Eocene dikes that widely intrude the crystalline core of the North Cascades, Washington can be used to understand the regional strain field during a period of postulated ridge subduction and slab window migration at ~50-45 Ma. For this study, the orientations and thicknesses of 244 Eocene dikes were measured in four domains in the east-central part of the Cascades core. These domains include the northern tip of the Cretaceous Seven-Fingered Jack pluton (SFJ; n=21), along the southern margin of the shallow (~2-4 km depth) part of the Eocene Duncan Hill pluton (DH; n=71), and two domains (CC; n=82 and CR; n=70) at the southern part of the Cretaceous Swakane Gneiss and Entiat pluton. Dikes range from felsic to mafic, and three types of dikes make up two-thirds of those in the study area: a quartz-phyric aplite (25%), a plagioclase-phyric andesite (23%), and a microdiorite (19%). Dike thickness ranges from ~30 cm to over 40 m. Detailed measurements across well-exposed 300 m to 2.7 km-long transects indicate that dike intrusion accommodated 3 to 15% extension across the study area and locally up to 45%. Orientations vary significantly across the study area: dikes strike 025° to 058° in the SFJ and DH domains in the north and 278° to 315° in CC and CR domains ~20 km to the south. Extension directions inferred from these strikes are parallel to oblique (328-148° to 295-115°) to the NW trend (~320°) of the Cascades core in the SFJ and DH domains, but are roughly perpendicular (008-188° to 045-225°) to the trend of the core in the CC and CR domains. The extension directions in the SFJ and DH domains are broadly similar to those of the voluminous, roughly coeval (~47 Ma), mafic Teanaway swarm to the southwest, those in part of the Skagit Gneiss Complex to the north, and to those E and SE of the Duncan Hill pluton (Bryant, 2014). The significant difference in extension direction in the CC and CR domains compared to the northern and regional directions implies either a very rapid shift in the strain field or a spatially complex strain field during the intrusion of similarly aged swarms. These changes may be related to the geometry of the inferred slab window.