PLUME-RELATED MODIFICATION OF THE COLORADO PLATEAU AT 1.1. GA - A RECIPE FOR MAKING CRATONIC LITHOSPHERE

The Colorado Plateau area is a “young craton”, an area that became tectonically stable and immune to orogenic deformation at some unresolved time within the period integrated under the “Great Unconformity” of the Grand Canyon (late Proterozoic). The mechanisms responsible for making the Colorado Plateau area an island of strong lithosphere within the North American Cordillera continuously escaping a variety of deformation styles throughout the Phanerozoic are also unresolved. Regions surrounding the plateau today, such as the Basin and Range transition zone, contain numerous mafic sills and dikes emplaced into the regional basement at about 1.1 Ga. Equivalent age sills and lava flows are found within and at the top of the basement exposures of the Grand Canyon. Howard (1991) argued that this so-called “diabase province” is regionally extensive, and can account for many kilometers (thickness) of the regional Precambrian basement. Since only some 7% of the exposed rocks in central Arizona are pre-1.1. Ga rocks, the diabase sills are deceptively limited among modern exposures. A compilation of regional geologic data shows that they are more abundant towards the Colorado Plateau. Geochemical data show that these rocks are plume-related, similar to other continental flood basalts, and a limited but growing age-data set shows that they were emplaced over a short period of time (<20 My and probably much less than that, at 1070 Ma). Although similar in age to mafic rocks formed in the mid-continental rift, the rocks described here were not emplaced during continental extension, as clearly demonstrated by lack of extensional faults at that time. Numerous thermo- and geochronologic data on exposed basement and regional crustal xenoliths supports the existence of a significant anorogenic crustal thermal event at 1.1 Ga. I suggest that this flood basalt province was centered on the area represented by the modern Colorado Plateau and had a significant role in modifying the Plateau lithosphere by addition of mantle-derived melts into the crust and the development of a strong depleted mantle lithospheric root. I further speculate that the strong, craton-like response of the area throughout the Phanerozoic is a direct result of this Proterozoic magmatic event.