Tracking the Australian plate motion through the Cenozoic: Constraints from 40Ar/39Ar geochronology

Here we use geochronology of Australian intraplate volcanoes to construct a high-resolution plate-velocity record and to explore how tectonic events in the southwest Pacific may have influenced plate motion. Nine samples from five volcanoes yield ages from 33.6 ± 0.5 to 27.3 ± 0.4 Ma and, when combined with published ages from 30 to 16 Ma, show that the rate of volcanic migration was not constant. Instead, the results indicate distinct changes in Australian plate motion. Fast northward velocities (61 ± 8 and 57 ± 4 km/Ma) prevailed from 34 to 30 (±0.5) and from 23 to 16 (±0.5) Ma, respectively, with distinct reductions to 20 ± 10 and 22 ± 5 km/Ma from 30 to 29 (±0.5) Ma and from 26 to 23 (±0.5) Ma. These velocity reductions are concurrent with tectonic collisions in New Guinea and Ontong Java, respectively. Interspersed between the periods of sluggish motion is a brief 29-26 (±0.5) Ma burst of atypically fast northward plate movement of 100 ± 20 km/Ma. We evaluate potential mechanisms for this atypically fast velocity, including catastrophic slab penetration into the lower mantle, thermomechanical erosion of the lithosphere, and plume-push forces; none are appropriate. This period of fast motion was, however, coincident with a major southward propagating slab tear that developed along the northeastern plate margin, following partial jamming of subduction and ophiolite obduction in New Caledonia. Although it is unclear whether such an event can play a role in driving fast plate motion, numerical or analogue models may help address this question. Key Points We determine nine 40Ar/39Ar ages from five Cenozoic volcanoes in Australia Slow velocities correlate with New Guinea and Ontong Java collisions Anomalously fast velocity of 100 +/- 20 km/Ma is identified from 29-26 M

Influence of the subducting plate velocity on the geometry of the slab and migration of the subduction hinge

Geological observations indicate that along two active continental margins (East Asia and Mediterranean) major phases of overriding plate extension, resulting from subduction hinge-retreat, occurred synchronously with a reduction in subducting plate velocity. In this paper, results of fluid dynamical experiments are presented to test the influence of the velocity of the subducting plate on the hinge-migration velocity and on the geometry of the slab. Results show that hinge-retreat decreases with increasing subducting plate velocity. In addition, phases of hinge-retreat alternate with phases of hinge-advance for relatively high subducting plate velocities due to interaction of the slab with the bottom of the box, simulating the upper-lower mantle discontinuity. Such slab kinematics could explain the episodic behaviour of back-arc opening observed in convergent settings. The geometry of the slab and the kinematics of subduction are significantly affected by the velocity of the subducting plate. Three subduction modes with accompanying slab geometry can be recognized. A relatively low subducting plate velocity is accompanied by relatively fast hinge-retreat with backward sinking of the slab and a backward draping slab geometry. With increasing subducting plate velocity hinge-migration is relatively small, resulting in subvertical sinking of the slab and a folded slab piling geometry. For a very high velocity the hinge migrates forward, resulting in forward oriented subduction vectors and a forward draping slab geometry

North-eastward subduction followed by slab detachment to explain ophiolite obduction and Early Miocene volcanism in Northland, New Zealand

Oligocene-Miocene models for northern New Zealand, involving south-westward subduction to explain Early Miocene Northland volcanism, do not fit within the regional Southwest Pacific tectonic framework. A new model is proposed, which comprises a north-east-dipping South Loyalty basin slab that retreated south-westward in the Eocene-earliest Miocene and was continuous with the north-east-dipping subduction zone of New Caledonia. In the latest Oligocene, the trench reached the Northland passive margin, which was pulled it into the mantle by the slab, resulting in obduction of the Northland allochthon. During and after obduction, the slab detached from the unsubductable continental lithosphere, inducing widespread calc-alkaline volcanism in Northland. The new model further explains contemporaneous arc volcanism along the Northland Plateau Seamount Chain and sinking of the Northland basement, followed by uplift and extension in Northland

Overriding plate shortening and extension above subduction zones:A parametric study to explain formation of the Andes Mountains

