An outlet for Pacific mantle: The Caribbean Sea?

AbstractThe Pacific Ocean is surrounded by subduction zone systems leading to a decreasing surface area as well as sub-surface mantle domain. In contrast, the Atlantic realm is characterized by passive margins and growing in size. To maintain global mass balance, the Caribbean and the Scotia Sea have been proposed as Pacific-to-Atlantic transfer channels for sub-lithospheric shallow mantle. We concentrate on the Caribbean here and test this idea by calculating the present-day regional dynamic topography in search of a gradual decrease from west to east that mirrors the pressure gradient due to the shrinkage of the Pacific. To calculate the dynamic topography, we isostatically correct the observed topography for sediments and crustal thickness variations, and compare the result with those predicted by lithospheric cooling models. The required age-grid was derived from our recently published reconstruction model. Our results confirm previous geochemical and shear-wave splitting studies and suggest some lateral asthenosphere flow away from the Galapagos hotspot. However, they also indicate that this flow is blocked in the Central Caribbean. This observation suggests that rather than through large scale Pacific-to-Atlantic shallow mantle flow, the global mass balance is maintained through some other process, possibly related to the deep mantle underneath Africa

Structure and Dynamics of Lithosphere and Asthenosphere in Asia: A Seismological Perspective

Knowledge of lithospheric structure is essential for understanding the impact of continental collision and oceanic subduction on surface tectonic configurations. Full-waveform tomographic images reveal lateral heterogeneities and anisotropy of the lithosphere and asthenosphere in Asia. Estimating lithospheric thickness from seismic velocity reductions at depth exhibits large variations underneath different tectonic units. The thickest cratonic roots are present beneath the Sichuan, Ordos, and Tarim basins and central India. Radial anisotropy signatures of 11 representative tectonic provinces uncover the different nature and geodynamic processes of their respective past and present deformation. The large-scale continental lithospheric deformation is characterized by low-velocity anomalies from the Himalayan Orogen to the Baikal rift zone in central Asia, coupled with the post-collision thickening of the crust. The horizontal low-velocity layer of ∼100–300 km depth extent below the lithosphere points toward the existence of the asthenosphere beneath East and Southeast Asia, with heterogeneous anisotropy indicative of channel flows

Global mantle flow retrodictions for the early Cenozoic using an adjoint method: Evolving dynamic topographies, deep mantle structures, flow trajectories and sublithospheric stresses

During the Cenozoic, the Earth experienced multiple first-order geological events that are likely mantle flow related. These include the termination of large-scale marine inundation in North America in the Palaeocene, the late Tertiary rise of Africa relative to other continents and the long-wavelength tilting of Australia since the late Cretaceous, which occurred when the continent approached the southeast Asia subduction systems on its northward passage from Antartica. Here we explore a suite of eight high-resolution, compressible, global mantle flow retrodictions going back to 50 Ma, using an adoint method with ≈670 million finite elements. These retrodictions show for the first time that these events emerge jointly as part of global Cenozoic mantle flow histories. Our retrodictions involve the dynamic effects from an upper mantle low-viscosity zone, assimilate a past plate-motion model for the tangential surface velocity field, probe the influence of two different present-day mantle state estimates derived from seismic tomography, and acknowledge the rheological uncertainties of dynamic Earth models by taking in four different realizations for the radial mantle viscosity profile, two of which were published previously.We find the retrodicted mantle flow histories are sensitive to the present-day mantle state estimate and the rheological properties of the Earth model, meaning that this input information is testable with inferences gleaned from the geological record. For a deep mantle viscosity of 1.7 × 1022Pa s and a purely thermal interpretation of seismic structure, lowermantle flow velocities exceed 7 cmyr-1in some regions,meaning they are difficult to reconcile with the existence of a hotspot reference frame. Conversely, a deep mantle viscosity of 1023Pa s yields modest flow velocities (< 3 cmyr-1) and stability of deep mantle heterogeneity for much of the retrodiction time, albeit at the expense that African uplift is delayed into the latest Neogene. Retrodictions allow one to track material back in time from any given sampling location, making them potentially useful, for example, to geochemical studies. Our results call for improved estimates on non-isostatic vertical motion of the Earth's surface-provided, for instance, by basin analysis, seismic stratigraphy, landform studies, thermochronological data or the sedimentation record-to constrain the recent mantle flow history and suggest that mantle flow retrodictions may yield synergies across different Earth science disciplines

Hotspot motion caused the Hawaiian-Emperor Bend and LLSVPs are not fixed

Controversy surrounds the fixity of both hotspots and large low shear velocity provinces (LLSVPs). Paleomagnetism, plate-circuit analyses, sediment facies, geodynamic modeling, and geochemistry suggest motion of the Hawaiian plume in Earth's mantle during formation of the Emperor seamounts. Herein, we report new paleomagnetic data from the Hawaiian chain (Midway Atoll) that indicate the Hawaiian plume arrived at its current latitude by 28 Ma. A dramatic decrease in distance between Hawaiian-Emperor and Louisville chain seamounts between 63 and 52 Ma confirms a high rate of southward Hawaiian hotspot drift (similar to 47 mm yr(-1)), and excludes true polar wander as a relevant factor. These findings further indicate that the Hawaiian-Emperor chain bend morphology was caused by hotspot motion, not plate motion. Rapid plume motion was likely produced by ridge-plume interaction and deeper influence of the Pacific LLSVP. When compared to plate circuit predictions, the Midway data suggest similar to 13 mm yr(-1) of African LLSVP motion since the Oligocene. LLSVP upwellings are not fixed, but also wander as they attract plumes and are shaped by deep mantle convection

Are splash plumes the origin of minor hotspots?

