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Imaging the Colorado Plateau - Basin and Range Transition Zone Using Basalt Geochemistry, Geochronology and Geographic Information Systems
In southwestern Utah the boundary between the Colorado Plateau and the Basin and Range trends north-south and is characterized by a narrow transition zone. Faulting in the transition zone began in Tertiary time (15 to 20 Ma) and continued to the present. New 40Ar/39Ar ages demonstrate that the Grand Wash, Washington and Hurricane faults, three major structures of the transition zone, formed between >5 Ma and ~ 280 ka. Changing composition of Pliocene to Recent alkali basalts defines a boundary between asthenospheric and lithospheric mantle in the general vicinity of the transition zone; however, the relationship between this mantle boundary and transition zone remains an important question. In order to address this problem, we collected 27 samples from seven closely spaced volcanic centers (~ 10 km apart) along an 80 km long northeast transect across the boundary between the Basin and Range and Colorado Plateau from the Grand Wash trough, Nevada to Crater Hill, Utah. We combined these samples with 25 previously published analyses that cover a broad area to the north and south of the transect. All 52 samples were used to construct inverse distance weighted (IDW) surfaces using Arc-View GIS.
Arc-View plots of geochemical and isotopic data define a smooth and broad boundary between asthenospheric and lithospheric mantle that trends to the northwest and lies nearly perpendicular to the crustal boundary. South of the boundary, alkali basalts formed by the partial melting of asthenospheric mantle, whereas to the north melting occurred in the lithospheric mantle. We suggest that this mantle boundary formed during Tertiary extension but its position was controlled by an older (perhaps Precambrian) lithospheric boundary. Differences in lithospheric strength across the Precambrian boundary may have controlled the degree of thinning during Tertiary extension; more thinning occurred to the southwest of the boundary. Although variations in element concentration are broadly continuous from Basin and Range to Colorado Plateau, islands of high or low element concentration occur in the Basin and Range-Colorado Plateau transition zone. These chemical islands may reflect interlayering of Basin and Range and Colorado Plateau mantle types or differing depths of magma generation.
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