Change In Pacific Plate Motion Research Articles

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.

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.

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

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.

Reconstruction of Subduction and Back‐Arc Spreading in the NW Pacific and Aleutian Basin: Clues to Causes of Cretaceous and Eocene Plate Reorganizations

AbstractThe Eocene (~50–45 Ma) major absolute plate motion change of the Pacific plate forming the Hawaii‐Emperor bend is thought to result from inception of Pacific plate subduction along one of its modern western trenches. Subduction is suggested to have started either spontaneously, or result from subduction of the Izanagi‐Pacific mid‐ocean ridge, or from subduction polarity reversal after collision of the Olyutorsky arc that was built on the Pacific plate with NE Asia. Here we provide a detailed plate‐kinematic reconstruction of back‐arc basins and accreted terranes in the northwest Pacific region, from Japan to the Bering Sea, since the Late Cretaceous. We present a new tectonic reconstruction of the intraoceanic Olyutorsky and Kronotsky arcs, which formed above two adjacent, oppositely dipping subduction zones at ~85 Ma within the north Pacific region, during another Pacific‐wide plate reorganization. We use our reconstruction to explain the formation of the submarine Shirshov and Bowers Ridges and show that if marine magnetic anomalies reported from the Aleutian Basin represent magnetic polarity reversals, its crust most likely formed in an ~85‐ to 60‐Ma back‐arc basin behind the Olyutorsky arc. The Olyutorsky arc was then separated from the Pacific plate by a spreading ridge, so that the ~55‐ to 50‐Ma subduction polarity reversal that followed upon Olyutorsky‐NE Asia collision initiated subduction of a plate that was not the Pacific. Hence, this polarity reversal may not be a straightforward driver of the Eocene Pacific plate motion change, whose causes remain enigmatic.

How to Create New Subduction Zones: A Global Perspective

The association of deep-sea trenches—steeply angled, planar zones where earthquakes occur deep into Earth’s interior—and chains, or arcs, of active, explosive volcanoes had been recognized for 90 years prior to the development of plate tectonic theory in the 1960s. Oceanic lithosphere is created at mid-ocean ridge spreading centers and recycled into the mantle at subduction zones, where down-going lithospheric plates dynamically sustain the deep-sea trenches. Study of subduction zone initiation is a challenge because evidence of the processes involved is typically destroyed or buried by later tectonic and crust-forming events. In 2014 and 2017, the International Ocean Discovery Program (IODP) specifically targeted these processes with three back-to-back expeditions to the archetypal Izu-Bonin-Mariana (IBM) intra-oceanic arcs and one expedition to the Tonga-Kermadec (TK) system. Both subduction systems were initiated ~52 million years ago, coincident with a proposed major change of Pacific plate motion. These expeditions explored the tectonism preceding and accompanying subduction initiation and the characteristics of the earliest crust-forming magmatism. Lack of compressive uplift in the overriding plate combined with voluminous basaltic seafloor magmatism in an extensional environment indicates a large component of spontaneous subduction initiation was involved for the IBM. Conversely, a complex range of far-field uplift and depression accompanied the birth of the TK system, indicative of a more distal forcing of subduction initiation. Future scientific ocean drilling is needed to target the three-dimensional aspects of these processes at new converging margins.

Mid-Cenozoic Pacific plate motion change: Implications for the Northwest Hawaiian Ridge and circum-Pacific

Mid-Cenozoic Pacific plate motion change: Implications for the Northwest Hawaiian Ridge and circum-Pacific

Deciphering the origin of the Cenozoic intracontinental rifting and volcanism in eastern China using integrated evidence from the Jianghan Basin

