Change In Plate Motion Research Articles

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.

Transient Quaternary erosion and tectonic inversion of the Northern Range, Trinidad

Cosmogenic 10Be measured in sediments from Northern Range catchments on the island of Trinidad reveals low millennial-scale rates of erosion (average ~40mm/ka) that increase roughly eightfold, from 11 to 92mm/ka, eastward across the mountain range. These results, in conjunction with an analysis of mountain morphometrics, are consistent with Quaternary east-side-up tilting of the Northern Range, which has occurred recently (~100ka). The highest millennial-scale erosion rates coincide spatially with Quaternary east-side surface uplift (albeit not in magnitude), high modern rainfall rates, low topographic relief, and convex longitudinal stream profiles, indicating transient Quaternary erosion. We interpret that hillslope adjustment and erosion of the Northern Range is buffered from climatic and tectonic forcings by dense tropical vegetation cover, thick saprolite, and porous regolith. Compared with longer-term, thermochronology exhumation rates, we document that an order of magnitude deceleration of and reversal in the locus of erosion and exhumation has occurred during the Pliocene to the Holocene. We suggest that these combined data sets highlight distinct syn- and post-collisional phases of Northern Range development that are related to a major change in relative Caribbean-South American plate motion from oblique convergence to transform motion. Oblique collision during the mid-Miocene likely caused relatively higher rates of and asymmetric exhumation of the Northern Range. Post-collisional mountain-scale tilting is likely caused by a combination of crustal extension in the Gulf of Paria and by deep subsurface lithospheric detachment processes that drives dynamic topography.

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.

Facies architecture and provenance of a boulder-conglomerate submarine channel system, Panoche Formation, Great Valley Group: A forearc basin response to middle Cretaceous tectonism in the California convergent margin

Tectonic reorganization induced by a rapid increase in plate motion ­obliquity and rate beginning at ca. 100 Ma affected California’s Andean-style convergent margin, with concomitant changes in the accretionary prism of the Franciscan Complex, the Great Valley forearc basin, and the Sierran continental arc. Using facies analysis and a combined provenance approach, we suggest that this ca. 100 Ma tectonic signal is preserved in a Cenomanian (Upper Cretaceous) boulder-conglomerate outcrop along the San Luis Reservoir (SLR) in the southern Great Valley, which represents the thickest and coarsest deep-water deposit ever described in the Great Valley Group (GVG). We document a 1.8-km-thick by 4-km-long depositional-dip profile of an interpreted SE-directed (axial) submarine channel system that is part of a conglomeratic package that stretches 20 km along the east-central Diablo Range. Our facies analysis of the SLR area documents five facies associations within four aggradational channel complex sets, followed by regional abandonment. Sandstone petrography and mudrock geochemical data suggest a dissected continental Sierra Nevadan arc source. Conglomerate clast counts show abundant ophiolitic-type clasts that may be derived from the Coast Range Ophiolite and/or the Western Sierra Nevada Metamorphic Belt. Detrital-­zircon geochronology data also indicate western and central Sierra Nevadan sources; however, we interpret an anomalous (relative to other Cenomanian localities) 105–95 Ma zircon population to indicate the initial erosional products from the volcanic carapace associated with the Late Cretaceous magmatic flare-up within the eastern Sierran arc. This flare-up has been linked to an increase in arc-parallel plate motion that induced deformation along shear zones in the eastern Sierra Nevada, allowing for widespread plutonism. Our provenance interpretation makes the SLR area the earliest Upper Cretaceous GVG locality to receive significant detritus from the flare-up, effectively linking tectonic plate motion changes and coarse-grained, deep-water forearc sedimentation.

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.

