ALBERT

Description: Travouillon et al. (2012) challenge our interpretation of proxy records (Herold et al., 2011), citing five points for rainforest at Riversleigh and across northern and central Australia in the early to middle Miocene; points that we refute here. (1) Cenogram/body mass distribution patterns Travouillon et al. (2009) were equivocal in assigning some fauna sites to open forest or rainforest using cenograms alone, but using cenograms and body mass distribution (BMD) in combination, they interpreted the majority of the Riversleigh sites as rainforest. Yet, their Discriminant Function Analysis (DFA) of the same faunas identified a mix of open forest (2/6) and rainforest sites (4/6) for the early Miocene, and all five middle Miocene sites as open forest. Thus their own data imply a mosaic of habitats in space and time in the Riversleigh area of northern Australia.

Description: This study presents results from the Community Climate System Model 3 (CCSM3) forced with early to middle Miocene (~20–14 Ma) vegetation, topography, bathymetry, and modern CO2. A decrease in the meridional temperature gradient of 6.5°C and an increase in global mean temperature of 1.5°C are modeled in comparison with a control simulation forced with modern boundary conditions. Seasonal poleward displacements of the subtropical jet streams and storm tracks compared to the control simulation are associated with changes in Hadley circulation and significant cooling of the polar stratosphere, consistent with previously predicted effects of global warming. Energy budget calculations indicate that reduced albedo and topography were responsible for Miocene warmth in the high-latitude Northern Hemisphere while a combination of increased ocean heat transport and reduced albedo was responsible for relative warmth in the high-latitude Southern Hemisphere, compared to the present. Model–data analysis suggests Miocene climate was significantly warmer and wetter than simulated here, consistent with previous uncoupled Miocene models and supports recent reconstructions of Miocene CO2 substantially higher than present.

Description: A variety of paleogeographic atlases have been constructed, with applications from paleoclimate, ocean circulation and faunal radiation models to resource exploration; yet their uncertainties remain difficult to assess, as they are generally presented as low-resolution static maps. We present a methodology for ground-truthing paleogeographic maps, by linking the GPlates plate reconstruction tool to the global Paleobiology Database and a Phanerozoic plate motion model. We develop a spatio-temporal data mining workflow to compare a Phanerozoic Paleogeographic Atlas of Australia with biogeographic indicators. The agreement between fossil data and paleogeographic maps is quite good, but the methodology also highlights key inconsistencies. The Early Devonian paleogeography of southeastern Australia insufficiently describes the Emsian inundation that is supported by biogeography. Additionally, the Cretaceous inundation of eastern Australia retreats by 110 Ma according to the paleogeography, but the biogeography indicates that inundation prevailed until at least 100 Ma. Paleobiogeography can also be used to refine Gondwana breakup and the extent of pre-breakup Greater India can be inferred from the southward limit of inundation along western Australia. Although paleobiology data provide constraints only for paleoenvironments with high preservation potential of organisms, our approach enables the use of additional proxy data to generate improved paleogeographic reconstructions.

Description: We describe a novel method implemented in the GPlates plate tectonic reconstruction software to interactively reconstruct arbitrarily high-resolution raster data to past geological times using a rotation model. The approach is based on the projection of geo-referenced raster data into a cube map followed by a reverse projection onto rotated tectonic plates on the surface of the globe. This decouples the rendering of a geo-referenced raster from its reconstruction, providing a number of benefits including a simple implementation and the ability to combine rasters with different geo-referencing or inbuilt raster projections. The cube map projection is accelerated by graphics hardware in a wide variety of computer systems manufactured over the last decade. Furthermore, by integrating a multi-resolution tile partitioning into the cube map we can provide on-demand tile streaming, level-of-detail rendering and hierarchical visibility culling, enabling researchers to visually explore essentially unlimited resolution geophysical raster data attached to tectonic plates and reconstructed through geological time. This capability forms the basis for interactively building and improving plate reconstructions in an iterative fashion, particularly for tectonically complex regions.

Description: Large tectonic plates are known to be susceptible to internal deformation, leading to a~range of phenomena including intraplate volcanism. However, the space and time dependence of intraplate deformation and its relationship with changing plate boundary configurations, subducting slab geometries, and absolute plate motion is poorly understood. We utilise a buoyancy-driven Stokes flow solver, BEM-Earth, to investigate the contribution of subducting slabs through time on Pacific plate motion and plate-scale deformation, and how this is linked to intraplate volcanism. We produce a series of geodynamic models from 62 to 42 Ma in which the plates are driven by the attached subducting slabs and mantle drag/suction forces. We compare our modelled intraplate deformation history with those types of intraplate volcanism that lack a clear age progression. Our models suggest that changes in Cenozoic subduction zone topology caused intraplate deformation to trigger volcanism along several linear seafloor structures, mostly by reactivation of existing seamount chains, but occasionally creating new volcanic chains on crust weakened by fracture zones and extinct ridges. Around 55 Ma, subduction of the Pacific-Izanagi ridge reconfigured the major tectonic forces acting on the plate by replacing ridge push with slab pull along its northwestern perimeter, causing lithospheric extension along pre-existing weaknesses. Large-scale deformation observed in the models coincides with the seamount chains of Hawaii, Louisville, Tokelau and Gilbert during our modelled time period of 62 to 42 Ma. We suggest that extensional stresses between 72 and 52 Ma are the likely cause of large parts of the formation of the Gilbert chain and that localised extension between 62 and 42 Ma could cause late-stage volcanism along the Musicians volcanic ridges. Our models demonstrate that early Cenozoic changes in Pacific plate driving forces only cause relatively minor changes in Pacific absolute plate motion directions, and cannot be responsible for the Hawaiian–Emperor bend (HEB), confirming previous interpretations that the 47 Ma HEB does not primarily reflect an absolute plate motion event.

