Sierras higher during Late Cretaceous?

["King, Norm" <nking.ucs@smtp.usi.edu>]

The Late Cretaceous was when the so-called "roots" of the Sierra Nevada 
were emplaced.  These roots are granitic batholiths associated with a 
marginal volcanic arc (like the present-day Cascades).  The volcanoes of 
that marginal arc are not preserved, so we don't know how high they 
were--only that there was a substantial volcanic chain there.

The volcanoes became extinct when the locus of subduction changed, and a 
strike-slip (tranform) fault formed along the West Coast.  This transform 
fault eventually evolved into our San Andreas fault, which has existed in 
recognizeable form for about 30 million years.  Because the older 
mountain roots were isostatically coupled to adjacent parts of the crust, 
the whole thing was held down--it didn't rebound as much as it might have 
if it wasn't coupled like that.  The area was one of rolling hills.  
Then, about 5-8 million years ago (I don't remember all the details), a 
major normal fault developed in what is now eastern California.  That 
fault destroyed the coupling, so the mountain roots were able to break 
loose and rapidly rise to the expected level.  Presto!--the Sierra Nevada 
Mountains.

This isostatic uplift has gone past its peak.  The Sierras were a little 
higher a million years ago, or so.  But now erosional denudation has 
become dominant.  There is still isostatic uplift, but as erosion 
proceeds, the amount of uplift is never quite enough to re-establish the 
earlier elevations.  However, some of the crest of the Sierras exceeds 
14,000' today (e.g., Mt. Whitney), and a significant stretch of the crest 
is over 12,000-13,000'. 

Bet that's more than you ever wanted to know about the Sierra Nevadas!




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Norman R. King                                       tel:  (812) 464-1794
Department of Geosciences                            fax:  (812) 464-1960
University of Southern Indiana
8600 University Blvd.
Evansville, IN 47712                      e-mail:  nking.ucs@smtp.usi.edu