Intracontinental rifting and low-volume volcanism are a globally common phenomenon, yet the underlying driving mechanisms and whether they can be explained through classic plate tectonic concepts, remain hotly debated. A prominent example is the Cenozoic rift and volcanic province in eastern China. Using an integration of geological, geophysical and geochemical data, we unravel the spatial and temporal variations of the rifting and volcanism in the Jianghan Basin. Both rifting and volcanism in the Jianghan Basin show two intense-to-weak cycles (65–50 Ma and 50–26 Ma, respectively) with significant enhancement in activity during the late rift phase. Moreover, rifting and depocentres progressively migrated eastward. The Jianghan basalts all share an asthenospheric origin while the source of the late phase basalts is slightly more enriched and heterogenous in Nd-Hf isotopes than that of the early phase basalts. The late phase basalts also display a smaller extent of partial melting even under a thinner lithosphere, likely indicating a significant decrease of volatile content in the mantle source. Based on regional tectonic correlations, the main stages of tectonic evolution of the Jianghan Basin and eastern China are not synchronous with changes in Pacific plate motion, while they are coincident with India-Asia collision processes. These observations lead us to propose that the asthenospheric flow driven by India-Asia collision rather than the rollback of the subducted Pacific slab has caused the widespread rifting and volcanism in eastern China. The variations of rifting and volcanism in the Jianghan Basin suggest a multiphase and eastward asthenospheric flow beneath eastern China driven by India-Asia collision, with an intense upwelling when passing through the North-South Gravity Lineament (NSGL). The much more intense rifting and volcanism during the late rift phase may indicate a much larger scale of volatile-poor asthenospheric flow than the early rift phase which could result in a more intense erosion of ancient enriched lithospheric mantle and the volatile content in the mantle source dropping sharply. This study provides an improved model based on our multidisciplinary observations for asthenospheric flow which may be an alternative driving mechanism for intracontinental rifting and low-volume volcanism in the regions where there are step changes in lithospheric thickness globally.

Sr–Pb–Nd–Hf isotopes and 40Ar/39Ar ages reveal a Hawaii–Emperor-style bend in the Rurutu hotspot

Current Pacific absolute plate motion (APM) models include two major, long-lived hotspot tracks: the ∼80 Ma Hawaii–Emperor and ∼79 Ma Louisville seamount tracks. Prior to ∼50 Ma, these two tracks show significant inter-hotspot drift, most likely due to a strong southern motion of the Hawaiian plume. With the addition of a third long-lived Pacific hotspot track, featuring a well-constrained hotspot location and a Hawaii–Emperor-style bend, we can achieve a more robust evaluation of the relationship between APM models and inter-hotspot drift. Such a candidate exists in the Rurutu hotspot track. The youngest volcanism (∼10 to 0.2 Ma) associated with the Rurutu hotspot is located in the Cook-Austral Islands and is known as the “Atiu” or “Young Rurutu” trend. Here we show with 40Ar/39Ar ages and Sr–Pb–Nd–Hf “isotopic fingerprints” that islands and seamounts ranging between ∼70 to 40 Ma fit the compositional trend and APM model-predicted locations of the Rurutu hotspot.The Rurutu hotspot has a distinctive HIMU- to FOZO-like composition, characterized by high 206Pb/204Pb (≥20.0) and low 87Sr/86Sr (≤0.704) ratios. HIMU is a rare composition, found in only two other hotspots worldwide. Uniquely, elevated Rurutu 208Pb/204Pb ratios relative to 206Pb/204Pb define a steeper regression that more closely reproduces the Northern Hemisphere Reference Line than the shallower regression defined by traditional HIMU. Rurutu also is the only HIMU-type hotspot known to continue into the West Pacific, allowing its unique composition to effectively be used as a fingerprint. The Rurutu signature can be traced through non-Samoan “interloper seamounts” in the Samoan region, to a hotspot bend in the southern Tuvalu Islands, continuing through the ∼74 to 63 Ma Gilbert Ridge and the ∼120 to 78 Ma Wake seamounts. However, precisely locating the Hawaii–Emperor style bend in Rurutu is complicated by the younger Samoan hotspot partially overlapping the Rurutu hotspot track, and Cretaceous volcanic structures predating regional Rurutu volcanism. Therefore, we present a computational method that tests a range of preexisting APM models, all generating Hawaii–Emperor style bend locations for Rurutu within error of each other. Our results confirm the presence of a third major hotspot track in the Pacific as originally proposed by W.J. Morgan (1972), with a ∼50.5–49.0 Ma bend at 178.48°E and 8.56°S.Paleomagnetic data for Hawaii indicate that changes in southern hotspot motion may explain its bend, while negligible latitudinal motion in Louisville and an unchanging distance to Rurutu (Tarduno et al., 2003; Koppers et al., 2012; Konrad et al., 2018) are consistent with subduction-driven plume motion explaining the Hawaii–Emperor bend. The presence of a defined ∼135° bend in the Rurutu hotspot track, coupled with minimal inferred latitudinal motion at that time (Konrad et al., 2018), leaves room for a small change in Pacific plate motion at ∼50 Ma.

