Spatial and temporal patterns of Cenozoic dynamic topography around Australia

[1] Despite its importance, the spatial and temporal pattern of dynamic topography generated by mantle convective circulation is poorly known. We present accurate estimates of dynamic topography from oceanic basins and continental margins surrounding Australia. Our starting point is measurement of residual depth anomalies on the oldest oceanic floor adjacent to the continental shelf. These anomalies were determined from a combined dataset of ~200 seismic reflection and wide-angle images of well-sedimented oceanic crust. They have amplitudes of between −1 km and +0.5 km, and their spatial variation is broadly consistent with long-wavelength free-air gravity and shallow seismic tomographic anomalies. Along the Northwest Shelf, a regional depth anomaly of −300 to −700 m intersects the adjacent continental shelf. The temporal evolution of this anomaly was determined by analyzing the stratigraphic architecture of an extensive carbonate platform, which fringes the shelf and records a dramatic switch from progradation to aggradation during Neogene times. Three-dimensional seismic mapping calibrated by boreholes was used to calculate water-loaded subsidence histories at rollover points of clinoforms along the shelf. At 9 ± 3 Ma, the rate of subsidence increases from 5 to up 75 m Myr−1, generating a subsidence anomaly of −300 to −700 m. The amplitude of this anomaly varies along the shelf and cannot be generated by glacio-eustatic sea-level variation. Instead, we propose that a regional subsidence episode, which affects both the proximal shelf and the distal oceanic basin, was generated by convective drawdown. By combining our results with other published estimates of uplift and subsidence, a map of Australia, which shows the spatial and temporal pattern of dynamic topography is presented. Most, but not all, of Australia's epeirogeny can be attributed to rapid northward motion of the Australian plate over a pre-existing pattern of convective circulation

Global dynamic topography observations reveal limited influence of large-scale mantle flow

Convective circulation of the Earth’s mantle maintains some fraction of surface topography that varies with space and time. Most predictive models show that this dynamic topography has peak amplitudes of about ±2km, dominated by wavelengths of 10⁴km. Here, we test these models against our comprehensive observational database of 2,120 spot measurements of dynamic topography that were determined by analysing oceanic seismic surveys. These accurate measurements have typical peak amplitudes of ±1km and wavelengths of approximately 10³km, and are combined with limited continental constraints to generate a global spherical harmonic model whose robustness has been carefully tested and benchmarked. Our power spectral analysis reveals significant discrepancies between observed and predicted dynamic topography. At longer wavelengths (such as 10⁴km), observed dynamic topography has peak amplitudes of about ±500m. At shorter wavelengths (such as 10³km), significant dynamic topography is still observed. We show that these discrepancies can be explained if short-wavelength dynamic topography is generated by temperature-driven density anomalies within a sub-plate asthenospheric channel. Stratigraphic observations from adjacent continental margins show that these dynamic topographic signals evolve quickly with time. More rapid temporal and spatial changes in vertical displacement of the Earth’s surface have direct consequences for fields as diverse as mantle flow, oceanic circulation and long-term climate change

