Wednesday, October 18th, 2017

Wednesday Seminar

4:10 PM, 55 Roessler
Tea and cookies at 3:45 in the aviary - (2110 EPS)

“The Significance of the Sierran Orogen”

      – by Dr. Craig Jones, Department of Geological Sciences, CU Boulder

Why study the geology of a place? As earth scientists, we focus on things that tell us how the earth works, but another way of looking at it is to understand the underpinnings of modern civilization. The Sierra Nevada, for example, hosted the Gold Rush, arguably a globally unique event that defined California, changed settlement practices throughout the West, and changed laws regarding the public lands. It also hosts Yosemite, whose soaring cliffs and waterfalls led to the first park set aside by the national government, which also changed land laws and practices. And from Yosemite ventured John Muir, whose writings changed America's outlook on wilderness and spawned environmental lobbying groups. Historians chronicling these events rarely look to see why they occurred here, yet as earth scientists we can see that all these arose from recent erosion in the range, exposing and reworking the gold deposits while carving the glacial gorges and aretes higher in the range. That erosion could only occur if the range was high enough, and so seeking the means by which the High Sierra is supported, we also plumb the depths of our own historical understanding. Investigations into the geology and geophysics of the range have turned up a number of surprises: the range lacks a thick crust, yet had one 10 million years ago. Unusually high seismic wavespeed material is in an inclined cylinder under the southwestern part of the range, a plausible if controversial repository for that old high density lower crust. Ongoing seismological work reveals unexpected complexity in this body (the Isabell anomaly), complexity most in keeping with the descent of petrologically heterogenous lower crust and upper mantle. If that lower crust is now foundering into the deeper mantle, study of the range is revealing a poorly understood process that could be globally significant.

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