A Tracer-Based Algorithm for Automatic Generation of Seafloor Age Grids from Plate Tectonic Reconstructions

The age of the ocean floor and its time-dependent age distribution control fundamental features of the Earth, such as bathymetry, sea level and mantle heat loss. Recently, the development of increasingly sophisticated reconstructions of past plate motions has provided models for plate kinematics and plate boundary evolution back in geological time. These models implicitly include the information necessary to determine the age of ocean floor that has since been lost to subduction. However, due to the lack of an automated and efficient method for generating global seafloor age grids, many tectonic models, most notably those extending back into the Paleozoic, are published without an accompanying set of age models for oceanic lithosphere. Here we present an automatic, tracer-based algorithm that generates seafloor age grids from global plate tectonic reconstructions with defined plate boundaries. Our method enables us to produce the first seafloor age models for the Paleozoic's lost ocean basins. Estimated changes in sea level based on bathymetry inferred from our new age grids show good agreement with sea level record estimations from proxies, providing a possible explanation for the peak in sea level during the assembly phase of Pangea. This demonstrates how our seafloor age models can be directly compared with observables from the geologic record that extend further back in time than the constraints from preserved seafloor. Thus, our new algorithm may also aid the further development of plate tectonic reconstructions by strengthening the links between geological observations and tectonic reconstructions of deeper time

MORGen: an Algorithm to Compute Spreading Centre and Transform Geometries from Simple Initial Plate Boundaries and Euler Rotations

The age structure of the global ocean floor is a key feature in paleogeographic reconstructions, which in turn forms the quantitative basis for Earth System Science. However, much of the ocean floor in paleogeographic reconstructions has been lost to subduction. The age structure of such lost ocean floor is constructed from the reconstructions of adjacent continents, using the relative rotations, around Euler poles to predict the geometry of spreading centres and transform faults. Building such mid-ocean ridge features in paleogeographic reconstructions is laborious, as it requires redrawing of ridge-transform systems upon every Euler pole shift in the model. In this paper, we present the Mid-Ocean Ridge Generator (MORGen) algorithm, based on pyGPlates. MORGen reduces the laborious task by automating the drawing of mid-ocean ridge geometries from geometrically simple plate boundary input assuming ridge-perpendicular spreading and adjusts ridge geometries in a simplest-scenario fashion by gradually adjusting ridge orientation and transform fault length upon Euler pole shifts, inspired by observations from the modern sea floor. The code takes as input curved line features, representing approximate divergent plate boundaries, and a set of Euler poles. These are then converted into spreading centre-transform geometries. Upon Euler pole shifts, the geometries are adjusted to fit the set of small circles and great circles dictated by the new Euler pole. For studies of paleo-environment and paleo-oceanography MORGen can be used in combination with other algorithms for full reconstructions of ocean floors, including their age, bathymetry, and roughness. For in-situ preserved ocean floor, the paleo-age distribution can be reconstructed directly in high resolution from geophysical and geological data from the modern ocean floor and MORGen would not normally be the option of choice. In cases where models contain ocean floor that has now been subducted, MORGen straightforwardly facilitates mid-ocean ridge geometry reconstruction. To illustrate how well the MORGen algorithm reproduces real ocean floor age structure, we show a synthetic ridge evolution for the South Atlantic and Southern Oceans and compare this to geophysically constrained ocean floor geometry. In addition, we show examples of use cases where direct (re)construction of mid ocean ridges is not possible: now-subducted ocean basins in the Mediterranean region and an ocean in a future supercontinent scenario

Phanerozoic evolution of mantle convection, long-wavelength dynamic topography and long-term sea level in a global tectonic framework

The changing pattern of convective circulation in the Earth’s mantle induces slowly developing, vertical displacements of the crustal surface resulting in dynamic topography; which influences erosion, sedimentation, eustatic sea-level change and continental flooding. Given the importance of dynamic topography, attempts have been made to constrain its spatiotemporal pattern, wavelength and amplitude. Thus, tracking the influence of mantle flow over geologic time is one of the important scientific endeavours. To achieve this aim, geodynamic experiments of mantle convection can be coupled with plate tectonic reconstructions to predict upwellings and downwellings that in turn elevate or depress the Earth’s surface. The overarching aim of this thesis is to understand and quantify the effect of mantle dynamics on the Earth’s surface throughout the Phanerozoic by combining global plate tectonic reconstructions, dynamic Earth models and geological observations

Long-term Phanerozoic global mean sea level: Insights from strontium isotope variations and estimates of continental glaciation