Cenozoic Epeirogeny of Arabian Peninsula from Drainage Modeling

It is generally accepted that the Arabian Peninsula has been uplifted by subcrustal processes. Positive residual depth anomalies from oceanic crust in the Red Sea and in the Gulf of Aden suggest that a region surrounding this peninsula is dynamically supported. Admittance calculations, surface wave tomography studies, and receiver function analyses all imply that regional topography is generated and maintained by some combination of mantle convective circulation and lithospheric thickness changes. Despite these significant advances, the spatial and temporal uplift rate history of the Arabian Peninsula is not well known. Here we show that a regional uplift rate history can be obtained by jointly inverting 225 longitudinal river profiles that drain this peninsula. Our strategy assumes that shapes of individual river profiles are controlled by uplift rate history and moderated by erosional processes. We used local measurements of incision rate to calibrate the relevant erosional parameters. In our inverse algorithm, uplift rate is permitted to vary smoothly as a function of space and time but upstream drainage area remains invariant. We also assume that knickzone migration is not lithologically controlled. Implications of these important assumptions have been investigated. Our results suggest that the Arabian Peninsula underwent two phases of asymmetric uplift during the last 20–30 Ma at rates of 0.05–0.1 mm a−1. The southwestern flank of the peninsula has been uplifted by 1.5–2.5 km. Regional stratigraphic constraints, the age and composition of volcanism, paleosol formation, incised peneplains, emergent marine terraces, and thermochronometric measurements corroborate our calculated patterns of uplift. Progressive development of three domal swells along the western margin of the peninsula is consistent with localized upwelling of hot asthenospheric mantle

Pliocene shorelines and the epeirogenic motion of continental margins: a target dataset for dynamic topography models

Global mean sea level during the mid-Pliocene epoch (∼3 Ma), when CO2 and temperatures were above present levels, was notably higher than today due to reduced global ice sheet coverage. Nevertheless, the extent to which ice sheets responded to Pliocene warmth remains in question owing to high levels of uncertainty in proxy-based sea level reconstructions as well as solid Earth dynamic models that have been used to evaluate a limited number of data constraints. Here, we present a global dataset of 10 wave-cut scarps that formed by successive Pliocene sea level oscillations and which are observed today at elevations ranging from ∼6 to 109 m above sea level. The present-day elevations of these features have been identified using a combination of high-resolution digital elevation models and field mapping. Using the MATLAB interface TerraceM, we extrapolate the cliff and platform surfaces to determine the elevation of the scarp toe, which in most settings is buried under meters of talus. We correct the scarp-toe elevations for glacial isostatic adjustment and find that this process alone cannot explain observed differences in Pliocene paleo-shoreline elevations around the globe. We next determine the signal associated with mantle dynamic topography by back-advecting the present-day three-dimensional buoyancy structure of the mantle and calculating the difference in radial surface stresses over the last 3 Myr using the convection code ASPECT. We include a wide range of present-day mantle structures (buoyancy and viscosity) constrained by seismic tomography models, geodynamic observations, and rock mechanics laboratory experiments. Finally, we identify preferred dynamic topography change predictions based on their agreement with scarp elevations and use our most confident result to estimate a Pliocene global mean sea level based on one scarp from De Hoop, South Africa. This inference (11.6 ± 5.2 m) is a downward revision and may imply that ice sheets were relatively resistant to warm Pliocene climate conditions. We also conclude, however, that more targeted model development is needed to more reliably infer mid-Pliocene global mean sea level based on all scarps mapped in this study

Task Based Learning and Oral Production in EFL Reading Classes

In Japan, English for Academic Purposes (EAP) holds its own place of importance in tertiary English education. However, with a recent lean towards more communicative learning styles, the effects of using task based activities to teach reading in communicative learning environments are yet to be fully comprehended. This paper examines strategies for the teaching of reading in such classrooms. The effectiveness, benefits and suitability of using communicative based activities in such teaching contexts are also discussed. This study arrives at the conclusion that finding the appropriate methodology for the context should achieve precedence over trying to meld current teaching trends into classroom practices irrespective of their suitability, both for the learner and for the teacher.研究ノートResearch Not

To What Extent can Questionnaires be used to Investigate Willingness to Communicate in Large Classes in Japanese Universities?

