Change In Plate Motion Research Articles

Zinc on the edge—isotopic and geophysical evidence that cratonic edges control world-class shale-hosted zinc-lead deposits

The North Australian Zinc Belt is the largest zinc-lead province in the world, containing three of the ten largest known individual deposits (HYC, Hilton-George Fisher, and Mount Isa). The Northern Cordillera in North America is the second largest zinc-lead province, containing a further two of the world’s top ten deposits (Red Dog and Howards Pass). Despite this world-class endowment, exploration in both mineral provinces during the past 2 decades has not been particularly successful, yielding only two significant discoveries (Teena, Australia, and Boundary, Canada). One of the most important aspects of exploration is to choose mineral provinces and districts within geological belts that have the greatest potential for discovery. Here, we present results from these two zinc belts that highlight previously unused datasets for area selection and targeting. Lead isotope mapping using analyses of mineralized material has identified gradients in μ (238U/204Pb) that coincide closely with many major deposits. Locations of these deposits also coincide with a gradient in the depth of the lithosphere-asthenosphere boundary determined from calibrated surface wave tomography models converted to temperature. Furthermore, gradients in upward-continued gravity anomalies and a step in Moho depth correspond to a pre-existing major crustal boundary in both zinc belts. A spatial association of deposits with a linear mid- to lower-crustal resistivity anomaly from magnetotelluric data is also observed in the North Australian Zinc Belt. The change from thicker to thinner lithosphere is interpreted to localize prospective basins for zinc-lead mineralization and to control the gradient in lead isotope and geophysical data. These data, when combined with data indicative of paleoenvironment and changes in plate motion at the time of mineralization, provide new exploration criteria that can be used to identify prospective mineralized basins and define the most favorable parts of these basins.

Insights on Formation of the Gulf of Mexico by Rayleigh Surface Wave Imaging

AbstractWe used cross‐correlation of ambient noise records from seismic stations in the US, Mexico, and Cuba to extract dispersion data of Rayleigh surface wave. Our derived 3D shear‐wave velocity model of the greater Gulf of Mexico (GOM) region captures variations in the crustal and lithospheric structures across the continental margins of the US Gulf Coast and Yucatan, Mexico. The model shows a zone of reduced velocity in the mantle lithosphere underlying the extended continental margin of the northwestern GOM. We attributed this velocity reduction to extensional deformation and melt‐induced weakening of the lithosphere during the Triassic continental rifting that preceded the seafloor spreading that formed the GOM. Melt extraction might have been hindered by the greater lithospheric thickness in the western region along the US Gulf Coast margin that resulted in the westward decrease of rift‐related volcanism/magmatism reported from previous studies. The clear asymmetry between the US Gulf Coast and its conjugate Yucatan margin suggests extension along a shear‐zone that focused more deformation on the North American plate prior to breakup. In contrast to the counterclockwise rotation of the Yucatan block during seafloor‐spreading, our analyses using deformable plate models demonstrate that continental rifting occurred in a predominantly northwest‐southeast direction. This change in plate motion is attributed to the development of mantle shear‐zones in the western part of the rift. We estimated the depth of the lithosphere‐asthenosphere boundary and determined that the extended continental and oceanic lithospheres have mostly regained their thickness since the time of breakup.

The formation of the 8˚20’ N seamount chain, east pacific rise

Near-axis seamounts provide a unique setting to investigate three-dimensional mantle processes associated with the formation of new oceanic crust and lithosphere. Here, we investigate the characteristics and evolution of the 8˚20’N Seamount Chain, a lineament of seamounts that extends ~ 175 km west of the East Pacific Rise (EPR) axis, just north of the fracture zone of the Siqueiros Transform Fault. Shipboard gravity, magnetic, and bathymetric data acquired in 2016 are utilized to constrain models of seamount emplacement and evolution. Geophysical observations indicate that these seamounts formed during four distinct episodes of volcanism coinciding with changes in regional plate motion that are also reflected in the development of intra-transform spreading centers (ITSCs) along the Siqueiros transform fault (Fornari et al. 1989; Pockalny et al. 1997). Although volcanism is divided into distinct segments, the magnetic data indicate continuous volcanic construction over long portions of the chain. Crustal thickness variations along the chain up to 0.75 km increase eastward, inferred from gravity measurements, suggest that plate reorganization has considerably impacted melt distribution in the area surrounding the Siqueiros-EPR ridge transform intersection. This appears to have resulted in increased volcanism and the formation of the 8˚20’N Seamounts. These findings indicate that melting processes in the mantle and subsequently the formation of new oceanic crust and lithosphere are highly sensitive to tectonic stress changes in the vicinity of fast-spreading transform fault offsets.

