Christopher D. Connors

Effect of the brittle‐ductile transition on the topography of compressive mountain belts on Earth and Venus

The Coulomb critical taper model has been very successful in explaining the large-scale topography of a number of terrestrial accretionary wedges; however, this model is limited to cases of purely brittle-frictional deformation. In this paper we extend the range of applicability of the critical taper model by explicitly including the effects of temperature-dependent ductile deformation. The new model includes temperature-dependent power law flow, an assumed velocity field, and linear thermal gradients in the atmosphere and within the crust. Flexural isostasy is also incorporated so that the decollement geometry is computed as a response to the applied load of the wedge material. We assume that ductile deformation within the decollement zone is controlled primarily by diffusion flow, whereas ductile deformation within the wedge itself is controlled by dislocation creep. The topographic profiles predicted by the model are very similar to those of a number of fold-and-thrust belts on both Earth and Venus. A typical wedge profile includes three distinctive topographic regions: a narrow taper toe, where both the wedge and the decollement zone deform in a brittle-frictional manner; a region of relatively steep slope, where the wedge base deforms ductilely and the decollement zone is still frictional; and a flat plateau region, where both the wedge base and the decollement zone are deforming by ductile flow. We have applied the model to two fold-and-thrust belts on Venus (Maxwell Montes and Uorsar Rupes) and to the Andes on Earth, and we find good agreement between observed and predicted topography using reasonable parameter values. The model accounts for the observed positive correlation between relief and elevation of Venusian fold-and-thrust belts on the basis of different thermal environments at different elevations. It is also able to explain the first-order differences between terrestrial and Venusian fold-and-thrust belts; fundamentally, this difference is due to a combination of the lower temperatures and the presence of water on Earth.

Critical taper wedge mechanics of fold-and thrust belts on Venus, Initial results from Magellan

Fold-and-thrust belts exist on Venus at the margins of crystal blocks such as plateaus, tesserae, and coronae and as ridge belts within the plains. These fold belts display a number of key features that are consistent with their formation by critical taper wedge mechanics, a mechanics that is well known for fold-and-thrust belts and accretionary wedges on Earth. For example, an analysis of fold geometry at the toe of the Artemis Chasma fold belt indicates fault-bend folding above a regionally extensive decollement horizon at a depth of about 1.5 km. Near-surface deformation on Venus is interpreted to be brittle and is anticipated to be dominated by cohesive strength in the upper 1–2 km. Critical taper wedge mechanics under anticipated Venus conditions suggests that brittle wedges should have maximum surface slopes in the range 10–20°, which is similar to some estimated slopes in the steep parts of the fold belts. The low taper toes of fold belts may be cohesion-dominated toes on either brittle or plastic decollement horizons. Once the base of the wedge undergoes the brittle-plastic transition, the surface slope is expected to flatten to near horizontal, in qualitative agreement with many topographic profiles of fold-and-thrust belts on Venus. The estimated depth of the brittle-plastic transition is uncertain based on rock mechanics data but is expected to be close enough to the surface to be affected by the atmospheric-temperature gradient. The relief of fold belts (measured between the toe of the wedge and the flat crest) displays a remarkable linear dependence on absolute elevation (Figure 15), ranging from 6 km for Maxwell Montes at an elevation of +10 km to a few hundred meters at the lowest planetary elevations (0 to −2 km). This remarkable phenomenon appears to reflect an absolute elevation dependence of the depth of the brittle-plastic transition, possibly controlled by an isostatic coupling of elevation, lithospheric thickness, and geothermal gradient.

Determining heights and slopes of fault scarps and other surfaces on Venus using Magellan stereo radar

This report presents relationships and techniques for determining the heights and slopes of discretely dipping surfaces, such as normal fault scarps, on the surface of Venus from measurements of their widths in Magellan stereo synthetic aperture radar (SAR) images. These surfaces are clearly recognizable as distinct bands of increased or decreased radar backscatter relative to flat-lying areas in the 120 to 280-m resolution Magellan SAR images, but are not generally imaged in the >10-km resolution Magellan altimetry. Our methods take into account radar distortion effects, and allow one to determine whether a slope is foreshortened, laid over, elongated, or in radar shadow. The techniques make use of graphs constructed for the Magellan incidence angle profiles so that investigators can determine the local height and slope of an individual surface in a straightforward manner, but the techniques are applicable to any stereo radar data set if the radar incidence angles are known. Additionally, the techniques can be used to improve digital elevation models constructed using radar stereoscopy in areas that have steep local terrain where stereoscopic fusion may be impossible due to the effects of layover or radar shadow. Using these techniques we show that there are normal fault scarps on Venus with heights in the 700-m range that have remarkable topographic slopes of close to 60°, something unheard of on Earth and which suggests a high effective cohesive strength of the Venusian crust.

