Syn‐Drift Plate Tectonics

The paradigm of plate tectonics holds that ocean plates are rigid during drift and only experience tectonic deformation at subduction zones, but new findings from the Pacific challenge this idea. Geological and geophysical evidence from the Ontong Java, Shatsky, Hess, and Manihiki oceanic plateaux indicates that extensional deformation during plate drift is a widespread phenomenon across the Pacific plate. These anomalously thick oceanic plateaux are weaker regions of the ocean lithosphere and more prone to tectonic deformation. Numerical geodynamic models demonstrate that a slab pull force from distant subduction plate boundaries can be effectively transmitted to oceanic plateaux through strong ocean lithosphere and cause substantial extension during plate drift. Our findings reveal that a wide expanse of the Pacific has experienced syn‐drift plate tectonics linked to pull from the western Pacific subduction factory.

• Seismic and petrologic data indicate that oceanic plateaux on the Pacific Plate are undergoing extensional deformation during plate drift • Numerical modeling finds a slab pull force may be causing the extension as the force is transmitted far away from the subduction zone • Oceanic plates can experience substantial tectonic deformation during their drift to subduction

Supporting Information:
Supporting Information may be found in the online version of this article.
at a subduction trench.This is outside of the normal plate tectonic paradigm and assumption of the rigidity of drifting ocean plates.
We reconcile these observations with numerical experiments of the oceanic plate/plateaux and western Pacific subduction system.The experiments test the magnitude of extensional deformation in the modeled oceanic plateaux with respect to their distance to the subduction trench.The findings suggest that there may be a genetic link between these enigmatic syn-drift tectonics (deformation that occurs during or together the plate drift phase of plate tectonics) and the consumption of the Pacific plate itself at the western Pacific subduction factory.

