Abstract Title: Burning bridges while hinterlands collapse around you

Abstract Author(s): Jones, Craig H. (Univ. Colorado, Boulder) - Farmer, G. Lang 
(Univ. Colorado, Boulder) - Bartley, John M. (Univ. Utah)  

Abstract: Differences in geologic histories of the northern and southern Basin 
and Range provinces (NBR, SBR) are puzzling. Extension has persisted from 40-50 
Ma to the present in the NBR but was more short-lived between about 15-25 Ma in 
the SBR. Both experienced broadly synextensional silicic volcanism that 
migrated northwestward across the SBR and southward across the NBR toward the 
central Basin and range (CBR), which formed an unextended bridge from the 
Colorado Plateau to the Sierra Nevada until the volcanic belts joined there. 
ÒBurningÓ the CBR bridge c. 14 Ma permitted westward movement Sierra/Great 
Valley block and formation of the Eastern California Shear Zone. Understanding 
differences between the CBR, NBR, and SBR might reveal why extension came last 
to the CBR; why extension continues in the NBR but not the CBR; and why 
Phanerozoic magmatism was rare in the CBR until 14 Ma, when southward magmatic 
migration in the NBR restarted after stalling near 38¡N since 32 Ma. The 
CBR/NBR boundary at ~37¡N parallels the 87Sr/86Sr=0.706 line, suggesting that 
Precambrian boundaries might dictate the differences. Alternatively, the 
boundary could represent the southern limit of lithospheric erosion by 
asthenospheric upwelling. Elevation and gravity differences across the boundary 
also could reflect crustal or mantle sources. Because the boundary near 37¡ N 
is defined by contrasts in many geologic, geophysical and geochemical processes 
characteristic of the Basin and Range, study of this area is likely to provide 
important insight into the causes and mechanisms of extensional deformation and 
volcanism.