Changes in Plate Motions Caused by Increases in Gravitational Potential Energy of Mountain Belts

AbstractReconstructions of motions of the Nazca, South American, and Indian plates record short‐duration (≲10 Myr) variations in angular velocity, which enable a vector‐based test of the hypothesis that mountain uplift can cause changes in plate motion. Reductions in velocity of Nazca and South America between ∼12 and 6 Ma coincide with a phase of rapid surface uplift in the Central Andes. Decrease in the rate of India's convergence with Eurasia between ∼20 and 10 Ma corresponds to an increase in gravitational potential energy per unit area (GPE) within Tibet, marked by a transition from crustal thickening to thinning. The vectorial test shows that, in each case, the only change in driving force capable of balancing the change in basal drag is an increased resistance along the convergent boundary to the plate. Changes in GPE associated with mountain uplift provide a calibration for basal drags on plates. Basal tractions of ∼0.1–1 MPa provide resisting forces comparable in magnitude to driving forces from GPE variation in ocean lithosphere. The rapid change in motion of the Indian plate between about 48 and 41 Ma is explained by the juxtaposition of the Indian continent against the Andean‐type margin of the Transhimalaya and reduction in driving force due to loss of the slab. The net slab driving force lost was ∼2–4 TN m−1, in agreement with previous studies suggesting that forces resisting slabs' penetration into the mantle largely offset their negative buoyancy.

Structure and Evolution of Northern Juan de Fuca Crust and Uppermost Mantle Over the Last 8 Ma From an Active‐Source Seismic Tomography Study

AbstractWe present results from a two‐dimensional wide‐angle controlled source seismic transect designed to characterize the velocity structure of the oceanic crust and uppermost mantle spanning the northern Juan de Fuca (JdF) plate from near Endeavor ridge to the Cascadia margin. Reflection and refraction travel time inversion is used to derive a tomographic Vp model of sediments, crust, and upper mantle. Velocity model results are compared to baseline reference Vp values for unaltered crustal and upper mantle rocks at temperatures assuming plate cooling. Effective medium theory is used to infer the degree of hydration of the crust and mantle. Results indicate a somewhat fractured and hydrated upper crust (≤2.5 wt% water), a near dry lower crust (≤0.7 wt%) and upper mantle (≤0.5 wt%) west of the deformation front, and an ∼75 km wide region of modestly lower velocity in the mid‐plate. Comparison with prior results from a complementary transect offshore Oregon indicates significant differences in Vp of the upper crust, with lower Vp‐inferred porosity along most of the Washington transects that may reflect different extents of fault‐related alteration and sediment burial histories. Approaching the deformation front, Washington transect Vp structure indicates dryer conditions than offshore Oregon, consistent with differences in extent of subduction bend faulting found in reflection imaging studies. On both transects, quasi‐abrupt changes in plate properties at ages of 8, 6, and 3.4/4 Ma are found. Distinct crustal accretion modes are recognized, aligning with changes in JdF plate motion and recent history of Cobb‐Eickelberg hotspot influence on crustal accretion at the JdF ridge.

The long-lived transcrustal magmatic systems of Southeast China in response to paleo-Pacific plate subduction, recorded by the Cretaceous volcanic sequences in southeastern Zhejiang Province