Late Miocene Pacific plate kinematic change explained with coupled global models of mantle and lithosphere dynamics

AbstractThe timing and magnitude of a Pacific plate motion change within the past 10 Ma remains enigmatic, due to the noise associated with finite‐rotation data. Nonetheless, it has been hypothesized that this change was driven by the arrival of the Ontong Java Plateau (OJP) at the Melanesian arc and the consequent subduction polarity reversal. The uncertainties associated with the timing of this event, however, make it difficult to quantitatively demonstrate a dynamical association. Here, we first reconstruct the Pacific plate's absolute motion since the mid‐Miocene (15 Ma), at high‐temporal resolution, building on previous efforts to mitigate the impact of finite‐rotation data noise. We find that the largest change in Pacific plate‐motion direction occurred between 10 and 5 Ma, with the plate rotating clockwise. We subsequently develop and use coupled global numerical models of the mantle/lithosphere system to test hypotheses on the dynamics driving this change. These indicate that the arrival of the OJP at the Melanesian arc, between 10 and 5 Ma, followed by a subduction polarity reversal that marked the initiation of subduction of the Australian plate underneath the Pacific realm, were the key drivers of this kinematic change.

Pacific plate motion change caused the Hawaiian-Emperor Bend

A conspicuous 60° bend of the Hawaiian-Emperor Chain in the north-western Pacific Ocean has variously been interpreted as the result of an abrupt Pacific plate motion change in the Eocene (∼47 Ma), a rapid southward drift of the Hawaiian hotspot before the formation of the bend, or a combination of these two causes. Palaeomagnetic data from the Emperor Seamounts prove ambiguous for constraining the Hawaiian hotspot drift, but mantle flow modelling suggests that the hotspot drifted 4–9° south between 80 and 47 Ma. Here we demonstrate that southward hotspot drift cannot be a sole or dominant mechanism for formation of the Hawaiian-Emperor Bend (HEB). While southward hotspot drift has resulted in more northerly positions of the Emperor Seamounts as they are observed today, formation of the HEB cannot be explained without invoking a prominent change in the direction of Pacific plate motion around 47 Ma.

The concurrent emergence and causes of double volcanic hotspot tracks on the Pacific plate.

Mantle plumes are buoyant upwellings of hot rock that transport heat from Earth's core to its surface, generating anomalous regions of volcanism that are not directly associated with plate tectonic processes. The best-studied example is the Hawaiian-Emperor chain, but the emergence of two sub-parallel volcanic tracks along this chain, Loa and Kea, and the systematic geochemical differences between them have remained unexplained. Here we argue that the emergence of these tracks coincides with the appearance of other double volcanic tracks on the Pacific plate and a recent azimuthal change in the motion of the plate. We propose a three-part model that explains the evolution of Hawaiian double-track volcanism: first, mantle flow beneath the rapidly moving Pacific plate strongly tilts the Hawaiian plume and leads to lateral separation between high- and low-pressure melt source regions; second, the recent azimuthal change in Pacific plate motion exposes high- and low-pressure melt products as geographically distinct volcanoes, explaining the simultaneous emergence of double-track volcanism across the Pacific; and finally, secondary pyroxenite, which is formed as eclogite melt reacts with peridotite, dominates the low-pressure melt region beneath Loa-track volcanism, yielding the systematic geochemical differences observed between Loa- and Kea-type lavas. Our results imply that the formation of double-track volcanism is transitory and can be used to identify and place temporal bounds on plate-motion changes.

