Sunday, January 30, 2011

Pangea supercontinent breakup rifting in Arizona

AZGS senior geologists Jon Spencer and Steve Richard (and 3 co-authors) have a new paper published online in the GSA Bulletin, that describes a Jurassic-aged rift basin that came close to breaking apart Arizona, during the breakup of the Pangea supercontinent [right, credit, C.R. Scotese]. Jon offered the history of their work on this project:

This study began in the early 1980s with Steve Richard’s Master’s thesis (1982), continued with mapping the Little Harquahala Mountains (OFR 85-09), Granite Wash Mountains (OFR 89-04 and AZGS Map 30), Plomosa Mountains (OFR 93-9), and New Water Mountains (94-14). That work did not answer basic questions about the origin of the McCoy Mountains Formation, including its age (Upper Jurassic or Upper Cretaceous?) so we went after the geochemistry of the interbedded basalts (OFR-99-1). When detrital-zircon dating became affordable, Steve and I did a sample collecting trip in 2004. We even got a date on a basalt from samples collected on that trip, which, along with the detrital zircon data, solved the big problems of age and origin of the McCoy Mountains Formation.

These new results led to the paper which looks at the breakup of the Pangea supercontinent, which consisted of all the earth's continent, by late in the Paleozoic Era (at about 250 Ma). "Africa separated from the eastern North America, and northern South America separated from southern North America. Arizona almost broke apart too in the late Jurassic (at about 155 Ma). At that time the McCoy Mountains Formation was first deposited in western Arizona (mostly La Paz County) and the Bisbee Group was deposited in southeastern Arizona. These two formations filled the rift valley that stopped rifting before southernmost Arizona was carried away by plate tectonic drift. The new study identified distinctive geochemical characteristics of rift-related basalts in the McCoy Mountains Formation, reported the first geochronologic date on the basalt (154 Ma), and determined the basic sources of the sands that were shed into the basin.

Ref: Spencer, J.E., Richard, S.M., Gehrels, G.E., Gleason, J.D., and Dickinson, W.R., 2011, Age and tectonic setting of the Mesozoic McCoy Mountains Formation in western Arizona, U.S.A.: Geological Society of American Bulletin, (electronic version published on January 26, 2011, doi:10.1130/B30206.1)

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