Description: An International Ocean Discovery Program (IODP) workshop was held at Sydney University, Australia, from 13 to 16 June 2017 and was attended by 97 scientists from 12 countries. The aim of the workshop was to investigate future drilling opportunities in the eastern Indian Ocean, southwestern Pacific Ocean, and the Indian and Pacific sectors of the Southern Ocean. The overlying regional sedimentary strata are underexplored relative to their Northern Hemisphere counterparts, and thus the role of the Southern Hemisphere in past global environmental change is poorly constrained. A total of 23 proposal ideas were discussed, with 12 of these deemed mature enough for active proposal development or awaiting scheduled site survey cruises. Of the remaining 11 proposals, key regions were identified where fundamental hypotheses are testable by drilling, but either site surveys are required or hypotheses need further development. Refinements are anticipated based upon regional IODP drilling in 2017/2018, analysis of recently collected site survey data, and the development of site survey proposals. We hope and expect that this workshop will lead to a new phase of scientific ocean drilling in the Australasian region in the early 2020s.

Description: Even though it is well accepted that the Earth's surface topography has been affected by mantle-convection induced dynamic topography, its magnitude and time-dependence remain controversial. The dynamic influence to topographic change along continental margins is particularly difficult to unravel, because their stratigraphic record is dominated by tectonic subsidence caused by rifting. We follow a three-fold approach to estimate dynamic topographic change along passive margins based on a set of seven global mantle convection models. We first demonstrate that a geodynamic forward model that includes adiabatic and viscous heating in addition to internal heating from radiogenic sources, and a mantle viscosity profile with a gradual increase in viscosity below the mantle transition zone, provides a greatly improved match to the spectral range of residual topography end-members as compared with previous models at very long wavelengths (spherical degrees 2–3). We then combine global sea level estimates with predicted surface dynamic topography to evaluate the match between predicted continental flooding patterns and published paleo-coastlines by comparing predicted versus geologically reconstructed land fractions and spatial overlaps of flooded regions for individual continents since 140 Ma. Modelled versus geologically reconstructed land fractions match within 10% for most models, and the spatial overlaps of inundated regions are mostly between 85% and 100% for the Cenozoic, dropping to about 75–100% in the Cretaceous. Regions that have been strongly affected by mantle plumes are generally not captured well in our models, as plumes are suppressed in most of them, and our models with dynamically evolving plumes do not replicate the location and timing of observed plume products. We categorise the evolution of modelled dynamic topography in both continental interiors and along passive margins using cluster analysis to investigate how clusters of similar dynamic topography time series are distributed spatially. A subdivision of four clusters is found to best reveal end-members of dynamic topography evolution along passive margins and their hinterlands, differentiating topographic stability, long-term pronounced subsidence, initial stability over a dynamic high followed by moderate subsidence and regions that are relatively proximal to subduction zones with varied dynamic topography histories. Along passive continental margins the most commonly observed process is a gradual motion from dynamic highs towards lows during the fragmentation of Pangea, reflecting the location of many passive margins now over slabs sinking in the lower mantle. Our best-fit model results in up to 500 (± 150) m of total dynamic subsidence of continental interiors while along passive margins the maximum predicted dynamic topographic change over 140 million years is about 350 (± 150) m of subsidence. Models with plumes exhibit clusters of transient passive margin uplift of about 200 ± 200 m, but are mainly characterised by long-term subsidence of up to 400 m. The good overall match between predicted dynamic topography to geologically mapped paleo-coastlines makes a convincing case that mantle-driven topographic change is a critical component of relative sea level change, and indeed the main driving force for generating the observed geometries and timings of large-scale continental inundation through time.

Description: We present a regional model of plate geometry and kinematics of Southeast Asia since the Late Cretaceous, embedded in a global plate model. The model involves subduction polarity reversals and sheds new light on the origin of the subduction polarity reversal currently observed in Taiwan. We show that this subduction zone reversal is inherited from subduction of the proto–South China Sea plate and owes its current location to triple junction migration and slab rollback. This analysis sheds new light on the plate tectonic context of the Taiwan orogeny and questions the hypothesis that northern Taiwan can be considered an older, more mature equivalent of southern Taiwan.

Description: Lateral viscosity variations (LVVs) in the mantle influence geodynamic processes and their surface expressions. With the observed long-wavelength geoid, free-air anomaly, gravity gradient in three directions and discrete, high-accuracy residual topography, we invert for depth- and temperature-dependent and tectonically regionalized mantle viscosity with a mantle flow model. The inversions suggest that long-wavelength gravitational and topographic signals are mainly controlled by the radial viscosity profile; the pre-Cambrian lithosphere viscosity is slightly (~ one order of magnitude) higher than that of oceanic and Phanerozoic lithosphere; plate margins are substantially weaker than plate interiors; and viscosity has only a weak apparent, dependence on temperature, suggesting either a balancing between factors or a smoothing of actual higher amplitude, but short wavelength, LVVs. The predicted large-scale lithospheric stress regime (compression or extension) is consistent with the world stress map (thrust or normal faulting). Both recent compiled high-accuracy residual topography and the predicted dynamic topography yield ~1 km amplitude long-wavelength dynamic topography, inconsistent with recent studies suggesting amplitudes of ~100 to ~500 m. Such studies use a constant, positive admittance (transfer function between topography and gravity), in contrast to the evidence which shows that the earth has a spatially and wavelength-dependent admittance, with large, negative admittances between ~4000 and ~10 4 km wavelengths.