Magnetic fabrics of arc plutons reveal a significant Late Jurassic to Early Cretaceous change in the relative plate motions of the Pacific Ocean basin and North America

Contrasting magnetic fabrics in five successively emplaced syntectonic plutons reveal temporal and spatial variations in tectonic strain in the oceanic terranes of the Blue Mountains province, northeastern Oregon, during the Late Jurassic to Early Cretaceous. The inferred strain regimes changed from: (1) thrusting and sinistral shearing at ca. 160 Ma, to (2) horizontal stretching at ca. 147 Ma (in the forearc–accretionary wedge Baker terrane), to (3) dextral transpression that started from ca. 140 Ma onward and was associated with progressive anticlockwise rotation of the principal horizontal shortening direction from ca. 130 Ma to ca. 126 Ma (in the Wallowa oceanic arc terrane). These progressive strain reorientations are interpreted in terms of an outboard Wallowa-­Baker terrane collision, lateral extrusion, docking of the amal­gamated Blue Mountains superterrane into a continental-margin reentrant, and onset of oroclinal bending, respectively. The changes in crustal strains are then interpreted as recording a progressive change in relative motions between the Pacific Ocean basin and North America and suggest a transition from Late Jurassic sinistral deformation to Early Cretaceous dextral terrane translations along the paleo-Pacific margin. We speculate that these events may have been linked along large portions of the North American Cordillera, from central California to Blue Mountains, and may have culminated in the onset of accretion in the Franciscan complex and voluminous plutonism in the Sierra Nevada magmatic arc. A similar plate-kinematic change is inferred to have occurred in British Columbia several tens of millions of years later (at ca. 100 Ma), implying that these kinematic transitions may have varied in space and time along the length of the Cordilleran orogen.

Episodic Jurassic to Lower Cretaceous intraplate compression in Central Patagonia during Gondwana breakup

From Lower Jurassic to Lower Cretaceous, several intraplate compression events affected discrete sectors of Central Patagonia, under a general context of crustal extension associated with Gondwana breakup. This was demonstrated by means of 2D and 3D seismic and borehole data, which show partial inversion of Lower and Middle Jurassic extensional structures of the Chubut and Cañadón Asfalto basins, during the earliest stages of breakup. A comparison with surrounding areas in Patagonia, where similar Jurassic intraplate compression was described, allowed the discrimination of three discrete pulses of subtle compression (C1: ∼188–185 Ma; C2: ∼170–163; C3: ∼157–136? Ma). Interestingly, episodic intraplate compressional events are closely followed by high flux magmatic events linked to the westward expansion of the Karoo-Ferrar thermal anomaly, which impacted on the lithosphere of southwest Gondwana in Lower Jurassic. In addition, we determined the approximate direction of the main compressive strain (σ1) compatible with other Jurassic intraplate belts of South America. These observations led us to propose a linkage between a thermo mechanically weakened continental crust due to LIPs activity, changes in plate motions and ridge-push forces generated by the opening of the Weddell Sea, in order to explain intraplate shortening, interrupted while Karoo LIPs magmatic invigoration took place.

Provenance of bentonite layers in the Palaeocene strata of the Central Basin, Svalbard: implications for magmatism and rifting events around the onset of the North Atlantic Igneous Province

A fold-and-thrust belt developed between Greenland and Svalbard during the Palaeogene, with an associated foreland basin forming in what is now Spitsbergen. This Central Basin is comprised of the Van Mijenfjorden Group, a ~2.3km thick sandstone-shale dominated succession that contains prominent and laterally continuous bentonite layers in the lower formations. These altered tephra layers can be used as stratigraphic markers that connect the basin development with regional explosive volcanism and changes to relative plate motions. We sampled and analysed bentonites from nine borehole cores across the Central Basin. Each layer shows evidence of alteration, with mobile elements such as alkali and alkali earth metals particularly disrupted. However, immobile elements including rare earth elements (REE) and preserved igneous minerals retain a magmatic signature, allowing for comparisons with potential volcanic sources to be made. The majority of bentonites are both evolved and strongly alkaline, with chemical signatures that are much closer to the continental rift events around Ellesmere Island and North Greenland than to the early activity of the North Atlantic Igneous Province (NAIP). There is a clear difference between tephra layers in the mid Palaeocene versus late Palaeocene strata. The early bentonites have a REE signature comparable to the volcanics of the Kap Washington Group exposed in North Greenland. The later bentonites have likely come from volcanic centres in the Nares Strait that are also the source of abundant volcaniclastic sediments in the Judge Daly Promontory, Ellesmere Island. These findings suggest that a mid to late Palaeocene change in locus of volcanic provenance may reflect changes in relative plate motions related to the formation of the West Spitsbergen fold-and-thrust belt and the emplacement of the NAIP. However, the lack of bentonites matching NAIP sources suggests that explosive volcanism was of insufficient magnitude to lead to deposition in the Central Basin at this time.