Extension and Related Magmatism in the Pacific Oceanic Plateaux
Among the Pacific plateaux, the Ontong Java Plateau (OJP) preserves the most extensive petrologic and structural evidence for extension.It was formed by flood basalts at ∼122 and ∼90 Ma on the ocean floor (Figure 1) (Mahoney et al., 1993;Tejada et al., 1996) and drifted together with the Pacific plate until arrival at the subduction trench near the Solomon Islands at ∼5 Ma (Mann & Taira, 2004).Petrologic, structural, and seismic studies suggest that the OJP underwent a number of episodes of major post-emplacement and syn-drift (pre-collisional) magmatic activity concurrently with extensional deformation prior to its arrival to the subduction trench.
Post-emplacement plateau magmatism started at the Lyra Basin about 65 Ma (Shimizu et al., 2015) and was followed by the intrusion of alnöites (kimberlite-like rocks, but originating from the mantle lithosphere) into the basement rocks and sedimentary cover of the OJP near Malaita Island at ∼52-35 Ma (Simonetti & Neal, 2010) (Figure 1).Contemporaneously, the OJP experienced extrusion of the Maramasike alkaline basalts at 44 Ma (Petterson et al., 1997).The latest known magmatism of the OJP at 20-25 Ma is at a seamount near the trench parallel Stewart Arch (Figure 2a) (Hanyu et al., 2017).Although these various magmatic events took place at different stages of the OJP's drift to the subduction trench, a comparison of trace element compositions of these magmatic rocks shows mostly a similar isotopic fingerprint of a deep lithospheric origin (Hanyu et al., 2017).
Several studies have linked the 44 and 65 Ma events to the passage of the OJP over the Samoan and Rarotonga hotspots (Shimizu et al., 2015;Tejada et al., 1996Tejada et al., , 2015)).However, there is considerable uncertainty in the paleogeographic reconstructions in addition to the similarities in the trace element geochemistry of the OJP's various magmatic episodes.Therefore, lithospheric scale extension and resulting decompression melting (Mckenzie & Bickle, 1988) of the OJP's lithospheric mantle is a simpler explanation for the source of the episodic magmatism rather than associating them to a number of different hotspots.
A number of seismic surveys and interpretations of structural features of the OJP corroborate an interpretation for extensional tectonics suggested by the magmatism from 65 to 20 Ma (Figure 2).Geophysical studies show that the Lyra Basin has horst and graben structures as a result of extension (Kroenke, 1972;Tejada et al., 2015) that may have thinned the crust by up to ∼50% (Gladczenko et al., 1997).The observed structures are synchronous with the basaltic eruptions at ∼65 Ma (Shimizu et al., 2015).Petterson et al. (1997) find extension related normal faults, graben structures, and basin development coincident with the intrusion of the alnöites (52-35 Ma) and extrusion of the Maramasike basalts (44 Ma).The development of normal faulting along the Stewart Arch as observed in seismic data (Figure 2e) has been connected to seamount volcanism from 20 to 25 Ma (Hanyu et al., 2017).Other normal faults in the multichannel seismic reflection profiles (Figures 2a, 2b, and 2f) terminate at the R34 horizon in the OJ3 megasequence (Late Cretaceous to recent), indicating extension in the Middle to Late Miocene (∼15 Ma) (Phinney et al., 1999).
Several authors have posited that subduction related elastic lithospheric flexure may have yielded the observed extension (Coleman & Kroenke, 1981;Hanyu et al., 2017;Neal et al., 1997).Figures 2e and 2f indeed shows normal faults along the Stewart Arch displacing sedimentary strata above the R34 horizon that are likely due to subduction bending effects at ∼4.6-4.0Ma (Phinney et al., 1999).However, this group of faults postdate the many other older syn-drift extensional structures cited above, and the OJP was not in the vicinity of the trench to have experienced an elastic flexural response from subduction during the earlier recorded episodes of extension (∼65-15 Ma).
The Shatsky Rise (SR) oceanic plateau in the northwest Pacific (Figure 1) manifests a similar extensional record to the OJP, with active tectonic stretching as the SR drifts toward the distant (∼1,200 km) Japan and Izu-Bonin trenches.Multi-channel seismic reflection profiles show that large extension-related and trench-parallel/subparallel normal faults, especially on the western flanks of the SR are a common structural feature of the plateau (Figures 1  and 2) (Zhang et al., 2015).The profiles (Figures 2g and 2h) illustrate that normal faults displaced both the igneous basement and young sediment cover, implying that they occurred contemporaneously.Differential subsidence of the plateau flanks related to the volcanic center was initially proposed to explain the normal faults (Ito & Clift, 1998), whereby a buoyant mass (late volcanism) underplated the center massif of the plateau and creating uplift relative to the plateau flanks.However, Zhang et al. (2015) conclude that this model cannot account for the nonuniform distribution of the large normal faults as they are observed only on the western flanks of the plateau.
A buoyant volcanic mass should also create a low velocity lower crustal zone beneath the plateau massifs which is not observed for the SR according to seismic tomography (Korenaga & Sager, 2012).Sea floor spreading on the western side of the plateau can create normal faults, but the orientation of the ocean floor magnetic lineations do not show such correlations (Zhang et al., 2015).
The Hess Rise (HR) oceanic plateau, about ∼900 km to the east of the Shatsky Rise (Figure 1), formed along the Pacific-Farallon ridge at ∼110 Ma (Deep Sea Drilling Project site 464) (Thiede & Vallier, 1981;Vallier et al., 1980).The HR is extensively deformed by grabens and normal faults (Thiede & Vallier, 1981).Based on seismic reflection data, the vertical offset along the normal faults can be up to up to 3,000 m and they create horst-graben and stair-step structures (Figure 2i) which are not necessarily parallel to the general morphologic trends of the plateau.The timing of the extensional deformation postdates the initial formation of the HR and is proposed to have occurred in the mid-Eocene (85 to 32 Ma) (Kroenke & Nemoto, 1982;Thiede & Vallier, 1981).Owing to the observed changes in the thickness of the strata near the faults (growth faults; Figure 2i), it is clear that these extensional structures were active during sedimentation (Vallier et al., 1983).The extensional deformation in the HR continued later in the Cenozoic, but on a smaller scale than the earlier extensional tectonics (Kroenke & Nemoto, 1982).
The Manihiki Plateau (MP) is ∼850 km distant from the Tonga-Kermadec subduction zone (Figure 1), and has been interpreted to have formed at ∼123 Ma together with the Hikurangi and Ontong Java plateaux as a  , 2015).(e and f) Seismic reflection profiles (a-a´) from the OJP (Phinney et al., 1999).OJ1, OJ2, and OJ3 represent early Cretaceous basement, Cretaceous mudstone and carbonates, late Cretaceous through pelagic carbonate, respectively (Phinney et al., 1999).(g and h) Seismic reflection profiles (b-b´ and c-c´) from the SR.Large normal faults are located mainly on the trench/western side of the SR and cut young sediment layers (Sager et al., 2013;Zhang et al., 2015).(i) Seismic reflection profile (d-d´) from the HR.Wide grabens and normal faults started to form at 82 Ma and are most probably still active since the young strata is displaced by the extensional structures (Vallier et al., 1981(Vallier et al., , 1983)).(j and k) Seismic reflection profile (e-e´) from the MP.The cross section cuts the Suvarov Trough, grabens, and normal faults (Pietsch & Uenzelmann-Neben, 2016).
part of the greater Ontong Java (Taylor, 2006;Timm et al., 2011;Winterer et al., 1974).The MP fragmented into three sub-plateaux by faulting soon after emplacement: the North Plateau, Western Plateaux, and High Plateau (Winterer et al., 1974).The Danger Island Trough, a syn-emplacement sinistral fault, marks the separation between the Western and North plateaux from the High Plateau (Nakanishi et al., 2015).The High Plateau is riven by a 220 km long and 10-15 km wide extensional graben, the Suvarov Trough, and another 70 km long unnamed parallel depression (Figure 2d) and widely distributed normal faults and graben systems that focus on the southeastern side of the MP (Pietsch & Uenzelmann-Neben, 2016).It has been suggested that the Suvarov and unnamed troughs formed in response to the cooling and subsidence of the MP from ∼120 Ma to present (Ai et al., 2008).However, more recent seismic evidence suggests that these formed much later during the Paleocene (65-45 Ma) as a result of a large-scale extensional tectonic regime (Pietsch & Uenzelmann-Neben, 2016).
We considered many other ocean plateau regions in the Pacific for the analyses, but only these four had sufficient or robust geological or geophysical data for analyses of syn-drift events.The four Pacific Ocean plateaux share an important geologic characteristic: seismic and petrologic signatures of major extension during the drift phase of the Pacific plate to the subduction-i.e., at variable times post-dating their genesis, but definitively prior to collision at a subduction trench.Further, the temporal coincidence of these documented syn-drift extensional events that span a wide part of the entire western Pacific indicate that this is a plate-wide tectonic event during drift.

