanding of how subduction drives plate motions. Using numerical models, we propose that eastward migration of the Lau Ridge concurrent with trench retreat explains both the narrow width and thickened crust of the Lau Basin. To match the slab geometry and basin width along the Tonga‐Kermadec trench, our models suggest that fast trench retreat rate of 16 cm/yr might start ~15 Ma. Tonga slab rollback induced vigorous mantle flow underneath the South Fiji Basin which is driving the extension and thinning of the basin and contributing to its observed deeper bathymetry compared to neighboring basins.more » « less
-
Subduction initiation remains one of the least understood global processes of plate tectonics. Prominent models have been cast in terms of two broad classes: spontaneous cases due to lithospheric gravitational instabilities and induced cases due to forced plate convergence. Yet gravitationally unstable lithosphere is old, strong, and difficult to begin to bend into a subduction zone and convergent forces necessary to begin subduction are often too large given the plates involved. These models also consider the asthenospheric mantle as passive, even though relative motion between slabs and the asthenosphere has long been regarded as a strong control on subduction dynamics. Here I propose that subduction-transform edge propagator (STEP) faults can initiate subduction depending on the absolute motion of the STEP fault with respect to the asthenosphere. STEP faults form where subduction zones end and the subducting plate tears forming a down flexed transcurrent plate boundary at the surface shearing against the adjacent rear arc lithospheric plate. However, STEP faults are not simple transcurrent boundaries. Absolute motion of the down flexed STEP fault edge with respect to the surrounding asthenosphere can produce a strong sea anchor force that either continues to bend the edge downward, initiating subduction, or opposes slab bending, inhibiting subduction. In the south Pacific, the southern end of the New Hebrides Trench and the northern end of the Tonga Trench are type-example STEP faults with opposite senses of dip but both moving northward with respect to the asthenosphere. The northward dipping New Hebrides STEP fault moves northward in a mantle reference frame creating a strong asthenospheric flow against the STEP fault edge, inducing active subduction at the Matthew-Hunter trench. In contrast, the Tonga STEP fault dips southward but also has a northward component of motion with respect to the mantle. Asthenosphere thus flows southward beneath the down flexed Tonga STEP fault edge opposing further bending. Subduction does not initiate at the Tonga STEP fault despite a ~100 Myr age contrast between the Pacific and north Fiji and Lau basin lithospheres. Since absolute plate motions reflect the sum of all forces acting on the entire lithospheric plate, a strong sea anchor mantle force may be generated at a STEP fault edge, initiating subduction (or inhibiting it), even where lithosphere is old, strong, and resists bending and without requiring large convergent forces between plates, overcoming these objections to previous models.more » « less
- Free Publicly Accessible Full Text
-
Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1029/2024JB029240