Eastern Indian Ocean microcontinent formation driven by plate motion changes

The roles of plate tectonic or mantle dynamic forces in rupturing continental lithosphere remain controversial. Particularly enigmatic is the rifting of microcontinents from mature continental rifted margins, with plume-driven thermal weakening commonly inferred to facilitate calving. However, a role for plate tectonic reorganisations has also been suggested. Here, we show that a combination of plate tectonic reorganisation and plume-driven thermal weakening were required to calve the Batavia and Gulden Draak microcontinents in the Cretaceous Indian Ocean. We reconstruct the evolution of these two microcontinents using constraints from new paleontological samples, 40Ar/39Ar ages, and geophysical data. Calving from India occurred at 101–104 Ma, coinciding with the onset of a dramatic change in Indian plate motion. Critically, Kerguelen plume volcanism does not appear to have directly triggered calving. Rather, it is likely that plume-related thermal weakening of the Indian passive margin preconditioned it for microcontinent formation but calving was triggered by changes in plate tectonic boundary forces.

The Jan Mayen microcontinent: an update of its architecture, structural development and role during the transition from the Ægir Ridge to the mid-oceanic Kolbeinsey Ridge

Abstract We present a revised tectonostratigraphy of the Jan Mayen microcontinent (JMMC) and its southern extent, with the focus on its relationship to the Greenland–Iceland–Faroe Ridge area and the Faroe–Iceland Fracture Zone. The microcontinent's Cenozoic evolution consists of six main phases corresponding to regional stratigraphic unconformities. Emplacement of Early Eocene plateau basalts at pre-break-up time (56–55 Ma), preceded the continental break-up (55 Ma) and the formation of seawards-dipping reflectors (SDRs) along the eastern and SE flanks of the JMMC. Simultaneously with SDR formation, orthogonal seafloor spreading initiated along the Ægir Ridge (Norway Basin) during the Early Eocene (C24n2r, 53.36 Ma to C22n, 49.3 Ma). Changes in plate motions at C21n (47.33 Ma) led to oblique seafloor spreading offset by transform faults and uplift along the microcontinent's southern flank. At C13n (33.2 Ma), spreading rates along the Ægir Ridge started to decrease, first south and then in the north. This was probably complemented by intra-continental extension within the JMMC, as indicated by the opening of the Jan Mayen Basin – a series of small pull-apart basins along the microcontinent's NW flank. JMMC was completely isolated when the mid-oceanic Kolbeinsey Ridge became fully established and the Ægir Ridge was abandoned between C7 and C6b (24–21.56 Ma).

Seamounts and oceanic igneous features in the NE Atlantic: a link between plate motions and mantle dynamics

