Wednesday, January 16, 2008

Rock of the (mid) week: Wasatch Fault rock

When I first started planning work for Advanced Structural Geology, I considered focusing the entire course on faults and shear zones. In part, I was trying to make up for things that I worried were deficiencies of my own - I still remember staring blankly at my field camp TA as he sat on the Darby Thrust, asking leading questions until I finally realized that the stratigraphic section was not in the correct order.

Now, I've got some spectacular exposures of minor faults in my backyard. But they aren't quite the same thing. So I decided I wanted to take students to see some of the rocks found in major fault zones.

My brittle example was the Wasatch Fault, the big normal fault east of Salt Lake City. It's active, and there are good geomorphic signs that it's capable of earthquakes, including a nicely offset glacial moraine at the mouth of Little Cottonwood Canyon.

This outcrop, of an actual slip surface, is on the edge of Provo. The rock is limestone, but it's been broken into tiny pieces, and then cemented back together by more calcite. The striations on the surface may be slickenlines, but they may also be scratches from the equipment that stripped this surface bare (in an aborted plan to build a ski lift).

Is this rock the result of earthquakes on the fault? In the past, the only dead-certain evidence for earthquakes on exhumed fault rock was pseudotachylite, rock that melted and then froze during an earthquake. But at last fall's GSA meeting, during a session on Co-Seismic Fault Zone Structures, a number of groups suggested that maybe cataclasites form during earthquakes, as well.

No comments: