Tuesday, June 25, 2013

Geo 365: June 25, Day 176: These Are Some Features in My Neighborhood

Looking east out over the Willamette Valley from the top of Marys Peak, the fault marking the boundary between the valley floor (forearc basin) and the Coast Range (forearc ridge) is pretty obvious. The fault itself actually sits about 1 1/2 km (1 mile) in front (east) of the prominent ridge. This sort of landform, where the escarpment runs parallel to the fault, but not on the fault- typically because erosion has beveled the ridge back- is called a fault line scarp. This differentiates it from a fault scarp, which is taken to mean that the ridge accurately marks the location of movement.

The Corvallis Fault is stitched by gabbroic/diabasic intrusions of Oligocene age, contemporaneous with and similar to the Marys Peak Sill. These intrusions have not been broken tectonically, providing good evidence that our neighborhood fault has not been active for roughly 30 million years. Despite its proximity to town, it's not one we need to worry about with respect to seismic hazards.
1st photo unmodified; in 2nd, contrast ramped up and annotated in Paint.net. June 22, 2013. FlashEarth location.

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