Abstract A new regional compilation of seamount-like oceanic igneous features (SOIFs) in the NE Atlantic points to three distinct oceanic areas of abundant seamount clusters. Seamounts on oceanic crust dated 54–50 Ma are formed on smooth oceanic basement, which resulted from high spreading rates and magmatic productivity enhanced by higher than usual mantle plume activity. Late Eocene–Early Miocene SOIF clusters are located close to newly formed tectonic features on rough oceanic crust in the Irminger, Iceland and Norway basins, reflecting an unstable tectonic regime prone to local readjustments of mid-ocean ridge and fracture zone segments accompanied by extra igneous activity. A SOIF population observed on Mid-Miocene–Present rough oceanic basement in the Greenland and Lofoten basins, and on conjugate Kolbeinsey Ridge flanks, coincides with an increase in spreading rate and magmatic productivity. We suggest that both tectonic/kinematic and magmatic triggers produced Mid-Miocene–Present SOIFs, but the Early Miocene westwards ridge relocation may have played a role in delaying SOIF formation south of the Jan Mayen Fracture Zone. We conclude that Iceland plume episodic activity combined with regional changes in relative plate motion led to local mid-ocean ridge readjustments, which enhanced the likelihood of seamount formation. Supplementary material: Figures detailing NE Atlantic seamounts and SOIF distribution, and the location of earthquake epicentres are available at https://doi.org/10.6084/m9.figshare.c.3459729

Late Neogene changes in North America and Antarctica absolute plate motions inferred from the Mid‐Atlantic and Southwest Indian Ridges spreading histories

AbstractReconstructions of absolute plate motions underpin our understanding of the plate torque balance, but are challenging due to difficulties in inferring well‐dated rates and directions of plate movements from hot spot tracks. Useful information about plate dynamics can be inferred from rapid absolute plate motion changes, as these are linked only to the torque(s) that changed. Here we infer late Neogene changes in the absolute motions of North America and possibly Antarctica from changes in the easier‐to‐determine relative plate motions recorded along the Arctic, northern Mid‐Atlantic and Southwest Indian Ridges. We show that Eurasia/North America and Nubia/North America motions changed by the same amount between 8 and 5 Ma, as may have Nubia/Antarctica and Somalia/Antarctica plate motions. By considering additional, independent constraints on Somalia/India plate motion, we argue that a scenario in which North America and Antarctica absolute motions changed is the simplest one that explains the observed changes in relative motions. We speculate that these changes are linked to the late Neogene dynamics of the Pacific plate.

Abrupt plate accelerations shape rifted continental margins.

Rifted margins are formed by persistent stretching of continental lithosphere until breakup is achieved. It is well known that strain-rate-dependent processes control rift evolution, yet quantified extension histories of Earth's major passive margins have become available only recently. Here we investigate rift kinematics globally by applying a new geotectonic analysis technique to revised global plate reconstructions. We find that rifted margins feature an initial, slow rift phase (less than ten millimetres per year, full rate) and that an abrupt increase of plate divergence introduces a fast rift phase. Plate acceleration takes place before continental rupture and considerable margin area is created during each phase. We reproduce the rapid transition from slow to fast extension using analytical and numerical modelling with constant force boundary conditions. The extension models suggest that the two-phase velocity behaviour is caused by a rift-intrinsic strength--velocity feedback, which can be robustly inferred for diverse lithosphere configurations and rheologies. Our results explain differences between proximal and distal margin areas and demonstrate that abrupt plate acceleration during continental rifting is controlled by the nonlinear decay of the resistive rift strength force. This mechanism provides an explanation for several previously unexplained rapid absolute plate motion changes, offering new insights into the balance of plate driving forces through time.

Increased rates of large-magnitude explosive eruptions in Japan in the late Neogene and Quaternary.

Tephra layers in marine sediment cores from scientific ocean drilling largely record high‐magnitude silicic explosive eruptions in the Japan arc for up to the last 20 million years. Analysis of the thickness variation with distance of 180 tephra layers from a global data set suggests that the majority of the visible tephra layers used in this study are the products of caldera‐forming eruptions with magnitude (M) > 6, considering their distances at the respective drilling sites to their likely volcanic sources. Frequency of visible tephra layers in cores indicates a marked increase in rates of large magnitude explosive eruptions at ∼8 Ma, 6–4 Ma, and further increase after ∼2 Ma. These changes are attributed to major changes in tectonic plate interactions. Lower rates of large magnitude explosive volcanism in the Miocene are related to a strike‐slip‐dominated boundary (and temporary cessation or deceleration of subduction) between the Philippine Sea Plate and southwest Japan, combined with the possibility that much of the arc in northern Japan was submerged beneath sea level partly due to previous tectonic extension of northern Honshu related to formation of the Sea of Japan. Changes in plate motions and subduction dynamics during the ∼8 Ma to present period led to (1) increased arc‐normal subduction in southwest Japan (and resumption of arc volcanism) and (2) shift from extension to compression of the upper plate in northeast Japan, leading to uplift, crustal thickening and favorable conditions for accumulation of the large volumes of silicic magma needed for explosive caldera‐forming eruptions.

