Optimal, scalable forward models for computing gravity anomalies

We describe three approaches for computing a gravity signal from a density anomaly. The first approach consists of the classical ‘summation' technique, while the remaining two methods solve the Poisson problem for the gravitational potential using either a finite-element (FE) discretization employing a multilevel pre-conditioner, or a Green′s function evaluated with the fast multipole method (FMM). The methods using the Poisson formulation described here differ from previously published approaches used in gravity modelling in that they are optimal, implying that both the memory and computational time required scale linearly with respect to the number of unknowns in the potential field. Additionally, all of the implementations presented here are developed such that the computations can be performed in a massively parallel, distributed memory-computing environment. Through numerical experiments, we compare the methods on the basis of their discretization error, CPU time and parallel scalability. We demonstrate the parallel scalability of all these techniques by running forward models with up to 108 voxels on 1000s of core

Numerical modelling of magma dynamics coupled to tectonic deformation of lithosphere and crust

Many unresolved questions in geodynamics revolve around the physical behaviour of the two-phase system of a silicate melt percolating through and interacting with a tectonically deforming host rock. Well-accepted equations exist to describe the physics of such systems and several previous studies have successfully implemented various forms of these equations in numerical models. To date, most such models of magma dynamics have focused on mantle flow problems and therefore employed viscous creep rheologies suitable to describe the deformation properties of mantle rock under high temperatures and pressures. However, the use of such rheologies is not appropriate to model melt extraction above the lithosphere-asthenosphere boundary, where the mode of deformation of the host rock transitions from ductile viscous to brittle elasto-plastic. Here, we introduce a novel approach to numerically model magma dynamics, focusing on the conceptual study of melt extraction from an asthenospheric source of partial melt through the overlying lithosphere and crust. To this end, we introduce an adapted set of two-phase flow equations, coupled to a visco-elasto-plastic rheology for both shear and compaction deformation of the host rock in interaction with the melt phase. We describe in detail how to implement this physical model into a finite-element code, and then proceed to evaluate the functionality and potential of this methodology using a series of conceptual model setups, which demonstrate the modes of melt extraction occurring around the rheological transition from ductile to brittle host rocks. The models suggest that three principal regimes of melt extraction emerge: viscous diapirism, viscoplastic decompaction channels and elasto-plastic dyking. Thus, our model of magma dynamics interacting with active tectonics of the lithosphere and crust provides a novel framework to further investigate magmato-tectonic processes such as the formation and geometry of magma chambers and conduits, as well as the emplacement of plutonic rock complexe

Continuum approximation of dyking with a theory for poro-viscoelastic–viscoplastic deformation

To reach Earth’s surface, magma must ascend from the hot, ductile asthenosphere through cold and brittle rock in the lithosphere. It does so via fluid-filled fractures called dykes. While the continuum mechanics of ductile asthenosphere is well established, there has been little theoretical work on the cold and brittle regime where dyking and faulting occurs. Geodynamic models use plasticity to model fault-like behaviour; plasticity also shows promise for modelling dykes. Here we build on an existing model to develop a poro-viscoelastic–viscoplastic theory for two-phase flow across the lithosphere. Our theory addresses the deficiencies of previous work by incorporating (i) a hyperbolic yield surface, (ii) a plastic potential with control of dilatancy and (iii) a viscous regularization of plastic failure. We use analytical and numerical solutions to investigate the behaviour of this theory. Through idealized models and a comparison to linear elastic fracture mechanics, we demonstrate that this behaviour includes a continuum representation of dyking. Finally, we consider a model scenario reminiscent of continental rifting and demonstrate the consequences of dyke injection into the cold, upper lithosphere: a sharp reduction in the force required to rift

Evolution of Rift Architecture and Fault Linkage During Continental Rifting: Investigating the Effects of Tectonics and Surface Processes Using Lithosphere-Scale 3D Coupled Numerical Models

Continental rifts grow by propagation, overlap and linkage of individual fault segments. These processes are influenced by erosion and sedimentation and generate complex three-dimensional fault-interaction patterns. We use a 3D thermo-mechanical model of lithosphere deformation coupled with surface processes to investigate the coupling between erosion and tectonics, fault interaction and rift linkage, and evaluate the respective characteristics of crustal strength, inherited structures and erosional efficiency. We find that (a) weaker crust limits interactions between individual rift segments, (b) inherited structures are a major control for fault overlap and linkage except if they are too far apart and prevent interaction, and (c) efficient surface processes prolong fault activity, increase accommodated offset and in doing so, limit fault segment propagation and interactions. From these individual feedbacks, we identify five types of characteristic rift architectures: (a) for strong crust and intermediate erosional efficiency, fault segments link and form a horst between the propagating rifts. (b) Decreasing notch offset leads to segmentation of the central horst. (c) In case of reduced crustal strength no fault linkage occurs and a continuous central horst is promoted. (d) If inherited structures are too far apart, irrespective of crustal strength and erosional efficiency, rift basins do not link and a wide plateau-like horst forms between the propagating rifts. (e) In case of efficient erosion, fault linkage is achieved by the formation of strike-slip faults connecting the individual rift segments. Several of these simulated rift architectures can be identified in the western branch of the East African Rift.publishedVersio

