Vein formation during progressive Paleogene faulting and folding within the lower Cook Inlet basin, Alaska

Regionally persistent vein sets cut Early Jurassic through late Paleogene(?) strata throughout a study area >2000 km(2) in the lower Cook Inlet forearc basin of Alaska. Using field, aerial, and GIS-based studies, we document vein orientations, group them into four dominant sets, and present relative timing observations to demonstrate their development during regional faulting and folding of Cook Inlet basin strata. All veins were restored about regional folds by first removing the bedding dip, and then rotating the bedding strike into parallelism with the regional structural trend (038 degrees). The most dominant vein set strikes similar to 310 degrees, orthogonal to the regional structural trend, and is present in all strata in the field area. The other sets strike 210 degrees, 360 degrees, and 260 degrees. Fold-test results show that variations in the vein set orientations throughout the study area are correlated with the changes in bedding attitudes that define regional folds, indicating that the veins formed progressively with the folds. We document abutting and crosscutting relations between sets, and present a new ca. 52 Ma Ar-40/Ar-39 age of a dike that parallels the dominant set (310 degrees) and is crosscut by others. Based on field relations, relative timing constraints, and the fold-test results, we suggest a sequence of vein development and its relationship to fold growth within the Bruin Bay fault system during Paleogene deformation along the southern margin of Alaska. Our results may serve as a case study for linking vein development to tectonic events in other ancient and modern forearc basins.