Ocean Basin Evolution and Global-Scale Plate Reorganization Events Since Pangea Breakup

We present a revised global plate motion model with continuously closing plate boundaries ranging from the Triassic at 230 Ma to the present day, assess differences among alternative absolute plate motion models, and review global tectonic events. Relatively high mean absolute plate motion rates of approximately 9–10 cm yr−1 between 140 and 120 Ma may be related to transient plate motion accelerations driven by the successive emplacement of a sequence of large igneous provinces during that time. An event at ∼100 Ma is most clearly expressed in the Indian Ocean and may reflect the initiation of Andean-style subduction along southern continental Eurasia, whereas an acceleration at ∼80 Ma of mean rates from 6 to 8 cm yr−1 reflects the initial northward acceleration of India and simultaneous speedups of plates in the Pacific. An event at ∼50 Ma expressed in relative, and some absolute, plate motion changes around the globe and in a reduction of global mean plate speeds from about 6 to 4–5 cm yr−1 indicates that an increase in collisional forces (such as the India–Eurasia collision) and ridge subduction events in the Pacific (such as the Izanagi–Pacific Ridge) play a significant role in modulating plate velocities.

Subduction controls the distribution and fragmentation of Earth’s tectonic plates.

The theory of plate tectonics describes how the surface of Earth is split into an organized jigsaw of seven large plates of similar sizes and a population of smaller plates whose areas follow a fractal distribution. The reconstruction of global tectonics during the past 200 million years suggests that this layout is probably a long-term feature of Earth, but the forces governing it are unknown. Previous studies, primarily based on the statistical properties of plate distributions, were unable to resolve how the size of the plates is determined by the properties of the lithosphere and the underlying mantle convection. Here we demonstrate that the plate layout of Earth is produced by a dynamic feedback between mantle convection and the strength of the lithosphere. Using three-dimensional spherical models of mantle convection that self-consistently produce the plate size–frequency distribution observed for Earth, we show that subduction geometry drives the tectonic fragmentation that generates plates. The spacing between the slabs controls the layout of large plates, and the stresses caused by the bending of trenches break plates into smaller fragments. Our results explain why the fast evolution in small back-arc plates reflects the marked changes in plate motions during times of major reorganizations. Our study opens the way to using convection simulations with plate-like behaviour to unravel how global tectonics and mantle convection are dynamically connected.

Miocene uplift of the NE Greenland margin linked to plate tectonics: Seismic evidence from the Greenland Fracture Zone, NE Atlantic

Tectonic models predict that, following breakup, rift margins undergo only decaying thermal subsidence during their post-rift evolution. However, post-breakup stratigraphy beneath the NE Atlantic shelves shows evidence of regional-scale unconformities, commonly cited as outer margin responses to inner margin episodic uplift, including the formation of coastal mountains. The origin of these events remains enigmatic. We present a seismic reflection study from the Greenland Fracture Zone – East Greenland Ridge (GFZ-EGR) and the NE Greenland shelf. We doc- ument a regional intra-Miocene seismic unconformity (IMU), which marks the termination of syn-rift deposition in the deep-sea basins and onset of: (i) thermo-mechanical coupling across the GFZ, (ii) basin compression, and (iii) contourite deposition, north of the EGR. The onset of coupling across the GFZ is constrained by results of 2-D flexural backstripping. We explain the thermo-mechanical coupling and the deposition of contourites by the forma- tion of a continuous plate boundary along the Mohns and Knipovich ridges, leading to an accelerated widening of the Fram Strait. We demonstrate that the IMU event is linked to onset of uplift and massive shelf-progradation on the NE Greenland margin. Given an estimated middle-to-late Miocene (ca. 15-10 Ma) age of the IMU, we speculate that the event is synchronous with uplift of the East and West Greenland margins. The correlation between margin uplift and plate-motion changes further indicates that the uplift was triggered by plate tectonic forces, induced perhaps by a change in the Iceland plume (a hot pulse) and/or by changes in intra-plate stresses related to global tectonics.

