Eric Horsman

Characterizing and Improving Spatial Visualization Skills

Three-dimensional spatial visualization is an essential skill for geoscientists. We conducted two evaluations of students’ spatial skills to examine whether their skills improve after enrollment in a geology course or courses. First, we present results of pre- and post-course survey of abstract visualization skills used to characterize the range of spatial abilities in the student population at Carleton College. In Introductory Geology, there was a correlation between those who score very poorly on the spatial survey and those who receive a grade of C or lower. Students in higher-level courses had better developed visualization skills than those in Introductory Geology. Gender differences disappeared in upper-level courses except for the spatial relations (mental rotation) task, where male students consistently outperformed females. Second, we describe the efficacy of instructional materials designed for a Structural Geology course at the University of Wisconsin. This study included a qualitative controlled experiment investigating whether frequent use of stereographic projections affected student performance on exam questions requiring spatial skills. The results of both the survey-based quantitative study and materials-based qualitative study suggest that students’ spatial abilities can improve through practice provided in geology courses.

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 vertical axis rotations to characterize off-fault deformation across the San Andreas fault system, central California

We use vertical axis rotations from new paleomagnetic data to constrain off-fault deformation within the San Andreas fault system in central California. Samples were collected from 177 sites in the Miocene Monterey Formation adjacent to the Rinconada fault. Reliable means from 57 sites have 3 prominent patterns: (1) the largest clockwise rotations are close to the Rinconada and Nacimiento faults; (2) no significant rotation is observed near the San Andreas fault; and (3) counterclockwise rotations are observed at several sites northwest of Paso Robles. These paleomagnetic results are compared to two other measures of off-fault deformation where rotation can be calculated. Results from a fold-based kinematic model show increasing clockwise rotations toward the Rinconada fault, consistent with pattern 1. Few folds are observed in rocks on Salinian basement near the San Andreas fault, suggesting that little deformation has occurred, and providing an explanation for the negligible paleomagnetic rotations in pattern 2. Rotations calculated from the global positioning system velocity field predict counterclockwise rotations that coincide with those observed from paleomagnetic data in pattern 3. Broad patterns in the velocity field appear to be controlled by the transition from creeping to locked behavior along the San Andreas fault, and the region of counterclockwise rotation is linked to this transition. Thus, we suggest that the creeping segment has been aseismic over geologic time scales in order to produce the observed paleomagnetic rotations. The integration of all three data sets demonstrates that the San Andreas fault borderlands record an important portion of fault-parallel plate motion over geologic and geodetic time scales.

Magma sheets defined with magnetic susceptibility in the Maiden Creek sill, Henry Mountains, Utah, USA

In the ∼20-m-thick Maiden Creek sill of the Henry Mountains (Utah, USA) intrusive complex, two magma sheets are locally separated by a 1.5-m-thick lens of sandstone. We studied the boundary between these sheets at the termination of this sandstone lens, where the upper sheet directly overlies the lower sheet, in order to test the reliability of using magnetic susceptibility in delineating internal magmatic contacts. The contact between these two sheets is along a cliff face and defined by a thin (<1 cm) brittle-ductile shear zone. Measurements of magnetic susceptibility (K) were collected within a grid every 20 cm across this contact. Drill cores (72) were also collected along four traverses across the shear zone. Mapping K across the cliff face reveals an abrupt decrease immediately below the shear zone contact. 1 m below the contact, K unexpectedly increases again to the same levels observed above the contact. This lower boundary coincides with a 1–2-mm-thick minor fracture zone. The 1-m-thick low-K zone (LKZ) is characterized by more intense microfracturing and is bleached compared to the surrounding igneous rock. Plotting the magnetic foliation from the drill cores reveals abrupt changes to the orientation across both the shear zone and fracture zone. We hypothesize that the LKZ was the original magma sheet that intruded the sandstone. The high-K zones above and below the LKZ represent later sheets that intruded above and below the original sheet, fracturing the partially or wholly crystallized original intrusion. These later sheets exsolved fluids that were injected into the original sheet, resulting in more advanced oxidation of magnetite and thus lowering the K. Alternatively, it is possible that the LKZ is simply the altered zone at the top of a thicker older sheet that was modified by the intrusion of a younger overlying sheet.

Progressive Construction of Laccolithic Intrusive Centers: Henry Mountains , Utah, U.S.A

The intrusions of the Henry Mountains of southern Utah provide an exceptional setting for the study of igneous emplacement processes in the shallow crust. The five separate intrusive centers intruded the flat-lying stratigraphy of the Colorado Plateau at 2–4 km depth. The intrusions are Oligocene in age and postdate the minor Laramide orogenic activity that affected this part of the Colorado Plateau. These intrusions can therefore be interpreted as having formed through purely magmatic processes, with no tectonic involvement or modification. Each of the five separate intrusive centers in the Henry Mountains preserves a different stage in the evolution of an igneous system constructed in the shallow crust. Each intrusive center is comprised of numerous small intrusive bodies surrounding a large laccolithic body assembled from several magma pulses. Collectively, the five intrusive centers provide a series of snapshots of the progressive growth of an igneous system in the shallow crust. A compilation of data from these intrusive centers allows development of a generalized model for progressive construction of a magmatic system in the shallow crust. This model involves three main stages. First, an early network of dikes and sills is intruded. Second, a relatively voluminous laccolithic central igneous body begins to form. The central laccolith may initiate though inflation of a sill that grew to a radius sufficient to lift the overburden, as hypothesized in traditional growth models. However, field evidence suggests progressive laccolith growth in the Henry Mountains involved numerous rapidly emplaced magma pulses separated by periods of no appreciable activity. In the final stage, satellite intrusions, many with a tongue-like geometry, are emplaced radially outward from the margin of the main laccolith, initiating in the lower hinge region where bending and fracturing of overburden is most intense. The step-wise assembly of these intrusive centers from multiple discrete pulses of magma calls into question the applicability of theoretical models of laccolith growth that presuppose the entire intrusion remains in a liquid state throughout the full emplacement history.