Mechanics of Syn-Drift Extension
The geological record from the eastern Mediterranean and western Alps indicates that drifting microcontinental terranes underwent extensional deformation prior to their collision at the Tethyan subduction zone (Avigad, 1996;Gün et al., 2021;Topuz et al., 2017).It has been suggested that such pre-collisional extension may be the consequence of a "subduction pulley" (Figure 3c) where the subducted slab pulls on the lithosphere and localizes deformation at a weaker microcontinent embedded in the ocean plate (Gün et al., 2021).The syn-drift tectonics observed in the Pacific may be a plate-scale process akin to these in the Tethyan realm.
We developed numerical experiments to investigate how the extensional deformation may arise in oceanic plateaux at their remote locations from subduction zones (Figure 3; see Supporting Information S1 for the experiment setup details and explanation of the modeling parameters).In this representative group of experiments, we placed oceanic plateaux at distant locations-750, 1,000, 1,250, and 1,500 km-from the subduction trench at the experiment initiation to avoid near-trench lithospheric bending and related deformation.Figure 3a stacks and aligns snapshot frames of EXP-1 through 11.1 Myr and demonstrates the deformation of the model oceanic plateau as it traverses from its initial position 1,500 km from the trench toward the subduction zone.To quantify progressive extension, we calculated stretching factors at each snapshot by dividing the current width (l f ) of the oceanic plateau by its initial width (l i = 300 km) and plotted them against the current distance from the trench (Figure 3d).
A major finding of our experiments is syn-drift extension of oceanic plateaux regardless of their distance from the trench.The model frames (EXP-1) and the stretching factor data show that the oceanic plateau undergoes continuous extensional deformation from the beginning of its drift to the end of the experiment (Figures 3a  and 3d).For example, by 11.1 Myr of model evolution, the plateau stretching factor is ∼1.45, representing 45% extension.We attribute this extension to the pull of the subducted slab, as articulated by the subduction pulley analog (Figure 3c).In this, the weaker ocean plateau regions are prone to pre-collisional damage during their drift to subduction within the strong oceanic (Pacific) lithosphere.Notably, the model evolution also shows that the oceanic plateau extension develops non-uniformly, especially in its crustal section, favoring more thinning on its trench side-thinning factors β = 1.63 and 1.28 for the plateau front and back, respectively (Figure 3b).This is an important behavior of our experiments because it is consistent with the observations from the Shatsky Rise and Manihiki oceanic plateaux (referenced above), where the large normal faults, troughs, and grabens preferentially develop on the subduction side of the plateaux (Figures 1 and 2).
The development of extensional deformation in EXP-2, 3, and 4-where the initial plateau distance from the trench is decreased to 1,250, 1,000, and 750 km, respectively-is expressed similarly (Figure 3d).That is, progressive extension of the drifting ocean plateaux by up to 45% (stretching factor of 1.45) in EXP-2 and EXP-3, and 51% in EXP-4.The stretching factor time series are similar among the different models, suggesting that the subduction pulley mechanism can cause extensional deformation of plateaux located thousands of kilometers away from subduction plate boundaries and that strong oceanic lithosphere is equally efficient at transmitting tensile stress over these long distances (Figure 3d).Some additional extensional deformation manifests in the latest stage of EXP-4.At this stage the plateau has almost completed its drift and has arrived at the subduction zone (at ∼100 km distance, Figure 3d), and is being affected by near-trench processes such as plate bending which can aggrandize the magnitude of extension.
The experiments explain the observed extensional deformation in the scattered Pacific oceanic plateaux.As cited, the Ontong Java Plateau records a number of episodes of syn-drift magmatic and volcanic activity linked to synchronous extensional deformation and normal faulting.Decompression melting of the OJP's mantle lithosphere due to lithospheric extension by a subduction pulley during its drift is a viable mechanism for this cumulative magmatic activity.The large normal faults on the northwestern side of the Shatsky Rise oceanic plateau are parallel to the strike of the Japan Trench and those on the southwestern end of the plateau are oriented parallel with the southern continuation of the subduction zone, the Izu-Bonin trench (Figure 1).The alignment of the normal fault orientations with these subduction trenches is consistent with slab-pull extension linked to the Japan-Izu-Bonin subduction activity.The thinning factor calculations from our experiments yield a higher magnitude of extensional deformation on the trench-side of oceanic plateaux (Figure 3b), which can account for the enhanced number of normal faults on the west side of the Shatsky Rise (Zhang et al., 2015).
For the Manihiki Plateau, it has been suggested that the extensional Suvarov and unnamed troughs and grabens reflect a change in the Pacific plate motion and slab-pull regimes (Pietsch & Uenzelmann-Neben, 2016).We agree with this general interpretation, but more specifically identify the subduction pulley of the nearby Tonga-Kermadec subduction zone as a pull effect for the observed extension.The NNE-SSW elongated grabens (Figure 2d) to the east of the Suvarov Trough are parallel to the Tonga-Kermadec subduction trench, suggesting that the slab-pull can account for the observed depressions.In this interpretation, there is no need for a complex plate reorganization to form these extensional structures, but rather they can all be generated by the pull of the subducting slab at Tonga-Kermadec.Again, these large extensional structures are biased toward the Tonga-Kermadec trench (western) side of the plateau, consistent with the findings from the numerical experiments.
The unexplained extensional structures in the Hess Rise oceanic plateau can similarly be accounted for by pull exerted from nearby slabs, such as the Japan or Aleutian trench.The orientation of the grabens in the Hess Rise are parallel or sub-parallel to the observed large normal faults in the Shatsky Rise.This implies a possible common tectonic regime for the extensional deformation in both plateaux and we favor the Japan subduction as the major pull force in the northwestern Pacific.