Changes in Relative Plate Motion during the Isan Orogeny (1670–1500 Ma) and Implications for Pre‐Rodinia Reconstructions

AbstractFoliation inflexion/intersection axes (FIAs) preserved within porphyroblasts that grew throughout Isan orogenesis reveal significant anticlockwise changes in the direction of bulk horizontal shortening between 1670 and 1500 Ma from NE–SW, N–S, E–W to NW–SE. This implies an anticlockwise shift in relative plate motion with time during the Isan orogeny. Dating monazite grains amongst the axial planar foliations defining three of the four FIAs enabled an age for the periods of relative plate motion that produced these structures to be determined. Averaging the ages from monazite grains defining each FIA set revealed 1649±12 Ma for NE–SW shortening, 1645±7 Ma for N–S shortening, and 1591±10 Ma for that directed E‐W. Inclusion trail asymmetries indicate shear senses of top to the SW for NW–SE FIAs and dominantly top to the N for E–W FIAs, reflecting thrusting towards the SW and N. No evidence for tectonism related to early NE–SW bulk horizontal shortening has previously been detected in the Mount Isa Inlier. Amalgamation of the Broken Hill and possibly the Gawler provinces with the Mount Isa province may have taken place during these periods of NE–SW and N–S‐directed thrusting as the ages of tectonism are similar. Overlapping dates, tectonic, metamorphic, and metallogenic similarities between eastern Australia (Mount Isa and Broken Hill terranes) and the southwest part of Laurentia imply a most probable connection between both continental masses. Putting Australia in such position with respect to North America during the Late‐Paleo‐to‐Mesoproterozoic time is consistent with the AUSWUS model of the Rodinia supercontinent.

Petrogenesis of the early Cretaceous volcanic rocks in the North Huaiyang tectono-magmatic unit of the Dabie Orogen, eastern China: Implications for crust–mantle interaction

New elemental and isotopic data are presented for the early Cretaceous felsic to mafic volcanic rocks in the North Huaiyang tectono-magmatic unit (NHY) of the Dabie Orogen, in order to investigate their petrogenesis and provide insights into the nature of the late Mesozoic lithosphere mantle beneath the region and its tectonic relationship with neighboring blocks. LA-ICP-MS zircon U–Pb dating reveals that volcanic rocks of the Jingangtai Formation erupted in a quite short interval about 5Mys during the Early Cretaceous (128–123Ma). The rocks have wide ranges of SiO2 (48–68wt.%) and MgO (0.6–5.6wt.%) contents. They are enriched in large-ion-lithophile-elements (LILE) (e.g. Rb, Ba) and light rare-earth-elements (LREE), and depleted in high field strength elements (e.g. Nb, Ta and Ti) with weak negative Eu anomalies (Eu/Eu∗=0.71–0.94). Meanwhile, the rocks show relatively high whole-rock initial 87Sr/86Sr ratios (0.7074–0.7094), strong negative εNd(t) (−19.1 to −15.8) and zircon εHf values (−20.7 to −14.1). Such typical “continental” geochemical characteristics did not result from crustal contamination during magma ascent, but from an enriched mantle source modified by materials from the subducted Yangtze Craton during the Triassic continental collision. We propose that the petrogenesis of the large-scale contemporaneous magmatism of Dabie Orogen including felsic to mafic volcanic rocks in the NHY reflects an intensive lithospheric thinning and extension during the early Cretaceous as a tectonic response to the change of plate motion of westward subducted Pacific Plate beneath the Eurasian continent.