ABSTRACT Paleocene Lower Wilcox Group sedimentation rates are three times the Cenozoic average for the Gulf of Mexico region and are attributed to Laramide tectonism within the Laramide–Rocky Mountains region. These increased rates likely represent the erosion of easily weathered Phanerozoic strata that blanketed the Laramide-age basement-cored uplifts. Geologic observations and U-Pb geochronology are not sufficient to fully address this hypothesis alone, so we conducted 439 Lu-Hf isotopic analyses on detrital zircons from eight samples from the San Juan Basin and five samples from the Gulf of Mexico Basin. Focusing on the zircons younger than 300 Ma allowed us to make direct comparisons to the eight principal components that comprise the North American Cordilleran magmatic arc: (1) Coast Mountains batholith; (2) North Cascades Range; (3) Idaho batholith; (4) Sierra Nevada batholith; (5) Laramide porphyry copper province; (6) Transverse Ranges; (7) Peninsular Ranges; and (8) Sierra Madre Occidental. The εHf ( t ) results range from +8.9 to –27.0 for the San Juan Basin samples and from +13.0 to –26.6 for the Gulf of Mexico samples. Using the San Juan Basin samples as a proxy for the eroded Mesozoic cover that was shed from the Laramide uplifts, we show that much of the sediment entering the Gulf of Mexico through the Houston and Mississippi embayments during the late Paleocene was derived from reworked cover from the greater Laramide–Rocky Mountains region. However, the Gulf of Mexico samples also include a distinct juvenile suite (εHf [ t ] ranging from +13 to +5) of zircons ranging in age from ca. 220 to 55 Ma that we traced to the Coast Mountains batholith in British Columbia, Canada. This transcontinental connection indicates an extension to the headwaters of the previously defined paleo-Mississippi drainage basin from ca. 58 to 56 Ma. Therefore, we propose a through-going fluvial system (referred to here as the “Coast Mountains River”) that was routed from the Coast Mountains batholith to the Gulf of Mexico. This expands the previously defined paleo-Mississippi drainage basin area by an estimated 280,000 km 2 . Our comprehensive Hf isotopic compilation of the North American Cordilleran magmatic arc also provides a benchmark εHf ( t ) versus U-Pb age plot, which can be used to determine provenance of detrital zircons (85–50 Ma) at the scale of specific region(s) within the Cordillera based on their εHf ( t ) values.

ABSTRACT This paper reviews the Mesozoic terranes in the central Cascades, south of the Windy Pass thrust and east of the Straight Creek–Fraser River fault, and provides a guide to field locations for these units. These include the Easton Metamorphic Suite, Hicks Butte complex and higher-grade tectonic zone, the Peshastin Formation, and the Ingalls ophiolite complex (also known as the Ingalls terrane). Age data, whole rock and mineral chemistry, and structural data are reviewed. These oceanic- and arcaffinity terranes formed outboard of the North American craton during the Jurassic and accretion likely occurred during the Late Jurassic or Early Cretaceous. They were then dextrally translated north and emplaced in Washington State during the Late Cretaceous. A better understanding of these Mesozoic terranes will more closely constrain the tectonic development of the North American Cordillera.