Subduction initiation with vertical lithospheric heterogeneities and new fault formation

How subduction initiates with mechanically unfavorable lithospheric heterogeneities is important and rarely studied. We investigate this with a geodynamic model for the Puysegur Incipient Subduction Zone (PISZ) south of New Zealand. The model incorporates a true free surface, elasto-visco-plastic rheology and phase changes. Our predictions fit the morphology of the Puysegur Trench and Ridge and the deformation history on the overriding plate. We show how a new thrust fault forms and evolves into a smooth subduction interface, and how a preexisting weak zone can become a vertical fault inboard of the thrust fault during subduction initiation, consistent with two-fault system at PISZ. The model suggests that the PISZ may not yet be self-sustaining. We propose that the Snares Zone (or Snares Trough) is caused by plate coupling differences between shallower and deeper parts, the tectonic sliver between two faults experiences strong rotation, and low-density material accumulates beneath the Snares Zone

Optimal, scalable forward models for computing gravity anomalies

We describe three approaches for computing a gravity signal from a density anomaly. The first approach consists of the classical "summation" technique, whilst the remaining two methods solve the Poisson problem for the gravitational potential using either a Finite Element (FE) discretization employing a multilevel preconditioner, or a Green's function evaluated with the Fast Multipole Method (FMM). The methods utilizing the PDE formulation described here differ from previously published approaches used in gravity modeling in that they are optimal, implying that both the memory and computational time required scale linearly with respect to the number of unknowns in the potential field. Additionally, all of the implementations presented here are developed such that the computations can be performed in a massively parallel, distributed memory computing environment. Through numerical experiments, we compare the methods on the basis of their discretization error, CPU time and parallel scalability. We demonstrate the parallel scalability of all these techniques by running forward models with up to $10^8$ voxels on 1000's of cores.Comment: 38 pages, 13 figures; accepted by Geophysical Journal Internationa

Continuous subcutaneous insulin infusion therapy in type 1 diabetes: 2013 clinical guidelines and recommendations from the Association of Clinical Endocrinologists of South Africa (ACE-SA)

No Abstract

Subduction Initiation With Vertical Lithospheric Heterogeneities and New Fault Formation

How subduction initiates with mechanically unfavorable lithospheric heterogeneities is important and rarely studied. We investigate this with a geodynamic model for the Puysegur Incipient Subduction Zone (PISZ) south of New Zealand. The model incorporates a true free surface, elasto-visco-plastic rheology and phase changes. Our predictions fit the morphology of the Puysegur Trench and Ridge and the deformation history on the overriding plate. We show how a new thrust fault forms and evolves into a smooth subduction interface, and how a preexisting weak zone can become a vertical fault inboard of the thrust fault during subduction initiation, consistent with two-fault system at PISZ. The model suggests that the PISZ may not yet be self-sustaining. We propose that the Snares Zone (or Snares Trough) is caused by plate coupling differences between shallower and deeper parts, the tectonic sliver between two faults experiences strong rotation, and low-density material accumulates beneath the Snares Zone

Adaptation of EPEC-EM™ Curriculum in a Residency with Asynchronous Learning

Objective: The Education in Palliative and End-of-life Care for Emergency Medicine Project (EPEC™-EM) is a comprehensive curriculum in palliative and end-of-life care for emergency providers. We assessed the adaptation of this course to an EM residency program using synchronous and asynchronous learning.Methods: Curriculum adaptation followed Kern’s standardized six-step curriculum design process. Post-graduate year (PGY) 1-4 residents were taught all EPEC™-EM cognitive domains, divided as seven synchronous and seven asynchronous modules. All synchronous modules featured large group didactic lectures and review of EPEC™-EM course materials. Asynchronous modules use only EPEC™-EM electronic course media for resident self-study. Targeted evaluation for EPEC™-EM knowledge objectives was conducted by a prospective case-control crossover study, with synchronous learning serving as the quasi-control, using validated exam tools. We compared de-identified test scores for effectiveness of learning method, using aggregate group performance means for each learning strategy.Results: Of 45 eligible residents 55% participated in a pre-test for local needs analysis, and 78% completed a post-test to measure teaching method effect. Post-test scores improved across all EPEC™-EM domains, with a mean improvement for synchronous modules of +28% (SD=9) and a mean improvement for asynchronous modules of +30% (SD=18). The aggregate mean difference between learning methods was 1.9% (95% CI -15.3, +19.0). Mean test scores of the residents who completed the post-test were: synchronous modules 77% (SD=12); asynchronous modules 83% (SD=13); all modules 80% (SD=12).Conclusion: EPEC™-EM adapted materials can improve resident knowledge of palliative medicine domains, as assessed through validated testing of course objectives. Synchronous and asynchronous learning methods appear to result in similar knowledge transfer, feasibly allowing some course content to be effectively delivered outside of large group lectures. [West J Emerg Med. 2010; 11(5):491-498.