Processes taking place in subduction zones are highly debated. The Cretaceous volcanic rocks are voluminously distributed along the coastal area of southeastern (SE) China. To elucidate their petrogenesis and relationship with subduction, we use new zircon U-Pb ages and Hf-O isotopes for the representative Cretaceous volcanic sequences in Zhejiang Province, China. According to stratigraphic investigations, zircon U-Pb ages, and Hf-O isotopes, these volcanic rocks can be divided into different groups corresponding to three stages of volcanic activity: the early (Stage 1, 136−129 Ma), middle (Stage 2, 125−115 Ma), and late (Stage 3, 110−94 Ma) stages. Diverse zircon populations (including antecrysts, autocrysts, and xenocrysts) are recognized. Hf-O isotopes of autocrysts suggest different protoliths for the identified three stages of volcanic activity. Xenocrysts show obvious different compositions from autocrysts. Antecrysts share similar compositions with autocrysts, which favor similarities in the magmas from which they were generated. Our observations (very small age intervals between antecrysts and autocrysts, sources for volcanic rocks throughout the entire crust, and volcanic magmatism with long-term and discontinuous characteristics), were inconsistent with the traditional melt-dominated magma chamber model. Consequently, we propose that those volcanic rocks were derived from long-lived transcrustal magmatic systems (TCMS), dominated by crystal mush, instead of melt-dominated magma chambers, maintained and recharged by a discontinuous contribution of contemporaneous underplated mantle-derived magmas, triggered by paleo-Pacific plate subduction. We suggest the different stages of volcanic activity and corresponding long-lived TCMS were produced by the change of Pacific plate motion beneath SE China during the Cretaceous period.

Evidence for active upper mantle flow in the Atlantic and Indo-Australian realms since the Upper Jurassic from hiatus maps and spreading rate changes

Histories of large-scale horizontal and vertical lithosphere motion hold important information on mantle convection. Here, we compare continent-scale hiatus maps as a proxy for mantle flow induced dynamic topography and plate motion variations in the Atlantic and Indo-Australian realms since the Upper Jurassic, finding they frequently correlate, except when plate boundary forces may play a significant role. This correlation agrees with descriptions of asthenosphere flow beneath tectonic plates in terms of Poiseuille/Couette flow, as it explicitly relates plate motion changes, induced by evolving basal shear forces, to non-isostatic vertical motion of the lithosphere. Our analysis reveals a timescale, on the order of a geological series, between the occurrence of continent-scale hiatus and plate motion changes. This is consistent with the presence of a weak upper mantle. It also shows a spatial scale for interregional hiatus, on the order of 2000–3000 km in diameter, which can be linked by fluid dynamic analysis to active upper mantle flow regions. Our results suggest future studies should pursue large-scale horizontal and vertical lithosphere motion in combination, to track the expressions of past mantle flow. Such studies would provide powerful constraints for adjoint-based geodynamic inverse models of past mantle convection.

Analysis of Pacific Hotspot Chains

AbstractSeamount trails created by mantle plumes are often used to establish absolute reference frames for plate motion. When plume drift is considered, changes in seamount trail direction and age progression cannot be attributed to plate motion change alone. Here, improvements to age‐progressive models of eleven Pacific hotspot chains are made independently of plate motion models. Our approach involves bathymetry processing to robustly predict a smooth, continuous hotspot path by connecting maxima in filtered seamount bathymetry, with across‐track uncertainties from seamount trail width and amplitude. Published ages from seamount samples are projected orthogonally onto these paths. We determine best‐fit models of age as functions of along‐track distance, giving continuous age‐progression models for each seamount chain with uncertainties in age and geometry. Different sources of paleolatitudes are examined by incorporating data from the magnetization of seamount drill‐core samples, paleo‐poles from marine magnetic anomaly skewness inversions, and paleo‐spin‐axes inferred from shifts of equatorial sediments. Improved paleolatitude models for the Hawaiian‐Emperor and Louisville chains are determined by combining these different types of data. Paleolatitude models are predicted for other chains during periods when sufficient amounts of paleo‐pole or paleo‐spin‐axis data are available. Analysis of the eleven Pacific seamount chains provide constraints for future plate and plume motion models. We analyze the temporal change in distance between coeval seamounts, reflecting relative drifts between the hotspots at different times in the past. The observed changes imply systematic relative hotspot drifts compatible with paleolatitude trends.