Mountain building above subduction zones, such as observed in the Andes, is enigmatic, and the key parameter controlling the underlying dynamics remains a matter of considerable debate. A global survey of subduction zones is presented here, illustrating the correlation between overriding plate deformation rate and twelve physical parameters: overriding plate velocity, subducting plate velocity, trench velocity, convergence velocity, subduction velocity, subduction zone accretion rate, subducting plate age, subduction polarity, shallow slab dip, deep slab dip, lateral slab edge proximity, and subducting ridge proximity. All correlation coefficients are low ( R ≤ 0.39), irrespective of the global reference frame, relative plate motion model, or overriding plate deformation model, except for the trench velocity (0.33-0.68, exact value depends on adopted global reference frame) and subduction velocity, which shows an anticorrelation (0.55-0.57). This implies that no individual parameter can explain overriding plate deformation, except that trench retreat generally corresponds to extension while an approximately stable trench or trench advance generally corresponds to shortening. Understanding of the variety of strain patterns is obtained when slab edge proximity and overriding plate velocity are combined. Orogenesis occurs in overriding plates bordering central regions of wide subduction zones (≥∼4000 km) when the overriding plate is moving trenchward at 0-2 cm/yr (e.g., Andes, Japan). Because the center of a wide slab offers large resistance to lateral migration, the overriding plate effectively collides with the subduction hinge, forcing the slab to attain a shallow dip angle (e.g., Nazca and Japan slabs). Overriding plate extension is only found close to lateral slab edges or during overriding plate motion away from the center of a wide subduction zone, but in the latter scenario, maximum extension velocities are much lower than in the former scenario. For subduction settings close to lateral slab edges, overriding plate motion plays no significant role in overriding plate deformation. Thus, for rapid overriding plate extension, the key ingredient is rapid trench retreat, which only occurs close to lateral slab edges, while for overriding plate shortening, the key ingredients are (1) the resistance to rapid trench and hinge retreat, which occurs far from lateral slab edges, and (2) trenchward overriding plate motion

Alpine deformation at the western termination of the axial zone, Southern Pyrenees

Detailed structural and sedimentological research has been conducted in order to unravel the local tectono-sedimentary history of an area located at the western termination of the axial zone, in the Southern Pyrenees. Two cross-sections have been constructed perpendicular to the axis of the orogen, situated above the westward dipping axial zone antiform and the northern part of the south Pyrenean zone, striking perpendicular to the axis of the orogen. The sediments of this area date from Late Cretaceous to Middle Eocene age and display syntectonic characteristics. The cross-sections, together with detailed analysis of key outcrops, reveal that there have been two main phases of deformation. The first phase (D1) is related to the movement along the Lakhoura basement thrust and can be subdivided into three sub-phases, related to activity of two main thrusts, which splay of the basement thrust. The first sub-phase is related to activity along the Lakhoura thrust (D1a), the second sub-phase to the Larra thrust (D1b) and the third sub-phase to reactivation of the Lakhoura thrust (D1c). The second phase (D2) is related to the Gavarnie basement thrust, which resulted in the formation of the axial zone antiform. D1 and D2 show their distinct type of deformation. The first phase is characterised by south vergent ramp-flat thrusting and fault propagation folding, whereas the second phase is characterised by upright to overturned folding and steep reverse faulting. One main structure, the Urzainqui fault propagation fold, shows a synsedimentary relationship and its activity can be dated at Early and/or Middle Lutetian. The total amount of shortening has been estimated at ~ 16.5 km, which represents ~ 49% of the undeformed length. The Lakhoura thrust (D1a, D1c) accounts for ~ 3.5 km of the total amount of shortening. Approximately 8.0 km of shortening can be attributed to the Larra thrust (D1b), whereas the remaining 5.0 km of shortening can be related to the Gavarnie thrust (D2)

Evolution of 3-D subduction-induced mantle flow around lateral slab edges in analogue models of free subduction analysed by stereoscopic particle image velocimetry technique

We present analogue models of free subduction in which we investigate the three-dimensional (3-D) subduction-induced mantle flow focusing around the slab edges. We use a stereoscopic Particle Image Velocimetry (sPIV) technique to map the 3-D mantle flow on 4 vertical cross-sections for one experiment and on 3 horizontal depth-sections for another experiment. On each section the in-plane components are mapped as well as the out-of-plane component for several experimental times. The results indicate that four types of maximum upwelling are produced by the subduction-induced mantle flow. The first two are associated with the poloidal circulation occurring in the mantle wedge and in the sub-slab domain. A third type is produced by horizontal motion and deformation of the frontal part of the slab lying on the 660 km discontinuity. The fourth type results from quasi-toroidal return flow around the lateral slab edges, which produces a maximum upwelling located slightly laterally away from the sub-slab domain and can have another maximum upwelling located laterally away from the mantle wedge. These upwellings occur during the whole subduction process. In contrast, the poloidal circulation in the mantle wedge produces a zone of upwelling that is vigorous during the free falling phase of the slab sinking but that decreases in intensity when reaching the steady-state phase. The position of the maximum upward component and horizontal components of the mantle flow velocity field has been tracked through time. Their time-evolving magnitude is well correlated to the trench retreat rate. The maximum upwelling velocity located laterally away from the subducting plate is ~18-24% of the trench retreat rate during the steady-state subduction phase. It is observed in the mid upper mantle but upwellings are produced throughout the whole upper mantle thickness, potentially promoting decompression melting. It could thereby provide a source for intraplate volcanism, such as Mount Etna in the Mediterranean, the Chiveluch group of volcanoes in Kamchatka and the Samoan hotspot near Tonga

