D. Y. Wyrick

Large-scale troughs on Vesta: A signature of planetary tectonics

Abstract Images of Vesta taken by the Dawn spacecraft reveal large-scale linear structural features on the surface of the asteroid. We evaluate the morphology of the Vesta structures to determine what processes caused them to form and what implications this has for the history of Vesta as a planetary body. The dimensions and shape of these features suggest that they are graben similar to those observed on terrestrial planets, not fractures or grooves such as are found on smaller asteroids. As graben, their vertical displacement versus length relationship could be evaluated to describe and interpret the evolution of the component faults. Linear structures are commonly observed on smaller asteroids and their formation has been tied to impact events. While the orientation of the large-scale Vesta structures does imply that their formation is related to the impact events that formed the Rheasilvia and Veneneia basins, their size and morphology is greatly different from impact-formed fractures on the smaller bodies. This is consistent with new analyses that suggest that Vesta is fully differentiated, with a mantle and core. We suggest that impact into a differentiated asteroid such as Vesta could result in graben, while grooves and fractures would form on undifferentiated asteroids.

Ground‐penetrating radar sounding in mafic lava flows: Assessing attenuation and scattering losses in Mars‐analog volcanic terrains

We conducted low-frequency (16 to 100 MHz) ground-penetrating radar surveys on the eroded lava flows at Craters of the Moon (Idaho, USA) volcanic field to evaluate the potential of future radar-sounding investigations on Mars to map shallow subsurface features. Radar-sounding profiles were obtained from three locations: above a lava tube, across a volcanic rift, and over a scoria cone. Results were combined with laboratory permittivity and magnetic permeability measurements of field-collected samples to deconvolve the electromagnetic attenuation and scattering losses from the total losses and therefore separately quantify both effects on the radar penetration depth. Our results demonstrate a constrained performance for low-frequency sounding radars to characterize mafic, arid volcanic terrains that contain a significant amount of ferro-oxides (∼14%), mainly in the form of olivine and magnetite. Penetration depths of 35 m were achieved at a frequency of 100 MHz, and depths of 80 m were achieved at 16 MHz, with an effective dynamic range of 60 dB. Results indicate that for frequencies below 100 MHz, the electromagnetic attenuation dominated the signal losses while above this frequency threshold the volume scattering dominated the losses. Over our frequency range, the observed electromagnetic attenuation and penetration depths were strongly dependent on the magnetic losses, ground porosities, and degree of heterogeneity rather than the sounding frequency. In light of these results, we suggest average attenuation and scattering losses measured in terms of dB/m and discuss the expected penetration depth for the Mars orbital radar-sounding instruments SHARAD and MARSIS in mafic volcanic terrains.

Coseismic, dilational-fault and extension-fracture related pit chain formation in Iceland: Analog for pit chains on Mars

Pit crater chains are common topographic features on Mars and several other planetary bodies, and a wide range of mechanisms has been proposed for their origin. Two rifting-related seismic events in 1975–1976 and 1978 along the Mid-Atlantic Ridge near the northern coast of Iceland, associated with the Krafla volcanic eruptions to the south, produced an array of pit chains in unconsolidated sediments overlying Holocene basalt flows. Fault scarps and extension fractures in basaltic lava flows are traceable laterally into overlying unconsolidated fluvial deposits, revealing contrasting deformation styles in the two mechanical layers. Map-scale structures in basalt with little or no sedimentary cover include (1) fault scarps, (2) extension fractures and fracture swarms, (3) faulted monoclines, (4) widened fractures with caverns, and (5) localized circular or elongate collapse pits. Where unconsolidated fluvial sand and gravel deposits >3 m thick cover the basaltic lava flows, structural geomorphic features are dominated by (1) grabens bounded by normal faults with ∼1 m displacement, (2) cone- to bowl-shaped pit craters with depths up to 2.8 m, and (3) elongate troughs. Formation of these structures in fluvial sediment was triggered by reactivation of faults and extension fractures in the underlying basalt. Pit craters are readily explained by downward “draining” of poorly consolidated material into subterranean cavities produced by fault and extension fracture dilation in underlying cohesive material (basalt). High-resolution imagery on Mars shows geomorphic patterns that are directly analogous to these Icelandic pit chains, suggesting similar processes have occurred on Mars.