Decadal change of the Apulia microplate motion preceding the MW 6.4, 26 November 2019 Durrës (Albania) earthquake

It is commonly assumed that the motions of tectonic plates remain steady over the earthquake cycle. This assumption is based on the notion that stresses associated with the cycle may not be sufficient to overcome the asthenosphere viscous resistance at the lithosphere base, which counters plate-motion changes. However, this remains to be verified. Recent inferences on the asthenosphere viscosity/thickness from modelling of glacial rebound data constrain the amount of viscous resistance needed to alter plate motions. In light of these recent findings, it is conceivable that stresses associated with the earthquake cycle are large enough to impact motions of microplates or small-/medium-sized plates. Here we show that the contemporary motion of the Apulia microplate slowed down by ∼20% and turned westward by ∼10∘ during the decade preceding the MW 6.4, 26 November 2019 Durrës (Albania) earthquake, which struck the Apulia eastern margin with the Eurasia plate. We make a statistical comparison between estimates of the torque variation required for Apulia to generate the observed slowdown, and parameterised estimates of the torque variation imparted to Apulia by the interseismic stress buildup during the decade before the 2019 Durrës earthquake. We conclude that the Apulia motion change likely resulted from the interseismic stress buildup associated with the Durrës earthquake cycle.

Northwest Pacific-Izanagi plate tectonics since Cretaceous times from western Pacific mantle structure

Northwest Pacific-Izanagi subduction histories along Eurasia are poorly constrained due to extensive subduction, which partially consumed the western Pacific plate and the entire Izanagi plate, its hypothesized conjugate margin. Here we reconstruct NW Pacific-Izanagi plate tectonics since Cretaceous times by mapping and structurally restoring (i.e. unfolding) the subducted western Pacific slabs from regional and global tomography, and re-creating the vanished Izanagi plate as its conjugate rift flank. Unfolding of the western Pacific slabs based on their cross-sectional areas, corrected for ‘tomographic smearing’, reveals that 2230 to 5000 km of western Pacific plate was subducted between Kamchatka and the southern Marianas. We add our restored western Pacific and Izanagi plates to a global plate model to reveal that Izanagi subduction under Eurasia after the mid-Cretaceous was limited between the present Bohai Bay-Yellow Sea, China, and northern Russia. The southern limit of Izanagi subduction was a NW-SE sinistral transform that intersected Eurasia near present Qingdao, China, and segmented eastern Eurasia continental magmatism during the late Cretaceous; we call this transform the ‘Qingdao line’. We reconstruct a low-angle Izanagi-Pacific ridge-trench intersection with Eurasia at ∼50 ± 10 Ma between Bohai Bay-Yellow Sea and northern Russia. The ∼50 Ma Pacific plate motion change initiated subduction along the Qingdao line transform, forming the Izu-Bonin-Marianas arcs and reorganizing the Bohai Bay-Yellow Sea faults. We show tomographic and geodynamic modeling evidence that a 4000 km-long, laterally-continuous, NE-SW trending, seismically-slow ‘slab gap’ at 1000 ± 250 km depth between present northern Sakhalin and central China is the tomographic signature of the subducted Izanagi-Pacific ridge.

Cretaceous Rift‐Drift Tectonics Then Paleogene Prearc Subsidence Related to Subduction Initiation: Aotea Basin, Zealandia, Southwest Pacific

AbstractStratal stacking patterns and termination relationships of Upper Cretaceous and Paleogene strata of Aotea Basin, which is at the southern end of New Caledonia Trough and adjacent to Taranaki Basin, are used to infer vertical movements before, during, and after Tonga‐Kermadec subduction initiation. Regional 2D seismic‐stratigraphic units are tied to petroleum exploration wells, including recently drilled Romney‐1. We identify: (a) an uppermost Cretaceous to Paleocene (70–56 Ma) aggradational shelf sequence, with at least two regressional events linked to eustatic sea‐level falls; (b) a diachronous deepening of the basin from shelfal to bathyal environments, that progressed northwest to southeast during the early to middle Eocene (53–46 Ma); (c) that was contemporaneous with small‐scale volcanism; (d) a brief phase of middle to late Eocene (46–35 Ma) delta development at the western basin margin (Challenger Plateau) that is evidence for local uplift; and (e) an onlapping sequence deposited since the latest Eocene (35–0 Ma) that requires ∼1,300 m of subsidence at the western margin. Paleogeographic maps and a generalized stratigraphic chart summarize our observations. The Eocene phase of tectonic subsidence, uplift, and diffuse volcanism (53–46 Ma) was a precursor to plate motion at the Cenozoic plate boundary through New Zealand and suggests a causal relationship between prearc subsidence, Tonga‐Kermadec subduction initiation, and plate motion change. Delamination of lower crust beneath Aotea Basin during initial subducted slab formation might explain our observations.

