Mapping extensional crack arrays in asphalt as normal fault system physical analogues

John Weber
Grand Valley State University
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Students map extensional crack arrays in asphalt, at full-scale, using long pieces of inexpensive craft paper and markers, map the connections between individual cracks in the arrays, and study (descriptively) crack arrays patterns e.g., en-echelon zones of right-stepping cracks, left-stepping cracks; relay ramp to crack connections; strike-slip fault to crack connections. Individual cracks in the arrays end, step, and connect in ways that mimic well-known and well-studied regional-scale normal fault systems.

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I use this exercise as an structural data collection and pattern-recognition in our required undergraduate (junior-level) structure course.

Skills and concepts that students must have mastered

Before the exercise students need to be introduced to basic fault terminology, the basic set of map-view fault patterns, the concept of relay ramps and strike-slip to normal fault connections, the difference between dilation (opening) faults and normal faults, the possibility of hybrid fault types (e.g., dilational-normal faults), and one or two examples of what regional-scale normal fault systems look like in map view.

How the activity is situated in the course

I use this exercise as an "kick start" activity early in the semester.


Content/concepts goals for this activity

The objectives of the activity are to: (1) study the descriptive, kinematic, and dynamic aspects of normal fault system analogues; (2) develop intuitive structural pattern recognition skills through a hands-on inquiry; (3) map and recognize different types of normal fault linkages: i.e., relay ramps and right- and left-stepping en-echelon connections; (4) learn about different fault types; (5) and to begin developing a working, practical, and readily accessible structural geology (fault) vocabulary.

Higher order thinking skills goals for this activity

This activity helps students develop pattern recognition skills, perhaps the most fundamental skill we apply over and over as geoscientists, and learn how to combine the descriptive, kinematic, and dynamic aspects the structural geology of a small "study area".

Other skills goals for this activity

Description of the activity/assignment

In this lab, students map extensional crack arrays, at full-scale, using long pieces of inexpensive craft paper and markers. I had previously tried this lab having students draw at a reduced-scale in their field notebooks; they seemed to focus on dealing with the scale factor, rather than recognizing patterns. Mapping at full scale obviates this problem. Students then highlight or color segments of their maps to emphasize the patterns they recognize; e.g., zones of right-stepping cracks; zones of left-stepping cracks; relay ramp crack connections, strike-slip fault connections, etc. Students work in groups of two. In addition to the maps, I ask them to draw one cross-section and to think about and talk about the underlying kinematics and dynamics. We then lay out all of the maps on the ground, then walk through and discuss them group-by-group. Each group talks the class through the descriptive, kinematic, and dynamic (sometimes speculative) aspects of their "study area".

Determining whether students have met the goals

I generally do this as a credit/no credit activity; students have consistently generated high-quality maps and enthusiastic and thoughtful discussions about what they saw and what it means. (I put them into "pre-assigned" field areas where the patterns are clear and interesting.)

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