PRECISION DIGITAL MAPPING OF GRANITIC DIKES WITH IMPLICATIONS FOR INTRUSIVE PROCESSES IN TRANSPRESSIONAL SHEAR ZONES: AN EXAMPLE FROM RAGGED ISLAND, CASCO BAY, MAINE

Ragged Is., SE of the Norumbega Fault Zone in Casco Bay, ME, exposes Ordovician felsic to intermediate, metavolcanic granofels and porphyroblastic gneisses. The Island is located within the Hen Cove Anticline, a gently south-plunging F2 upright fold structure oblique to the transpressional Norumbega zone. Coarse to pegmatitic granite dikes compose 15% of bedrock exposure. Granite dike contacts were mapped using survey-grade RTK GPS, Total Station, and on-screen digitizing in the field with HP iPaq PDAs equipped with ArcPad, LiDAR-based DEMs and high-resolution orthographic imagery. Structural orientations of gneissic layers, lineation, granite contacts, and quartz vein contacts were positioned using handheld GPS with m-scale precision. Gneissic layering is shallowly dipping and traces the nose of the SW plunging Hen Cove Anticline, rotating from strikes of N40W in the north to N75W in the south, with a consistent stretching lineation at a mean of 17⁰ S11W. Granite intrusions as dikes up to 10’s of meters thick occur in a dominant set striking NW-SE generally parallel to the gneiss layers, and two cross dike sets striking NE-SW and E-W. The NW-SE layer-parallel set dips 30-55 SW whereas the NE-SW and E-W cross dikes cut gneissic layers and have contacts dipping 65-85⁰ SE. The NW-SE striking dikes are structurally controlled by gneissic layering at the north end of the island but not the south, where they cut across the nose of the plunging Hen Cove Anticline. Therefore, these dikes post-date folding. However, the regional stretching direction is also evident within the granites as quartz veins that average N64W 30 NE perpendicular to the gneissic lineation, indicating that the granites are syntectonic. A three stage intrusion model by Crawford et al. (2008) has been reinterpreted as a multi-phase “flooding” model with steeply dipping, E-W and NE-SW cross dikes feeding the layer-parallel, shallow dipping dikes in a feeder-dike-to-sill type relationship. A similar geometry has been mapped on-strike at Yarmouth Is. However, cross dikes at Yarmouth Is. are orthogonal-to-lineation whereas those of Ragged Is. are oblique. Instead, quartz veins on Ragged Is. maintain this orthogonal-to-lineation geometry. This variation may be due to Ragged Is.’s position at the nose of the plunging Hen Cove Anticline.