Lithospheric extension appears to be responsible for erosional and tectonic evolution of the western margin of the Colorado Plateau. Late Cenozoic (20 Ma to 10,000 years) basaltic flows, scoria cones, necks, dikes, and sills exposed in the Western Grand Canyon region offer a unique opportunity to place age constraints on the landscape evolution. New K-Ar ages show that these basaltic rocks become progressively younger to the northeast from the Hualapai and Shivwits Plateaus onto the Uinkaret Plateau, a distance of about 100 km. Magmatism appears to have migrated at a rate of about 0.45 cm/yr until sometime between 13.5 and 9.2 Ma, when the rate changed to about 1.15 cm/yr. Recent regional geologic mapping indicates that extensional faulting, with ubiquitous downdrop to the west, accompanied this magmatism; the Hurricane Fault offsets lavas as young as 0.29 Ma. Young fault scarps in sedimentary rocks, and historic seismic activity along these faults, indicate that the region is still undergoing faulting. The basalts form three relatively distinct groups based on their age, chemistry, and geographic distribution: (1) 20--11 Ma, (2) 11--4.5 Ma, and (3) ≤4.5 Ma.

The older basalts (>11 Ma) are petrographically and compositionally distinct from younger basalts; their olivine and augite are more Mg rich with lower TiO2 and higher Ni, Cr, and incompatible element contents. Group 1 basalts have more radiogenic Sr and less radiogenic Nd isotopic compositions that younger group 2 and 3 basalts. Overall, present-day Sr isotopic compositions range from 0.70344 to 0.70505, 143Nd/144Nd from 0.51291 to 0.51227 (&egr;Nd=+5.4 to -7.1), and 206Pb/204Pb from 17.951 to 19.091. Group 1 basalts were probably derived from asthenospheric mantle melts that underwent crustal contamination. These basalts appear to have been generated originally from more primitive melts than groups 2 and 3, which were probably also derived from asthenospheric mantle-derived magmas but underwent only minor crustal contaminations. Group 2 basalts appear to be transitional between groups 1 and 3. The intervals 13--9 Ma and 4.6--4.3 Ma temporally separate the three chemically distinct groups of basalts in the Western Grand Canyon. These periods correspond closely with major Miocene and Pliocene plate reorganizations in the western United States at about 12.5--10 Ma and 3.9--3.4 Ma. These changes in plate motion were reflected in changes in the rate of magmatism migration in the Western Grand Canyon.

Recent movements on the Hurricane and Toroweap Faults appear directly related to the <4.5 Ma volcanism and are probably a result of recent plate reorganization. ¿ American Geophysical Union 1995