Geochemistry, Sr–Nd–Pb–Hf isotopes systematics and geochronological constrains on petrogenesis of the Xishan A-type granite and associated W–Sn mineralization in Guangdong Province, South China

The Xishan deposit, located in the western Guangdong Province in South China, is a quartz-vein type W-Sn deposit with an average Sn grade of 0.1–0.4wt%. The deposit is temporally and spatially associated with Xishan alkali feldspar granite. The W–Sn mineralization is present mainly as veins that are hosted by the granite. In this paper we present new zircon U–Pb age, whole-rock geochemical data, Sr–Nd–Pb–Hf isotopic data and Re–Os age in order to constrain the nature and timing of magmatism and mineralization in the Xishan mining district with implications on geodynamic settings. LA–ICP–MS zircon U–Pb analyses yielded an age of 79.14±0.31Ma for the alkali feldspar granite, consistent with the molybdenite Re–Os age of 79.41±1.11Ma. The alkali feldspar granite shows high contents of SiO2 (71.52–76.25wt%), high total alkalis (Na2O+K2O=9.35–13.51wt%), high field strength elements (e.g. Zr=95.4–116ppm, Y=97.1–138ppm, Nb=36.1–55.5ppm, Ga=97.1–138ppm), and rare earth elements (total REE=171.8–194.0ppm) as well as high Ga/Al ratios (10,000×Ga/Al=3.23–3.82) suggesting that it has the geochemical characteristics of A-type granite and shows an A2 subtype affinity. Sr–Nd isotopes of the alkali feldspar granite show that (87Sr/86Sr)i values range from 0.7111 to 0.7183, and the εNd(t) values and Nd model ages (T2DM) vary from −6.8 to −6.5 and 1414 to 1433Ma, respectively. The Pb isotopic compositions are variable, with 206Pb/204Pb, 207Pb/204Pb and 208Pb/204Pb values ranging from 18.783 to 18.947, 15.709 to 15.722 and 38.969 to 39.244, respectively, indicating that the alkali feldspar granite was derived from a mantle-crust mixed source. In situ Hf isotopic analyses reveal that the alkali feldspar granite has εHf(t) values ranging from −9.69 to −0.04 and two-stage Hf model ages from 1145Ma to 1755Ma, indicating that the alkali feldspar granite was formed by the partial melting of Mesoproterozoic crusts of the Cathaysia Block with additions of mantle-derived materials. These results, together with previously presented regional geological relationships, suggest that the formation of the Xishan granite and associated W–Sn mineralization is related to lithospheric extension and asthenospheric upwelling that are attributed to a directional change of Pacific plate motion.

Quantifying the forces needed for the rapid change of Pacific plate motion at 6 Ma

Studies have documented several rapid changes along the Pacific–North American, Pacific–Antarctic and Pacific–Australian plate boundaries in latest Miocene to earliest Pliocene time consistent with a sudden clockwise rotation of Pacific plate velocity relative to hotspots during this period. We test the hypothesis that this change in plate motion was initiated by cessation of subduction along the northern Melanesian arc due to the collision between the arc and the Ontong Java plateau. This hypothesis has long been formulated but never tested quantitatively. We use a geomechanical model of the lithosphere to determine the changes in plate boundary forces that are necessary to obtain the observed change in the Pacific plate motion. Our model results show that the change in motion can be explained by a clockwise rotation of the slab-related (basal-strength) component of plate driving force. The change of slab-related force from a post-6 Ma to a pre-6 Ma setting is perpendicular to the arc and points towards the Australian plate. The force per unit length is in the range of currently accepted values for subduction zones. Since there have been no other relevant changes at subduction zones along the Pacific plate boundary during the latest Miocene, we relate this change in slab-related force to the former southward-dipping Pacific plate slab along the northern Melanesian arc system which is now detached. Our model results suggest that rapid changes in plate motion can be triggered by slab detachment, with consequences for plate boundary processes even at great distances from the event.