Analog modeling of normal faulting above Middle East domes during regional extension

We study the effects of planform dome shape on fault patterns developing with and without concurrent regional extension oriented oblique to the long axis of the dome. The motivation was the need to understand fault and fracture patterns in two adjacent mature hydrocarbon fields in the Middle East: one, an elliptical dome, and one, an irregularly shaped dome. The largest faults have throws of approximately 30 m (98 ft), which is close to the resolution limit of older two-dimensional seismic reflection data. The known fault trends are not parallel to the highest transmissivity direction but could form compartment boundaries. Fault and fracture patterns developed over the modeled domes provide insight into the populations of faults and fractures that are likely to exist in the reservoirs but have been undetected because they are at or below the resolution limit of reflection seismic data. Major domal structural elements, crestal fault systems, end splay systems, and radial faults are observed in modeled domes rising both with and without concurrent regional extension. Experimental results indicate that fault and fracture patterns are influenced by the effects of dome shape, regional extension, and relative timing of uplift with respect to regional extension. The expression of particular sets of faults and fractures associated with concurrent doming and regional extension depends on the interaction among regional extension, outer arc extension over the dome, and tangential extension around the dome margins. Our results also indicate that the two adjacent natural domes possibly experienced different kinematic histories from those previously interpreted.

Physical models of grooved terrain tectonics on Ganymede

Grooved terrain on Ganymede consists of distinct areas of parallel to subparallel ridges and troughs at a variety of spatial scales. Grooved terrain has been interpreted as the product of tectonism in the form of fault-accommodated distributed lithospheric extension. We use physical analog methods to test the formation of grooved terrain by imbricate normal faulting in response to distributed extension. Faults and fault systems produced in the models are geometrically and kinematically similar to patterns inferred for some grooved terrains on Ganymede. The high degree of similarity between model structures and those observed on Ganymede indicates that rotational half-graben brittle block faulting can explain at least some tectonic resurfacing on Ganymede and that 20% extension is sufficient to form structures analogous to grooved terrain.

Physical analogue modelling of Martian dyke-induced deformation

Abstract The Tharsis region of Mars is characterized by large volcanic and tectonic centres that have been active throughout Martian geological history, including distinct sets of graben that extend radially for distances of hundreds to thousands of kilometres. Formation of these graben has been attributed to crustal extension and/or dyke propagation. Physical analogue models using layered sand and liquid paraffin wax were constructed to test the magnitude and style of deformation in the host rock associated with dyke injection. A variety of igneous morphologies was produced, including dykes and plugs. Results suggest that, in the absence of pre-existing faults, vertical dykes do not produce significant deformation in the surrounding rock. Deformation associated with other magmatic intrusions produced primarily contractional features rather than extensional features, similar to previous numerical studies and terrestrial field investigations.

Tectonism and magmatism identified on asteroids

Abstract Linear features generally accepted as tectonic structures have been observed on several asteroids and their presence has implications for the internal structure, strength and evolution of these various bodies. Lineaments observed on the Martian moon Phobos led to the prediction that other cratered small bodies would be similarly lineated. Observations of several small bodies, including Gaspra, Ida, Mathilde, Eros, Itokawa, Steins, Lutetia and Vesta, have identified different physical mechanisms by which linear features can be formed. Analysis shows that asteroid lineaments appear to have different origins, including impact, and that lineament orientations and magnitudes provide important constraints on the interior structure of these bodies. Being a differentiated proto-planet, Vesta is a unique body with which to study the role that internal rheologies and structures play on surface tectonic expression. Vesta presents fractures and grooves similar to the other observed asteroids, but also large-scale graben and trough structures more characteristic of terrestrial planet tectonics. Unlike many terrestrial planets, Vestas main stressors have, however, been primarily exogenic (e.g. impacts) rather than internally driven. Nevertheless, while the search for volcanic features on Vesta has not been successful, the evaluation of the Brumalia Tholus feature suggests that the geological history of Vesta may have included magmatism.

Particle Shape Influences Settling and Sorting Behavior in Microfluidic Domains

We present a new numerical model to simulate settling trajectories of discretized individual or a mixture of particles of different geometrical shapes in a quiescent fluid and their flow trajectories in a flowing fluid. Simulations unveiled diverse particle settling trajectories as a function of their geometrical shape and density. The effects of the surface concavity of a boomerang particle and aspect ratio of a rectangular particle on the periodicity and amplitude of oscillations in their settling trajectories were numerically captured. Use of surrogate circular particles for settling or flowing of a mixture of non-circular particles were shown to miscalculate particle velocities by a factor of 0.9–2.2 and inaccurately determine the particles’ trajectories. In a microfluidic chamber with particles of different shapes and sizes, simulations showed that steady vortices do not necessarily always control particle entrapments, nor do larger particles get selectively and consistently entrapped in steady vortices. Strikingly, a change in the shape of large particles from circular to elliptical resulted in stronger entrapments of smaller circular particles, but enhanced outflows of larger particles, which could be an alternative microfluidics-based method for sorting and separation of particles of different sizes and shapes.