Abstract Global geophysical observations constrain all theories of terrestrial dynamics. We jointly interpret EGM2008 gravity, RET2014 topography and the Global Centroid Moment Tensor database from a structural point of view. We hypothesize that lateral variations of gravity and topography reflect the scale-dependent competence of rocks. We compare the spectral and spatial characteristics of the observed fields with structural predictions from the mechanics of differential grade-2 (DG-2) materials. The results indicate that these viscoelastic materials are a powerful tool for exploring dynamic processes in the Earth. We demonstrate that the known spectral range of Earth's gravity and topography can be explained by the folding, shear banding, faulting and differentiation of the crust, lithosphere and mantle. We show that the low-amplitude long-wavelength bias apparent in the disturbance field can be explained by perturbations to Earth's overall ellipsoidal shape, induced by internal slab loading of the mantle. We find by examining the directional isotropy of the data that the zonal energy in Earth's gravity disturbance is maximized about an axis coincident with the shape-perturbation minimum. The symmetry of tectonic features about this axis, extending from eastern Borneo to Brazil, and its coincidence with the equator suggest the coupling of current plate motions to true polar wander.

The Lewiston Structure is located in southeastern Washington and west-central Idaho and is a generally east-west–trending (~075°), asymmetric, noncylindrical anticline in the Columbia River Basalt Group that transfers displacement into the Limekiln fault system to the southeast and the Silcott fault system to the southwest. A serial cross-section analysis and three-dimensional (3-D) construction of this structure show how the fold varies along its trend and shed light on the deformational history of the Lewiston Basin. Construction of the fold’s 3-D form shows that the fold’s wavelength increases and amplitude decreases near its eastern and western boundaries. Balanced cross sections show ~5% shortening across the structure, which is consistent with the Yakima Fold Belt. An angular unconformity below the Grande Ronde Basalt N1 magnetostratigraphic unit, in addition to a variation of N1 unit thickness across the structure, suggests that the fold was forming before N1 time. Analysis of structural data using the Gauss method for heterogeneous fault-slip data indicates north-south (~350°) shortening prior to and after N1 emplacement. The presence of a reverse fault on the southern limb of the Lewiston Structure is controversial. This fault crops out to the east of the field area where Grande Ronde Basalt magnetostratigraphic unit R2 is thrust over Pliocene(?) gravels. However, better control on unit thicknesses and map contacts rules out an exposed reverse fault on the southern limb of the fold west of the Washington-Idaho border, suggesting the fault either dies out or becomes blind.

Abstract Tectonic and orogenic processes, reflecting the dynamic nature of the planet, provide myriad examples of the failure of Earth materials under load. Despite this wealth of data, the shear localization process remains a difficult physical modelling problem, lying at the frontiers of complex and non-linear systems research. We present a non-conventional continuum-physics approach to address this problem, based on the mathematical properties of differential grade-2 (DG-2) materials. We choose this material because it is both frame-indifferent, and general enough to include other, simpler materials as special cases. DG-2 materials in pure shear exhibit a dynamic rescaling mechanism, associated with localized shearing, which links the spatial and temporal scales of this process in a self-consistent manner, independent of the observer. On typical thermal timescales, the thermomechanical competence of DG-2 materials depends on the ratio of thermal to mechanical diffusivities, κ/χ. On this basis, we hypothesize the effective rigidity of Earth materials, pertaining when the thermomechanical competence is greater than unity. This theory, applied to the whole Earth, suggests the existence of isopycnal ‘detachment’ zones at systematic, globally correlated depths beneath orogens, consistent with a variety of geological data.

Deviation between velocity trajectories from global positioning system (GPS) networks and strain trajectories from earthquake focal mechanisms and fault-slip inversion within the central Walker Lane are reconciled as the consequence of non–plane strain (constriction) within a transtensional zone separating the Sierra Nevada and central Great Basin. Dextral transtension within the central Walker Lane is produced by differential displacement of the Sierra Nevada with respect to the central Great Basin, and it is partitioned into domains exhibiting simple shear–dominated and pure shear–dominated strain. From east to west across the central Walker Lane, GPS velocities change orientation from west-northwest to northwest and increase from 2–3 to 12–14 mm/yr as the incremental-strain elongation axis changes from west-northwest to west-southwest. The deviation between strain and velocity trajectories increases to 50° as the Sierra Nevada Range is approached from the east. This deviation in strain and velocity trajectories is consistent with analytical models linking kinematic vorticity, particle velocity paths, and incremental non–plane strain during transtensional deformation. We link field observations to the analytical models using a polar Mohr construction in a system that conserves kinematic boundary conditions to graphically demonstrate that the relationship between velocity and strain fields is a consequence of constrictional deformation.

Ontologies constitute one of the most important and enabling components of the semantic Web, allowing geoscientists to explicitly and formally model their knowledge base for sharing and reuse over a global network. The geoscience community, realizing the need for such knowledge-enhancing technologies, has started developing a series of geoontologies for specific fields. We analyze the problems of designing geoontologies, emphasizing issues related to conceptual modeling and system architecture, and present the preliminary conceptual model of part of the structural geology ontology (StructuralGeoOntology). We discuss the ontology development process and identify a set of useful steps and activities that enhance and facilitate the development of any geoontology. Developing ontologies for any field in the geosciences becomes complex if all the concepts and their relationships in the field are included in a single, large ontology. To reduce complexity, we design the StructuralGeoOntology with a modular architecture involving multiple ontologies. This component-based ontology merges several homogeneous subontologies from allied subdisciplines in structural geology, and integrates ontologies from other fields. Each of the shared subontologies will have its corresponding relational database. The modular architecture leads to simplified and more efficient modeling, development, integration, maintenance, reasoning, and future extensibility for the ontologies and databases.