Transform and rift structure of Paleogene crust near Resolution Ridge, Tasman Sea, southwest New Zealand

Multibeam bathymetry, seismic reflection, magnetic anomaly, and gravity anomaly data show that most of the Resolution Ridge System is Tasman Sea oceanic crust, deformed by Eocene rift faulting associated with the initiation of the modern Australia‐Pacific plate boundary. Resolution Ridge, the most eastern ridge of the system, is inferred to be continental crust that was plucked from the southwest corner of the Campbell Plateau during Eocene rift propagation. Rift propagation proceeded through sequential northward steps between weak points in the lithosphere: young fossil spreading centers, fracture zones, and the western margin of the continental Campbell Plateau. Fracture zones guided the large‐scale geometry of the rift, but there is not always a close surface correspondence between transform and rift faults. This may be because transform faults were subvertical, whereas rift faults evolved to be moderately dipping. The very good correspondence of fracture zones, rift faults, and magnetic anomalies from the Resolution Ridge System with their conjugates southwest of the Campbell Plateau allows precise prerift reconstruction of the Australia‐Pacific plate boundary at a time (circa 48 Ma) of significant global plate reorganization and profound change in Pacific plate motion. This precise plate reconstruction better constrains plate motion chains that connect the Pacific and Indo‐Atlantic hemispheres, thus contributing to debate of global scientific issues such as what caused the bend in the Emperor‐Hawaii seamount chain.

Nonlinear 40Ar/39Ar age systematics along the Gilbert Ridge and Tokelau Seamount Trail and the timing of the Hawaii‐Emperor Bend

Over the last three decades the first‐order correlation in morphology and orientation of seamount trails has been called upon to support the concept of a “fixed” Pacific hot spot frame of reference and to explain the Hawaii‐Emperor bend (HEB) by a dramatic change in Pacific plate motion. In this paper, however, we present 40Ar/39Ar ages for the Gilbert Ridge and Tokelau Seamounts (Pacific) that show similar changes or bends in their orientation, but at different geological times, up to 20 Myr earlier than the HEB. Changes in Pacific plate motion alone cannot explain these observations, because these asynchronous bends should have been reflected in the morphology of each of these seamount trails. Together with the lack of (linear) age progressions and inconsistent apparent local plate velocities of 131 and 87 mm/yr, we rule out a fixed hot spot origin for the Gilbert Ridge and Tokelau seamount trails. Instead we invoke secondary or alternate processes to explain the complex age systematics and morphologies in these seamount trails. We propose here that the HEB‐type bends in these seamount trails were likely formed by short‐term “jerk‐like” plate extensions in the studied southwestern region of the Pacific plate, reactivating a preconditioned lithosphere that can be characterized by a complex structure and precursory magmatic impingements. The remarkable differences observed in these colinear seamount trails fundamentally question the existence of HEB‐type bends in the formation of Pacific volcanic lineaments. They also show us that applying geometric and morphologic observations alone is insufficient in constraining past plate motions. Nevertheless, the need and search for alternate volcano‐tectonic mechanisms offer opportunities to better understand intraplate volcanism in general.

50-Ma Initiation of Hawaiian-Emperor Bend Records Major Change in Pacific Plate Motion

The Hawaiian-Emperor bend has played a prominent yet controversial role in deciphering past Pacific plate motions and the tempo of plate motion change. New ages for volcanoes of the central and southern Emperor chain define large changes in volcanic migration rate with little associated change in the chain's trend, which suggests that the bend did not form by slowing of the Hawaiian hot spot. Initiation of the bend near Kimmei seamount about 50 million years ago (MA) was coincident with realignment of Pacific spreading centers and early magmatism in western Pacific arcs, consistent with formation of the bend by changed Pacific plate motion.

Age progression along the Society hotspot chain (French Polynesia) based on new unspiked K-Ar ages