Description: We describe a novel method implemented in the GPlates plate tectonic reconstruction software to interactively reconstruct arbitrarily high-resolution raster data to past geological times using a rotation model. The approach is based on the projection of geo-referenced raster data into a cube map followed by a reverse projection onto rotated tectonic plates on the surface of the globe. This decouples the rendering of a geo-referenced raster from its reconstruction, providing a number of benefits including a simple implementation and the ability to combine rasters with different geo-referencing or inbuilt raster projections. The cube map projection is accelerated by graphics hardware in a wide variety of computer systems manufactured over the last decade. Furthermore, by integrating a multi-resolution tile partitioning into the cube map we can provide on-demand tile streaming, level-of-detail rendering and hierarchical visibility culling enabling researchers to visually explore essentially unlimited resolution geophysical raster data attached to tectonic plates and reconstructed through geological time. This capability forms the basis for interactively building and improving plate reconstructions in an iterative fashion, particularly for tectonically complex regions.

Description: Large tectonic plates are known to be susceptible to internal deformation, leading to a range of phenomena including intraplate volcanism. However, the space and time dependence of intraplate deformation and its relationship with changing plate boundary configurations, subducting slab geometries, and absolute plate motion is poorly understood. We utilise a buoyancy driven Stokes flow solver, BEM-Earth, to investigate the contribution of subducting slabs through time on Pacific Plate motion and plate-scale deformation, and how this is linked to intraplate volcanism. We produce a series of geodynamic models from 62 to 42 Ma in which the plates are driven by the attached subducting slabs and mantle drag/suction forces. We compare our modelled intraplate deformation history with those types of intraplate volcanism that lack a clear age progression. Our models suggest that changes in Cenozoic subduction zone topology caused intraplate deformation to trigger volcanism along several linear seafloor structures, mostly by reactivation of existing seamount chains, but occasionally creating new volcanic chains on crust weakened by fracture zones and extinct ridges. Around 55 Ma subduction of the Pacific-Izanagi ridge reconfigured the major tectonic forces acting on the plate by replacing ridge push with slab pull along its north-western perimeter, causing lithospheric extension along pre-existing weaknesses. Large scale deformation observed in the models coincides with the seamount chains of Hawaii, Louisville, Tokelau, and Gilbert during our modelled time period of 62 to 42 Ma. We suggest that extensional stresses between 72 and 52 Ma are the likely cause of large parts of the formation of the Gilbert chain and that localised extension between 62 and 42 Ma could cause late-stage volcanism along the Musicians Volcanic Ridges. Our models demonstrate that early Cenozoic changes in Pacific plate driving forces only cause relatively minor changes in Pacific absolute plate motions, and cannot be responsible for the Hawaii-Emperor Bend (HEB), confirming previous interpretations that the 47 Ma HEB does not reflect an absolute plate motion event.

Description: Tectonic reconstructions of Southeast Asia have given rise to numerous controversies which include the accretionary history of Sundaland and the enigmatic tectonic origin of the Proto South China Sea. We assimilate a diversity of geological and geophysical observations into a new regional plate model, coupled to a global model, to address these debates. Our approach takes into account terrane suturing and accretion histories, the location of subducted slabs imaged in mantle tomography in order to constrain the opening and closure history of paleo-ocean basins, as well as plausible absolute and relative plate velocities and tectonic driving mechanisms. We propose a scenario of rifting from northern Gondwana in the Late Jurassic, driven by northward slab pull, to detach East Java, Mangkalihat, southeast Borneo and West Sulawesi blocks that collided with a Tethyan intra-oceanic subduction zone in the mid Cretaceous and subsequently accreted to the Sunda margin (i.e. southwest Borneo core) in the Late Cretaceous. In accounting for the evolution of plate boundaries, we propose that the Philippine Sea Plate originated on the periphery of Tethyan crust forming this northward conveyor. We implement a revised model for the Tethyan intra-oceanic subduction zones to reconcile convergence rates, changes in volcanism and the obduction of ophiolites. In our model the northward margin of Greater India collides with the Kohistan-Ladakh intra-oceanic arc at ∼53 Ma, followed by continent-continent collision closing the Shyok and Indus-Tsangpo suture zones between ∼42 and 34 Ma. We also account for the back-arc opening of the Proto South China Sea from ∼65 Ma, consistent with extension along east Asia and the emplacement of supra-subduction zone ophiolites presently found on the island of Mindoro. The related rifting likely detached the Semitau continental fragment from east China, which accreted to northern Borneo in the mid Eocene, to account for the Sarawak Orogeny. Rifting then re-initiated along southeast China by 37 Ma to open the South China Sea, resulting in the complete consumption of Proto South China Sea by ∼17 Ma when the collision of the Dangerous Grounds and northern Palawan blocks with northern Borneo choked the subduction zone to result in the Sabah Orogeny and the obduction of ophiolites in Palawan and Mindoro. We conclude that the counterclockwise rotation of Borneo was accommodated by oroclinal bending consistent with paleomagnetic constraints, the curved lithospheric lineaments observed in gravity anomalies of the Java Sea and the curvature of the Cretaceous Natuna paleo-subduction zone. We complete our model by constructing a time-dependent network of continuously closing plate boundaries and gridded paleo-ages of oceanic basins, allowing us to test our plate model evolution against seismic tomography. In particular, slabs observed at depths shallower than ∼1000 km beneath northern Borneo and the South China Sea are likely to be remnants of the Proto South China Sea basin.