Global mean sea level is a key component within the fields of climate and oceanographic modelling in the Anthropocene. Hence, an improved understanding of eustatic sea level in deep time aids in our understanding of Earth's paleoclimate and may help predict future climatological and sea level changes. However, long-term eustatic sea level reconstructions are hampered because of ambiguity in stratigraphic interpretations of the rock record and limitations in plate tectonic modelling. Hence the amplitude and timescales of Phanerozoic eustasy remains poorly constrained. A novel, independent method from stratigraphic or plate modelling methods, based on estimating the effect of plate tectonics (i.e., mid-ocean ridge spreading) from the 87Sr/86Sr record led to a long-term eustatic sea level curve, but did not include glacio-eustatic drivers. Here, we incorporate changes in sea level resulting from variations in seawater volume from continental glaciations at time steps of 1 Myr. Based on a recent compilation of global average paleotemperature derived from δ18O data, paleo-Köppen zones and paleogeographic reconstructions, we estimate ice distribution on land and continental shelf margins. Ice thickness is calibrated with a recent paleoclimate model for the late Cenozoic icehouse, yielding an average ∼1.4 km thickness for land ice, ultimately providing global ice volume estimates. Eustatic sea level variations associated with long-term glaciations (>1 Myr) reach up to ∼90 m, similar to, and is at times dominant in amplitude over plate tectonic-derived eustasy. We superimpose the long-term sea level effects of land ice on the plate tectonically driven sea level record. This results in a Tectono-Glacio-Eustatic (TGE) curvefor which we describe the main long-term (>50 Myr) and residual trends in detail

7th Annual Jackson School of Geosciences Student Research Symposium, February 3, 2018

ConocoPhillipsGeological Science

8th Annual Jackson School of Geosciences Student Research Symposium, February 2, 2019

ConocoPhillipsGeological Science

The influence of subduction zone deformation and geometry on the genesis of megathrust earthquakes and tsunamis

The results presented in this cumulative ‘Habilitation’ document a nearly decade-long effort to image the interior regime of convergent margins. Subduction zones are formed by the underthrusting of a lithospheric plate (commonly of oceanic nature) underneath the overriding plate along the subduction thrust fault or ‘megathrust’. The subducting plate moves into the earth’s mantle at rates typically measured in centimeters per year. Stress accumulation in this high-friction setting may be released through seismic slip during an earthquake. The interplate environment hosts the seismogenic zone (typically at depths ranging from 5-40 km) where megathrust earthquakes are generated. These interplate earthquakes are the largest occurring on the planet. Due to the marine setting of subduction zones, tsunami waves are a common phenomenon in relation with megathrust events. As 60% of the global population lives within 50 km of the coast, damage and casualties related to subduction zone earthquakes or earthquaketriggered events rank highest among all geohazard-associated losses. The 2004 Sumatra earthquake and associated secondary processes served as a reminder that these hazards are major threats to society not only in regions from which they originate, but also on a global scale. Against this backdrop, the studies summarized here were initiated to improve our knowledge on the geological framework of the different types of subduction zones and on the control of the geometry of the subduction zone on seismic rupture and tsunami hazard. Data acquisition during seven research cruises in the Indian Ocean, Caribbean, and Pacific yielded a wealth of information that was analyzed in the course of these investigations. The results have been published in a series of inter-related scientific papers presented in the Appendix and referenced in bold font in the main text. The umbrella for these investigations is provided in the following chapters: Chapter 1 sets the stage by introducing the motivation and aims of the studies. Based on the observation that some segments of subduction zones produce large megathrust events of moment magnitudes > 8.5, whereas other portions of the same margin experience only moderate size earthquakes, the implication is drawn that individual subduction zones must show a high variability in their structure and geometry to induce such diverse seismicity. Elucidating the structural diversity between different margin segments using exemplary field data from Indonesia’s Sunda Margin is therefore one of the goals of this work. Chapter 2 summarizes the current knowledge on convergent margin kinematics. The notion that the subduction channel, which hosts the material sandwiched between upper and lower plate, influences megathrust seismogenesis is introduced here in relation to the role of subducted basement relief serving as earthquake nucleation or termination patches. These concepts are expanded and discussed in Chapter 3, which presents the original research documented in the contributions comprising the Appendix and puts them in context. Chapter 3.1 reviews the framework concept on margin structure and geometry, which was developed based on the investigations published in the ten papers discussed in Chapter 3.1.1.1-3.1.4.2. The traditional perception of ‘steady-state’ accretion is disproved by the recognition of multiple kinematic boundaries in a single subduction complex, resulting in across-strike forearc segmentation. Comparison with erosive systems shows that many elements present in accretionary settings are also recognized in erosive margins. Furthermore, variations in lower to upper plate mass transfer result in along-strike margin segmentation. This chapter also inspects new analysis schemes developed during the course of the studies. Chapter 3.2 then lays the bridge between kinematics and dynamics and links three papers, which examine oceanic plateau and ridge subduction at locations in the Atlantic, Pacific, and Indian Ocean. The last section of Chapter 3 extracts the essence of the previous investigations and reviews the notion that subducting oceanic relief may act as ‘asperities’ or ‘barriers’ to seismic rupture and relates this to the observed absence of magnitude > 8 subduction earthquakes along the Java Trench. A synopsis is provided in Chapter 4, which also inspects the results of a number of PhD-theses that emerged from the studies. The final Chapter 5 discusses unresolved issues and problems in subduction zone research and provides an outlook on future research strategies to tackle these open questions