Field-based Instruction as Part of a Balanced Geoscience Curriculum at Washington and Lee University

Traditionally at Washington and Lee University teaching in the field has been the core of our geology curriculum. We emphasize fieldwork at all levels of our instruction from the field-based introductory courses to our senior theses. We are fortunate to be located in a geologically diverse location (in the Valley and Ridge of Virginia and within minutes of the Blue Ridge Mountains). The close proximity of geologic variety allows us to spend nearly every class or laboratory period outside. We view fieldwork, however, as just the beginning of geoscience education. A crucial aspect of field geology is making observations and synthesizing the data collected. It is equally important for students to have well-developed skills in field methods, in analytical techniques, in computation and modeling, and in synthesis and presentation. To emphasize all of these aspects, our coursework is largely focused on emulating the process of research. Because we have had such a strong field emphasis, we are striving to strike a balance in our curriculum. We will present 3 examples of integrated exercises in our geology courses (including introductory geology, sedimentary geology, and geochemistry).

Pre-Mississippian tectonic affinity across the Canada Basin–Arctic margins of Alaska and Canada

New and reprocessed seismic reflection data on the Alaskan and Canadian Arctic margins of the Canada Basin, together with geologic constraints from exploration wells and outcrops, reveal structural and stratigraphic relationships in pre-Mississippian rocks that constrain models of Canada Basin opening. Lithostratigraphic age and acoustic character indicate that the Devonian and older passive-margin to foreland-basin succession in the Canadian M’Clure Strait is also found on the central Alaska margin. This succession also displays similar structural geometry and relief as well as deformational age on both margins. Moreover, Middle Devonian to Early Mississippian tectonic vergence—north directed on the central Alaska margin and east directed in the Canadian M’Clure Strait—indicates a common direction of tectonic transport if the two margins were conjugate. All of these observations demonstrate that pre-Mississippian rocks of the Alaskan and Canadian Arctic margins share a common tectonic history of uplift and exhumation and that the two margins were conjugates prior to opening of the Canada Basin.

Constraints on magnitudes of extension on Venus from slope measurements

Slopes related to normal fault scarps on Venus appear in Magellan synthetic aperture radar (SAR) images as tonal bands of i reased or decreased radar backscatter relative to adjacent regions. Calculations, through measurements on Magellan stereo SAR, of 170 of these surfaces in plains regions show that their mean topographic slope is 36.4° ± 1.2°, regardless of height. Because there is little to no erosion on Venus, we infer from this mean slope that most faults have collapsed to talus slopes, approximately at an angle of repose, and thus the effective cohesive strength of the crust near most fault surfaces is low, probably because of secondary faulting and jointing. In areas well imaged by Magellan stereo SAR we show that simple, balanced cross sections through individual grabens on Venus can be made, assuming a Coulomb shear angle for fault orientation, thus constraining slip on causitive faults and extension across the grabens. In rifted areas imaged by one Magellan SAR viewing geometry, constraints on extension across rift zones can be made by measuring the tonal bands representing slopes related to individual normal faults and using the mean topographic slope calculated for similar slopes imaged in stereo SAR. With this technique we show that crustal extension due to faulting and folding for the most recent deformation in the Beta rift zone is <20 km. These modest strains, across one of the best developed rift zones on Venus, put severe limits on tectonic models of Venus that invoke significant lateral displacements of the Venusian crust.

Introduction to special section: Balancing, restoration, and palinspastic reconstruction

Methods to quantify deformation and reverse the process of strain as a mode to illustrate geologic evolution through time have been previously used for a number of decades. Early efforts on the quantification of bed reconstruction were completed either by manually weighing the sections on delicate