Dyanna M. Czeck

Testing models for obliquely plunging lineations in transpression: a natural example and theoretical discussion

Theory predicts that stretching lineations in an ideal vertical transpressional zone should be either vertical or horizontal. Many field descriptions of transpressional zones, however, indicate a range of lineation orientations between these extremes. Several theoretical models have been developed to explain such departures from expected lineation orientation, and we discuss these in the context of a field example from the Archean Superior Province in the North American craton. Existing models are insufficient to explain obliquely plunging lineations in this example because: (1) obliquely plunging lineations cannot be accounted for by shear zone boundary effects imposed by a no-slip condition, (2) foliations and lineations vary independently, (3) the vorticity-normal section is subhorizontal, limiting possibilities for inclined simple shear, (4) high vorticity is needed for finite strains and lineations to match previously proposed triclinic models, but vorticity is relatively low, and (5) juxtaposed east and west plunging lineations are unlikely in the previously proposed triclinic models. Because existing theoretical models are not applicable to our field example, we contemplate a new model to explain obliquely plunging lineations within quasi homogeneous transpression.

Rheological implications of heterogeneous deformation at multiple scales in the Late Cretaceous Sierra Nevada, California

Late Cretaceous deformation in the east-central Sierra Nevada arc of California was heterogeneous at multiple scales. We quantify this heterogeneous deformation at centimeter, meter, and kilometer scales in the vicinity of the Gem Lake shear zone and infer variations in effective viscosity from our data. At the centimeter scale, variations in strain of different clast types in conglomerate suggest that effective viscosities varied by less than an order of magnitude. Lithology controlled the magnitude and nature of deformation recorded by clasts. At the meter scale, cleavage refraction between stratigraphic layers records variations in finite strain. Comparison of our observations with cleavage refraction models suggests a maximum effective viscosity contrast of ∼10 between layers. Bulk composition controls variations in deformation of the different layers. At the kilometer scale, variation in finite-strain magnitude and orientation in similar rock types both within and outside the shear zone demonstrates that deformation inside the zone was relatively intense. Comparing these results to numerical models of heterogeneous regional deformation, we estimate that the angle of oblique convergence inside the zone was ∼15 ± 10°, while outside it was greater than 60°. These kilometer-scale results imply regional deformation was moderately strike-slip partitioned during the Late Cretaceous and suggest regional effective viscosity varied by a factor between 6 and 17. At each scale of observation, the apparent range of effective viscosity varies by an order of magnitude or less. Consequently, we infer that relatively modest strength variations produced the structures observed at hand sample to tectonic scales.

Using network analyses within geographic information system technologies to quantify geometries of shear zone networks

Shear zones often exhibit anastomosing network geometries. Previous work has shown that the detailed geometries of shear zone networks may partially control strain localization, fluid flow, rheology, and deformation mechanisms. However, there are currently no reliable tools to quantify network geometries such as the distribution of individual small shear zones and the connectedness of the network. Geographic information systems (GIS) provide a potential method for quantifying network geometries. GIS-based networking analyses have been used to quantify many different types of other networks, and here they are applied to shear zones. Many parameters within GIS-based networking analyses are useful for quantifying shear zone network geometries, including the connectivity parameters gamma and alpha, sinuosity, and vertex distribution patterns. Sets of these parameters are useful to quantitatively distinguish geometrical patterns of shear zone networks over a variety of conditions. Further quantification of shear zone network geometries may allow us to link those geometries to shear zone mechanisms, strain accumulation, and rheology.

Using image analysis and ArcGIS ® to improve automatic grain boundary detection and quantify geological images

Geological images, such as photos and photomicrographs of rocks, are commonly used as supportive evidence to indicate geological processes. A limiting factor to quantifying images is the digitization process; therefore, image analysis has remained largely qualitative. ArcGIS^(R), the most widely used Geographic Information System (GIS) available, is capable of an array of functions including building models capable of digitizing images. We expanded upon a previously designed model built using Arc ModelBuilder^(R) to quantify photomicrographs and scanned images of thin sections. In order to enhance grain boundary detection, but limit computer processing and hard drive space, we utilized a preprocessing image analysis technique such that only a single image is used in the digitizing model. Preprocessing allows the model to accurately digitize grain boundaries with fewer images and requires less user intervention by using batch processing in image analysis software and ArcCatalog^^^(R). We present case studies for five basic textural analyses using a semi-automated digitized image and quantified in ArcMap^(R). Grain Size Distributions, Shape Preferred Orientations, Weak phase connections (networking), and Nearest Neighbor statistics are presented in a simplified fashion for further analyses directly obtainable from the automated digitizing method. Finally, we discuss the ramifications for incorporating this method into geological image analyses.

The tectonic significance of dikes of irregular fold-like shape

The effects of postemplacement deformation on originally nonplanar dikes were tested experimentally and compared to field data from the Rainy Lake zone (Ontario, Canada) where irregular fold-like structures are observed. The analyses reveal that two processes are responsible for the irregular dike geometries. First, dikes intrude with varying orientations as they propagate across layers with rheology contrasts, thus developing nonplanar profiles. Second, with postemplacement ductile deformation, dikes that are more competent than the host rocks are folded and refracted in variable amounts, with the amount depending on the initial dike geometry, the heterogeneities in the host, and the regional strain. Deformed dikes and veins are often used as kinematic markers to evaluate regional tectonics, using methods that generally assume that veins had planar walls prior to deformation. The experimental results indicate that caution should be taken when using this approach, and they provide criteria to distinguish between intrusive and deformation structures in intensely deformed areas where distinction between the two is not readily apparent.

Using Candies to Demonstrate Concepts of Weathering and Sedimentary Processes in Lecture-Based Introductory Earth Science Courses

Students enrolling in undergraduate level introductory earth science courses often have little or no science background. For lecture format courses, demonstrations or hands-on activities used to illustrate geologic concepts may be valuable teaching tools to facilitate student learning. Demonstrations using materials with which students have familiarity can be especially effective. We used peanut M&Ms® in a series of classroom demonstrations to illustrate concepts of physical and chemical weathering, sediment transportation, and deposition. Student response to this and other demonstrations has been favorable. The demonstrations have fostered student interest in lectures. Results from student learning surveys indicate that most students found such classroom demonstrations aided their understanding of the course material. In particular, when asked about specific aspects of the course and the relationship to learning, most students considered physical activities to be more effective in their learning than other course-related activities such as reading the textbook or working on non-physical group activities.