STRUCTURE OF EOCENE DIKES SOUTH OF THE DUNCAN HILL PLUTON, NORTH CASCADES, WASHINGTON—IMPLICATIONS FOR REGIONAL STRAIN DURING POSTULATED RIDGE SUBDUCTION

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.