VARIATIONS IN LATE CRETACEOUS BATHOLITH EXHUMATION PATTERNS ALONG THE WESTERN NORTH AMERICAN MARGIN

The western North American margin exhibits one of the largest medial Cretaceous magmatic arcs in the world and includes (S to N) the Peninsular Ranges batholith (PRB), the Sierra Nevada batholith (SNB), and the Coast Plutonic Complex (CPC). Comparative analysis of detrital zircon populations from Late Cretaceous forearc basins flanking these arc segments provides a first-order assessment of extent and style of exhumation along the margin.

Detrital zircon U-Pb age results from Turonian to Maastrichtian strata from both ends of the 800 km long PRB demonstrate that its forearc is dominated by 120-90 Ma grains that closely match the crystallization ages of adjacent plutons. These results are sensible in terms of the rapid and deep exhumation of the batholith that closely followed emplacement of the voluminous ~100-90 Ma La Posta intrusive suite.

In contrast, Cenomanian to Campanian forearc strata of the northern Great Valley Group astride the 600 km SNB segment yield mostly Early Cretaceous and Jurassic zircons with peaks at ~150 and 120 Ma. The lack of younger grains is surprising, as the most voluminous plutons in the SNB are 110-85 Ma. Both the relative paucity of 110-85 Ma detrital zircons and the preservation of coeval subaerial volcanic strata atop the SNB are consistent with comparatively limited Late Cretaceous exhumation of this batholithic segment, except at its southernmost end.

Basins formed astride the southern CPC reflect a more complex evolution from a forearc to a two-sided intra-arc basin due to Late Cretaceous accretion of allochthonous terranes. Like the SNB, detrital zircons from the older sediments (Methow basin - Albian to Santonian) are dominantly JuraCretaceous (peaks at ~160 Ma and ~110 Ma). We believe this distribution reflects initial unroofing of the 100-120 Ma Okanogan arc system. Younger sediments (Nanaimo basin - Turonian to Maastrichtian) display a strong peak at 90-75 Ma and significant numbers of Proterozoic-Archean grains. Although the Cretaceous zircon population is distinctly younger than that of the PRB forearc, the results from the Nanaimo basin are similar in that they reflect rapid and extensive Late Cretaceous unroofing of the CPC and flanking supracrustal rocks. Proterozoic-Archean zircons reflect detritus derived from the Proterozoic Belt Supergroup further east.