Description: Subduction initiation may unfold via different pathways in response to plate strength, plate age, and driving mechanism. Such pathways influence volcanism on the overriding plate and may be preserved in the sequence of erupted volcanic products. Here, we parameterize melting in a mechanical model to determine the volcanic products that form in response to different subduction initiation modes. We find that with a mode of continuous initiation with infant-arc spreading, the foundering of the subducting slab and water release from the slab govern a succession from basalts with compositions similar to mid-ocean-ridge basalts (MORB) to boninites. The modeled transition from MORB-like to boninite composition typically occurs within a few million years. When plate strength is reduced, the subducting slab tends to segment, with extensive melting occurring when the slab breaks; most melting occurs close to the trench. When plate strength increases, subduction initiation becomes continuous without infant-arc spreading; such a mode leads to a limited, very low degree of melting occurring during a long interval of plate convergence before subduction initiation starts, although extensive melting near the trench is still possible when subduction initiation starts after a protracted period of plate convergence (~10 m.y.). If the subduction initiation is driven by constant stresses, such as through ridge push, the slab subducts rapidly in response to continuous acceleration of the plate under action of the far-field push; significant melting, including boninite eruption, can be generated within a few million years with no trench migration. Based on the tectonic and volcanic evolution, these different modes may be applicable to the initiation of the Izu-Bonin-Mariana arc (infant-arc spreading and a sequence from MORB-like to boninites), the New Hebrides arc (slab segments in the upper mantle), the Puysegur Trench in New Zealand (scarce distribution of volcanism and no infant-arc spreading), and the Aleutian Trench (strong volcanism and no infant-arc spreading).

Description: The putative Pliocene–Quaternary removal of mantle lithosphere from beneath the southern Sierra Nevada region (California, USA) is investigated by the iteration of thermal-mechanical models that incorporate and are tested against a range of data that are geologically observable, including rock uplift and basin subsidence data, structural and compositional data on crustal architecture, and a synthesis of seismic data that image lower crust–upper mantle structure of the region. The primary focus is testing model results with rock uplift and basin subsidence data. The initial state of our models recognizes that (1) the sub–Sierra Nevada batholith mantle lithosphere, including a substantial thickness of eclogitic cumulates that were produced during high magma flux arc activity, termed arclogite, was cooled to a conductive geotherm by amagmatic flat slab subduction at the end of the Cretaceous; and (2) the gravitationally metastable mantle lithosphere was thermally mobilized from beneath in the Neogene by the opening of an underlying slab window. Based on a detailed synthesis of appropriate rheologies of the multiphase system, a preferred class of models correctly predicts (1) the ca. 10 Ma inception of the Sierra Nevada microplate due to a lithospheric separation event along the eastern Sierra Nevada region as a result of the mobilization of the mantle lithosphere as a Rayleigh-Taylor instability; and (2) the subsequent delamination of the arclogite root of the Sierra Nevada batholith that appears to be in progress. Our preferred model also predicts focused rock uplift and basin subsidence resulting from delamination, both of which are anomalous to uplift and subsidence patterns of all other regions of the microplate. The rheology of the Great Valley crust is found to control rock uplift patterns across the Sierra Nevada, and tectonic subsidence in the Tulare Basin of the Great Valley. The Tulare Basin is uniquely situated over the region where the principal residual arclogite root remains attached to batholithic crust. The anomalous rock uplift and tectonic subsidence data are best satisfied by modeling a bulk rheology for the Great Valley crust that is similar to that of the Sierra Nevada batholith. These results are consistent with a recent synthesis of basement core and geophysical data showing that much of the Great Valley basement consists of the western Early Cretaceous zones of the Sierra Nevada batholith. The existence of this batholithic domain within the Great Valley subsurface is also in agreement with recent seismic data that resolve additional residual arclogite root materials along the base of the crust of this region.

Description: Recent seismic imaging of the mantle beneath western North America reveals complexities interpreted as structures ranging from plumes to lithospheric drips and slab fragments. A prominent high-velocity "curtain" beneath Idaho has been interpreted as a remnant of the subducted Farallon plate left dangling within the upper mantle since 〉40 Ma. Consequently, using numerical models, we explore the rheological, chemical, geometrical, and dynamic conditions under which a slab fragment might persist in the mantle for tens of millions of years. With thermal buoyancy alone, stalled slabs extending to 500 km depth tend to detach and sink vertically within ~17 m.y. for the slab age and rheologic conditions explored here, and shorter slabs 〈300 km deep have the greatest impact on delaying detachment up to 28 m.y. Otherwise, we find that an unrealistic chemical density contrast of 90 kg/m 3 with respect to the mantle is required for the stalled slab to remain attached to the lithosphere 〉40 m.y. An increase in upper- to lower-mantle viscosity contrast (1.4 x to 100 x ) can slow sinking velocities and extend slab dangling time by up to 5 m.y. Dynamic effects such as those arising from active nearby subduction only slightly delay or do not affect stalled slab detachment timing but do affect the geometry of the slabs as they respond to suction pressures within the wedge. Overall, a combination of buoyant, viscous, geometric, and dynamic factors may allow cases of extended slab stalling, and conditions we explore here within realistic ranges can so far account for a delay of up to 28 m.y.

Description: A new generation, parallel adaptive-mesh mantle convection code, Rhea, is described and benchmarked. Rhea targets large-scale mantle convection simulations on parallel computers, and thus has been developed with a strong focus on computational efficiency and parallel scalability of both mesh handling and numerical solvers. Rhea builds mantle convection solvers on a collection of parallel octree-based adaptive finite element libraries that support new distributed data structures and parallel algorithms for dynamic coarsening, refinement, rebalancing and repartitioning of the mesh. In this study we demonstrate scalability to 122 880 compute cores and verify correctness of the implementation. We present the numerical approximation and convergence properties using 3-D benchmark problems and other tests for variable-viscosity Stokes flow and thermal convection.