The importance of slab–slab interactions is manifested in the kinematics and geometry of the Philippine Sea Plate and western Pacific subduction zones, and such interactions offer a dynamic basis for the first-order observations in this complex subduction setting. The westward subduction of the Pacific Sea Plate changes, along-strike, from single slab subduction beneath Japan, to a double-subduction setting where Pacific subduction beneath the Philippine Sea Plate occurs in tandem with westward subduction of the Philippine Sea Plate beneath Eurasia. Our 3-D numerical models show that there are fundamental differences between single slab systems and double slab systems where both subduction systems have the same vergence. We find that the observed kinematics and slab geometry of the Pacific–Philippine subduction can be understood by considering an along-strike transition from single to double subduction, and is largely independent from the detailed geometry of the Philippine Sea Plate. Important first order features include the relatively shallow slab dip, retreating/stationary trenches, and rapid subduction for single slab systems (Pacific Plate subducting under Japan), and front slabs within a double slab system (Philippine Sea Plate subducting at Ryukyu). In contrast, steep to overturned slab dips, advancing trench motion, and slower subduction occurs for rear slabs in a double slab setting (Pacific subducting at the Izu–Bonin–Mariana). This happens because of a relative build-up of pressure in the asthenosphere beneath the Philippine Sea Plate, where the asthenosphere is constrained between the converging Ryukyu and Izu–Bonin–Mariana slabs. When weak back-arc regions are included, slab–slab convergence rates slow and the middle (Philippine) plate extends, which leads to reduced pressure build up and reduced slab–slab coupling. Models without back-arcs, or with back-arc viscosities that are reduced by a factor of five, produce kinematics compatible with present-day observations.
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Subduction Duration and Slab Dip
The dip angles of slabs are among the clearest characteristics of subduction zones, but the factors that control them remain obscure. Here, slab dip angles and subduction parameters, including subduction duration, the nature of the overriding plate, slab age, and convergence rate, are determined for 153 transects along subduction zones for the present day. We present a comprehensive tabulation of subduction duration based on isotopic ages of arc initiation and stratigraphic, structural, plate tectonic and seismic indicators of subduction initiation. We present two ages for subduction zones, a long‐term age and a reinitiation age. Using cross correlation and multivariate regression, we find that (1) subduction duration is the primary parameter controlling slab dips with slabs tending to have shallower dips at subduction zones that have been in existence longer; (2) the long‐term age of subduction duration better explains variation of shallow dip than reinitiation age; (3) overriding plate nature could influence shallow dip angle, where slabs below continents tend to have shallower dips; (4) slab age contributes to slab dip, with younger slabs having steeper shallow dips; and (5) the relations between slab dip and subduction parameters are depth dependent, where the ability of subduction duration and overriding plate nature to explain observed variation decreases with depth. The analysis emphasizes the importance of subduction history and the long‐term regional state of a subduction zone in determining slab dip and is consistent with mechanical models of subduction.
more » « less- Award ID(s):
- 1645775
- NSF-PAR ID:
- 10456998
- Publisher / Repository:
- DOI PREFIX: 10.1029
- Date Published:
- Journal Name:
- Geochemistry, Geophysics, Geosystems
- Volume:
- 21
- Issue:
- 4
- ISSN:
- 1525-2027
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Abstract The along‐strike variations of the velocity, thickness, and dip of subducting slabs and the volcano distribution have been observed globally. It is, however, unclear what controls the distribution of volcanoes and the associated magma generation. With the presence of nonuniform volcanism, the Aleutian‐Alaska subduction zone (AASZ) is an ideal place to investigate subduction segmentation and its relationship with volcanism. Using full‐wave ambient noise tomography, we present a high‐resolution 3‐D shear wave velocity model of the AASZ for the depths of 15–110 km. The velocity model reveals the distinct high‐velocity Pacific slab, the thicker, flatter, and more heterogeneous Yakutat slab, and the northeasterly dipping Wrangell slab. We observe low velocities within the uppermost mantle (at depth <60 km) below the Aleutian arc volcanoes, representing partial melt accumulation. The large crustal low‐velocity anomaly beneath the Wrangell volcanic field suggests a large magma reservoir, likely responsible for the clustering of volcanoes. The Denali volcanic gap is above an average‐velocity crust but an extremely fast mantle wedge, suggesting the lack of subsurface melt. This is in contrast with the lower‐velocity back‐arc mantle beneath the adjacent Buzzard Creek‐Jumbo Dome volcanoes to the east. The back‐arc low velocities associated with the Pacific, the eastern Yakutat, and the Wrangell slabs may reflect subduction‐driven mantle upwelling. The structural variation of the downgoing slabs and the overriding plate explains the change of volcanic activity along the AASZ. Our findings demonstrate the combined role of the subducting slab and the overriding plate in controlling the characteristics of arc magmatism.
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Abstract Subduction zones host some of Earth's most damaging natural hazards, including megathrust earthquakes and earthquake‐induced tsunamis. A major control on the initiation and rupture characteristics of subduction megathrust earthquakes is how the coupled zone along the subduction interface accumulates elastic strain between events. We present results from observations of slow slip events (SSEs) in Cascadia occurring during the interseismic period downdip of the fully coupled zone, which imply that the orientation of strain accumulation within the coupled zone can vary with depth. Interseismic GPS motions suggest that forces derived from relative plate motions across a shallow, offshore locked plate interface dominate over decadal timescales. Deeper on the plate interface, below the locked (seismogenic) patch, slip during SSEs dominantly occurs in the updip direction, reflecting a dip‐parallel force acting on the slab, such as slab pull. This implies that in subduction zones with obliquely convergent plate motions, the seismogenic zone of the megathrust is loaded by forces acting in two discrete directions, leading to a depth‐varying orientation of strain accumulation on the plate interface.
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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.
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SUMMARY Spatio-temporal variability in arc geochemistry and the conditions recorded by exhumed rocks suggest subduction zone thermal structure evolves in time and along-strike. Although much effort has been dedicated to studying subduction zone thermal structure, we lack an understanding of spatio-temporal temperature variability during time-dependent subduction. We model 3-D, dynamic subduction and examine the time evolution of the along-strike temperature difference of the slab’s upper surface (‘slab-top’) at the centre relative to the edge of the subduction zone. We examine this slab-top temperature variability for subduction systems of different widths and with different plate mobilities (i.e. fixed versus free subducting and overriding plates). In all of our models, the main control on slab-top temperature is convergence rate; either by simply controlling the rate of slab sinking or via the effect it has on the decoupling depth (DD). In the early stages of subduction, more rapid convergence at the plate centre produces a cooler slab relative to warmer slab edges. For mature subduction, this flips; a shallower DD at the slab centre produces warmer temperatures with respect to the edge. Importantly, our maximum along-strike temperature changes are reduced (≤50 °C) relative to previous kinematically driven modelling studies, due to a reduced role for slab-top heating via toroidal flow. Our dynamic subduction models, therefore, point towards a strong time dependence in the sense of along-strike temperature variation, but with relatively low absolute values in geometrically simple subduction zones.
