DGGS) has conducted topical stratigraphic, structural, and geochronologic studies along the western margin of lower Cook Inlet basin since 2009 to improve knowledge of its Mesozoic geologic framework and help spur industry interest in this under-explored region of the petroleum basin. The publicly-available geological and geochronologic data augment stateand federally-funded geologic mapping along the basin margin performed since 2013. This report presents zircon U-Pb LA-ICP-MS and 40 Ar/ 39 Ar step-heating geochronology results for one mafic and two felsic dikes in the Iliamna B-3 and C-2 quadrangles at Ursus Head and near Contact Point, lower Cook Inlet. The dikes intrude the Bruin Bay fault zone, a regional-scale system of faults that extend approximately 450 km from the upper Alaska Peninsula northeastward to upper Cook Inlet (e.g. Magoon and others, 1976), and their ages help to constrain the timing of deformation within the fault zone. The fault system is an important element of the Cook Inlet basin, as it forms the structural boundary between exhumed Mesozoic and Cenozoic magmatic arc from contemporaneous forearc basin strata along most of its length, and it likely influenced basin subsidence, sedimentation, and the growth and distribution of structural hydrocarbon traps. A mafic sill and felsic dike cut by subsidiary faults at Ursus Head returned zircon U-Pb ages of 206.9 ±0.96 (2 sigma), and 212.5 ±2.28 (2 sigma), respectively, constraining deformation associated with the fault system to younger than late Triassic. A felsic dike intruding the principal Bruin Bay fault plane near Contact Point produced biotite and plagioclase 40 Ar/ 39 Ar ages of 36.5 ±0.8 Ma and 31.5 ±0.7 Ma, respectively. The dike preserves well-developed, undeformed cooling columns oriented perpendicular to its margins, indicating that most slip along the main plane was complete by late Eocene to early Oligocene time. However, the columnar dike and the dominant fault plane are cut by one or more higher-angle faults, suggesting a change in how the fault system accommodated stress after early Oligocene time. Recent investigation of the Bruin Bay fault system by DGGS, including detailed kinematic analysis of numerous small faults within, and proximal to the structural boundary of the basin, has revealed a polyphase deformation history that occurred under separate states of stress (Betka and Gillis, 2014;2015;2016). A synthesis of these results and tectonic interpretation, including the new age constraints contained in this report, are published concurrently in a separate paper (Betka and others, in press).
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