Dynamics of the abrupt change in Pacific Plate motion around 50 million years ago

Dynamics of the abrupt change in Pacific Plate motion around 50 million years ago

Tectonostratigraphic framework in the eastern Korean continental margin, East Sea: Implication for evolution of the Hupo Basin

AbstractIntegrating seismic interpretation facilitates the discernment of tectonostratigraphic evolution in the eastern Korean continental margin, East Sea. The sedimentary succession of this margin is divided into three major seismic units based on distinct unconformities. These unconformities and their associated seismic characteristics indicate that the eastern Korean continental margin has experienced four evolutionary stages through extensional and subsequent two‐phase tectonic inversions. Early Miocene back‐arc opening of the East Sea triggered the extension of NNW–SSE and N–S trending rifts, resulting in non‐marine to deep‐marine deposition of typical rift‐related linked sedimentological systems in the eastern Korean continental margin. In the early Late Miocene, changes in plate motion and the subduction mode of the Japanese island (NW–SE compression) caused the positive inversion of extensional fault‐bounded half‐grabens in the eastern Korean continental margin, and the Hupo Basin was likely created by the regional flexural response. During this depositional period, hemipelagic sedimentation accompanied by episodic gravity‐controlled slope failures was predominant in the deep‐water environment. The subsidence of the Hupo Basin was enhanced by crustal shortening (E–W compression) that was induced by subduction initiation at the western margin of the Ulleung Basin during the late Early Pliocene (ca. 3.8 Ma). At that time, sedimentary environment gradually became shallower with time and coarse‐grained terrigenous input into the Hupo Basin began so that shallow‐ to deep‐marine sedimentation occurred. In the Quaternary period, the uplift of the Hupo Bank and coeval subsidence of the Hupo Basin were maintained by continued compressive neotectonics. The shallow‐ to deep‐marine deposition continued, but greater quantities of coarse‐grained terrestrial sediment were transported into the Hupo Basin due to combined effects on tectonics and eustasy. Based on the tectonostratigraphic reconstruction developed in this study, we propose that the Hupo Basin is likely a Late Miocene–Quaternary compression‐related basin induced by crustal or thrust loading.

Mechanism of progressive broad deformation from oceanic transform valley to off-transform faulting and rifting

Oceanic transform faults (TFs) are commonly viewed as single, narrow strike-slip seismic faults that offset two mid-ocean ridge segments. However, broad zones of complex deformation are ubiquitous at TFs. Here, we propose a new conceptual model for the progressive deformation within broad zones at oceanic TFs through detailed morphological, seismic, and stress analyses. We argue that, under across-transform extension due to a change in plate motion, plate deformation occurs first along high-angle transtensional faults (TTFs) within the transform valleys. Off-transform normal faults (ONFs) form when across-transform deviatoric extensional stresses exceed the yield strength of the adjacent oceanic lithosphere. With further extension, these normal faults can develop into off-transform rift zones (ORZs), some of which can further develop into transform plate boundaries. We illustrate that such progressive complex deformation is an inherent feature of oceanic TFs. The new conceptual model provides a unifying theory to explain the observed broad deformation at global transform systems.

Provenance and depositional history of the Mesozoic Sanjiang Basin (northeastern China): implications for the uplift history of the northeastern Asian continental margin

AbstractThe uplift history of northeastern Asian continental margin has been the subject of much debate. The Sanjiang Basin is an ideal area to investigate the uplift history of this margin. We present new data from sandstone petrology, whole-rock geochemistry, U–Pb geochronology and Hf isotopy of detrital zircons to trace the provenance of the basin and to unravel the uplift history of the northeastern Asian continental margin. We investigated the chemical and plagioclase indices of alteration and index of compositional variability (CIA, PIA and ICV, respectively) in samples from two formations. The geochemical proxies indicate that source rocks were subjected to an intensified weathering process. Based on the high SiO2/Al2O3 values and low K2O, we infer that the two basinal strata experienced silicification and insignificant potassium metasomatism. Sandstone petrography is indicative of low degrees of sedimentary sorting, suggesting a proximal deposition. The provenance fingerprints of light rare earth, high-field-strength and transition metal elements indicate that parts of the provenance included recycled sediments, and that first-cycle sediments were derived mainly from felsic rocks with a minor contribution from intermediate and mafic rocks. The combined detrital zircon U–Pb ages and Hf isotopic results constrain the contributions of different source terranes over time. The provenance results, in combination with seismic profile interpretations from the Sanjiang Basin, suggest that the northeastern Asian continental margin experienced at least three stages of uplift, which were driven by subduction initiation of the Paleo-Pacific Ocean during the Jurassic Period, a plate motion change during the Early Cretaceous Epoch, and a shallowing of the slab dip angle during the Late Cretaceous Epoch.

