LINKING DEEP AND SHALLOW CRUSTAL PROCESSES IN EXTENDING CONTINENTAL CRUST: EXAMPLES FROM THE CENOZOIC BASIN AND RANGE AND THE CRETACEOUS OF THE BERING STRAIT

The Basin and Range province with its distributed deformation, high heat flow and uniform elevation presents key questions about the rheology of the middle and lower crust and its capacity to flow in extensional settings. The concept of crustal flow leads to additional questions such as how elevated temperatures are reached and maintained, the role of magmatism, and whether magmatism must be driven by heat input from the mantle. Metamorphic core complexes in the Basin and Range provide tantalizing glimpses of mid-crustal processes during extension but those operating in the deeper crust can only be inferred from surface geology and geophysical data.

The Albion-Raft River-Grouse Creek metamorphic core complex, Idaho-Utah, exhibits Tertiary crustal depths of at least 15 km, where Ky+Stt+Gnt bearing assemblages progress into the sillimanite field near Miocene plutons. Lower plate rocks in the Ruby Mountains, NV, may have been at depths greater than 20-25 km in the Oligocene (Snoke et al., 1999). Peak metamorphic conditions indicate much deeper crustal levels in core complexes of the Canadian Cordillera and in the Cretaceous gneiss domes of the Bering Strait Region. These are likely hotter and/or deeper versions of Basin and Range core complexes. P-T conditions in the Kigluaik and Koolen domes, Alaska and Russia, for instance, range from 8-9 kb and 800°C to less than Al-silicate triple point conditions, the array of values a function of isothermal decompression during syn-magmatic diapirism. Xenolith suites from Neogene basalts in the region contain crustal rocks that experienced coeval (U-Pb SHRIMP zircon studies) granulite facies conditions that overlap and exceed metamorphic conditions represented in the gneiss domes. Seismic data provide independent information on crustal thickness, composition and reflectivity that are similar to those of the Basin and Range.

These data sets suggest a critical inter-relationship between magmatism, metamorphism and deformation at the scale of the crust. Mantle heat input is necessary to explain observed magmas and T's which in turn enable large-scale flow of crust during extension. Xenoliths indicate granulite facies conditions in the lower crust of extended regions and provide insight into the wholesale nature of the reconstitution of continental crust during magmatism and extension.