Abstract New K-Ar dates of volcanic rocks from five of the nine islands of the Society Archipelago (Moorea, Huahine, Raiatea, Bora Bora and Maupiti), confirm a Pacific plate velocity of around 11 cm/a during the last 4.3 m.y. These new data allow us to analyse the age-distance relationship along the chain and to evaluate possible temporal variations in the activity of the Society hotspot. A clear increase of ages is observed along the linear chain away from the present Society hotspot location. The time-space relationship between Taiarapu, Tahiti-Nui and Moorea can be explained by a simple hotspot model. Nevertheless, the simple fixed hotspot model assuming constant Pacific plate velocity may need adjustments to fully explain the age progression along the Archipelago. The slight departures from a linear age distribution can be explained by changes in Pacific plate motion which occurred at 5 and 3 Ma. In addition, the contemporaneous magmatic activities in the pairs Bora-Bora/Tahaa, Raiatea/Huahine, Maiao/Moorea require additional lithospheric control on magma transport. Combined with the hotspot activity, lithospheric loading may have produced extension and triggered volcanism along already existing fractures linking paired islands. The most likely model for the Society chain, proposed by McNutt [1998], involves a plume originating from a wide deep thermally anomalous zone (the Pacific Superswell) as a rising diapir (hotspot of secondary type according to the classification of Courtillot et al. [2003]). It melted during ascent and ponded beneath the Pacific plate to form short linear island chains showing rather good age vs. distance correlations.

Radiometric ages for basement rocks from the Emperor Seamounts, ODP Leg 197

The Hawaiian‐Emperor seamount chain is the “type” example of an age‐progressive, hot spot‐generated intraplate volcanic lineament. However, our current knowledge of the age distribution within this province is based largely on radiometric ages determined several decades ago. Improvements in instrumentation, sample preparation methods, and new material obtained by recent drilling warrant a reexamination of the age relations among the older Hawaiian volcanoes. We report new age determinations (40Ar‐39Ar incremental heating method) on whole rocks and feldspar separates from Detroit (Sites 1203 and 1204), Nintoku (Site 1205), and Koko (Site 1206) Seamounts (Ocean Drilling Program (ODP) Leg 197) and Meiji Seamount (Deep Sea Drilling Project (DSDP) Leg 19, Site 192). Plateaus in incremental heating age spectra for Site 1203 lava flows give a mean age of 75.8 ± 0.6 (2σ) Ma, which is consistent with the normal magnetic polarity directions observed and biostratigraphic age assignments. Site 1204 lavas produced discordant spectra, indicating Ar loss by reheating and K mobilization. Six plateau ages from lava flows at Site 1205 give a mean age of 55.6 ± 0.2 Ma, corresponding to Chron 24r. Drilling at Site 1206 intersected a N‐R‐N magnetic polarity sequence of lava flows, from which six plateau ages give a mean age of 49.1 ± 0.2 Ma, corresponding to the Chron 21n‐22r‐22n sequence. Plateau ages from two feldspar separates and one lava from DSDP Site 192 range from 34 to 41 Ma, significantly younger than the Cretaceous age of overlying sediments, which we relate to postcrystallization K mobilization. Combined with new dating results from Suiko Seamount (DSDP Site 433) and volcanoes near the prominent bend in the lineament [Sharp and Clague, 2002], the overall trend is increasing volcano age from south to north along the Emperor Seamounts, consistent with the hot spot model. However, there appear to be important departures from the earlier modeled simple linear age progression, which we relate to changes in Pacific plate motion and the rate of southward motion of the Hawaiian hot spot.

Cenozoic plate boundary evolution in the South Island of New Zealand: New thermochronological constraints

Thermochronological investigations of samples collected west of the Alpine Fault zone provide new insight into the early development of the Australian‐Pacific (AUS‐PAC) plate boundary through New Zealand that is not preserved elsewhere in the modern orogenic system of the South Island. The40Ar/39Ar, fission track, and (U‐Th)/He ages for these samples span the Cenozoic and provide direct constraints on the timing and character of two discrete episodes in the evolution of this tectonic boundary. The initial propagation of the AUS‐PAC boundary through the South Island in the early Miocene is expressed as short‐lived, geographically localized cooling, which we infer to be due to exhumation, beginning at 23–25 Ma. This is consistent with development of this boundary as an initially distributed zone of deformation, progressively localizing onto discrete fault structures developing primacy of strain accommodation over time. Regionally synchronous development of rapid cooling at 8 Ma corroborates the association of the present obliquely convergent character of the AUS‐PAC boundary through the South Island with a marked change in Pacific plate motion at that time. Subsequent increases in cooling rate and a regionally prominent 5 Ma thermochronologic signal illuminate the progressive adaptation of the plate boundary to this new tectonic system over a period of ∼3 Myr, culminating in development of the extant tectonic regime.