Yellowstone Plume Drives Neogene North American Plate Motion Change

AbstractPlate motion changes provide powerful constrains on plate tectonic forces. North America is ideal to explore these forces. We use recently published high‐temporal resolution plate reconstructions of North America, which reveal a velocity slow‐down at 17 Ma, roughly coeval with the eruption of the Yellowstone plume, together with a stratigraphic analysis of hiatus surfaces across the continent, which provides proxy information for paleotopography. Using a simple Couette/Poiseuille flow models we estimate asthenosphere flow beneath North America and its impact on Neogene plate motion. We find that North America's Neogene plate motion change can be explained by Poiseuille flow in the asthenosphere generated upon the arrival of the Yellowstone plume and that the flow length‐scale matches the extent of hiatus surfaces. While plume driven upper mantle flow constitutes a geodynamically viable model to explain North America's Neogene plate motion change it provides an intrinsic link between vertical and horizontal plate motions.

Deciphering paleogeography from orogenic architecture: Constructing orogens in a future supercontinent as thought experiment

Orogens that form at convergent plate boundaries typically consist of accreted rock units that form an incomplete archive of subducted oceanic and continental lithosphere, as well as of deformed lithosphere of the former upper plate. Reading the construction of orogenic architecture forms the key to decipher the pre-orogenic paleogeographic distribution of oceans and continents, as well as bathymetric and topographic features that existed thereon such as igneous plateaus, seamounts, microcontinents, or magmatic arcs. Current classification schemes of orogens divide between settings associated with termination of subduction [continent-continent collision, continent-ocean collision (obduction)] and with ongoing subduction (accretionary orogenesis), alongside intraplate orogens. Perceived diagnostic features for such classifications, particularly of collisional orogenesis, hinge on dynamic interpretations linking downgoing plate paleogeography to upper plate deformation, plate motion changes, or magmatism. Here, we show, however, that Mesozoic-Cenozoic orogens that undergo collision almost all defy these proposed diagnostic features and behave as accretionary orogens instead. To reconstruct paleogeography of subducted and upper plates, we therefore propose an alternative approach to navigating through orogenic architecture: subducted plate units comprise nappes (or mélanges) with Ocean Plate Stratigraphy (OPS) and Continental Plate Stratigraphy (CPS) stripped from their now-subducted or otherwise underthrust lower crustal and mantle lithospheric underpinnings. Upper plate deformation and paleogeography respond to the competition between absolute motions of the upper plate and the subducting slab. Our navigation approach through orogenic architecture aims to avoid <i>a priori</i> dynamic interpretations that link downgoing plate paleogeography to deformation or magmatic responses in the upper plate, to provide an independent basis for geodynamic analysis. From our analysis we identify ‘rules of orogenesis9 that link the rules of rigid plate tectonics with the reality of plate deformation. We use these rules for a thought experiment, in which we predict orogenic architecture that will result from subducting the present-day Indian Ocean and colliding the Somali, Madagascar, and Indian margins using a published continental drift scenario for a future supercontinent as basis. We illustrate that our inferred rules (of thumb) generate orogenic architecture that is analogous to elements of modern orogens, unlocking the well-known modern geography as inspiration for developing testable hypotheses that aid interpreting paleogeography from orogens that formed since the birth of plate tectonics.

Tectonic constraints on formation and evolution of microplates in the Indian and Pacific Oceans: reviews and statistical inferences