A subduction and mantle plume origin for Samoan volcanism

The origin of Samoan volcanism in the southwest Pacific remains enigmatic. Whether mantle melting is solely caused by a mantle plume is questionable because some volcanism, here referred to as non-hotspot volcanism, defies the plume model and its linear age-progression trend. Indeed, non-hotspot volcanism occurred as far as 740 km west of the predicted Samoan hotspot after 5 Ma. Here we use fully-dynamic laboratory subduction models and a tectonic reconstruction to show that the nearby Tonga-Kermadec-Hikurangi (TKH) subduction zone induces a broad mantle upwelling around the northern slab edge that coincides with the non-hotspot volcanic activity after 5 Ma. Using published potential mantle temperatures for the ambient mantle and Samoan mantle plume, we find that two geodynamic processes can explain mantle melting responsible for intraplate volcanism in the Samoan region. We propose that before 5 Ma, the volcanism is consistent with the plume model, whereas afterwards non-hotspot volcanism resulted from interaction between the Subduction-Induced Mantle Upwelling (SIMU) and Samoan mantle plume material that propagated west from the hotspot due to the toroidal component of slab rollback-induced mantle flow. In this geodynamic scenario, the SIMU drives decompression melting in the westward-swept plume material, thus producing the non-hotpot volcanism

The development of sheath folds in viscously stratified materials in simple shear conditions: An analogue approach

Sheath folds are highly non-cylindrical folds occurring in a variety of geological settings, and have been studied using different approaches. With the present work, we provide a quantitative analysis of the generation and development of sheath folds in a viscously layered system in simple shear conditions. The sheath folds develop from an initial non-cylindrical deflection located on the highly viscous layer. The analogue experiments investigated the influence of (1) variations in the viscosity ratio between the high viscosity layer and the matrix (ηhvl/ηm), (2) variations in the ratio between the amplitude of the initial deflection and the thickness of the high viscosity layer (Af/Thvl), and (3) progressive simple shear (γ). The results show that increases in ηhvl/ηm will produce progressively less elongated sheath folds, while increases in Af/Thvl will result in more elongated sheath folds. We present regime diagrams with ηhvl/ηm and Af/Thvl for different shear strains illustrating under which conditions sheath folds form. In case the original deflection amplitude and layer thickness as well as γ can be retrieved for sheath folds in nature, then their geometry can be used to quantify the effective ηhvl/ηm

A geological map of the Scotia Sea area constrained by bathymetry, geological data, geophysical data and seismic tomography models from the deep mantle

The Scotia Sea is one of the tectonically most complex and least understood back-arc basins on Earth, which partly results from its remote location making the acquisition of data challenging. Here, we provide a review of current, publicly available geophysical and geological data in the Scotia Sea realm, including magnetic, Bouguer gravity anomaly, high-resolution bathymetric, heat flow and reflection seismic data and rock type data from cored and dredged samples. With this inter-disciplinary data-set, we performed an offshore geological mapping exercise that allowed us to identify lithologies in the predominantly submerged Scotia Sea domain. Cross-sections combining crustal structure and mantle tomography enabled us to address some of the still persisting geological challenges in this tectonically complex area. The data-review revealed that basalt is the dominant lithology in the Scotia Sea area, occupying most of the West and East Scotia Sea (WSS and ESS). Andesitic and more felsic lithologies are identified in the Central Scotia Sea (CSS) and the northern East Scotia Sea (ESS). Mesozoic/Palaeozoic metamorphic/sedimentary lithologies are reported from the highs along the North and South Scotia Ridges (NSR and SSR). These highs originate from a land bridge that, until the late-Mesozoic, connected the South American and Antarctic continents. Scarcely available and contradicting data prevent the age determination of several structural units surrounding and located on the Scotia plate, but our mapping exercise allowed us to confirm the presence of the early Oligocene - late Miocene Ancestral South Sandwich Arc (ASSA) in the east of the CSS, setting the minimum age of the older segment of the CSS crust to Eocene-earliest Oligocene. Three cross-sections cross-cutting the Scotia Sea reveal two high velocity zones, indicating cold mantle material. One is situated below the structural highs along the SSR, which we interpreted as remnant slab material of the ASSA and another below South Georgia and the CSS, which we interpreted as the front of the slab that currently subducts at the South Sandwich Trench (SSaT). The neutral velocity discontinuity below the WSS implies mantle conditions of a recently extinct spreading centre. The upper mantle low velocity anomaly below the CSS is interpreted as warmer toroidal flows around the slab edges of the subducting plate at the SSaT. We have demonstrated that the geological and geophysical data publicly available today allows us to create offshore geological maps in remote, inaccessible offshore domains. This is a less time-consuming, economically advantageous exercise, which re-uses existing geological and geophysical data for a new purpose. It is the most data-inclusive study there is today of the Scotia Sea region and serves as a guideline for future expeditions targeting the CSS and the structural features along the NSR to identify their age and origin. A georeferenced version of the map is provided in the supplementary material