SEGMENTATION OF THE CONTINENTAL ARC: LATE CRETACEOUS OROGENIC EXHUMATION PATTERNS IN THE NORTH AMERICAN CORDILLERA

The magnitude, style and rate of arc exhumation along the western N. American margin can be constrained by detrital zircon analysis of Upper Cretaceous forearc strata. Segmentation of the arc is evidenced by variable detrital patterns from forearc strata adjacent to (S to N) the Peninsular Ranges (PRB), the Sierra Nevada (SNB), and the Coast Plutonic Complex (CPC) batholiths. These patterns reflect contrasting styles of arc exhumation, probably resulting from slab segmentation in the Late Cretaceous, providing an ancient analog to the modern S. American margin.

Detrital zircon from Turonian to Maastrichtian strata along the 800km PRB are dominated by a unimodal population of 125-90 Ma grains that mimic crystallization ages of adjacent plutons. Progressive domination of detritus from 100-90 Ma granitoids from the eastern PRB illustrates that emplacement and uplift of the massive La Posta TTG suite was the mechanism driving basin subsidence.

North of the PRB, structurally disrupted Upper Cretaceous strata contain a heterogeneous population reflecting mixing of JuraCretaceous detritus from western SNB with both latest Cretaceous grains from the eastern SNG and older grains from the Mojave region. This signature results from rapid exhumation of the southern eastern SNB coupled with dissection of the arc by drainages sourced in the craton to the east.

Cenomanian to Campanian strata of the northern Great Valley Group yield a population of Early Cretaceous and Jurassic zircons with peaks at ~150 and 120 Ma. The paucity of 110-85 Ma detrital zircons and preservation of coeval subaerial volcanic strata are consistent with comparatively limited Late Cretaceous exhumation of the northern SNB.

Basin evolution adjacent to the southern CPC reflects a two-stage denudation history. Older Cretaceous sediments (Methow basin - Albian to Santonian) east of the CPC are dominated by JuraCretaceous grains (peaks at ~160 Ma and ~110 Ma) due to erosional denudation of an eastern arc. The Methow basin inverted by contractional tectonism in Late Cretaceous time; younger rocks (Nanaimo basin - Turonian to Maastrichtian) west of the CPC display a bimodal population of 90-75 Ma and Proterozoic-Archean grains, reflecting rapid exhumation of CPC and flanking supracrustal rocks, coupled with older detritus derived from Proterozoic strata in the hinterland fold-thrust belt.