Investigating the attitudes, motivations, and orientations towards second language learning is a valuable step on the path to improving pedagogical practice. This paper discusses the merits of using a questionnaire based quantitative approach to studying willingness to communicate in a second language among students in large, non-elective classes. In particular, the suitability of questionnaires for gathering data regarding attitudes from a large number of participants is discussed. The conclusion this paper reaches is that for researchers with limited time and access to participants, questionnaires can be an efficient and meaningful method of data collection.研究ノートResearch Not

Diagnostic criteria for primary autoimmune cerebellar ataxia—guidelines from an international task force on immune-mediated cerebellar ataxias

Aside from well-characterized immune-mediated ataxias with a clear trigger and/or association with specific neuronal antibodies, a large number of idiopathic ataxias are suspected to be immune mediated but remain undiagnosed due to lack of diagnostic biomarkers. Primary autoimmune cerebellar ataxia (PACA) is the term used to describe this later group. An International Task Force comprising experts in the field of immune ataxias was commissioned by the Society for Research on the Cerebellum and Ataxias (SRCA) in order to devise diagnostic criteria aiming to improve the diagnosis of PACA. The proposed diagnostic criteria for PACA are based on clinical (mode of onset, pattern of cerebellar involvement, presence of other autoimmune diseases), imaging findings (MRI and if available MR spectroscopy showing preferential, but not exclusive involvement of vermis) and laboratory investigations (CSF pleocytosis and/or CSF-restricted IgG oligoclonal bands) parameters. The aim is to enable clinicians to consider PACA when encountering a patient with progressive ataxia and no other diagnosis given that such consideration might have important therapeutic implications

Reply to “Geochemical Characteristics of Anatolian Basalts: Comment on ‘Neogene Uplift and Magmatism of Anatolia: Insights from Drainage Analysis and Basaltic Geochemistry’ by McNab et al.”

Uslular and Gençalioglu-Kuscu [2018] have written a lengthy, and highly critical, comment about McNab et al. [2018] which states that our data compilation for Neogene (and Quaternary) volcanic rocks from Anatolia is selective, inconsistent, and not fit for purpose. We state for the record that our compilation is not based on analyses from the published GEOROC database. Uslular and Gençalioglu-Kuscu [2018] also state that our sub-division of this database into three broad longitudinal categories is unrealistic since it does not consider the full range of different tectonic units. They conclude that our interpretation of the link between Neogene-Quaternary volcanism and uplift of Anatolia is erroneous. We refute this rather strongly worded comment by carefully addressing the five substantive issues raised

Reply to “Geochemical Characteristics of Anatolian Basalts: Comment on ‘Neogene Uplift and Magmatism of Anatolia: Insights from Drainage Analysis and Basaltic Geochemistry’ by McNab et al.”

Uslular and Gençalioğlu‐Kuşcu (2018) have written a lengthy, and highly critical, comment about McNab et al. (2018, https://doi.org/10.1002/2017GC007251) which states that our data compilation for Neogene (and Quaternary) volcanic rocks from Anatolia is selective, inconsistent, and not fit for purpose. We state for the record that our compilation is not based on analyses from the published GEOROC database. Uslular and Gençalioğlu‐Kuşcu (2018) also state that our subdivision of this database into three broad longitudinal categories is unrealistic since it does not consider the full range of different tectonic units. They conclude that our interpretation of the link between Neogene‐Quaternary volcanism and uplift of Anatolia is erroneous. We refute this rather strongly worded comment by carefully addressing the five substantive issues raised

Rapid postglacial rebound amplifies global sea level rise following West Antarctic Ice Sheet collapse

Earth_Model_Data is a zipped folder containing the Earth model data for both the standard model (V3D_SD and V3D_RH). A readme file is in this folder. FPRINT_CODE is a zipped folder containing the fingerprint code. A readme file for the code is also in this folder. WAmask_512.gz is a mask for West/East Antarctica, used for masking out changes in East Antarctica. All other files are sea-level outputs for each of the runs on a degree 512 Gauss-Legendre grid (uniform longitudes but unevenly spaced latitudes, as described in the readme for FPRINT_CODE). Files are named SLt_??? and numbered from 0 (elastic response) to 105 (10 ky). They have a 1D layout, with the first line being the time tag in years followed by 512*1024 row entries. A time array is included (tt_v10.dat). More details are in README.tx