Discussion
The experiments explain the observed syn-drift extensional deformation in the scattered Pacific oceanic plateaux.Namely, the OJP, SR, HR, and MP share common deformational and magmatic features that indicate substantial extensional tectonics during their drift to the Pacific subduction zones.The recorded syn-drift extension is oriented parallel or sub-parallel with the western Pacific plate boundary-either the Japan-Izu-Bonin (for the OJP, SR, HR) or Tonga-Kermadec (for the MP) subduction segments.Even at distant locales (1,500 km), a subduction pulley is effective at triggering syn-drift extension and related magmatism focussed at these tectonically susceptible oceanic plateaux.Further, the models suggest that the mechanism may be somewhat more efficient at inducing extension on the trench-ward side of the plateaux, and this may account for the enhanced occurrence of such features on the trench side for the Manihiki Plateau and Shatsky Rise (Zhang et al., 2015).
The oceanic slabs in the western Pacific are steep (Isacks & Barazangi, 1977), which may yield a higher slab pull force and hence be especially effective for causing extensional deformation of the Pacific oceanic plateaux.Low angle slabs such as in the eastern Pacific (Isacks & Barazangi, 1977) may not be as effective at damaging oceanic plate, but the lack of geological data on the plateaux toward the eastern side of the plate prevents such a comparison.Further, the inherent challenges of geological surveying on the seafloor mean that our interpretations are limited to the four identified plateaux in the western Pacific where there are available data.Although the findings are consistent across these four sites spanning the Pacific, ongoing geophysical and petrologic exploration of the seafloor lithosphere will help refine the analyses.
The observations show that in the case of the Pacific, with a vast subduction factory along the western boundary, these syn-drift tectonics can span a major tectonic plate, and the mechanism provides an elegant unifying interpretation for the deformational and magmatic events.The geological and geophysical data and quantitative experiments demonstrate that oceanic plate interiors are not impervious to tectonic damage during plate drift.

