Tuesday, April 29, 2014

Re: [Geology2] The thin-crusted U.S. Sierra Nevada Mountains: Where did the Earth go?

This might be the connection with the mysterious partially subducted slab under the eastern US I couldn't think of a few days ago.

Eman

On Tuesday, April 29, 2014 9:45 AM, Lin Kerns <linkerns@gmail.com> wrote:

The thin-crusted U.S. Sierra Nevada Mountains: Where did the Earth go?

Date:

April 28, 2014

Source:

Geological Society of America

Summary:

Scientist have examined the seismological study of the entire extent of the Sierra Nevada range using seismograms collected in the Sierra Nevada EarthScope field experiment from 2005 to 2007. The southern Sierra Nevada is known to have unusually thin crust for mountains with such high elevations (peaks higher than 4 km/14,000 ft, and average elevations near 3 km/10,000 ft). Scientists have used measurements of the arrival times of seismic waves (called P-waves) from earthquakes around the globe to image the earth under the Sierra Nevada and neighboring locations.

Geologic map showing seismic stations used in the study by C.H. Jones and colleagues, "P-wave tomography of potential convective down-wellings and their source regions, Sierra Nevada, California."

Credit: Image courtesy of Geological Society of America

In an addition to Geosphere's ongoing themed issue series, "Geodynamics and Consequences of Lithospheric Removal in the Sierra Nevada, California," Craig H. Jones of the University of Colorado Boulder and colleagues examine the seismological study of the entire extent of the U.S. Sierra Nevada range using seismograms collected in the Sierra Nevada EarthScope field experiment from 2005 to 2007.

The southern Sierra Nevada is known to have unusually thin crust for mountains with such high elevations (peaks higher than 4 km/14,000 ft, and average elevations near 3 km/10,000 ft). Jones and his team use measurements of the arrival times of seismic waves (called P-waves) from earthquakes around the globe to image Earth under the Sierra Nevada and neighboring locations.

Their results reveal that the entire eastern Sierra overlies low-velocity upper mantle and lacks the dense, quartz-poor lower crust that they say must have existed 80 million years ago when the granites of the range were created.

Jones and colleagues write that this missing dense material probably was removed within the past 10 million years. "Previous workers," they note, "have suggested it might be within a high-velocity mantle anomaly under the southeastern San Joaquin Valley," which is "the right size to be the old, dense rock previously under the eastern Sierra."

They argue, however, that the geometry and extent of earth within the anomaly does not appear to be consistent with it being a piece of old subducted ocean floor. This would mean that a long strip of dense rock under the Sierra somehow deformed into a steeply plunging ellipsoid at the southwestern edge of the range. This conclusion suggests that the range rose within the past 10 million years as this dense material fell away to the west and south. Finally, Jones and colleagues note that something similar might be underway at the northern edge of the range.

Story Source:

The above story is based on materials provided by Geological Society of America. Note: Materials may be edited for content and length.

Journal Reference:

C. H. Jones, H. Reeg, G. Zandt, H. Gilbert, T. J. Owens, J. Stachnik. P-wave tomography of potential convective downwellings and their source regions, Sierra Nevada, California. Geosphere, 2014; DOI: 10.1130/GES00961.1

Geological Society of America. "The thin-crusted U.S. Sierra Nevada Mountains: Where did the Earth go?." ScienceDaily. ScienceDaily, 28 April 2014. <www.sciencedaily.com/releases/2014/04/140428155820.htm>.