Oceanic plates are growing through narrow boundaries, such as mid-ocean ridges and transform faults. However, the discovery of diffuse plate boundary suggests another type of plate boundary that accommodates difference in plate motion via internal deformation. Along the Central and Southeast Indian ridges, for example, the Capricorn and Macquarie microplates exhibit widespread diffuse boundaries and hence divide the Indo-Australian Plate further into the Indian, Australian, Capricorn, and Macquarie plates. As for microplates distributed along the East Pacific Rise and Pacific-Antarctic Ridge in the Pacific Ocean, however, the typical plate boundaries surrounding the given microplate are distinctly established. Global plate reorganization involving the changes in plate motion or in spreading direction can be accommodated by forming a microplate through ridge extinction, ridge propagation, and pseudofault formation. However, relations between these tectonic processes have not been quantitatively assessed. In particular, we aim to examine tectonic constrains on the formation processes of microplates with diffuse plate boundary. In this study, we compare plate size, plate age, full-spreading rates, thermal structures, total rotation, and rotation rate for the 9 microplates including extinct plates (i.e., Capricorn, Macquarie, and Mammerickx* microplates in the Indian and Southern Oceans; Galapagos, Easter, Juan Fernandez, Bauer*, Friday*, and Selkirk* microplates in the Pacific Ocean; extinct plates are denoted with asterisks). From this comparison, we find that the microplate formation would require certain tectonic conditions (e.g., full-spreading rates faster than 70–80 mm/yr and rotation rates faster than 5–6°/m.y.) to evolve into an independent and rigid plate with respect to the neighboring plates. If the conditions are not met, the same tectonic reorganization would result in a microplate with diffuse plate boundaries.

The trials and tribulations of the Hawaii hot spot model

The Hawaiian-Emperor volcanic chain (H-E chain) is located in the middle of the North Pacific Ocean. It extends from northwest to southeast, including two segments, the older Emperor chain and the younger Hawaiian chain between which is a 60o change in strike, here termed the H-E bend. The H-E chain is the clearest and most intensively researched hot spot track in terms of plate motion, mantle plumes, tectonics, geochemical evolution, and lithospheric studies. However, debates on the formation of the H-E chain, in particular the H-E bend, concerning its origin in hot spot drift and/or Pacific plate motion change, have been ongoing for several decades. In this paper, we review current understanding and ideas concerning this debate and suggest ways forward. So far, neither hot spot southerly drift nor Pacific plate motion change can perfectly account for the geometry and progression of the H-E chain. In this review, we put forward a joint model where these two competing processes together can reasonably explain the evolution of the H-E chain and the H-E bend. In addition, we proposed three stages for formation of the H-E chain, including: 1) A ridge-plume interaction stage: Meiji~Detroit seamounts and a possible subducted section; 2) A combination of hot spot-Pacific plate motion: South of Detroit seamount ~ H-E bend; and 3) Pacific plate motion with a fixed hot spot: Hawaiian volcanic chain. In addition, any plate movement at the surface must be balanced by motion deeper in the mantle. Therefore, we consider that the surface Pacific plate motion and the state of deep mantle plume at 47–55 Ma are not totally separated but co-evolved. Furthermore, reconstructions of the Pacific plate and its boundaries should be considered if Hawaiian hot spot motion makes great contributions to the formation of the H-E chain. Nevertheless, establishing the causal links between these events and their underlying dynamic triggers requires further, more comprehensive work.

The structural evolution of pull‐apart basins in response to changes in plate motion

AbstractPull‐apart basins are structural features linked to the interactions between strike‐slip and extensional tectonics. Their morphology and structural evolution are determined by factors such as extension rate, the basin length/width ratio, and changes in extension direction. In this work, we investigate the effect of a change in the plate motion direction on a pull‐apart basin's structure, using analogue modelling experiments with a two‐layer ductile‐brittle configuration to simulate continental crust rheology. We initially impose orthogonal extension on an interconnected rift and strike‐slip system to drive pull‐apart development. Subsequently, we rotate the relative motion vector, imposing transtensional deformation and continuing with this new relative motion vector to the end of the experiment. To compare with natural examples, we analyse the model using seismic interpretation software, generating 3D fault structure and sedimentary thickness interpretations. Results show that the change in the direction of plate motion produces map‐view sigmoidal oblique slip faults that become normal‐slip when deformation adjusts to the new plate motion vector. Furthermore, sediment distribution is strongly influenced by the relative plate rotation, changing the locus of deposition inside the basin at each model stage. Finally, we compare our observations to seismic reflection images, sedimentary package thicknesses and fault interpretations from the Northern Gulf of California and find good agreement between model and nature. Similar fault arrays occur in the Bohai Basin in northern China, which suggests a rotational component in its evolution. More broadly, such similar structures could indicate a role for oblique extension and fault rotation in any pull‐apart basin.