It has been claimed that focused hot cylindrical upwelling plumes cause many of the surface volcanic hotspots on Earth. It has also been argued that they must originate from thermal boundary layers. In this paper, we present spherical simulations of mantle circulation at close to Earth-like vigor with significant internal heating. These show, in addition to thermal boundary layer plumes, a new class of plumes that are not rooted in thermal boundary layers. These plumes develop as instabilities from the edge of bowls of hot mantle, which are produced by cold downwelling material deforming hot sheets of mantle. The resulting bowl and plume structure can look a bit like the “splash� of a water droplet. These splash plumes might provide an explanation for some hotspots that are not underlain by thermal boundary layer–sourced plumes and not initiated by large igneous provinces. We suggest that in Earth's mantle, lithospheric instabilities or small pieces of subducting slab could play the role of the model downwelling material in initiating splash plumes. Splash plumes would have implications for interpreting ocean-island basalt geochemistry, plume fixity, excess plume temperature, and estimating core heat flux. Improved seismic imaging will ultimately test this hypothesis

Rapid plate motion variations through geological time: Observations serving geodynamic interpretation

Past and current plate motions are increasingly well mapped from high-temporal-resolution paleomagnetic and geodetic studies, revealing rapid variations that occur on short timescales relative to the time it takes for the large-scale structure associate

Tomographic images of a mantle circulation model

Sampling convection in the Earth's mantle by seismic tomography is difficult as evidenced by the uneven distribution of seismic stations and events with some regions being well imaged compared to others. Here we quantitatively explore tomographic filtering on Earth structure by tracing ISC P‐body‐wave data through a computer simulation of mantle circulation which accounts for internal heating of the mantle by radioactive decay, heatflux from the core, a depthwise increase in viscosity, and plate motion history of the past 120 million years. The travel time residuals are inverted by solving jointly for structure and hypocentral parameters with explicit damping and smoothing. We recover the Farallon and Tethys slabs as well as some low velocity anomalies associated with hot upwelling flow suggesting that tomographic filtering is probably minor in areas of high ray density

Monsoon speeds up Indian plate motion

Short-term plate motion variations on the order of a few Myr are a powerful probe into the nature of plate boundary forces, as mantle-related buoyancies evolve on longer time-scales. New reconstructions of the ocean-floor spreading record reveal an increasing number of such variations, but the dynamic mechanisms producing them are still unclear. Here we show quantitatively that climate changes may impact the short-term evolution of plate motion by linking explicitly the observed counter-clockwise rotation of the Indian plate since ~. 10. Ma to increased erosion and reduced elevation along the eastern Himalayas, due to temporal variations in monsoon intensity. By assimilating observations into empirical relations for the competing contributions of erosion and mountain building, we estimate the first-order decrease in elevation along the eastern Himalayas since initial strengthening of the monsoon. Furthermore, we show with global geodynamic models of the coupled mantle/lithosphere system that the inferred reduction in elevation is consistent with the Indian plate motion record over the same period of time, and that lowered gravitational potential energy in the eastern Himalayas following stronger erosion is a key factor to foster plate convergence in this region. Our study implicates lateral variations in plate coupling and their temporal changes as an efficient source to induce an uncommon form of plate motion where the Euler pole falls within its associated plate

Monsoon speeds up Indian plate motion

International audienceShort-term plate motion variations on the order of a few Myr are a powerful probe into the nature of plate boundary forces, as mantle-related buoyancies evolve on longer time-scales. New reconstructions of the ocean-floor spreading record reveal an increasing number of such variations, but the dynamic mechanisms producing them are still unclear. Here we show quantitatively that climate changes may impact the shortterm evolution of plate motion by linking explicitly the observed counter-clockwise rotation of the Indian plate since ~10 Ma to increased erosion and reduced elevation along the eastern Himalayas, due to temporal variations in monsoon intensity. By assimilating observations into empirical relations for the competing contributions of erosion and mountain building, we estimate the first-order decrease in elevation along the eastern Himalayas since initial strengthening of the monsoon. Furthermore, we show with global geodynamic models of the coupled mantle/lithosphere system that the inferred reduction in elevation is consistent with the Indian plate motion record over the same period of time, and that lowered gravitational potential energy in the eastern Himalayas following stronger erosion is a key factor to foster plate convergence in this region. Our study implicates lateral variations in plate coupling and their temporal changes as an efficient source to induce an uncommon form of plate motion where the Euler pole falls within its associated plate