Figure 1 .
Figure 1.Major oceanic plateaux and subduction zones in the western Pacific.The map shows locations of the Ontong Java, Shatsky, Hess, and Manihiki plateaux and their significant extensional features that are parallel or sub-parallel to the subduction trenches (see references in the text).The base map was made with GeoMapApp (www.geomapapp.org)under a Creative Commons license CC BY 4.0.

Figure 2 .
Figure 2. Extensional tectonics in seismic reflection profiles.(a,b, c, and d) Bathymetric maps of the OJP, SR, HR, and MP and locations of the marine seismic reflection profiles including major extensional structures(Phinney et al., 1999;Pietsch & Uenzelmann-Neben, 2016;Zhang et al., 2015).(e and f) Seismic reflection profiles (a-a´) from the OJP(Phinney et al., 1999).OJ1, OJ2, and OJ3 represent early Cretaceous basement, Cretaceous mudstone and carbonates, late Cretaceous through pelagic carbonate, respectively(Phinney et al., 1999).(g and h) Seismic reflection profiles (b-b´ and c-c´) from the SR.Large normal faults are located mainly on the trench/western side of the SR and cut young sediment layers(Sager et al., 2013;Zhang et al., 2015).(i) Seismic reflection profile (d-d´) from the HR.Wide grabens and normal faults started to form at 82 Ma and are most probably still active since the young strata is displaced by the extensional structures(Vallier et al., 1981(Vallier et al., , 1983)).(j and k) Seismic reflection profile (e-e´) from the MP.The cross section cuts the Suvarov Trough, grabens, and normal faults(Pietsch & Uenzelmann-Neben, 2016).

Figure 3 .
Figure 3. Syn-drift extension mechanism and experiment results.(a) Snapshot frames of EXP-1 from the experiment start to end.Frames are aligned along the plateau end to illustrate the progressive syn-drift extension through the model evolution.See text for detailed explanation.(b) A comparison of stretching factor (β = current crustal thickness/initial thickness) calculations from the front and back regions of the oceanic plateau crust during the EXP-1 development.(c) The subduction pulley analogy that explains the syn-drift ocean plateau extension tectonics (Gün et al., 2021).(d) Stretching factor (current ocean plateau width/initial width) evolution plots of EXP-1 to 4. Experiments show a similar plateau extension development regardless of the initial distance from trench.See text for details.